Fabiana Csukasi

Catharina Merchante

Professor Victoriano Valpuesta
email: valpuesta@uma.es

Modification of plant hormone levels and signaling as a tool in plant biotechnology

Keywords: plant hormones, agricultural traits, transgenic, mutants, signaling plant hormones, agricultural traits, transgenic, mutants, signaling

Plant hormones are signal molecules, present in trace quantities that act as major regulators of plant growth and development. They are involved in a wide range of processes such as elongation, flowering, root formation and vascular differentiation.

For many years, agriculturists have applied hormones to their crops to either increase the yield, or improve the quality of the commercial product. Nowadays, the knowledge of hormone biosynthesis, degradation and signaling pathways has allowed the utilization of biotechnological tools to improve even more the main agricultural crops.

Natural or artificial mutants, with impaired functioning of the corresponding genes, have been adopted by their superior phenotype in specific agricultural traits. Also, transgenic plants have been generated to regulate internal hormone levels, or their signaling pathways, resulting in some crops that have revolutionized the agriculture.

Introduction

Plant hormones are signal molecules, present in trace quantities that act as major regulators of plant growth and development. Changes in concentration and cell/tissue sensitivity have dramatic effects in the physiology of the plant. This is a consequence of the fact that they participate in many different developmental processes that range from general growth, to flowering, root formation and vascular differentiation [START_REF] Galston | Hormonal regulation in higher plants[END_REF]. External application of these hormones has been for many years the procedure used in many agricultural crops to either increase the yield, or to improve the quality of the commercial product [START_REF] Sakamoto | Phytohormones and rice crop yield: strategies and opportunities for genetic improvement[END_REF]. Nowadays, biochemical studies and genetic dissections have supported new insights into the molecular mechanisms of hormone biosynthesis and signal transduction pathways [START_REF] Mcsteen | Plant hormones and signaling: common themes and new developments[END_REF][START_REF] Spartz | Plant hormone receptors: new perceptions[END_REF]. This new information has allowed affording with new tools the improvement of main agricultural crops.

Known plant hormones are auxin, gibberellins, cytokinins, abscisic acid, ethylene, brassinosteroids, and more recently strigolactones [START_REF] Gómez-Roldán | Strigolactone inhibition of shoot branching[END_REF][START_REF] Umehara | Inhibition of shoot branching by new terpenoid plant hormones[END_REF]. Not all of them have received the same attention as putative targets for plant biotechnology. Effectively, structural diversity of hormones is high, and so is the complexity of the metabolic pathways for their synthesis and catabolism, and not all of them are completely known [START_REF] Mcsteen | Plant hormones and signaling: common themes and new developments[END_REF]. The biggest aim of plant hormones Biotechnology has been to modulate their internal concentrations in a tissue/development specific manner. This is achieved after modification of either synthesis or degradation pathways, or both, since steady state level results from the net balance of these two processes. The second limitation has been determined by the same complexity of the signaling pathways for the different hormones. Whereas metabolic routes for hormone syntheses are diverse, their signaling pathways might overlap/interact [START_REF] Kepinski | Integrating hormone signaling and patterning mechanisms in plant development[END_REF]. This means that simple changes have frequently ). TN-1 and IR-8 have been used also to produce some indica and japonica cultivars. The semi-dwarf phenotype is the result of different alleles of a single recessive gene, sd1 [START_REF] Hedden | The genes of the green revolution[END_REF]. The sd1 mutation is recessive and normal height could be restored by GA application, indicating that they were defective in GA production. In 2002, the sd1 gene was identified as encoding a GA20ox. Dee-geo-woo-gen rice, possessing the sd1 allele, presents a 383-bp deletion in the GA20ox gene (OsGA20ox2) that introduces a stop codon, producing a truncated enzyme [START_REF] Sasaki | A mutant gibberellinsynthesis gene in rice[END_REF]. As GA20ox are members of a gene family and present functional redundancy, loss-of-function GA20ox mutants present only a semi-dwarf phenotype compared with severely GA-deficient plants that are completely dwarfed and sometimes sterile [START_REF] Hedden | The genes of the green revolution[END_REF].

The rice Tan-Ginbozu (d35 Tan-Ginbozu ) is another example of increased yield by semi-dwarf traits. The d35 Tan-Ginbozu was first believed to be blocked in the synthesis of ent-kaurene or an earlier reaction. Years later, it was confirmed that the mutation in d35 Tan-Ginbozu inhibited the activity of ent-kaurene oxidase (KO), that catalyzes the three oxidative steps from ent-kaurene to ent-kaurenioc acid [START_REF] Itoh | A rice semi-dwarf gene, Tan-Ginbozu (D35), encodes the gibberellin biosynthesis enzyme, ent-kaurene oxidase[END_REF].

The study of GA biosynthesis has extended to fruit plants. Some fruit trees can grow very tall if they do it on their own roots, so many of them are grown as scions grafted on to a dwarfing rootstock. These trees are smaller and easier to handle, they also present earlier flowering time and reduced vegetative growth. However, for many crops, rootstocks are not enough and chemical growth retardants are used to prevent excessive vegetative growth. In apple, inhibition of MpGA20ox1 by sense or antisense suppression resulted in dwarf plants with much lower levels of GA 1 . The dwarf phenotype was maintained when the transgenic scions were grafted on to invigorating rootstocks. This technology could be very useful to improve the utilization of rootstocks to prevent plants to grow vigorous and to reduce the need for chemical growth retardant application [START_REF] Bulley | Modification of gibberellin biosynthesis in the grafted apple scion allows control of tree height independent of the rootstock[END_REF].

In the citrus hybrid, Carrizo citrange, used as a rootstock, plants overexpressing sense and antisense CcGA20ox gene produced shorter and taller phenotypes respectively [START_REF] Fagoaga | Engineering of gibberellin levels in citrus by sense and antisense overexpression of a GA 20-oxidase gene modifies plant architecture[END_REF]. Plants also showed thorns, organs of citrus in juvenile stages, with abnormal characteristics, they were much longer in sense and shorter in antisense transgenic plants. Internodes cell length was not altered in plants, indicating that in citrus GAs promote cell division rather than cell elongation. When transgenic plants were grafted on wild type rootstocks, the phenotype disappeared, contrary to what was described in apple. This suggests that GA produced in the roots can compensate the deficit of GA in the antisense transgenic aerial part and opens the possibility of using these plants as dwarfing rootstocks [START_REF] Fagoaga | Engineering of gibberellin levels in citrus by sense and antisense overexpression of a GA 20-oxidase gene modifies plant architecture[END_REF].

Although the biosynthesis of GA is very well understood since many years ago, the way that plants perceive the hormone and then transmit the signal to the whole plant is still not completely clear. The GA receptor, GID1, was discovered in rice in 2005 [START_REF] Ueguchi-Tanaka | Gibberellin insensitive dwarf1 encodes a soluble receptor for gibberellin[END_REF]. OsGID1 was characterized as a soluble receptor and loss-of-function mutations in the gene were shown to produce plants with a severe dwarf phenotype and loss of GA responsiveness. It was also demonstrated that the OsGID1 protein was able to bind bioactive GAs but not inactive GAs and that OsGID1 interacted with the rice DELLA protein, SLR1. [START_REF] Ueguchi-Tanaka | Molecular interactions of a soluble gibberellin receptor, GID1, with a rice DELLA protein, SLR1, and gibberellin[END_REF][START_REF] Nakajima | Identification and characterization of Arabidopsis gibberellin receptors[END_REF].

A few months after the discovery of rice GA receptor, Arabidopsis GID1 orthologs were identified. There are three Arabidopsis GID1 genes, AtGID1a, b and c, and all of them showed GA binding affinity to biologically active GAs. Arabidopsis GID1 receptors show partial redundancy, triple mutant plants show a very severe GAinsensitive phenotype, demonstrating a more important role for GA signaling in Arabidopsis growth and development than previously described. As in rice, Arabidopsis GID1s interact with DELLA proteins in a GA-dependent manner [START_REF] Nakajima | Identification and characterization of Arabidopsis gibberellin receptors[END_REF][START_REF] Griffiths | Genetic characterization and functional analysis of the GID1 receptors in Arabidopsis[END_REF].

DELLA proteins are repressors of GA signaling and modulate all aspects of GA-induced growth and development in plants [START_REF] Zentella | Global analysis of DELLA direct targets in early gibberellin signaling in Arabidopsis[END_REF]. DELLAs belong to the GRAS family of transcriptional regulators, presenting a C-terminal GRAS domain. A unique N-terminal region is what gives name to this family of proteins and is composed of five amino acids, DELLA, a short spacer region and other six amino acids, TVHYNP [START_REF] Ueguchi-Tanaka | Molecular interactions of a soluble gibberellin receptor, GID1, with a rice DELLA protein, SLR1, and gibberellin[END_REF][START_REF] Zentella | Global analysis of DELLA direct targets in early gibberellin signaling in Arabidopsis[END_REF][START_REF] Achard | DELLAs contribute to plant photomorphogenesis[END_REF]. This region is known to be essential to normal GA response [START_REF] Swain | Tall tales from sly dwarves: novel functions of gibberellins in plant development[END_REF]. DELLA proteins are rapidly degraded in response to GA treatment, and deletion or amino acid substitution in this conserved motif confers GA-insensitive phenotypes [START_REF] Zentella | Global analysis of DELLA direct targets in early gibberellin signaling in Arabidopsis[END_REF].

