Ela Frak
email: frak@clermont.inra.fr

Xavier Le Roux

Peter P Millard

Erwin E Dreyer

B. B. Saint-Joanis Gaëlle G Jaouen

R R Wendler

B Saint-Joanis

& R Wendler

Changes in total leaf nitrogen and partitioning of leaf nitrogen drive photosynthetic acclimation to light in fully developed walnut leaves

Keywords: Chlorophyll, electron transport capacity, maximum carboxylation rate, Rubisco

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Leaf photosynthetic characteristics exhibit remarkable adaptability to light levels [START_REF] Field | The photosynthesis-nitrogen relationship in wild plants[END_REF][START_REF] Evans | Partitioning of nitrogen between and within leaves grown under different irradiances[END_REF]. During expansion and growth, single leaves usually acclimate to the vertical gradient of mean irradiance encountered within plant stands [START_REF] Hirose | Maximizing daily canopy photosynthesis with respect to the leaf nitrogen allocation pattern in the canopy[END_REF][START_REF] Anten | The photosynthetic capacity and leaf nitrogen concentration as related to light regime in shade leaves of a montane tropical forest tree, Tetrorchidium rubrivenium[END_REF][START_REF] Anten | Canopy structure and nitrogen distribution in dominant and subordinate plants in a dense stand of Amaranthus dubius L with a size hierarchy of individuals[END_REF]Kull & Niinemets 1998) or to the heterogeneous light regime observed within individual tree crowns [START_REF] Dejong | Seasonal relationships between leaf nitrogen content (photosynthetic capacity) and leaf canopy light exposure in peach (Prunus persica)[END_REF]Le Roux, Sinoquet & Vandame 1999a;[START_REF] Roux | Photosynthetic light acclimation in peach leaves: importance of changes in mass : area ratio, nitrogen concentration and leaf nitrogen partitioning[END_REF]. Generally, leaves developed at high irradiance exhibit higher nitrogen (N) content and photosynthetic capacity per unit leaf area as compared with shade leaves [START_REF] Boardman | Comparative photosynthesis of sun and shade plants[END_REF][START_REF] Givnish | Adaptation to sun and shade: a whole-plant perspective[END_REF][START_REF] Sims | Photosynthetic characteristics of a tropical forest understorey herb, Alocasia macrorrhiza, and a related crop species, Colocasia esculenta grown in contrasting light environments[END_REF][START_REF] Niinemets | An analysis of light effects on foliar morphology, physiology and light interception in temperate deciduous woody species of contrasting shade tolerance[END_REF]. Many studies have analysed the changes in leaf anatomical and biochemical characteristics that drive photosynthetic acclimation to light [START_REF] Kappel | Effect of shade on photosynthesis, specific leaf weight, leaf chlorophyll content and morphology of young peach trees[END_REF][START_REF] Seemann | Environmental effects on photosynthesis, nitrogen-use efficiency and metabolite pools in leaves of sun and shade plants[END_REF][START_REF] Seemann | Light adaptation/acclimation of photosynthesis and the regulation of ribulose-1,5-bisphosphate carboxylase activity in sun and shade plants[END_REF][START_REF] Chazdon | Plasticity of leaf anatomy of two rain forest shrubs in relation to photosynthetic light acclimation[END_REF][START_REF] Hikosaka | Effects of leaf age, nitrogen nutrition and photon flux density on the organization of the photosynthetic apparatus in leaves of a vine (Ipomoea tricolor Cav) grown horizontally to avoid mutual shading of leaves[END_REF][START_REF] Rôças | Leaf anatomy of Alchornea triplinervia (Euphorbiaceae) under distinct light regimes in a Brazilian montane Atlantic rain forest[END_REF]Niinemets et al . 1998). Because more than 50% of total leaf N is allocated to the photosynthetic apparatus [START_REF] Evans | The allocation of protein nitrogen in the photosynthetic apparatus: costs, consequences and control[END_REF], photosynthetic acclimation to light has been described largely in terms of changes in total leaf N and/or in the partitioning of total leaf N between the different pools of the photosynthetic apparatus [START_REF] Evans | The relationship between electron transport components and photosynthetic capacity in pea leaves grown at different irradiances[END_REF][START_REF] Chow | Photosynthetic acclimation of Alocasia macrorrhiza (L.) G. Don to growth irradiance: structure, function and composition of chloroplasts[END_REF][START_REF] Hikosaka | A model of the acclimation of photosynthesis in the leaves of C 3 plants to sun and shade with respect to nitrogen use[END_REF].

