Journal of Experimental Botany, Vol. 53, No. 378, pp. 2207-2216,
November 1, 2002
© 2002 Oxford University Press
Spatial distribution of leaf nitrogen and photosynthetic capacity within the foliage of individual trees: disentangling the effects of local light quality, leaf irradiance, and transpiration
Received 14 February 2002; Accepted 18 June 2002
1 UMR-PIAF Integrated Tree Physiology, INRA-University Blaise Pascal, 234 avenue du Brézet, F-63039 Clermont-Ferrand Cedex 02, France
2 Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, Scotland, UK
3 UMR INRA-UHP ‘Ecologie-Ecophysiologie Forestières’, centre INRA de Nancy, F-54280 Champenoux, France
4 INRA-Unité d’Ecophysiologie des Plantes Fourragères, F-86600 Lusignan, France
6 To whom correspondence should be addressed. Fax: +33 4 73 62 44 54. E-mail: frak{at}clermont.inra.fr
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Abstract |
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There is presently no consensus about the factor(s) driving photosynthetic acclimation and the intra-canopy distribution of leaf characteristics under natural conditions. The impact was tested of local (i) light quality (red/far red ratio), (ii) leaf irradiance (PPFDi), and (iii) transpiration rate (E) on total non-structural carbohydrates per leaf area (TNCa), TNC-free leaf mass-to-area ratio (LMA), total leaf nitrogen per leaf area (Na), photosynthetic capacity (maximum carboxylation rate and light-saturated electron transport rate), and leaf N partitioning between carboxylation and bioenergetics within the foliage of young walnut trees grown outdoors. Light environment (quantity and quality) was controlled by placing individual branches under neutral or green screens during spring growth, and air vapour pressure deficit (VPD) was prescribed and leaf transpiration and photosynthesis measured at branch level by a branch bag technique. Under similar levels of leaf irradiance, low air vapour pressure deficit decreased transpiration rate but did not influence leaf characteristics. Close linear relationships were detected between leaf irradiance and leaf Na, LMA or photosynthetic capacity, and low R/FR ratio decreased leaf Na, LMA and photosynthetic capacity. Irradiance and R/FR also influenced the partitioning of leaf nitrogen into carboxylation and electron light transport. Thus, local light level and quality are the major factors driving photosynthetic acclimation and intra-canopy distribution of leaf characteristics, whereas local transpiration rate is of less importance.
Key words: Key words: Juglans, leaf irradiance, maximum carboxylation rate, maximum electron transport rate, photosynthetic acclimation, R/FR ratio, transpiration, walnut.
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Introduction |
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Under natural conditions, leaf anatomical, morphological, and physiological characteristics exhibit a large spatial variability at individual plant and/or canopy scale. Generally, leaves developed in lower canopy layers or in the centre of tree crowns exhibit a low amount of nitrogen (N) and a low photosynthetic capacity per unit leaf area compared with leaves of the upper canopy or at the edge of tree crowns (Hirose and Werger, 1994; Anten et al., 1996; Le Roux et al., 1999a, b). Several studies have reported the close relationship between the distribution of leaf characteristics and the mean irradiance experienced by the leaves within closed canopies (Chabot et al., 1979; Ellsworth and Reich, 1993; Evans, 1993; Pons et al., 1993; Pearcy and Sims, 1994; Anten and Werger, 1996) and/or within individual tree crowns (DeJong and Doyle, 1985; Le Roux et al., 1999a, b; Rosati et al., 1999; Frak et al., 2001; Le Roux et al., 2001a, b; Piel et al., 2002; Walcroft et al., 2002), and have suggested that mean irradiance distribution can be used as a predictor for the spatial variation of leaf properties in a given canopy. The empirical relationships between leaf irradiance (intercepted photosynthetic photon flux density, PPFDi) and leaf characteristics are widely used to scale-up photosynthesis from leaf to canopy levels (Leuning et al., 1995; Kull and Kruijt, 1999; Sinoquet et al., 2001). Nevertheless, there is still no consensus about the environmental factors and physiological mechanisms driving acclimation of leaf photosynthetic capacity within canopies.