As with biosynthesis mutants, characterization of mutants that are impaired in GA signaling has been crucial to understand the role of GAs in plant development, as well as their targeting for biotechnological achievements. In general, all GA-deficient mutants show a similar phenotype, they are dark-green dwarf plants, defective in leaf expansion, stem elongation and seed germination. In Arabidopsis, a rga null mutation (rga-24) partially suppressed the phenotype of the GA biosynthetic mutant ga1-3. The gai mutant was identified because it did not respond to GA application. In contrast to rga-24, gai-t6 null mutation did not suppress the ga1-3 phenotype, but in combination with rga-24 completely suppressed the plant defects [START_REF] Silverstone | Gibberellins and the green revolution[END_REF][START_REF] Thomas | Update in gibberellin signaling. A tale of the tall and the short[END_REF]. The triple mutant (ga1-3/ rga-24/ gai-t6) had a slender phenotype but mutant seeds were still non-germinating and mutant plants still showed GA deficient floral defects. This suggests that there must be other genes controlling developmental processes, and these genes are the RGL. A triple null mutant (rga/rgl1/rgl2) rescued the floral developmental defect and fertility of ga1-3 [START_REF] Cheng | Gibberellin regulates Arabidopsis floral development via suppression of DELLA protein function[END_REF]. The gai-1, rga∆-17 and rgl1 mutants all lack 17 amino acids of the DELLA domain, and have a GA-insensitive dwarf phenotype. The deletion in the protein makes 8 them constitutive repressors of the GA response; the proteins are not degraded in response to GA as in the wild type [START_REF] Silverstone | Gibberellins and the green revolution[END_REF][START_REF] Thomas | Update in gibberellin signaling. A tale of the tall and the short[END_REF].

Several mutants with GA-insensitive phenotypes have been identified in other species, like Rht1, Rht2 and Rht3 in wheat and d8 and d9 in maize. Many dwarf wheat lines, as Norin 10, were introduced in the USA from Japan. Norin 10 contained the Reduced height genes (Rht), responsible of the high-yielding semi dwarf wheat [START_REF] Silverstone | Gibberellins and the green revolution[END_REF][START_REF] Hedden | The genes of the green revolution[END_REF][START_REF] Peng | Green revolution genes encode mutant gibberellin response modulators[END_REF]. Maize mutant alleles d8 also showed these characteristics. In 1999, Peng et. al [START_REF] Peng | Green revolution genes encode mutant gibberellin response modulators[END_REF] demonstrated that Arabidopsis GAI, wheat Rht-1 and maize d8 were functional orthologs. They also demonstrated that the mutations of the N-terminal domain of these genes were responsible for the less sensitivity of the proteins to GA [START_REF] Peng | Green revolution genes encode mutant gibberellin response modulators[END_REF]. Although all of the DELLA genes were first identified in Arabidopsis and cereals, the study of GA signaling genes in fruit plants is becoming a priority.

Parthenocarpy, fruit development without seeds, is a much appreciated characteristic because it ensures high yields in difficult environments with low temperatures or absence of cross pollinators. It has been for many years a main goal in breeding programs of fruit crops. Hormones, particularly auxins and gibberellins, are known to promote fruit development without fertilization. In tomato there is one gene encoding for a DELLA protein, named SlDELLA. Silencing of this gene by antisense technique resulted in slender plants with elongated flower trusses. The fruits were smaller and elongated compared with the wild type. The silencing of this gene also produced pathenocarpic fruits, but this phenomenon was facultative because hand pollination restored wild type fruit phenotype [START_REF] Martí | Silencing of DELLA induces facultative parthenocarpy in tomato fruits[END_REF].

In grapevine, a dwarf mutant derived from de L1 layer of the apical meristem, produced inflorescences in the shoot instead of the tendrils that are normally formed.

The mutated gene was identified as an ortholog of wheat Rht1 and Arabidopsis GAI. The formation of inflorescences in the mutant demonstrates that in grapevine, GAs inhibit floral development [START_REF] Boss | Association of dwarfism and floral induction with a grape "green revolution" mutation[END_REF] instead of promoting it as in Arabidopsis [START_REF] Blázquez | Integration of floral inductive signals in Arabidopsis[END_REF]. The knowledge of the regulation of the flowering process can be useful to try to control fruit development and therefore to improve the production.

The introduction of the Arabidopsis gai mutant gene in apple, resulted in a clear reduction in stem length, internodes length and node number [START_REF] Zhu | Overexpression of the Arabidopsis gai gene in apple significantly reduces plant size[END_REF]. This is important since in modern apple production, dwarf trees are commonly used for achieving high production efficiency. However, transgenic plants showed reduced rooting ability. Production of new cultivars or rootstocks with dwarf characteristics has been an important objective for apple breeding.

Auxin

Since its discovery 70 years ago, the compound indole-3-acetic acid (IAA or auxin) has been the most studied plant hormone. It is known to be involved in almost every aspect of plant growth and development. Depending on the tissue, auxin can promote cell elongation or cell division. The exact mechanisms and sites of auxin biosynthesis remain unclear, but there are two accepted biosynthesis pathways, one dependent and one independent of tryptophan [START_REF] Benjamins | Auxin: the looping star in plant development[END_REF].

The first problem to study auxin biosynthesis was the inability to find auxin deficient mutants, since the mutants that presented high levels of free auxin were sterile or had very heterogeneous phenotypes. In addition, it was not possible to determine if the elevated levels of auxin were caused by an effect in auxin biosynthesis or conjugation. In 2001, Zhao, Y. et. al., [START_REF] Zhao | A role for flavin monooxygenase-like enzymes in auxin biosynthesis[END_REF] identified the Arabidopsis yucca mutants, they presented a phenotype characteristic of elevated auxin levels, as long hypocotyls, short roots with long root hairs. They demonstrated that YUCCA is a member of the tryptophan-dependent auxin biosynthesis. They also showed that the YUCCA family of proteins is composed in Arabidopsis plants by ten members [START_REF] Zhao | A role for flavin monooxygenase-like enzymes in auxin biosynthesis[END_REF]. There is another auxin biosynthesis mutant in Arabidopsis, the superroot (sur1) that presents high levels of both free and conjugated auxin, resulting in auxin related phenotypes. The SUR1 gene encodes a protein with homology to the tyrosine aminotransferases and may be involved in auxin Trp-dependent and independent biosynthesis pathways [START_REF] Benjamins | Auxin: the looping star in plant development[END_REF].

In rice, seven YUCCA-like genes have been identified. In plants overexpressing OsYUCCA1 IAA levels were increased while plants with antisense expression of the gene presented dwarfed shoots and short roots with long root hairs. The results provided evidence for a role of OsYUCCA in auxin biosynthesis as in Arabidopsis [START_REF] Yamamoto | Auxin biosynthesis by the YUCCA genes in rice[END_REF]. As with GA genes, the modification OsYUCCA genes in rice can generate the desired dwarf phenotype.

Auxin is known to play an important role in fruit development and ripening both in climacteric and non-climacteric fruits, presenting a peak after ovary fertilization [START_REF] Gillaspy | Fruits: A developmental perspective[END_REF].

In strawberry and raspberry, the introduction of DhfH9-iaaM gene fusion, composed of the regulatory region of DhfH9 gene from snapdragon and the iaaM coding region from Pseudomonas syringae pv savastanoi, that is known to promote auxin synthesis, confer parthenocarpic fruit development to eggplant and tomato and increase fruit production and quality, resulted in an increased number of fruits per plant and also a higher weight and bigger size of the transgenic fruits. The increase in fruit yield reached 180% in Fragaria x ananassa, the cultivated strawberry, and 140% in Fragaria vesca, the wild strawberry, while the increase was of 100% in raspberry. The use of DhfH9-iaaM gene must be considered to improve productivity in these two Rosaceae species [START_REF] Mezzetti | The defH9-iaaM auxin-synthesizing gene increases plant fecundity and fruit production in strawberry and raspberry[END_REF]. Thompson Seedless the number was doubled compared to the wild type plants [START_REF] Costantini | Auxin synthesis-encoding transgene enhances grape fecundity[END_REF].

As what happens with auxin biosynthesis, many aspects of auxin signaling are poorly understood. The auxin receptor TIR1 has recently been identified [START_REF] Dharmasiri | The F-box protein TIR1 is an auxin receptor[END_REF][START_REF] Kepinski | The Arabidopsis F-box protein TIR1 is an auxin receptor[END_REF]. In contrast, auxin regulated transcription is better understood since the past decade. Two big families of transcriptional regulators are known to be involved in this process, the Aux/IAA and ARF (auxin response factors) families. Auxin, after its binding to TIR1, promotes the interaction between Aux/IAA proteins and the ubiquitin protein ligase SCF TIR1 , stimulating the degradation of Aux/IAA proteins. Actually, the auxin receptor TIR1 is part of the SCF TIR1 ubiquitin ligase complex and binds auxin directly. The degradation of Aux/IAA proteins allows ARFs to bind to auxin regulated genes, activating or repressing their transcription [START_REF] Benjamins | Auxin: the looping star in plant development[END_REF].

In tomato, the diageotropica (dgt) mutant was identified as being defective in some auxin-mediated responses. It was demonstrated that the mutant presented an inhibition of the auxin induction of the only tomato Aux/IAA gene identified at that moment, LeIAA [START_REF] Mito | The diageotropica mutation and synthetic auxins differentially affect the expression of auxin-regulated genes in tomato[END_REF]. Years later, ten other Aux/IAA genes were identified in tomato, some of them also showed a reduced auxin stimulation of gene expression [START_REF] Nebenführ | The diageotropica mutation alters auxin induction of a subset of the Aux/IAA gene family in tomato[END_REF]. The inhibition of SlIAA9 produced plants with simple leaves instead of the compound leaves of the wild type, and most importantly it induced fruit parthenocarpy. Despite their parthenocarpic character, fruits presented normal size, skin color and flesh consistency and they showed no change in ripening process. SlIAA9 antisense lines were auxin hypersensitive, presenting hypocotyls elongation, reduced apical dominance and [START_REF] Wang | The tomato Aux/IAA transcription factor IAA9 is involved in fruit development and leaf morphology[END_REF]. Later, it was demonstrated that a single-base deletion mutation in SlIAA9 gene caused the tomato mutant phenotype [START_REF] Zhang | A single-base deletion mutation in SlIAA9 gene causes tomato (Solanum lycospersicum) entire mutant[END_REF].

In potato, auxins are known to play an important role in tuber development.