In contrast, the processes involved in the photosynthetic acclimation of mature leaves following a sudden change in light regime (as observed after green pruning in tree orchards or gap formation in forests, for instance) is documented only poorly. Some studies have measured changes in the light-saturated photosynthetic rate and respiration rate of leaves after a change in light regime without analysing the concurrent changes in the main photosynthetic functions (i.e. light harvesting, electron transport and carboxylation) and in the key biochemical pools of the photosynthetic apparatus [START_REF] Von Caemmerer | Effects of partial defoliation, changes of irradiance during growth, short-term water-stress and growth at enhanced P(CO 2 ) on the photosyn-thetic capacity of leaves of Phaseolus vulgaris L[END_REF][START_REF] Bauer | Photosynthetic light acclimation in fully developed leaves of the juvenile and adult life phases of Hedera helix[END_REF][START_REF] Turnbull | The dynamics of photosynthetic acclimation to changes in light quantity and quality in three Australian rain-forest tree species[END_REF][START_REF] Brooks | Acclimation responses of mature Abies amabilis sun foliage to shading[END_REF][START_REF] Naidu | Acclimation of shade-developed leaves on saplings exposed to late-season canopy gaps[END_REF]). In contrast, other studies have accurately quantified changes in a given photosynthetic function and/or a few biochemical characteristics of the leaves, such as the thylakoid compounds or Calvin cycle enzymes, generally under controlled conditions [START_REF] Prioul | Interaction between external and internal conditions in the development of photosynthetic features in a grass leaf[END_REF][START_REF] Besford | Changes in some Calvin cycle enzymes of the tomato during acclimation to irradiance[END_REF][START_REF] Davies | Acclimation of tomato leaves to changes in light intensity: effects on the function of the thylakoid membrane[END_REF][START_REF] De La | Acclimation of barley to changes in light intensity -chlorophyll organization[END_REF][START_REF] Burkey | Response of soybean photosynthesis and chloroplast membrane -function to canopy development and mutual shading[END_REF][START_REF] Evans | Photosynthetic acclimation and nitrogen partitioning within a lucerne canopy: II. Stability through time and comparison with a theoretical optimum[END_REF]). To our knowledge, very few studies have surveyed the relative importance of concurrent changes in total leaf N versus changes in the partitioning of leaf N between light harvesting, electron transport and carboxylation after a sudden change in light regime. [START_REF] Evans | Partitioning of nitrogen between and within leaves grown under different irradiances[END_REF] studied changes in photosynthetic capacity, N con-tent and N partitioning in fully developed leaves of Cucumis sativus subjected to changes in an irradiance regime. In Glycine max plants grown in a climate chamber, [START_REF] Pons | Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants[END_REF] showed that after a high (400 µ mol m -2 s -1 ) to low (50 µ mol m -2 s -1 ) light transfer, photosynthetic acclimation resulted both from changes in total leaf N and from changes in the partitioning of leaf N within the photosynthetic apparatus. Such a comprehensive analysis of photosynthetic acclimation has never been made, either for plants growing outdoors or for trees.

The objectives of this study were to test (i) the ability of photosynthetic acclimation in hybrid walnut ( Juglans nigra x regia ) growing outdoors after low-to-high and highto-low light transfers made early or late in the vegetation cycle, and (ii) the relative importance of changes in total leaf N versus changes in the partitioning of N between the different photosynthetic functions during a 2 month period after transfer. We determined changes in maximum carboxylation rate, light-saturated electron transport rate, respiration rate, non-structural carbohydrates, total leaf N, ribulose 1•5-bisphosphate carboxylase/oxygenase (Rubisco) and total chlorophyll before and after a change in light regime. The results are discussed in terms of the mechanisms involved in photosynthetic acclimation after a change in light regime.

MATERIALS AND METHODS

Plant material and growth conditions

One-year-old hybrid walnut trees ( Juglans nigra x regia ) were grown outdoors in 35 dm 3 pots filled with a soil/peat mixture (1/2 v/v). One lot of 16 1-year-old plants was grown during 1999; another lot of 14 1-year-old plants was grown during 2000. To avoid complete light or shade exposure of the whole plant, each tree had only two opposite and erected branches. This was obtained by removing the apical and axillary buds before bud burst, with the exception of two vigorous axillary buds. Immediately after bud burst, the lower branch was placed in a tunnel made with altuglass screens. Two types of screen, mimicking high-light and deep-shade conditions, were used. The first (neutral) screen transmitted 90% of incident radiation without modification of the light spectrum (red/far red = 1•25), as measured with a spectroradiometer (LI-1800; Li-Cor Inc., Lincoln, NE, USA). The other (green) screen transmitted 10% of incident radiation (red/far red = 0•22). Plants were watered automatically once or twice a day according to evaporative demand.

The lower branches of eight and seven plants were grown in each screen tunnel during 1999 and 2000, respectively. Four plants were transferred from high-to low-light conditions and four were transferred from low-to highlight conditions on 29 July (91 d after bud burst) in 1999 and on 30 June (58 d after bud burst) in 2000. Four treatments were thus identified: constant low light (LL), constant high light (HH), low-to-high light transfer (LH) and high-to-low light transfer (HL). All physiological and biochemical mea-surements were subsequently made on single leaflets from fully developed leaves of the treated branch. The oldest and youngest leaves were excluded from these measurements. Air temperature and incident global radiation were recorded during the experiment period for both years.