Several microclimatic (e.g. vapour pressure deficit, light quality) and physiological (leaf transpiration and carbon gain) parameters change concurrectly with light intensity within plant canopies (Denmead, 1969; Shuttleworth, 1989; Combes et al., 2000). In particular, under natural conditions, the local mean irradiance, light quality, carbon gain, and transpiration rate are well correlated (Combes et al., 2000; Sinoquet et al., 2001). This implies that leaf irradiance is not necessarily the main driving factor for photosynthetic acclimation. The influence of light quality (red/far red ratio or blue irradiance) on leaf characteristics has been investigated (Corré, 1983; Kwesiga and Grace, 1986; Milivojevic and Eskins, 1990; Lee, 1992; Turnbull et al., 1993). Large morphogenetic responses (e.g. shoot elongation, leaf area development) to the R/FR ratio have been frequently observed (Kasperbauer, 1992; Lee, 1992; Varlet-Grancher and Gauthier, 1995), but no general response has been obtained for photosynthetic characteristics (leaf photosynthetic capacity, amount of N per unit leaf area, Na). The R/FR ratio is known to have important effects at the molecular and biochemical levels, including translatable mRNA of the small subunit of Rubisco (Smith, 1995; Batschauer, 1998; Casal et al., 1998). However, light quality only played a minor role in leaf acclimation to shade in spite of large morphogenetic responses in seedlings of tropical tree species (Tinocoojanguren and Pearcy, 1995). A decrease in leaf mass to area ratio and maximum net assimilation rate under low R/FR was observed in the rainforest tree species Terminalia ivorensis, but not in Khaya senegalensis (Kwesiga and Grace, 1986). R/FR ratio did not influence leaf photosynthetic capacity in six herbaceous species but altered dark respiration (Corré, 1983). All these results explain why the actual importance of light quality for photosynthetic acclimation is still debated.
Some recent studies have outlined the possible role of the local transpiration flux in the control of leaf N allocation and photosynthetic capacity (Pons and Bergkotte, 1996; Jordi et al., 2000; Pons et al., 2001). Because transpiration rate (E) is related to air water vapour pressure deficit (VPD) and local leaf irradiance (PPFDi), photosynthetic acclimation at leaf level could be the result of changes of both variables. Pons and Bergkotte (1996) and Pons et al. (2001) showed that a reduced VPD partly mimicked the effects of shading on leaf N and photosynthesis in bean leaves. They suggested that the local rate of import of xylem sap and, in particular, the influx of cytokinins from roots to shoots play an important role in the perception of partial shading by plants. However, their study on bean leaves was carried out at low irradiance and VPD, resulting in very low E. In such conditions, changes in E could result in marked changes in fluxes of N and cytokinins to the leaves. By contrast, even shade leaves exhibit substantial E within canopies under natural conditions (Daudet et al., 1999), which could partly jeopardize the conclusions drawn by Pons and colleagues. In addition, further studies supported the hypothesis of a potential role of compounds in the transpiration stream in the herbaceous species Lysimachia vulgaris and Humulus lupulus, but suggested that these compounds play a much smaller role in the woody species Ficus benjamina and Hedera helix (Pons and Jordi, 1998). The effects of local leaf irradiance versus light quality and/or transpiration rate on the spatial distribution of leaf N and photosynthetic capacity within the foliage of individual trees have never been studied under outdoor conditions.
The objective of this study was to test the relative contribution of (i) local light quality, (ii) local leaf irradiance and (iii) local transpiration rate to the spatial variability of leaf mass:area ratio (LMA), leaf N, leaf photosynthetic capacity (maximum carboxylation rate and light-saturated electron transport rate) and leaf N partitioning between carboxylation (Pc) and bioenergetics (Pb) in the foliage of young walnut trees. The local light environment (quantity and quality) was controlled by placing individual branches under neutral or green screens during spring growth, and air vapour pressure deficit (VPD) was controlled and leaf transpiration measured at branch scale using a branch bag technique.