Endogenous auxin levels are high prior and during stolon swelling and after that they start to decrease gradually. The first potato Aux/IAA gene identified, named StIAA, was shown to be induced in potato tubers after fungal infection, but it was not clear if the infection by itself caused the accumulation of Aux/IAA mRNAs or if it was the result of other signals released by the fungus or the plant [START_REF] Zanetti | Molecular characterization of a potato cDNA encoding a stress regulated Aux/IAA protein[END_REF]. Another Aux/IAA gene was identified in potato, StIAA2. Transgenic plants overexpressing StIAA2 did not show any visible phenotype; in contrast, transgenic plants inhibiting the gene expression presented several dramatic defects. Plants were much taller, they contained two to three more internodes, presented petiole hyponasty and extreme curvature of growing leaf primordia in the shoot apex. These phenotypes are both common and unique to the ones found in other plants, like Arabidopsis and tomato, and evidenced the importance of studying the function of Aux/IAA genes individually, and in different plant species [START_REF] Kloosterman | Isolation and characterization of a novel potato Auxin/Indole-3-Acetic Acid family member (StIAA2) that is involved in petiole hyponasty and shoot morphogenesis[END_REF].

In addition, overlapping functions among different Aux/IAA family members must be considered when affording a biotechnological benefit of modifying a single gene.

ARF genes are expressed in a wide range of tissues and organs. Screens in Arabidopsis T-DNA insertion mutations have identified many ARF mutants, but most of them did not show a distinctive phenotype. That is not the case with ARF double mutants that usually present a clear phenotype, as delayed flower maturation, reduced fertility, reduced lateral root number, root cap defects and abnormal root gravitropism [START_REF] Guilfoyle | Auxin response factors[END_REF]. A mutation in the coding region of AtARF8 resulted in the production of parthenocarpic fruits in Arabidopsis [START_REF] Goetz | Auxin response factor 8 is a negative regulator of fruit initiation in Arabidopsis[END_REF]. This was also found to be the case in tomato, in which the expression of Atarf8-4, an aberrant form of the gene, in transgenic tomato (Monalbo) resulted in a significant increase in the number and also in the size of parthenocarpic fruits [START_REF] Goetz | Expression of aberrant forms of auxin response factor 8 stimulates parthenocarpy in Arabidopsis and tomato[END_REF]. Tomato transgenic plants inhibiting the expression of DR12 gene, a member of the ARF family, showed a pleiotropic phenotype, including darkgreen immature fruit, blotchy ripening, enhanced fruit firmness and increased hypocotyl growth [START_REF] Jones | Down-regulation of DR12, an auxinresponse-factor homolog, in the tomato results in a pleiotropic phenotype including dark green and blotchy ripening fruit[END_REF]. Some of these changes might be relevant for the improvement of this fruit, but it will be needed to dissect the functions of other genes involved in order to avoid the pleiotropic effects.

Ethylene

Ethylene is a gaseous hormone that regulates many aspects of plant growth and development, including seed germination, root and stem elongation, root hair development, root nodulation, flower senescence, abscission, sex determination, fruit ripening, programmed cell death, responses to biotic and abiotic stresses, gravitropism, and interaction between roots and rhyzosphere [START_REF] Stepanova | Ethylene signaling: from mutants to molecules[END_REF][START_REF] Delseny | Ethylene biology: a foreword[END_REF]. affinities for all Arabidopsis and tomato receptor proteins [START_REF] Giovannoni | Fruit ripening mutants yield insights into ripening control[END_REF]. In Arabidopsis, ethylene receptors play a negative role in the ethylene pathway [START_REF] Guo | The ethylene signaling pathway: new insights[END_REF]. It has been found that the REVERSION-TO-ETHYLENE-SENSITIVITY1 (RTE1) protein genetically interacts with ethylene receptors for the regulation of the ethylene response [START_REF] Resnick | REVERSION-TO-ETHYLENE SENSITIVITY1, a conserved gene that regulates ethylene receptor function in Arabidopsis[END_REF]. In the absence of ethylene, the receptors are continuously stimulating CONSTITUTIVE TRIPLE RESPONSE1, CTR1, which is a Ser/Thr kinase, and in this way, inhibits the rest of the downstream components of the pathway. When ethylene binds to the receptors, the MAPK cascade is released and results in the activation of EIN2, a positive key regulator in the ethylene transduction pathway, which transmits the signal to the EIN3/EIL family of transcription factors [START_REF] Guo | The ethylene signaling pathway: new insights[END_REF][START_REF] Czarny | Genetic modulation of ethylene biosynthesis and signaling in plants[END_REF]. At the end of the pathway are the ethylene inducible genes which expression is under the control of the ethylene response transcription factors (ERF). ERF-type transcription factors are specific to plants and belong to the large AP2/ERF family. ERFs provide a control point for integration of multiple signals and for diversification of ethylene responses [START_REF] Fujimoto | Arabidopsis ethylene-responsive element binding factors act as transcriptional activators or repressors of GCC boxmediated gene expression[END_REF].

Ethylene is produced in plants from

The role of ethylene in promoting fruit ripening and flower senescence provided a commercial incentive for early, successful approaches to down-regulation of its production. It is well described that the ripening of climacteric fruits is initiated by a peak of autocatalytic production of ethylene. It is assumed that control of ethylene production at the late stages of ripening will delay the processes occurring at this stage, and consequently will extend the shelf life of the fruit. Tomato has been the model plant for genetic manipulation, although similar strategies have been used in melon and apple.

In terms of synthesis, genes encoding for the biosynthetic steps as ACC synthase and ACC oxidase have been silenced, by both antisense technology and RNAi (RNA ). Antisense inhibition of LeACS2, which also down-regulated LeACS4, reduced ripening-related synthesis of ethylene to 0.1% of control fruit. The antisense fruit displayed an abnormal pattern of ripening such as reduced lycopene accumulation, delayed softening and a much reduced climacteric peak [START_REF] Oeller | Reversible inhibition of tomato fruit senescence by antisense RNA[END_REF]. This antisense strategy has also been used in apple transformed with the antisense versions of the apple ACC synthase and ACC oxidase with the aim of studying the role of ethylene in apple ripening [START_REF] Dandekar | Effect of down-regulation of ethylene biosynthesis on fruit flavor complex in apple fruit[END_REF]. They observed very low levels of ethylene synthesis in three year old apple trees grown in the field, and hence fruit that ripened more slowly. One side effect of this strategy was a decrease in the concentration of enzymes responsible for the formation of flavour esters, an effect that may adversely affect consumer acceptance.

RNAi strategy has also been used to silence ACO, these transgenic tomato plants produced fruit that released only traces of ethylene and had a shelf life of more than 120 days [START_REF] Xiong | Different effects on ACC oxidase gene silencing triggered by RNA interference in transgenic tomato[END_REF]. In addition, to interfere with the endogenous synthesis of ethylene the heterologous expression of ACC deaminase, cloned from soil bacteria, has been employed. Effectively, the microbial enzyme ACC deaminase converts ACC to ammonia and α-ketobutyrate, both of which are further metabolized. By inserting the ACC deaminase gene into tomato plants under the control of the CaMV 35S promoter, fruits with delayed ripening were readily produced [START_REF] Klee | Control of ethylene synthesis by expression of a bacterial enzyme in transgenic tomato plants[END_REF]. No other significant phenotypic differences were observed between plants with high levels of ACC deaminase activity and nontransformed plants.

Another strategy used to interfere with the ethylene action has been the use of the mutated ethylene receptor from Arabidopsis etr1-1. This is a dominant mutation that confers ethylene insensitivity when expressed in heterologous plant species. Thus, overexpression of etr1-1 has been used to produce transgenic plants with diminished sensitivity to the hormone [START_REF] Wilkinson | A dominant mutant receptor for Arabidopsis confers ethylene insensitivity in heterologous plants[END_REF]. Seedling growth, flower senescence, flower abscission, [START_REF] Lanahan | The Never-ripe mutation blocks ethylene perception in tomato[END_REF]. As NR and LeETR4 genes are up-regulated during ripening, they have both been targeted for antisense experiments. Antisense repression of the mutant NR gene in the Nr background produced fruit that ripened normally indicating that the NR ethylene receptor is not required for ripening and confirming the receptor inhibition model of ethylene signaling [START_REF] Hackett | Antisense inhibition of the Nr gene restores normal ripening to the tomato Never-ripe mutant, consistent with the ethylene receptor-inhibition model[END_REF]. By opposite, it is becoming apparent that LeETR4 plays an important role in ripening. Down regulation of LeETR4 produces an ethylene hypersensitive phenotype that includes accelerated ripening [START_REF] Tieman | The tomato ethylene receptors NR and LeETR4 are negative regulators of ethylene response and exhibit functional compensation within a multigene family[END_REF], whereas in Arabidopsis increased ethylene sensitivity is not observed until three receptors have been knocked out [START_REF] Hua | Ethylene responses are negatively regulated by a receptor gene family in Arabidopsis thaliana[END_REF]. The tomato dominant mutant Green ripe (Gr) is specifically defective in fruit ethylene responses. It has been found that the mutation is the consequence of a deletion in the 5´flanking region of the gene and causes ectopic expression of GR [START_REF] Barry | Ripening in the tomato Green-ripe mutant is inhibited by ectopic expression of a protein that disrupts ethylene signaling[END_REF]. Interestingly, GR and RTE1 are members of the same protein family, and GR could achieve the appearance of tissue-specific function of the different ethylene receptors if it interacts with specific receptors [START_REF] Klee | Highly conserved proteins that modify plant ethylene responses[END_REF].