Leaf mass : area ratio and non-structural carbohydrates

Leaflets were sampled on eight dates in 1999 and seven in 2000. One leaflet was harvested at random from each treated branch on each date. The leaflets' areas were measured with an area meter (LI-3100; Li-Cor), before they were freeze-dried and their dry mass measured. Samples were milled, and total non-structural carbohydrates (TNCs) [i.e. glucose fructose sucrose (GFS) and starch] were extracted prior to quantification. A hot ethanol : water buffer (80 : 20, v/v) was used for GFS, then extracts were purified on ion-exchange resins (Bio-Rad AG 1-X8; Bio-Rad Lab., Hercules, CA, USA in the carbonate form and Dowex 50 W (Supelco, Belle Fonte, PA, USA) in the H + form. The amount of GFS was determined spectrophotometrically after a hexokinase, glucose-6-phosphate linked assay [START_REF] Boehringer | Methods of Enzymatic Food Analysis Using Single Reagents[END_REF]. Starch was quantified by enzymatic assay after hydrolysis with amyloglucosidase [START_REF] Boehringer | Methods of Enzymatic Food Analysis Using Single Reagents[END_REF]. The TNC-free leaf mass : area ratio (LMA) was computed as (total leaf dry mass -TNC mass) / fresh leaf area.

Total leaf N, Rubisco and chlorophyll

The amount of total leaf N per unit dry mass (N m , %) was measured on the same dry samples with an elemental analyser (Carlo ERBA-1108;Carlo, Milan, Italy). Total leaf N was expressed per unit area (N a ) as (N m × leaf dry mass) / (fresh leaf area × 100).

Total chlorophyll was estimated with a portable chlorophyll meter (SPAD-502; Minolta Camera Co., Osaka, Japan) on the leaflets used for leaf gas exchange measurements. SPAD data were converted into the total amount of chlorophyll per unit leaf area using a calibration curve ( y = 0•0606 e 0•0502x , N = 64, r 2 = 0•87). This curve was established from destructive leaflet sampling, chlorophyll extraction in acetone and spectrophotometrical measurements according to [START_REF] Porra | Determination of accurate extinction coefficients and simultaneous equations for assaying chlorophylls a and b extracted with four different solvents: verification of the concentration of chlorophyll standards by atomic absorption spectroscopy[END_REF].

One 5 cm 2 disc sampled on a fresh leaflet was used to quantify Rubisco on each date and for each branch. Discs were frozen immediately in liquid N 2 and stored at -80 ° C. Sampled discs were ground in liquid N 2 until complete evaporation, then 5 cm 3 of extraction buffer (pH 7•6) containing 100 mol m -3 Tris and 200 mol m -3 sodium borate was added. Three protease inhibitors [10 mm 3 of each per cm 3 of extraction buffer: 2•3 mol m -3 Benzamidine, 2 mol m -3 ∈aminocarpic acid, 40 mol m -3 phenylmethylsulfonyl fluoride (PMSF)] and 500 mg polyvinylpolypyrrolidone (PVPP) were also added. Rubisco extracts were centrifuged for 2 min at 14 000 rpm and diluted 1 : 2000. Elisa assays were performed as described by [START_REF] Catt | The measurement of Ribulose 1,5bisphosphate carboxylase/oxygenase concentration in the leaves of potato plants by enzyme linked immunosorbtion assays[END_REF] by the double antibody technique, using a 1 : 1000 dilution of primary polyclonal antibody raised in sheep against wheat Rubisco, and a 1 : 300 dilution of secondary antibody against sheep conjugated to alkaline phosphatase (Sigma A -5187). Purified Rubisco from wheat was used as a standard. The second antibody reacted with p-nitrophenol phosphate (Sigma 104-0) by developing yellow coloration. When sufficient colour had developed (5-10 min), the reaction was stopped by the addition of 10 mm 3 5 mol m -3 NaOH and the optical density was read by a plate reader (Titertek Multiscan MCCl; Biolog Inc., Hayward, CA, USA).

Gas exchange measurements

Leaf gas exchange was measured with an infrared gas analyser-leaf chamber system (LI-6400, Li-Cor). Measurements were made on four dates in 1999 and seven in 2000. Leaf gas exchange was measured on the same leaflet of each plant throughout the experiment. Photosynthesis response curves to internal CO 2 partial pressure ( A -p i ) were established at a leaf temperature of 25 ° C and under high irradiance (1200 µ mol m -2 s -1 ) and different CO 2 partial pressures (190,150,110,90,70,50,35,25,20,15 and 10 Pa CO 2 ). Respiration rate was measured after 20 min of acclimation in darkness at 25 ° C and 35 Pa CO 2 . A version of the [START_REF] Farquhar | Modelling of photosynthetic response to environmental conditions[END_REF] photosynthesis model proposed by [START_REF] Harley | Modelling photosynthesis of cotton grown in elevated CO 2[END_REF] was used to analyse the A -p i curves. A fitting procedure (SAS software for Solaris version 6•12, SAS Institute Inc., Cary, NC, USA) was used to estimate the two key model parameters, i.e. maximum carboxylation rate ( V cmax ) and the light-saturated electron transport rate ( J max ) (see Le Roux et al . 1999b).