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Materials and methods |
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Plant material and growth conditions
In 1999, 1-year-old hybrid walnut trees (Juglans nigraxregia) were grown outdoors in 35 dm3 pots filled with a soil/peat mixture (1:2 by vol). Walnut generally exhibits only one leaf flush (i.e. the number of leaves of this flush is determined in the buds). Surveys of leaf characteristics derived from buds initiated in sun or shade conditions after reversing the light environment between two years show that the light environment experienced during the previous year does not influence leaf characteristics (E Frak et al., unpublished observations).
Before bud break, all but two buds were removed on each tree. Eighteen individuals exhibiting a close timing of bud break were selected. To allow for the existence of contrasts between the mean radiation regime experienced by leaves within the whole plant (as observed for isolated tree crowns), only one of the two developing branches was placed under controlled conditions while the other remained under full sunlight in free air. Immediately after bud burst, the lowest branch was placed under tunnels made with acrylic screens (Altuglas, ALTUMAX, Cergy Pontoise, France) that allow ventilation. These screens generated different light conditions ranging from high light (90% of incident PPFD, including the effect of plastic films used for branch bags) to deep shade (10% of incident PPFD, Table 1) and different light qualities (R/FR ratio ranging from 0.22 to 1.25, as measured in each tunnel with a spectroradiometer (LI-1800, Li-Cor Inc., Lincoln, NE, USA, Table 1). Approximately 3 weeks after bud burst (i.e. around 15 May), when branches were around 10 cm long and only 3–4 young leaves were visible, a branch bag system was installed and used to measure gas exchange (see below) and manipulate VPD by regulating air flow rate (flow rates were high enough to ensure high leaf-to-air coupling in all the branch bags according to Daudet et al., 1999) and air temperature. Five treatments were imposed on the individual branches (Table 1). Plants were grown from 15 May to 15 July under these conditions, and they were automatically watered once or twice a day depending on the evaporative demand. All physiological and biochemical parameters were measured on single leaflets from fully developed walnut leaves about two months after bud break. Leaflets from leaves of rank 6, 7 or 8 (i.e. that emerged and developed under the prescribed microclimate) were used. Air temperature and incident photosynthetic photon flux density (PPFD) (PAR-CBE sensors, SOLEMS, Palaiseau, France) were recorded during the study period.
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Survey of gas exchange at the shoot scale
Gas exchange (net CO2 assimilation rate, A, and transpiration rate, E) was surveyed at branch scale with a branch bag system. Each bag, enclosing an individual branch, was built with thick (30 µm) polyethylene film, transparent to solar and to infrared irradiance, thus limiting greenhouse effects. Branch bags were attached to the base of growing branches as soon as they were sufficiently lignified (around 3 weeks after bud break), in the tunnels in which the branches were growing.
Individual electric fans generated an air flow from a common chimney (air inlet at 5 m height) through large flexible pipes into each branch bag. Similar pipes were used for the air outlet of each branch bag. Air flow rate was controlled via fan speed and adjusted in the range of 60–150 l min–1 according to leaf development and intensity of gas exchange in order to ensure homogeneity of atmospheric CO2 concentration among bags (from 370 ppm during the night to 330 ppm at midday). Air flow was large enough to homogenize the atmosphere within each bag without additional fans in the bags.