The process of flower senescence, with the consequence of flower wilting and decay, is triggered in many plant species by ethylene. Thus, commercial plants of carnation, begonia, and torenia have been produced with an antisense ACC oxidase construct. In all the cases there was a dramatic decrease of endogenous ethylene production and the benefit of extended vase life of the cut flowers [START_REF] Czarny | Genetic modulation of ethylene biosynthesis and signaling in plants[END_REF]. Also, ethyleneinsensitive carnation flowers were produced after transformation with the Arababidopsis etr1-1 gene. Transgenic carnation cut flowers had three times the vase It has been established that ethylene response factor (ERF) proteins play important regulatory roles in plant response to abiotic and biotic stresses. Among abiotic stresses, drought is the most devastating constraint to crop productivity. Tobacco plants overexpressing JERF3, a tomato ERF protein, showed an enhanced resistance to drought, salinity and freezing [START_REF] Wu | Transcriptional modulation of ERF protein JERF3 in the oxidative stress response enhances tolerance of tobacco seedlings to salt, drought and freezing[END_REF]. Enhanced tolerance to drought and high salinity was also observed in rice plants overexpressing TERF1, another tomato ERF protein [START_REF] Gao | Expression of TERF1 in rice regulates expression of stress-responsive genes and enhances tolerance to drought and highsalinity[END_REF].

Regarding biotic stress, another field of biotechnological application of the control of ethylene production is in the plant defense system. Ethylene involvement in the plant defense is complex, and commonly coordinated to other plant hormones. Thus, it is known that ethylene production might be an early response of plants to pathogen attack, being associated to defense reaction, but it is also known that some pathogenic fungi and bacteria are capable of producing ethylene as a virulence factor. This contradictory role played by ethylene has resulted in quite variable effects of the diminution of ethylene production and sensitivity in the plant sensitivity to pathogens [START_REF] Van Loon | Ethylene as a modulator of disease resistance in plants[END_REF]. For example, tobacco plants transformed with the etr1-1 mutant gene from Arabidopsis are insensitive to ethylene and develop symptoms of wilting and stem rot when grown in nonautoclaved soil [START_REF] Geraats | Ethylene-insensitive tobacco shows differentially altered susceptibility to different pathogens[END_REF]. Regarding the signaling cascade, the Arabidopsis mutant ein2, impaired in ethylene signal transmission, exhibits increased susceptibility to the necrotrophic fungus B. cinerea and the necrotrophic bacterium Erwinia carotovora but shows no alteration in susceptibility to the biotrophic Oomycete H. parasitica and the biotrophic bacterium P. syringae pv tomato. Similar phenotypic effects, plants more resistant/susceptible to some pathogen species with no effect to other pathogen species, were obtained following other strategies, like the overexpression of some Arabidopsis, [START_REF] Broekaert | The role of ethylene in host-pathogen interactions[END_REF]. In general, these results indicate that ethylene and ERF-like transcription factors are involved in defense responses of various plant species, but that their effect on disease resistance depends on the specific plantpathogen interaction. However, it appears as an area for future developments of genetic strategy, transgenic or mutants, for improving plant response to pathogens. Success has been variable, mainly due to two facts. First is that usually endogenous enzymes are encoded by gene families, being the transcription of the various members organ-, tissue-, and developmental-dependent. This makes unpredictable the final result of the genetic engineering strategy. Second, which is related to the first, is that abolition of ethylene production in the whole plant frequently have deleterious effects. Best solution to these shortcomings has been the use of specific promoters [START_REF] Baudinette | Isolation and characterization of the carnation floral-specific MADS box gene[END_REF].

Cytokinins

Cytokinin (CK) was initially discovered as the hormone that promotes cell division; it is now known to influence many aspects of plant growth and development [START_REF] Mok | Cytokinin metabolism and action[END_REF]. Cytokinins (CKs) are a structurally diverse group of biological molecules whose chemistry has been extensively studied. However, some aspects of their metabolism and signaling pathways are still undefined. The natural cytokinins are adenine derivatives that can be classified by the configuration of their N6-side chain as isoprenoid or aromatic cytokinins [START_REF] Mok | Cytokinin metabolism and action[END_REF]. Biotechnological applications in relation to this hormone have rested on the possibility of changing their endogenous level in a tissue/organ specific manner. Local levels of CKs are controlled by synthesis, transport, inactivation, and oxidation. Among these, synthesis and oxidation have been the targets for biotechnological plant improvement. that encodes for a rate-limiting step. In Arabidopsis seven IPT genes have been identified, being the promoters of four of them suppressed by CKs [START_REF] Miyawaki | Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate[END_REF]. The only IPTencoding gene known in Petunia hybrida [START_REF] Zubko | A natural antisense transcript of the Petunia hybrida Sho gene suggests a role for an antisense mechanism in cytokinin regulation[END_REF] has been used to modify the internal level of CKs. This has been performed in potato where CKs are known to influence tuber initiation, growth and dormancy. Potato plants cultured in vitro showed a significantly increased tuber biomass after cytokinin application [START_REF] Romanov | Effect of indole-3-acetic acid and kinetin in tuberisation parameters of different cultivars and transgenic lines of potato in vitro[END_REF]. The Sho gene, from Petunia hybrida, encodes an enzyme for cytokinin synthesis [START_REF] Zubko | Modification of cytokinin levels in potato via expression of the Petunia hybrida Sho gene[END_REF]. Moderate expression of the Sho gene in potato produced and enhanced shoot production, delayed tuber formation with significant reduction in tuber size, and inhibition of tuber dormancy [START_REF] Zubko | Modification of cytokinin levels in potato via expression of the Petunia hybrida Sho gene[END_REF]. In general, stimulation of shoot and stolon development can be beneficial, jointly with the reduction of dormancy, for enhanced tuber production. However, as occurs with other transgenic plants, there are deleterious effects derived from ectopic expression of the transgene. This must be eventually overcome with the use of tissue/stage-specific promoters, as above indicated.

In the catabolic pathway there is a step catalyzed by a cytokinin oxidase/dehydrogenase enzyme (CKX) [START_REF] Mok | Cytokinin metabolism and action[END_REF]. This enzyme irreversibly degrades CKs by cleavage of the unsaturated N6-isoprenoid side chain. This step plays a main role in controlling CK levels in plants. Selection of the corresponding gene as a target for genetic engineering was based on previous studies aimed to characterize in many crops, including rice, quantitative trait loci (QTLs) responsible for grain number, and eventually for higher yield. Grain number is an important trait that contributes directly to crop yield. A QTL that increases grain number in rice, Gn1a, was identified. High resolution mapping of this Gn1a identified as a possible candidate gene the one with a predicted open reading frame with high similarity to cytokinin oxidase/dehydrogenase [START_REF] Ashikari | Cytokinin oxidase regulates rice grain production[END_REF]. This was further confirmed by producing transgenic rice plants expressing different levels of OsCKX2 and examining the grain yield. Lines with two copies of the sense strand of OsCKX2 showed reduced grain number compared to the control, whereas lines with antisense strands, which had reduced levels of expression, showed higher grain number [START_REF] Ashikari | Cytokinin oxidase regulates rice grain production[END_REF]. Grain number jointly with plant height is an important trait that directly contributes to grain productivity. Thus, the manipulation of OsCKX2 in rice is relevant since food shortage still remains as one of the most serious problems in this century. It is envisaged that crop grain production will need to increase by 50% by 2025 to meet the expanding food demands of the growing world population [START_REF] Khush | Challenges for meeting the global food and nutrient needs in the new millennium[END_REF].

Brassinosteroids

Brassinosteroids (BRs) constitute another class of plant hormones that participate in multiple plant physiological processes like cell division, cell elongation, vascular differentiation, root growth inhibition, and ethylene synthesis. Currently 42 brassinosteroid metabolites and conjugates are known [START_REF] Bajguz | Metabolism of brassinosteroids in plants[END_REF], among them the most physiologically active is Brassinolide (BL). Metabolic pathways for BRs biosynthesis might vary depending upon the plant species, the developmental stage, and the BR structure. Two categories of changes have been reported in BRs metabolism: 1) structural changes of the steroidal skeleton; and 2) structural changes in the side chain.

The most common modification in the side chain include hydroxylation and side chain cleavage. Less common modifications include dehydrogenation, glycosylation and demethylation [START_REF] Bajguz | Metabolism of brassinosteroids in plants[END_REF]. The synthesis of the steroidal skeleton starts from an initial precursor, campesterol, and de BR intermediates undergo a series of hydroxylations, reductions and epimerization, and a oxidation leading to the brassinolide (BL), the most was identified by positional cloning in a mutant of Arabidopsis (bri1) insensitive to brassinosteroids [START_REF] Wang | BRI1 is a critical component of a plasma-membrane receptor for plant steroids[END_REF].

The importance of brassinosteroids in cell elongation has generated interest in manipulating their levels. It was reported that exogenous application of brassinosteroids to a number of vegetables increased yields [START_REF] Maugh | New chemicals promise larger crops[END_REF]. Recently, main interest has been in rice where they are known to influence both plant height and leaf erectness [START_REF] Sakamoto | Generating high-yielding varieties by genetic manipulation of plant architecture[END_REF]. Erect leaves enhance light capture for photosynthesis, what allows more dense plantings and higher nitrogen storage. The result is an increase in yield.

In the screening of a mutant rice population, generated by γ-ray irradiation, retrotransposon-mediated, or chemical mutagenesis, a mutant was isolated, osdwarf4-1, with erect leaves but without abnormal leaf, flower or grain morphology. The gene was finally identified as OsDWARF4 after characterizing the site of insertion of the retrotransposon [START_REF] Sakamoto | Erect leaves caused by brassinosteroid deficiency increase biomass production and grain yield in rice[END_REF]. This gene is homologue of the DWARF4 gene from Arabidopsis that encodes a cytochrome P450 that catalyzes the C-22 hydroxylation rate limiting step of brassinolide biosynthesis [START_REF] Choe | The DWF4 gene of Arabidopsis encodes a CytochromeP450 that mediates multiple 22a-hydroxylation steps in Brassinosteroid biosynthesis[END_REF].