Calculations of leaf N partitioning between carboxylation, bioenergetics and light capture

The model proposed by [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF] was used to determine the coefficients for leaf N partitioning between carboxylation (mainly Rubisco) ( P r ), bioenergetics ( P b ) and thylakoid light-harvesting components ( P l ) based on measured N a values and estimated values of V cmax , J max and chlorophyll concentration. In this model, P r is the foliar N investment in carboxylation capacity (i.e. influencing V cmax ), P b is the N investment for the capacity of electron transport (i.e. influencing J max ) and P l is the N investment in light harvesting. P r [g N in Rubisco (g total leaf N) -1 ] and P b [g N in cyt f , ferredoxin NADP reductase and coupling factor (g total leaf N) -1 ] are given as follows:

(1)

(2) where V cr is the specific activity of Rubisco [i.e. the maximum rate of RuBP carboxylation per unit Rubisco protein in µ mol CO 2 (g Rubisco) -1 s -1 ], J mc is the potential rate of photosynthetic electron transport per unit cyt f [mol electrons (mol cyt f

) -1 s -1 ], 6•25 [g Rubisco (g N in Rubisco) -1 ] Pr = Vcmax 6◊25 ¥ Vcr Na ( ) Pb = Jmax 8◊06 ¥ Jmc Na ( ) ,
converts N content to protein content, and 8•06 [µmol cyt f (g N in bioenergetics) -1 ] is used assuming a constant 1 : 1 : 1•2 molar ratio for cyt f : ferredoxine NADP reductase : coupling factor [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF].

According to [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF], at a leaf temperature of 25 °C, V cr and J mc are equal to 20•2 µmol CO 2 [(g Rubisco) -1 s -1 ] and 156 mol electrons [(mol cyt f) -1 s -1 ], respectively. P l [g N in PSI, PSII and LCHII (g total leaf N) -1 ] is computed in a similar way, assuming scaling relationships between (i) J max and cyt f, and (ii) the different thylakoid chlorophyll-protein complexes and cyt f (for details, see Eqn 6 and appendix B in Niinemets & Tenhunen 1997; see also [START_REF] Hikosaka | A model of the acclimation of photosynthesis in the leaves of C 3 plants to sun and shade with respect to nitrogen use[END_REF]. Nitrogen investment in carboxylation was also estimated directly from measured total N and Rubisco amounts and was compared with values calculated according to [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF].

Statistical analyses

Statistical analyses were performed with the Genstat 4•2 software developed by Genstat Committee; IACR-Rothansted, Harpenden, UK. The residual maximum likelihood (REML) procedure was applied for the analysis of variance components on repeated measurements and unbalanced data sets. This procedure allowed overall tests of treatment (T), date (D) and treatment × date interaction (I). Because the various parameters were observed through time on the same individuals, ante-dependence analysis [START_REF] Gabriel | Ante-dependence analysis of an ordered set of variables[END_REF]) was used to assess the times at which treatment effects occurred.

RESULTS

Comparison of leaf stage between the two years

The seasonal courses of global incident radiation and air temperature were very similar for the two years (Fig. 1).

The average global irradiances from June to mid-September were 19•24 ± 6•31 and 19•66 ± 6•74 MJ m -2 during 1999 and 2000, respectively. The average air temperatures for the same period were 19•1 ± 2•7 and 18•9 ± 3•0 °C in 1999 and 2000, respectively.

The time and duration of bud break (late April and early May) was also similar for the two years (Fig. 1). N m decreased from 2•8 to 1•5% from the end of June to September. At a given date, values were similar in 1999 and 2000 (Fig. 2). Leaf N m was greater when plants were transferred in late June 2000 (2•8%) than in late July 1999 (2•0%). Therefore, leaves did not have a comparable age or N status when the treatments were imposed during the two years.

LMA, starch and soluble sugars

In 1999, the LMA and the amount of starch and soluble sugars per unit leaf area were higher for leaves developed under sun conditions than for those developed in the shade (Fig. 3). LMA and non-structural carbohydrate values of transferred leaves did not change after transfer. Starch exhibited a slight decrease with time in all treatments.

In 2000, LMA was constant or increased slightly in the HH, HL and LL treatments during the 2 month measurement period (Fig. 3). In contrast, LMA increased from 35 to 55 g m -2 during the same period in the LH treatment. During the first month after transfer, the amount of starch remained roughly constant for the HH, HL and LL treatments, whereas it increased from around 4 to 6 g m -2 in the LH treatment. The amount of starch per unit area decreased rapidly in all treatments after day 210. The amount of soluble sugars per unit leaf area decreased slightly in the HH and LL treatments. In contrast, soluble sugar content increased with time for the LH treatment and decreased for the HL treatment as compared with the constant LL-and HH-grown plants.

Total leaf N, Rubisco and chlorophylls

In 1999, N a was higher in sun than in shade leaves, and values decreased with time in all treatments (Fig. 4). N a of transferred leaves did not change as compared with constant light leaves. However, in 2000, N a increased by about During both years, total chlorophyll levels changed within a few days of transfer. Before transfer, the amount of chlorophyll per unit area was similar between shade and sun leaves. However, after transfer, chlorophyll decreased significantly in LH treatments and increased in HL treatments.

Photosynthetic capacities

During both years, V cmax and J max were higher in sun than in shade leaves. V cmax and J max decreased with time in all treatments (Fig. 5). In 1999, the leaf photosynthetic capacities of transferred leaves remained close to the values of the corresponding controls after transfer. In contrast, the respiration rate of HL leaves decreased rapidly after transfer and was similar to that of LL leaves 1 week after transfer. The respiration rate of LH leaves did not differ from values for LL leaves during the study period.