Air temperature was measured with a sheltered copper–constantan thermocouple located in the outlet pipe of each bag. Air vapour pressure was monitored sequentially from air samples from the inlet and outlet pipes using resistive sensors (type RHT-2, General Eastern, RFA). Differences of CO2 concentrations between inlet and outlet were monitored through differential analysis (non-dispersive infrared analyser BINOS-100, Rosemount GmbH & Co., RFA). Air was analysed for 3 min, sequentially in the different branch bags. Instantaneous photosynthesis (A) and transpiration (E) were expressed in µmol CO2 m–2 s–1 and µmol H2O m–2 s–1, respectively. The leaf area enclosed in the branch bag (La, m2) was surveyed during the study period, by measuring the length (L) and width (w) of each leaflet on each treated branch (at three dates: around 15 May, 15 June and 15 July), and using an allometric relationship between Lxw and the measured leaflet area (A): A=0.705 Lxw, n=50, r2=0.98.
Measurement of leaf photosynthetic capacity
Gas exchange at the leaf scale was measured with an infrared gas analyser-leaf chamber system (LI-6400, Li-Cor, Inc., Lincoln, NE, USA). Measurements were made on four or three leaflets per branch at the end of treatment period. Photosynthesis response curves to internal CO2 partial pressure (A–pi) were established at a leaf temperature of 25 °C and under high irradiance (1500 µmol m–2 s–1). Different CO2 partial pressures were used: 190, 150, 110, 90, 70, 50, 35, 25, 20, 15, and 10 Pa CO2. A version of the Farquhar and Von Caemmerer (1982) photosynthesis model proposed by Harley et al. (1992) was used to analyse the A–pi curves. A fitting procedure (SAS software for Solaris, version 6.12) was used to estimate the two key model parameters, i.e. maximum carboxylation rate Vcmax and the light-saturated electron transport rate Jmax (Le Roux et al., 1999a).
Leaf mass:area ratio, non-structural carbohydrates and total nitrogen
At the end of the experiment, leaflets used for A–pi curves were harvested. The areas of the leaflets were measured with an area meter (LI-3100, Li-Cor, Inc., Lincoln, NE, USA). Leaves were freeze-dried and their dry mass measured. Samples were milled, and total non-structural carbohydrates (TNC), i.e. glucose, fructose and 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 in the carbonate form and Dowex 50W in the H+ form). The amounts of GFS were determined spectrophotometrically after a hexokinase, glucose-6-phosphate linked assay (Boehringer, 1984). Starch was quantified by enzymatic assay after hydrolysis with amyloglucosidase (Boehringer, 1984). The TNC-free leaf mass:area ratio (LMA) was computed as (total leaf dry mass–TNC mass)/(fresh leaf area).
Total leaf nitrogen per unit dry mass (Nm) was measured on the same dry samples with an elemental analyser (Carlo ERBA-1108, Milan, Italy). Total leaf nitrogen was expressed per unit area (Na) as (Nmxleaf dry mass)/(fresh leaf area).
Calculations of leaf N partitioning between carboxylation and bioenergetics
The model proposed by Niinemets and Tenhunen (1997) was used to determine the coefficients for leaf N partitioning between carboxylation (mainly Rubisco) (Pc) and bioenergetics (Pb) based on measured Na values and estimated values of Vcmax and Jmax. In this model, Pc is the foliar N investment in carboxylation capacity (i.e. influencing Vcmax), and Pb is the N investment for the capacity of electron transport (i.e. influencing Jmax). Pc (g N in Rubisco {g total leaf N}–1) and Pb (g N in cytochrome f, ferredoxin NADP reductase, and coupling factor {g total leaf N}–1) are given by:
Pc=Vcmax/(6.25xVcr Na)(1)
Pb=Jmax/(8.06xJmc Na)(2)
where Vcr is the specific activity of Rubisco (i.e. the maximum rate of RuBP carboxylation per unit Rubisco protein in µmol CO2 {g Rubisco}–1 s–1), Jmc is the potential rate of photosynthetic electron transport per unit cytochrome f (mol electrons {mol cyt f}–1 s–1), 6.25 (g Rubisco {g N in Rubisco}–1) 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:ferredoxin NADP reductase:coupling factor (Niinemets and Tenhunen, 1997). According to Niinemets and Tenhunen (1997), at a leaf temperature of 25 °C, Vcr and Jmc are equal to 20.2 µmol CO2 {g Rubisco}–1 s–1 and 156 mol electrons {mol cyt f}–1 s–1, respectively. In walnut shade and sun leaves, values of nitrogen investment into carboxylation were found to be consistent with values estimated from measured total N and Rubisco amounts (Frak et al., 2001).