Most plant genes for brassinosteroid biosynthesis exist as single copies, however in rice there are two genes encoding the P450 C-22 hydroxylase, OsDWARF4L1 that acts during shoot elongation and reproductive development, and the OsDWARF4 gene that controls leaf inclination. Thus the osdwarf4-1 mutant displayed increased erectness of the leaves, without the severe phenotype of the double mutant osdwarf4-1/osdwarf4l1 plants. Small field trials involving the wild type and the osdwarf4-1 mutant revealed that yield was higher for mutants, being the difference higher with the increase of density of planting and the amount of nitrogen fertilizer [START_REF] Sakamoto | Erect leaves caused by brassinosteroid deficiency increase biomass production and grain yield in rice[END_REF]. Biomass production of the osdwarf4-1 plants was increased nearly 40% as compared with the Table 1 shows some prominent examples of biotechnological applications derived from changes either in the hormone levels or the signaling pathways.

Future prospects

It is accepted that rational approach to improve crop quality depends on a better understanding of plant biology, in this case of hormone biosynthesis and signaling.

Genomic projects have deciphered the sequences of tens of thousands of genes in plant genomes, but it is needed to know which genes control the quality traits of the crops by their involvement in hormone action. The functional genomics tools are being used for this task that will pave the way for biotechnological applications. Among these, those related to reverse genetics are becoming more important. We consider here the main advantages and disadvantages of two of them: the preparation of transgenic plants and the generation of chemical mutants.

Field growth of transgenic crops for commercial production has been continuously increasing to reach values over 120 M of hectares by 2007 (www.isaaa.org/resources/publications). However, most of them are restricted to few species (soybean, maize, cotton, and canola) and few traits (herbicide and insect resistance). In terms of hormone balance one of the most conspicuous events was performed in rice, where tissue-specific expression of the GA2ox gene caused a dwarf phenotype without affecting grain yield. Ectopic expression of the transgene, with the consequent pleiotropic effect, was avoided by using the promoter of another rice biosynthetic gene (OsGA3ox1) [START_REF] Sakamoto | Genetic manipulation of gibberellin metabolism in rice[END_REF]. The need of tissue/developmental-specific promoters is a great challenge for the future development of transgenic plants that lack of knowledge on the rules for promoter engineering; 3) unavailability of efficient systems for gene targeting/homologous recombination in plants [START_REF] Busov | Genes for control of plant stature and form[END_REF].

Future developments of commercially attractive transgenic plants would be probably restricted to main crops, like cereals and some legumes, improved in traits with benefits for developing countries. Rejection by most European countries of the technology of genetic modification, known as GMO, appears as a great shortcoming for future development of new transgenic plants, especially in quality traits appreciated by the consumers of the developed countries.

Most transgenic plants commonly present a complete loss of function, or constitutive gain of function, phenotypes that make them unacceptable as commercial crops [START_REF] Hedden | The genes of the green revolution[END_REF]. However, it should be reminded that there were two alleles in two species that were critical to bring about the Green Revolution [START_REF] Silverstone | Gibberellins and the green revolution[END_REF]. This indicates that the alleles that cause just a moderate variation in the physiology of the plant are the most important for the development of new commercial varieties. Since natural variation for new alleles is limited, once again chemical mutagenesis is being used to look for new improvements in plant performance. The use of the technical approach known as targeted induced local lesions in genomes (TILLING) [START_REF] Mccallum | Targeting induced local lesions IN genomes (TILLING) for plant functional genomics[END_REF] will facilitate the job. Using this technique, point mutations of selected genes can be produced and screened for their phenotypic effect. There are examples where allelic variants are produced and screened by TILLING to generate the desired phenotype [START_REF] Slade | A reverse genetic, nontransgenic approach to wheat crop improvement by TILLING[END_REF]. The possibility of modifying multiple alleles with non GMO technology will become more realistic in the future, as the genome sequencing projects and the refinement of what is known as system biology will increase the predictive value of specific genetic changes.

  FMO)-like family that catalyzes a limiting step in Arabidopsis

  the DhfH9-iaaM transgene in Thompson Seedless (low shoot fruitfulness) and Silcora (medium shoot fruitfulness) cultivars of grape resulted in an increased number of inflorescences per shoot in transgenic plants for both cultivars, in

  S-adenosyl-methionine (SAM), which is first converted in 1-aminocyclopropane-1-carboxylate (ACC) by the action of ACC synthase (ACS), and then oxidized to ethylene by the ACC oxidase (ACO). Both ACC synthase and ACC oxidase activities are encoded by multi-gene families, and the transcription of the different genes is induced under different environmental and physiological conditions [50, 51]. Ethylene is perceived by a family of membrane-localized receptors that are homologous to bacterial two-component histidine kinases. The number of isoforms as well as the expression profiles varies within the species. For instance, Arabidopsis harbors five ethylene receptors, ETHYLENE RECEPTOR1 (ETR1), ETR2, ETHYLENE INSENSITIVE4 (EIN4), ETHYLENE RESPONSE SENSOR1 (ERS1), compared to the six of tomato, LeETR1, LeETR2, NEVER RIPE (NR), LeETR4, LeETR5, and LeETR6; but ethylene-binding studies indicate similar ethylene

  were all affected by the presence of the etr1-1 transgene in both petunia and tomato plants. Regarding other receptors, NR was the first ethylene receptor discovered in tomato. The nr tomato mutant shows global and total ethylene insensitivity

  non-transformed flowers and lasted up to 16 days, which is longer than flowers treated with either inhibitors of ethylene biosynthesis or ethylene antagonists[START_REF] Bovy | Heterologous expression of the Arabidopsis etr1-1 allele inhibits the senescence of carnation flowers[END_REF][START_REF] Baudinette | Isolation and characterization of the carnation floral-specific MADS box gene[END_REF].

  The signaling pathway is being characterized. The receptor BRI1

  plants. Under normal or dense planting conditions the erect leaves of the mutants increased photosynthetic rates and as result the plants were more productive.

  important when hormone actions are targeted. Limitations to have access to these promoters are: 1) lack of characterization of 5' regulatory regions; 2)

  Victoriano Valpuesta is Professor at the Department of Biochemistry and Molecular Biology of the University of Málaga. He received a PhD in Biochemistry from the University of Sevilla and he gained a postdoc position in Michigan State University, working with Professor Martin J. Bukovac. In 1982 he was granted with a position as Professor in the University of Málaga, where he achieved the tenure as full Professor in 1990. Main research interest during this period was enzymology of the amino acids metabolism in plants. Afterwards he has spent two sabbaticals in the University of California in the laboratory of Dr. Michael S. Reid , and the University of Queensland in the laboratory of Dr. José R. Botella. His current research goals are centered on molecular studies of developmental processes in plants, mostly in fruits. His most recent research addresses the use of functional genomics tools in the ongoing research on plant development.

Table 1 .

 1

	Gene/Locus	Hormone involved	Species	Mutant/transgenic	Phenotype	Reference
	sd1 (GA20ox gene)	GA	rice	mutant	Semi-dwarfed plants	[12]
					Decreased
					plant height,
	Rht (DELLA gene)	GA	wheat	mutant	response to GA and	[25]
					increased GA
					levels
					Increased
					number of
	DhfH9-iaaM	auxin	strawberry, raspberry	transgenic (sense)	increased fruit fruits per plant,	[34]
					weight and
					size
	SlIAA9	auxin	tomato	transgenic (antisense)	Parthenocarpic fruits	[40]
					Reduced
					ripening-
					related
					synthesis of
	LeACS2	ET	tomato	transgenic (antisense)	ethylene to 0.1% of	[57]
					control fruit,
					abnormal
					pattern of
					ripening
					ET
					hypersensitive
	LeETR4	ET	tomato	transgenic (antisense)	phenotype including	[64]
					accelerated
					ripening
					Cut flowers
					had three
	etr1.1	ET	carnation	transgenic (sense)	times the vase	[68]
					life of non-
					transformed
					Increased
					shoot
					production,
	Sho	CK	potato	transgenic (sense)	delayed tuber	[77]
					formation,
					decreased
					tuber size and

Wiley-VCH Biotechnology Journal

Ackonowledgements

The authors are financially supported by the Grant BIO2007-67509-C02-01 (Ministerio de Educación y Ciencia, Spain).


Evangellos Bacharoudis

Michalis Gr Vrachopoulos

Maria K Koukou

Dionysios Margaris

Andronikos E Filios

Stamatis A Mavrommatis

Prof. M. Gr Vrachopoulos

Study of the natural convection phenomena inside a wall solar chimney with one wall adiabatic and one wall under a heat flux

Keywords: wall solar chimney, heat transfer, buoyancy, simulation, experiment

Four wall solar chimneys have been constructed and put at each wall and orientation of a small-scale test room so as to be used for the evaluation and measurement of their thermal behavior and the certification of their efficiency. At this stage, research focuses on the study of the buoyancy-driven flow field and heat transfer inside them. A numerical investigation of the thermo-fluid phenomena that take place inside the wall solar chimneys is performed and the governing elliptic equations are solved in a twodimensional domain using a control volume method. The flow is turbulent and six different turbulence models have been tested to this study. As the realizable k-model is likely to provide superior performance for flows boundary layers under strong adverse pressure gradients, it has been selected to be used in the simulations. This is also ACCEPTED MANUSCRIPT 2 confirmed by comparing with the experimental results. Predicted velocity and temperature profiles are presented for di erent locations, near the inlet, at different heights and near the outlet of the channel and they are as expected by theory. Important parameters such as average Nusselt number are also compared and calculated at several grid resolutions. The developed model is general and it can be easily customised to describe various solar chimney's conditions, aspect ratios, etc. The results from the application of the model will support the effective set-up of the next configurations of the system.