In 2000, V cmax decreased by around 40% in the HL treatment and increased by 25% in the LH treatment. Concurrently, J max decreased by around 35% in HL and increased by 45% in LH treatment. Respiration rate acclimated fully within 2 weeks of transfer in both LH and HL treatments. Full acclimation was defined as occurring when plants transferred to the new light condition produced values for the various parameters that were the same as those for plants grown constantly under this condition.

Leaf N partitioning within the photosynthetic apparatus

In 1999, leaf N partitioning between carboxylation and bioenergetics did not change significantly after transfer as compared with plants grown in constant LL and HH (not shown). However, in 2000, the fraction of leaf N invested into carboxylation decreased significantly for HL leaves as compared with HH leaves (Fig. 6). Concurrently, the frac-tion of N invested into carboxylation did not change between LL and LH treatments. The fraction of leaf N invested in bioenergetics increased significantly in LH and decreased in HL trees after transfer as compared with constant LL and HH, respectively. These changes were significant 2 weeks after transfer for LH leaves (6•9% versus 5•6%) and 3 weeks after transfer for HL leaves (5•6% versus 6•2%). In 1999 (not shown) and 2000 (Fig. 6), the fraction of leaf N invested into light capture decreased significantly in LH and increased in HL treatments after transfer as compared with controls. For HL leaves, the fraction of leaf N invested into light capture became similar to values for LL leaves (12% instead of 8% initially) within 2 weeks of transfer. The delay for full acclimation of P l in LH leaves was around 1•5 months.

After LH transfer, the relative changes in V cmax resulted mainly from an increase in N a , whereas P r decreased slightly with time (Fig. 7). However, after HL transfer, the relative changes in V cmax resulted from changes in N a and P r equally. Concurrently, changes in N a and P b drove the relative changes in J max after HL transfer equally. After LH transfer, the relative changes in J max resulted mainly from changes in P b initially (until 20 d after transfer), and were then driven mainly by an increase in N a .

For the LL, HH and HL treatments, the P r values computed according to [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF] were well correlated to those computed directly from measured total N and Rubisco amounts (Fig. 8). However, P r values estimated from [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF] were 45% lower than those computed directly. Furthermore, P r values of Figure 6. Temporal changes of the nitrogen investments into carboxylation (P r ), bioenergetics (P b ) and light harvesting (P l ) in 2000, computed according to [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF]. Symbols as in Fig. 3. T had a smaller effect on P r than D (P = 0•042 and P < 0•001), and P b was affected more by T than by D (P < 0•001 and P = 0•034). I was NS for P r and P b . T, D and I had a highly significant effect on P l (P < 0•001). Significant differences on a few dates are indicated by letters. LH leaves computed according to [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF] decreased from 30 to 19% after transfer, whereas those computed directly from N and Rubisco amounts increased from 41 to 59%.

DISCUSSION

Acclimation ability in fully developed walnut leaves

In 2000, photosynthetic acclimation occurred in fully developed walnut (Juglans nigra x regia) leaves after a sudden change in light regime, and was demonstrated by the increase in photosynthetic capacity in the LH treatment and the decrease in the HL treatment. Acclimation of photosynthetic capacity in adult leaves was reported to occur in response to sudden changes in light microclimate in Phaseolus vulgaris [START_REF] Von Caemmerer | Effects of partial defoliation, changes of irradiance during growth, short-term water-stress and growth at enhanced P(CO 2 ) on the photosyn-thetic capacity of leaves of Phaseolus vulgaris L[END_REF], Lolium multiflorum [START_REF] Sebaa | Acclimation of adult Lolium multiflorum leaves to changes in irradiance: effect on leaf photosynthesis and chloroplast ultrastructure[END_REF], C. sativus [START_REF] Evans | Partitioning of nitrogen between and within leaves grown under different irradiances[END_REF]), Abies amabilis [START_REF] Brooks | Acclimation responses of mature Abies amabilis sun foliage to shading[END_REF] and G. max [START_REF] Pons | Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants[END_REF]) after high-to-low light transfers, and in adult leaves of Hedera helix [START_REF] Bauer | Photosynthetic light acclimation in fully developed leaves of the juvenile and adult life phases of Hedera helix[END_REF], Quercus rubra and Acer saccharum [START_REF] Naidu | Acclimation of shade-developed leaves on saplings exposed to late-season canopy gaps[END_REF]) after low-to-high light transfers. In our study, photosynthetic capacity decreased by 35-40% in HL trees and increased by 25-45% in LH trees within 20 d of transfer, whereas full acclimation of V cmax and J max to the new light regime was not observed 50 d after transfer. The fact that the leaf structure is fixed after the completion of leaf development could explain the lack of full acclimation. This is consistent with the lack of full acclimation of photosynthetic capacity in Q. rubra and A. saccharum 1 month after a low-to-high light transfer [START_REF] Naidu | Acclimation of shade-developed leaves on saplings exposed to late-season canopy gaps[END_REF].

In A. amabilis, full acclimation was observed only 2-3 months after shading [START_REF] Brooks | Acclimation responses of mature Abies amabilis sun foliage to shading[END_REF]. However, abscission of needles with a less favourable carbon balance, together with the large gradient in needle properties within a single shoot, could lead to an illusion of perfect acclimation. In contrast, photosynthetic capacity acclimated almost fully to the new, low-light regime within 10 d in P. vulgaris [START_REF] Von Caemmerer | Effects of partial defoliation, changes of irradiance during growth, short-term water-stress and growth at enhanced P(CO 2 ) on the photosyn-thetic capacity of leaves of Phaseolus vulgaris L[END_REF] and L. multiflorum [START_REF] Sebaa | Acclimation of adult Lolium multiflorum leaves to changes in irradiance: effect on leaf photosynthesis and chloroplast ultrastructure[END_REF], and within 12 d in G. max [START_REF] Pons | Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants[END_REF]. As anticipated by [START_REF] Pearcy | Photosynthetic acclimation to a changing light environment: scaling from the leaf to the whole plant[END_REF], these results suggest that the time required for the completion of photosynthetic acclimation is higher for fully developed leaves of woody (more than 1 month) than herbaceous (less than 2 weeks) species. However, we anticipate that studies on fast-growing trees (e.g. poplar) and slow-growing herbaceous species could lead to a distinction between species exhibiting low versus high photosynthetic activities rather than between woody versus herbaceous species.

In contrast to the dynamics of acclimation observed for V cmax and J max , the dark respiration rate of fully developed walnut leaves acclimated fully within 1-2 weeks of the HL or LH transfer. Such a short delay in the acclimation of R d is consistent with results obtained in both woody [START_REF] Naidu | Acclimation of shade-developed leaves on saplings exposed to late-season canopy gaps[END_REF]) and herbaceous [START_REF] Sims | Photosynthesis and respiration in Alocasia macrorrhiza following transfers to high and low light[END_REF][START_REF] Pons | Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants[END_REF] species after low-to-high or high-to-low transfers. The increase and decrease in leaf dark respiration rate in LH and HL leaves, respectively, were probably due to the costs of carbohydrate processing and transport and/ or protein turnover [START_REF] Irving | The respiration of mature field bean (Vicia faba L.) leaves during prolongated darkness[END_REF][START_REF] Visser | Energy cost of protein turnover: theoretical calculation and experimental estimation from regression of respiration on protein concentration of full-grown leaves[END_REF].

Given that climatic conditions were similar in 1999 and 2000, the comparison of the results obtained in both years shows that the capacity for photosynthetic acclimation depended on leaf age at the time of change in light regime. In walnut, fully developed leaves exhibited a poor capacity for photosynthetic acclimation when the transfer was made 91 d after bud burst (1999), whereas they were largely able to acclimate when the transfer was made 58 d after bud burst (2000). Leaf age and/or ontogenic status has been shown to strongly influence photosynthetic acclimation capacity in young versus adult leaves of Fragaria virginiana [START_REF] Jurik | Ontogeny of photosynthetic performance in Fragaria virginiana under changing light regimes[END_REF], H. helix [START_REF] Bauer | Photosynthetic light acclimation in fully developed leaves of the juvenile and adult life phases of Hedera helix[END_REF] and Alocasia macrorrhiza [START_REF] Sims | Response of leaf anatomy and photosynthetic capacity in Alocasia macrorrhiza (Araceae) to a transfer from low to high light[END_REF]. In contrast, both 1-year-old and 4-year-old leaves of A. amabilis acclimated after shading [START_REF] Brooks | Acclimation responses of mature Abies amabilis sun foliage to shading[END_REF]. These results show that, along with leaf production and turnover, the acclimation capacity of leaves of different ages in response to changes in light regime should be taken into account when analysing the ecological roles of photosynthetic acclimation.

Changes in non-structural carbohydrates and leaf mass : area ratio

Foliar TNCs vary spatially along light gradients in tree canopies (Kull & Niinemets 1998;Niinemets & Kull 1998;Le Roux et al. 1999a). The amount of TNCs in leaves is generally assumed to reflect the carbon (C) balance between acquisition and export [START_REF] Chapin | The ecology and economics of storage in plants[END_REF]) in source-sink interactions. Changes in light regime during ) 1/1 LL, HH and HL 2000 induced a modification of TNCs, which suggests that source-sink relationships in LH and HL leaves were modified. However, a decrease in photosynthetic capacity in HL leaves did not reduce starch pool of the leaves, as reported by [START_REF] Thorne | Influence of assimilate demand on photosynthesis, diffusive resistances, translocation and carbohydrate levels of soybean leaves[END_REF] for shaded plants of G. max. The high amount of starch found in HL leaves after transfer was presumably because of a decrease in C demand (respiration rate and protein synthesis), as well as higher chlorophyll content and electron transport capacity as compared with 'true' shade leaves. In contrast to G. max [START_REF] Thorne | Influence of assimilate demand on photosynthesis, diffusive resistances, translocation and carbohydrate levels of soybean leaves[END_REF], the amount of GFS in HL leaves of walnut decreased through the experiment and was independent of starch content. In both the HL and the LH treatments, changes in GFS were concurrent with changes in N a , suggesting a strong interaction between these two pools. Although C demand for respiration and protein synthesis increased during photosynthetic acclimation in LH leaves, limitation of C exportation could be responsible for starch accumulation, as reported by [START_REF] Silvius | Photosynthate partitioning in soybean leaves at two irradiance levels. Comparative response of acclimated and unacclimated leaves[END_REF] for fully developed leaves of G. max transferred to bright light.