In Mediteranean countries, solar radiation during summer months is very intense and the ambient air temperature often rises up to 40°C or above. This fact in combination with the limitations of conventional energy sources, in terms of cost and availability, and the increased awareness of environmental issues, have led to renewed interest in passive building design. Passive solar heating, in which part or all of the building is a solar collector, has been widely examined, passive solar cooling, however, remains largely unexplored. Among the applications of these technologies, particularly appropriate for the hot-humid climates of Mediteranean region is solar chimney which is an effective technique to reduce the temperature inside a building as well as to provide natural ventilation, which helps in lowering the humidity and achieving comfortable conditions inside the space. A solar chimney generates air movement by buoyancy forces, in which hot air rises and exits from the top of the chimney, drawing cooler air through the building in continuous cycle. Its application in buildings may provide the required ventilation while simultaneously covers part of the heating and cooling requirements. The thermally-induced air flow depends on the difference in air density between the inside and outside of the solar chimney. In the ways to increase the solar heat absorption and ventilation rate, the replacement of the south-facing wall of the solar chimney with glazing, the blackening of the interior of other walls and the insulation of the exterior can be considered.

Solar chimneys have been investigated by a number of researchers and for different applications including passive solar heating and cooling of buildings, ventilation, power generation, etc. [START_REF] Barozzi | Physical and numerical modelling of a solar chimney-based ventilation system for buildings[END_REF][START_REF] Bansal | Solar chimney for enhanced stack ventilation[END_REF][START_REF] Gan | A numerical study of solar chimney for natural ventilation of buildings with heat recovery[END_REF][START_REF] Afonso | Solar chimneys: simulation and experiment[END_REF][START_REF] Chen | An experimental investigation of a solar chimney model with uniform wall heat flux[END_REF][START_REF] Ong | A mathematical model of a solar chimney[END_REF][START_REF] Ong | Performance of solar chimney[END_REF][START_REF] Khedari | Field study of performance of solar chimney with air conditioned[END_REF]. Experimental and theoretical studies have been conducted for the determination of the size of a solar chimney, confirming that the velocity of air flow and temperature of different parts are functions of the gap between absorber and walls, ambient air temperature, and the elevation of air exit above the inlet duct.

Although the behaviour of solar chimneys in their general form has been studied and certified both theoretically and experimentally [START_REF] Barozzi | Physical and numerical modelling of a solar chimney-based ventilation system for buildings[END_REF][START_REF] Bansal | Solar chimney for enhanced stack ventilation[END_REF][START_REF] Gan | A numerical study of solar chimney for natural ventilation of buildings with heat recovery[END_REF][START_REF] Afonso | Solar chimneys: simulation and experiment[END_REF][START_REF] Chen | An experimental investigation of a solar chimney model with uniform wall heat flux[END_REF][START_REF] Ong | A mathematical model of a solar chimney[END_REF][START_REF] Ong | Performance of solar chimney[END_REF][START_REF] Khedari | Field study of performance of solar chimney with air conditioned[END_REF], however, the wall solar chimney concept has been studied theoretically but it has not been fully certified at a laboratory level [START_REF] Aboulnaga | Improving night ventilation into low-rise buildings in hot-arid climates exploring a combined wall-roof solar chimney[END_REF][START_REF] Chantawong | Investigation on thermal performance of glazed solar chimney walls[END_REF]. AboulNaga and Abdrabboh [START_REF] Aboulnaga | Improving night ventilation into low-rise buildings in hot-arid climates exploring a combined wall-roof solar chimney[END_REF] made a theoretical investigation of a combined wall roof solar chimney to improve night time ventilation in buildings. They have developed a spreadsheet computer program for the parametric study to find out the optimum configuration of the wall roof chimney. Chantawong et al [START_REF] Chantawong | Investigation on thermal performance of glazed solar chimney walls[END_REF] made an experimental and numerical study of the thermal performance of a glazed solar chimney wall (GSCW). Experimental results conducted using a labscale GSCW 0.74 m high and 10 cm air gap were in good agreement with those obtained by solving the heat transfer equations using an explicit finite-di erence scheme and Gauss Seidel iterative method.

The wall solar chimneys are embodied in the building cell, they consist of integral parts and they do not modify the architectural view of the building in contrast with the classic "solar chimneys" that have a quite much larger width and contain quite larger air mass. Their operation induces the natural draw which causes the required under pressure in the area so as to be filled with fresh air e.g. from an underground and cooler place.

This under pressure causes air uptake conditions from other places having higher pressure (atmospheric) and such kind of places are the underground places which are in lower temperature conditions in comparison with the outside environment and with the places that will be air conditioned (the average temperature of underground places is equal to the average annual air temperature in an area and in Athens is equal to 19 o C which causes the cooling of a place that has conditioning requirements of 26 o C). This results in both the coverage of the air replenishment loads and (from the other side to) the qualitative replenishment of the air in the place. The wall solar chimneys are channels with quite high air velocity and intensity of natural draw inducing such conditions that it is required to perform a detailed thermo-fluid analysis. To meet this objective experimental and theoretical work has been scheduled so as to obtain a clear understanding of the system's operation.

Research work will be carried out through various stages assuming different configurations with various degree of complexity. At this current first stage, the air from the outside space enters the solar chimney from the bottom and escapes from the top and there is no connection with the room's interior. With this configuration research focuses on the flow field and temperature variation inside the wall solar chimney so as to understand the system behavior under various environmental conditions. At a second stage, various configurations will be applied where air will flow from or to the room interior through openings at the walls. With these configurations it is scheduled to study the solar chimney's performance focusing on the system capability to decrease temperature at the room interior providing thermal comfort as well as on room ventilation either with air uptake or with air recycling.

In the present study, a numerical investigation of the thermo-fluid phenomena that take place inside the wall solar chimneys is performed. The developed model faces the problem as a natural convection one between two vertical parallel plates and the governing elliptic equations are solved in a two-dimensional domain using a control volume method. It accounts for a detailed thermo-fluid analysis in contrast with the models proposed in other works which are quite simpler [START_REF] Aboulnaga | Improving night ventilation into low-rise buildings in hot-arid climates exploring a combined wall-roof solar chimney[END_REF][START_REF] Chantawong | Investigation on thermal performance of glazed solar chimney walls[END_REF]. Furthermore, as the flow conditions inside the wall solar chimneys studied in this work are in the turbulence regime special focus is given in the correct description of turbulence and various turbulence models are tested. In the wall solar chimneys studied the air channel has very small width and the surface of solar incidence is much larger in comparison with that of solar chimneys of older type [START_REF] Bansal | Solar chimney for enhanced stack ventilation[END_REF][START_REF] Gan | A numerical study of solar chimney for natural ventilation of buildings with heat recovery[END_REF][START_REF] Afonso | Solar chimneys: simulation and experiment[END_REF][START_REF] Chen | An experimental investigation of a solar chimney model with uniform wall heat flux[END_REF][START_REF] Ong | A mathematical model of a solar chimney[END_REF]. This causes the thermal intension of the air content and the development of higher velocities and turbulent flow conditions in contrast with the conventional (of older type) solar chimneys where the thermal intension of the air content is lower, air velocity values are much smaller and velocity profiles are quite different.

Predicted velocity and temperature profiles together with the average Nusselt number are presented for di erent locations, near the inlet, at different heights and near the outlet of the channel. The procedure is general and can be applied for the simulation of solar chimneys of different aspect ratios and conditions. First results show that the model predicts realistically the system behaviour for various environmental conditions.

Next steps focus on the extended verification of the current version of the model with experimental results and on the modification of the model to study the configurations where air will flow from or to the room interior through openings at the walls.

The physical problem

Experimental facility

A model room has been designed and constructed at the campus of the Technological Educational Institution of Chalkida located in the agricultural area of Psachna. The dimensions of the room are 4m x 6m x 4m and its roof is covered with roman tiles and a radiant barrier reflective insulation system (Fig. 1a). Four wall solar chimneys have been constructed and put at each wall and orientation (Fig. 1a). In Fig. 1b details on the flow through the wall solar chimney and their construction are shown together with the size of the air gap. The chimneys are constructed from plaster board. At this first stage, the air from the outside space enters the solar chimney from the bottom and escapes from the top and there is no connection with the room's interior. For the measurements in the current configuration of the wall solar chimneys small holes have been opened along the chimney height so as to obtain velocity and temperature measurements close to the inlet, close to the outlet and at the middle of the solar chimneys. Air velocity and temperature at the solar chimneys has been measured with KIMO VT 200 hot wire anemometer.

Characteristics of solar chimney

In Fig. 2 the geometry of the wall solar chimney studied is presented wherein L, is the height of the chimney and b, is the inter-plate spacing. The left and right walls are considered isothermal and heat transfer through the walls causes buoyancy-driven flow.

The solution of the governing conservation equations in their dimensionless form depends on the Rayleigh number based either on the channel length, L:

Pr L ) T T ( g Ra 2 3 2 w 1 w L ⋅ - = ν β (1)
or on the inter-plate spacing, b:

Pr b ) T T ( g Ra 2 3 2 w 1 w b ⋅ - = ν β ( 2
)
where , is the volumetric thermal expansion coefficient namely the change in the density of air as a function of temperature at constant pressure (K -1 ), g is the gravitational acceleration (9.81m 2 /s), ν is the kinematic viscosity (m 2 /s), T w1 ,T w2 are the left and right wall temperatures, respectively and Pr, is the Prandtl number.

Traditionally, in such kind of problems the dimensionless form of the equations is based on the channel width/inter plate spacing, b and thus the solution of the equations is a function of Ra b and the ratio b/L.

The modified Rayleigh, Ra * is also used defined as the ratio of the Rayleigh number to the aspect ratio of the channel:

⋅ = L b Ra Ra b * (3)
which is a very useful number for the integrated presentation of the results.

The Ra L and more specifically the Grashof number, Gr L where Gr L =Ra L /Pr is usually used in heat transfer for the definition of the flow regime to be laminar or turbulent. It has the same role with Reynolds number in forced convection flows and it indicates the ratio of the bouyancy force to the viscous force acting on the fluid. The buoyant forces are fighting with viscous forces and at some point they overcome the viscous forces and the flow is no longer laminar. For a vertical plate, the flow transitions to turbulent around a Grashof number of 10 9 [START_REF] Holman | Heat transfer, Fourth Edition[END_REF][START_REF] Patankar | Numerical heat transfer and fluid flow[END_REF].