In many species, LMA varies according to the light environment and largely drives the differences observed for some leaf photosynthetic characteristics (e.g. N a ) within the tree canopy [START_REF] Niinemets | Distribution patterns of foliar carbon and nitrogen as affected by tree dimensions and relative light conditions in the canopy of Picea abies[END_REF]Niinemets & Kull 1998;Le Roux et al. 1999a;[START_REF] Rosati | Distribution of leaf mass per unit area and leaf nitrogen concentration determine partitioning of leaf nitrogen within tree canopies[END_REF]. The TNCfree LMA quantifies the allocation of C to the leaf structures. Allocation of biomass to the leaf structures was higher in HH and HL than in LL and LH treatments, respectively. After transfer, TNC-free LMA in the HL treatment did not change as compared with values observed for the constant light-grown plants. This shows that physiological and biochemical, rather than structural, modifications controlled photosynthetic acclimation. However, in LH leaves, an increase in LMA occurred during photosynthetic acclimation. This increase was independent of the accumulation of TNC and protein, and could be due to a larger production of secondary metabolites like phenolics or flavonoids (Christian Jay-Allemand, personal communication).

Importance of concurrent changes in total leaf N and N partitioning between photosynthetic functions for photosynthetic acclimation

Generally, photosynthetic capacities per unit leaf area are tightly correlated to the amount of leaf N invested into the different photosynthetic functions [START_REF] Evans | The allocation of protein nitrogen in the photosynthetic apparatus: costs, consequences and control[END_REF]. Thus, photosynthetic light acclimation in fully developed leaves following a change in light regime can be driven either by changes in N a and/or by partitioning of total leaf N between the different pools of the photosynthetic machinery, as expressed by P r and P b . In our study, the relative changes in V cmax or J max resulted equally from changes in N a and P r or P b after a HL transfer. The qualitative agreement observed for the HL treatment between the P r values computed according to [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF] and those computed directly from the measured amounts of total N and Rubisco provided some confidence in the observed behaviour of P r . Only a few studies have actually surveyed the changes in photosynthetic capacity, N a and leaf N partitioning after a change in light regime. In G. max, the decrease in V cmax resulted from a decrease in both N a and P r after shading [START_REF] Pons | Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants[END_REF]. Concurrently, the decrease in J max resulted mainly from changes in N a , whereas P b remained constant. In A. amabilis, [START_REF] Brooks | Acclimation responses of mature Abies amabilis sun foliage to shading[END_REF] showed that acclimation of photosynthetic capacity resulted only marginally from changes in N a after a high-to-low light transfer. Therefore, changes in both N a and leaf N partitioning between the different photosynthetic functions can control photosynthetic acclimation to shading in fully developed leaves.

After the LH transfer, changes in V cmax resulted mainly from an increase in N a , whereas P r computed according to [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF] decreased with time. An increase in N a after a low-to-high light transfer was observed in Q. rubra and A. saccharum [START_REF] Naidu | Acclimation of shade-developed leaves on saplings exposed to late-season canopy gaps[END_REF]. Similarly, an increase in soluble proteins was observed in H. helix after an increase in light level [START_REF] Bauer | Photosynthetic light acclimation in fully developed leaves of the juvenile and adult life phases of Hedera helix[END_REF]. However, to our knowledge, changes in the partitioning of leaf N have not been quantified accurately after a transfer of mature shade leaves to bright light. The discrepancy observed for the LH treatment between the P r values computed according to [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF] (i.e. from the V cmax values estimated on the basis of the CO 2 partial pressure in the substomatal spaces) and those computed directly from measured total N and Rubisco amounts suggests that (i) the investment of leaf N into carboxylation actually increased, and (ii) an enhanced limitation of photosynthesis by CO 2 diffusion into the leaves occurred when the photosynthetic capacity of LH leaves increased. Several studies demonstrated that a significant diffusive resistance to CO 2 exists between the intercellular spaces and carboxylation sites, which leads to underestimation of maximal carboxylation rate [START_REF] Lloyd | Low conductances for CO 2 diffusion from stomata to the sites of carboxylation in leaves of woody species[END_REF][START_REF] Loreto | Estimation of mesophyll conductance to CO 2 flux by three different methods[END_REF][START_REF] Epron | Limitation of net CO 2 assimilation rate by internal resistances to CO 2 transfer in the leaves of two tree species (Fagus sylvatica L. and Castanea sativa Mill.)[END_REF]. In our study, the internal conductance (g i ) to CO 2 transfer measured before transfer was much lower in shade (around 100 mmol m -2 s -1 ) than in sun (around 250 mmol m -2 s -1 ) leaves (C. Piel et al., unpublished results). Using these g i values to calculate V cmax led to a 35-45% increase in both computed V cmax and N investment into carboxylation, calculated from Eqn 1. This underlines the importance of internal conductance for the computation of V cmax and N investment, as suggested by [START_REF] Niinemets | A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum[END_REF].

The strong increase in the fraction of leaf N invested into light harvesting after a HL transfer is consistent with results reported by [START_REF] Evans | Partitioning of nitrogen between and within leaves grown under different irradiances[END_REF] in C. sativus, by [START_REF] Pons | Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants[END_REF] in G. max and by [START_REF] Brooks | Acclimation responses of mature Abies amabilis sun foliage to shading[END_REF] in A. amabilis. In our study and that of [START_REF] Pons | Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants[END_REF], P l acclimated fully within <2 weeks of transfer. Such an increase in P l enhances the capture of the major limiting resource for shade leaves, i.e. light [START_REF] Evans | The relationship between electron transport components and photosynthetic capacity in pea leaves grown at different irradiances[END_REF][START_REF] Evans | The allocation of protein nitrogen in the photosynthetic apparatus: costs, consequences and control[END_REF]. In contrast, the decrease of P l in LH leaves was less rapid, and P l values only acclimated fully 50 d after transfer. P l decreased only weakly in leaves of Q. rubra and A. saccharum transferred to bright light [START_REF] Naidu | Physiological and morphological acclimation of shade-grown tree seedlings to late-season canopy gap formation[END_REF].

CONCLUSIONS

In this study, we demonstrated that concurrent changes in total leaf N and N investment in carboxylation, electron transport and light harvesting drove photosynthetic acclimation in mature leaves of hybrid walnut. The relative roles of the different changes differed between HL and LH transfers and depended on the time after transfer. Our results suggest that a strong limitation of photosynthesis by CO 2 diffusion occurred after a transfer to high light. The contrast between the data obtained in 1999 and in 2000 underlined the effect of leaf age on photosynthetic acclimation ability.

Figure 1 .

 1 Figure 1. Seasonal courses of global incident radiation and daily mean air temperature from 1 January to 15 September in 1999 and 2000. Horizontal bars indicate the time and duration of bud burst. Arrows indicate the dates of transfers.

Figure 2 .

 2 Figure 2. Temporal courses of nitrogen concentration (N m ) for leaves of the HH treatment in 1999 (᭛) and 2000 (᭺); LL treatments in 1999 (×) and 2000 (L); HL treatments in 1999 (᭛) and 2000 (᭹), and LH treatments in 1999 (×) and 2000 (᭝).Arrows indicate the dates of transfers. Bars indicate standard errors (n = 4 for all treatments in 1999; in 2000, n = 3 for HH and LL and n = 4 for HL and LH). Significant effect of treatments (T) was observed only in 1999 (P = 0•014); dates (D) always had a significant effect (P < 0•001), but the interaction (I) between treatments and dates was not significant (NS).

Figure 3 .

 3 Figure 3. Temporal courses of the TNC-free leaf mass : area ratio (LMA), and the amount of starch and soluble sugars (GFS) per unit leaf area. Values are presented for high-to-high (᭺) and low-to-low (L) (both with solid lines), and high-to-low (᭹) and low-to-high (᭝) (both with dashed lines) treatments in 1999 and 2000. Arrows indicate the dates of transfers. Bars indicate standard errors (n = 4 for all treatments in 1999; in 2000, n = 3 for HH and LL and n = 4 for HL and LH).In 1999 and 2000, significant T (P < 0•001) and D (P < 0•001) effects were observed on LMA (I = NS in both years). T, D (P < 0•001) and I (P = 0•001) had significant effects on starch amount in 1999. In 2000, T and D had a strong effect (P < 0•001) on starch (I = NS). In 1999, I was NS whereas T and D had significant effects (P < 0•001) on GFS. In 2000, T, D and I had strong effects on GFS (P < 0•001). Significant differences on a few dates are indicated by letters.

Figure 4 .Figure 5 .

 45 Figure 4. Temporal courses in the amount of total leaf nitrogen (N a ), Rubisco and total chlorophylls (Total chls) per unit leaf area in 1999 and 2000. Symbols as in Fig. 3; bars indicate standard errors.In 1999 and 2000, N a was affected significantly by T and D (P < 0•001), but I was NS. T (P < 0•001) and D (P = 0•001) had significant effects on Rubisco in both years; I was only significant in 2000 (P < 0•001). In both years, T, D and I had strong effects on total chls (P = 0•001). Significant differences on a few dates are indicated by letters.

)Figure 7 .

 7 Figure 7. Variations in V cmax and J max and in their components N a , P r and P b as a function of time after HL or LH transfer. Values for HL and LH leaves are normalized to those observed on control HH and LL leaves, respectively.

Figure 8 .

 8 Figure 8. Comparison between the nitrogen investment into carboxylation computed according to Niinemets & Tenhunen (1997) (P r ) and that computed directly from measured total nitrogen and Rubisco amounts (P r measured ). Symbols as in Fig. 3. The fitted curve and equation given in the graph correspond only to LL, HH and HL treatments

© 2001 Blackwell Science Ltd, Plant, Cell and Environment, 24, 1279-1288

ACKNOWLEDGMENTS

This work was a funded part of the twinning agreement between the Institut National de Recherche Agronomique (INRA) and the Macaulay Land Use Research Institute (MLURI). E.F., X.L.R., P.M and R.W. acknowledge financial support from the Alliance programme 99-120. The PhD grant to E.F. was funded by INRA and the Auvergne region. The help of Jacqueline Liebert (Plant ecology Unit, CNRS/University Paris II) for chlorophyll analyses, of Adrian Walcroft (Lancare Research, Massey University, Palmerston North) for SAS fitting procedure and of Mike Proe (MLURI Aberdeen) for manuscript reviewing is acknowledged gratefully.


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