In the following, the modified Ra b was used for the better presentation of the results and the Ra L was used for the definition of the flow regime to be laminar or turbulent.

From the engineering point of view an important characteristic of the flow is the rate of heat transfer through the solar chimney walls. Using Newton's law of cooling for the local convection coefficient h the Nusselt number for the left and right wall of Fig. 2 may be expressed as:

• Left wall: k L h Nu dy x T q and T T q h where k b h Nu l L x o w l b ⋅ = ∂ ∂ = - = ⋅ = = 0 1 (4) • Right wall: k L h Nu dy x T q and T T q h where k b h Nu r L x w o r b ⋅ = ∂ ∂ = - = ⋅ = = 0 2 (5)
3 Mathematical modeling of the wall solar chimney

The governing equations

The computational model of the wall solar chimney is a mathematical representation of the thermo-fluid phenomena governing its operation. A numerical investigation of the natural buoyancy-driven fluid flow and heat transfer in the vertical channel has been attempted. The simulations were conducted using the commercial, well-known, generalpurpose CFD code, Fluent ® . The steady, laminar, incompressible and two-dimensional form of the conservation equations [START_REF] Patankar | Numerical heat transfer and fluid flow[END_REF][START_REF] Markatos | Laminar and Turbulent Natural Convection in an Enclosed Cavity[END_REF] was solved for the fluid flow in the vertical channel using the Boussinesq approximation [START_REF] Gray | The validity of the Boussinesq approximation for liquids and gases[END_REF]. The latter imposes constant values in all thermophysical properties except for the density in the buoyancy force term of the momentum equation. It is also assumed that viscous dissipation is neglected.

For steady flow, the equations for continuity, velocity components and temperature take the following form [START_REF] Patankar | Numerical heat transfer and fluid flow[END_REF][START_REF] Markatos | Laminar and Turbulent Natural Convection in an Enclosed Cavity[END_REF]: continuity:

( ) 0 y ) v ( x u = ∂ ∂ + ∂ ∂ ρ ρ (6)
x-momentum:

( ) u S ) y u ( y ) x u ( x y ) vu ( x uu + ∂ ∂ ∂ ∂ + ∂ ∂ ∂ ∂ = ∂ ∂ + ∂ ∂ µ µ ρ ρ (7)
y-momentum:

( ) v S ) y v ( y ) x v ( x y ) vv ( x uv + ∂ ∂ ∂ ∂ + ∂ ∂ ∂ ∂ = ∂ ∂ + ∂ ∂ µ µ ρ ρ (8)
general transported fluid scalar, φ (e.g. T):

( )

φ φ φ φ Γ φ Γ φ ρ φ ρ S ) y ( y ) x ( x y ) v ( x u + ∂ ∂ ∂ ∂ + ∂ ∂ ∂ ∂ = ∂ ∂ + ∂ ∂ (9)
where: x,y are the coordinates in the Cartesian-coordinate system indicated in Fig. 2, , is air's density (kg.m -3 ), S u , S v , are momentum source terms in the x-, y-, directions, respectively, , is air's viscosity, is the exchange coefficient for the general transport fluid scalar .

For Ra L above 10 9 a two equation turbulence model should be used. In this context, the above equations become time-averaged equations and , are replaced by their effective values eff , eff as given by the turbulence model.

Boundary and internal conditions

Boundary conditions must be specified at the inlet, outlet and walls. Details on their specification are given below, for the application considered.

Numerical solution details

The solution of the set of the equations together with the boundary and internal conditions has been made with the segregated steady-state solver embodied in Fluent commercial software. Because the governing equations are non-linear (and coupled), several iterations of the solution loop must be performed before a converged solution is obtained. SIMPLE method has been used in all cases studied. Because of the nonlinearity of the problem the solution process is controlled via relaxation factors that control the change of the variables as calculated at each iteration. The convergence is checked by several criteria (e.g. the conservation equations should be balanced; the residuals of the discretised conservation equations must steadily decrease). Gridindependence studies and computer requirements are presented below, for the application considered.

Application of the model

The case considered

The wall solar chimney studied in this work is one of the wall solar chimneys of the test room. The values of the height and the inter-plate spacing of the latter solar chimney are 4m, 0.05m, respectively, and the aspect ratio is equal to

0125 . 0 L b =
. It is assumed that the chimney's walls are isothermal but they have different temperatures T w1 and T w2 , respectively. Actually, the left wall temperature varies with time and during the whole day however, it is realistic to assume that it is almost constant for a certain time period of the day. The thermo-fluid analysis performed describes the phenomenon during the above certain period of the day which is not random and it has been selected during noon when the requirements for indoors cooling reach their maximum. Furthermore, the existence of the reflective insulation may justify that way of approach because its thermal behavior causes unknown conditions and thus various approximate methods can be applied.

Boundary Conditions

Inlet: At the inlet section, it is obvious that there is a specific velocity profile because the fluid is moving with a specific mass flow rate. It is well-known that this velocity profile is the result of the pressure difference between two points inside and outside of the channel at the same height. Let us consider the points 0, 1 in Fig. 2. The fluid is motionless at the point 0 in ambient temperature T o =20 o C and static pressure p o . In point 1 the fluid obtains an unknown velocity profile which produces mass flow rate at inlet temperature T o and static pressure p. According to the literature [START_REF] Marcondes | Treatment of the inlet boundary conditions in natural-convection flows in open-ended channels[END_REF][START_REF] Kaiser | Correlation for Nusselt number in natural convection in vertical convergent channels at uniform wall temperature by a numerical investigation[END_REF][START_REF] Gadafalch | Natural convection in a large, inclined channel with asymmetric heating and surface radiation[END_REF][START_REF] Desrayaud | Laminar natural convection in a vertical isothermal channel with symmetric surface-mounted rectangular ribs[END_REF][START_REF] Zamora | Influence of variable property effects on natural convection flows in asymmetrically-heated vertical channels[END_REF][START_REF] Zamora | Influence of upstream conduction on the thermally optimum spacing of isothermal, natural convection-cooled vertical plates arrays[END_REF] it is assumed that the air moves from point 0 to point 1 with an adiabatic and reversible way.

Specifically, the Bernoulli equation holds at the entrance region outside the channel and the pressure difference between the two points is converted to kinetic energy. From Fig. . Moreover, it is assumed that the streamwise variations of temperature are neglected.

(Bernoulli

Furthermore, regarding inlet conditions for turbulence, the turbulence intensity has been assumed equal to 0.01% which is considered realistic as the fluid flow at the channel entrance is laminar and it is developed to the turbulent regime upwards while the length scale is equal to 0.07 D where D, in the general case, is the channel diameter.

Outlet:

At the outlet section the streamwise variations of velocity components and temperature are neglected. In addition, it is assumed that the fluid's pressure becomes equal to the ambient pressure [START_REF] Marcondes | Treatment of the inlet boundary conditions in natural-convection flows in open-ended channels[END_REF][START_REF] Kaiser | Correlation for Nusselt number in natural convection in vertical convergent channels at uniform wall temperature by a numerical investigation[END_REF][START_REF] Gadafalch | Natural convection in a large, inclined channel with asymmetric heating and surface radiation[END_REF][START_REF] Desrayaud | Laminar natural convection in a vertical isothermal channel with symmetric surface-mounted rectangular ribs[END_REF][START_REF] Zamora | Influence of variable property effects on natural convection flows in asymmetrically-heated vertical channels[END_REF][START_REF] Zamora | Influence of upstream conduction on the thermally optimum spacing of isothermal, natural convection-cooled vertical plates arrays[END_REF]. It is well-known that static pressure in an arbitrary point can be written as: . In this way according to Gadafalch et al [START_REF] Gadafalch | Natural convection in a large, inclined channel with asymmetric heating and surface radiation[END_REF] all the kinetic energy of the air is assumed to be converted to heat.

Finally, it is considered a Backflow Total Temperature=29.7 C in case the fluid entered to the chimney from the outlet. In this case, the incoming air is considered to be fresh air in a temperature =29. Furthermore, there are two approaches for modelling the near-wall region. In the first approach, the viscosity-affected inner region is not resolved. Instead a wall function is used to bridge the viscosity-affected region with the fully turbulent region. In the second approach, the turbulence model is modified to enable the viscosity-affected region to be resolved with a mesh all the way to the wall (enhanced wall treatment). If the near-wall mesh is fine enough to be able to resolve the laminar sublayer, then the enhanced wall treatment will be identical to the traditional two-layer zonal model. Because of the nature of the buoyancy-induced flow inside the wall solar chimney where special treatment should applied at the near wall region to account for the development of the boundary layers, it has been decided to follow the second approach in the simulations.

Grid independence study

A structure, mapped mesh with quadrilateral 2D elements has been built in the code. In order to ensure the accuracy of the numerical results, a grid independence study was performed by changing the number of the nodes in the horizontal (H) and in the vertical (V) direction (Table 1). As the flow was in the turbulent regime a thorough investigation has been attempted for each turbulence model applied. Successful computation of turbulent flow requires some consideration during the mesh generation. Due to the strong interaction of the mean flow and turbulence, the numerical results for turbulent flows tend to be more susceptible to grid dependency than those for laminar flows. In all the simulations y + <1 and it has been concluded that a grid consisting of 70×420 cells can provide sufficient spatial resolution giving a grid-independent solution for each of the turbulence models tested.

Turbulence modelling

For the numerical simulation of the turbulent flow inside the wall solar chimney six and in the viscous sub-layer adjacent to solid walls. However, these models have been certified for forced convection flows [START_REF]Fluent 6.2 User' s Guide[END_REF].

In Table 2 the average Nusselt number for the various turbulence models used is shown.

In Figures 3456typical results from the application of the above turbulence models are

shown. In all the cases studied the temperature of the left wall was T w1 =45 o C. In Figures 3a,b the streamwise velocity profile vs inter plate spacing at the outlet and middle of the wall solar chimney for various turbulence models is shown. By applying all the turbulence models except RSM, the predictions show that streamwise velocity increases close to the left wall at the outlet of the chimney which is expected by theory. Also, it is worthwhile mentioning that with the application of the Lam-Bremhost model an unexpected high increase of the velocity close to the left wall at the middle of the solar chimney is predicted (Figure 3b). In Figures 4a,b temperature profile vs inter plate spacing at the outlet and middle of the chimney for various turbulence models is shown.

It can be noticed that by applying the RSM model quite different profiles are predicted both at the outlet and middle of the solar chimney (Figure 4a,b) while by applying the Lam-Bremhost model a different temperature profile is predicted at the middle of the solar chimney.

Based on the above, it can be concluded that the use of the k-models and the use of the Abid Low-Re model assures the prediction of realistic velocity and temperature profiles as expected by theory. For the final selection it should be taken into account that the turbulence model should account for both the high and low (close to the chimney's walls) Reynolds areas of the computational domain. The k-model is robust and has reasonable accuracy for a wide range of turbulent flows, however, it is a high-Reynoldsnumber model. The RNG k-provides an analytically-derived differential formula for effective viscosity that accounts for low-Reynolds-number effects. Effective use of this feature does, however, depend on an appropriate treatment of the near-wall region. As the Abid Low-Re model has been certified for forced convection flows and the realizable k-model is likely to provide superior performance for flows boundary layers under strong adverse pressure gradients, the latter has been selected to be used in the simulations. Furthermore, this selection is confirmed from the comparison with the preliminary experimental results shown in Table 3 where experimental and predicted average velocity and temperature values for the various turbulence models used at the outlet of the wall solar chimney are presented. As it is shown the application of the kand the realizable k-models provides realistic predictions of the average velocity and of the temperature as well.

Typical results and discussion

In the following typical results from the performed simulations are presented and discussed. In all the cases studied the realizable k-model has been used.

In Figures 5,6 the predicted streamwise velocity and temperature profiles at the outlet of the wall solar chimney for various temperatures of the left wall are shown. It is well shown that as the left wall temperature increases the streamwise velocity at the channel increases and it is higher close to the wall with the higher temperature. The temperature profiles are as expected by theory (Figure 6). In Table 4 the predicted mass flow rates

CONCLUSIONS

In this work research focuses on the study of the thermo-fluid phenomena occurring inside wall solar chimneys that have been constructed and put at each wall and orientation of a small-scale test room. A numerical investigation of the buoyancy-driven flow field and heat transfer that take place inside the wall solar chimneys is performed.

The governing elliptic equations are solved in a two-dimensional domain using a control volume method. The procedure is general and can be applied for the simulation of solar chimneys of different aspect ratios and conditions. For the numerical simulation of the turbulent flow inside the wall solar chimney six turbulence models have been tested: the standard k-model, the RNG k-model, the realizable k-model, the Reynolds stress model (RSM), and two Low-Reynolds (Low-Re) models, namely, the Abid and the Lam-Bremhost. It is concluded that the use of the k-models and the use of the Abid Low-Re model assures the prediction of realistic velocity and temperature profiles as expected by theory. As the realizable k-model is likely to provide superior performance for flows boundary layers under strong adverse pressure gradients, the latter has been selected to be used in the simulations. Furthermore, this selection is confirmed from the comparison with the experimental results. Simulation results also show that the model predicts realistically the system behaviour for various environmental conditions while they support the evaluation of the air mass flow rate that can be achieved through this system and the turbulence effects.

LIST OF FIGURES

Fig. 1a. View of the in-house developed wall solar chimneys.

  coefficient for general transport fluid scalar (kg/m-s) Dynamic viscosity (kg/ms)

  The working fluid is air (Pr=0.713) coming into from the bottom of the channel (1) at a constant ambient temperature T o and gets out from the top (Fig.2). The fluid is motionless at the point 0 in ambient temperature =29.7 C. The estimated of turbulent flow conditions.

  , is the 'reduced' static pressure and p o, is the ambient pressure with

  7 C. Walls: It is assumed that the walls of the chimney have different temperatures. In the simulations for the left wall the following temperature values have been applied: T w1 =45 C, 50 C, 55 C, 60 C, 65 C, 70 C. In all cases studied the right wall temperature was equal to T w2 =27 C.

  turbulence models have been tested provided by Fluent ® : the standard k-model, the RNG k-model, the realizable k-model, the Reynolds stress model (RSM), and two Low-Reynolds (Low-Re) models, namely, the Abid and the Lam-Bremhost. The standard two-equation k-turbulence model involves the solution of two additional partial differential equations for the turbulent kinetic energy (k) and its dissipation rate ( )[START_REF] Launder | The Numerical Computation of Turbulent Flows[END_REF]. The values of the constants C , C 1 , C 2 , and applied are 0.09, 1.44, 1.92, 1.0 and 1.3 respectively[START_REF] Launder | The Numerical Computation of Turbulent Flows[END_REF]. The RNG k-model is essentially a variation of the standard k-model, with the used constants estimated rather through a statistical mechanics approach than from experimental data. The values of the constants C , C 1 and C 2 applied are 0.0845, 1.42 and 1.68, respectively[START_REF] Yakhot | Renormalization Group Analysis of Turbulence: I. Basic Theory[END_REF]. For the realizable model the term "realizable' ' means that the model satisfies certain mathematical constraints on the Reynolds stresses, consistent with the physics of turbulent flows. The realizable kmodel contains a new formulation for the turbulent viscosity. Also, a new transport equation for the dissipation rate, , has been derived from an exact equation for the transport of the mean-square vorticity fluctuation [24]. The RSM closes the Reynoldsaveraged Navier-Stokes equations by solving transport equations for the Reynolds stresses, together with an equation for the dissipation rate. It also requires additional memory and CPU time due to the increased number of the transport equations for Reynolds stresses. However, to account for the low-Re effects wall damping functions should be used in the -equation while the dissipation rate term in the transport equations should be modified to take into account the non-uniformity of turbulence. Finally, as concerns the Low-Re models, they are adequate for low-Re flows and if a very fine grid is used they can take into account the viscous sub-layer. Besides, wall damping functions are used in the equation of viscosity and the equation in both the production and the destruction term of the . Wall damping functions ensure that viscous stresses take over from turbulent Reynolds stresses at low Reynolds numbers

  and average Nu numbers for the left and right walls of the wall solar chimney vs. left wall temperature are shown. Increase in the left wall temperature causes an increase of the mass flow rate and of the heat transfer in the channel as well.In Figure7the predicted average Nu bl number for the left wall vs. modified Rayleigh number, Ra * is shown. In Figures8,9typical predicted streamwise velocity and temperature profiles across the solar chimney height are presented for the case where the left wall temperature is equal to 45 o C. The boundary layer development is well shown in both figures. At the inlet, the velocity profile, that has been created because of the pressure difference in the channel, is uniform. As the flow is developed upwards an acceleration close to the left wall is noticed. Furthermore, at the beginning and up to 1m height the flow is accelerated, however, at higher heights the profile becomes more uniform. This can be explained if it is considered that in the channel except the bouyancy forces that act on the fluid trying to accelerate it upwards, viscous forces also act opposing to the fluid flow. The contribution of turbulence should also been taken into account. Bouyancy forces play major role close to the wall, however, far from the wall laminar or turbulent shear stresses dominate. Furthermore, bouyancy forces tend to become important as temperature difference between fluid and the channel walls become high. As the fluid rises upwards, it is heated and the latter temperature difference decreases causing flow decelaration. It should be mentioned that similar trends occur in the rest of the performed simulations.

Fig. 1b .

 1b Fig. 1b. Air flow through the wall solar chimney and construction details.

Fig. 2 .

 2 Fig. 2. The geometry studied.

Fig. 3a .

 3a Fig. 3a. Streamwise velocity profile vs inter plate spacing at the chimney outlet for

Fig. 5 .

 5 Fig. 5. Predicted streamwise velocity profiles at the outlet of the solar chimney for

Fig. 6 .

 6 Fig. 6. Predicted temperature profiles at the outlet of the solar chimney for various

Fig. 7 .

 7 Fig. 7. Predicted average Nu bl number for the left wall vs. Modified Rayleigh number.

Fig. 8 .

 8 Fig. 8. Predicted streamwise velocity profiles across the solar chimney height.

Fig. 9 .Fig. 4a .Fig. 4b .Fig. 5 .Fig. 6 .Fig. 7 .Fig. 8 .Fig. 9 .

 94a4b56789 Fig. 9. Predicted temperature profiles across the solar chimney height.

Table 2

 2 Average Nusselt number for the various turbulence models used.

		Standard k-	RNG k-	Realizable k-	RSM Abid	Lam
							Bremohorst
	Nu l	5.12	5.35	5.13	5.56	5.30	5.51
	Nu r	9.71	10.56	9.73	12.85 10.28	9.41

Table 3

 3 Experimental and predicted average velocity and temperature for the various turbulence models used at the outlet of the wall solar chimney. Inlet air temperature= 29.7 o C, T w1 =45 o C, T w2 =27 o C.

		Standard	RNG	Realizable	RSM Abid	Lam	Experimental
		k-	k-	k-		Bremhorst
	Average	0.3395	0.3438	0.3395	0.2893 0.3322	0.3731	0.3381
	velocity
	Temperature
	at the	33.55	33.68	33.55	34.50 33.30	33.88	32.05
	middle of
	the profile
	( o C)

Table 4 .

 4 Predicted mass flow rates and average Nu numbers for left and right walls of the solar chimney vs. Left wall temperature.

		45 o C	50 o C	55 o C	60 o C	65 o C	70 o C
	m (kg/s)	0.02298	0.02763	0.03161	0.03524	0.03856	0.04165
	Nu bl	5.13	5.69	6.20	6.67	7.11	7.51
	Nu br	9.73	11.92	14.05	16.29	18.57	20.89

ACKNOWLEDGEMENT

This research work is co-funded by the European Social Fund and National resources (EPEAEK II) Archimedes I.

LIST OF TABLES