Shoot digitizing and calculation of leaf irradiance
At the end of the experiment, leaf location and orientation in individual shoots were recorded with an electromagnetic 3D digitizer (Fastrak, Polhemus Inc., Colchester, VT, USA) and the software 3A (Adam et al., 1999). The spatial coordinates and the orientation of each leaflet were recorded at the point of insertion between the central nerve and leaflet lamina, by setting the digitizer pointer parallel to the leaflet lamina (Sinoquet et al., 1998). Concurrently, leaflet length and width were measured with a ruler to compute leaflet area (see above).
For simulation of leaf irradiance, the VegeSTAR 1.0 software (Adam et al., 2000) was used. VegeSTAR computes light interception properties of 3D digitized plants from processing of virtual plant images (Fig. 1). Virtual leaves are given false colours in order to locate them on the image, and image processing consists of counting coloured pixels as an estimation of the projected sunlit leaf area in a given view direction (Sinoquet et al., 1998). In this experiment, for each individual shoot, mean leaf irradiance was estimated assuming diffuse light conditions. The incident diffuse radiation was approximated by a set of 46 light sources according to the turtle discretization proposed by Den Dulk (1989). Each light direction was given a fraction of total incident radiation according to the Standard Overcast Sky distribution (SOC, Moon and Spencer, 1942). For each turtle direction, the Silhouette to Total Area Ratio (STAR) was calculated as the ratio of projected sunlit leaf area to total leaf area. Mean leaf irradiance was then estimated as an average of the 46 directional STAR values weighted by the SOC distribution and multiplied by the screen-transmitted PPFD.
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Time integration and statistical analyses
In this study, leaf characteristics were correlated to PPFDi integrated over a 10 d period preceding leaf sampling. The rationale for this choice is that, on a time-scale of days to weeks, the turnover of labile C, starch and proteins allow leaf pools and photosynthetic capacity to track the average environmental conditions on that time-scale (Dewar et al., 1998).
The effects of VPD on transpiration rate and leaf characteristics (treatments III versus IV) were tested with a one-way ANOVA (Genstat Committee, 1993). The relationships between leaf characteristics and PPFDi were tested using an ANOVA analysis with R/FR and VPD as covariates (SPSS 9.0.1, SPSS Inc.).
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Results |
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Environmental variables and gas exchange of whole branches
Gas exchange data recorded over two consecutive days with contrasting irradiance are shown in Fig. 2. During sunny days, ambient VPD reached high values (up to 6 kPa, e.g. 4 July 1999). During cloudy days (e.g. 5 July 1999), ambient VPD remained below 2 kPa. VPD in branch bags of treatments I, II, III, and V were close to ambient throughout the study period, whereas it remained 30% lower than ambient in treatment IV. Air temperature was always close between treatments I, II, III, and V (difference less than 0.5 °C). On average, air temperature was around 1.5–2.0 °C lower in treatment IV than in other treatments (Table 1). The highest value for net CO2 assimilation rate (A, 11 µmol CO2 m–2 s–1) and transpiration (E, 3.3 mmol H2O m–2 s–1) were recorded on branches grown under high light (treatment I), whereas the lowest ones (A, around 5 µmol CO2 m–2 s–1 and E, around 2.2 mmol H2O m–2 s–1) were observed in treatment V. Treatment IV (low VPD) resulted in slightly higher A and lower E compared with III and V treatments.
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Effects of VPD on E, Na, LMA, Vcmax and Jmax
The effect of VPD on the different leaf characteristics was tested at low irradiance (relative PPFD 12.5%) and for a neutral shading (R/FR around 1.0) by comparing results from treatments III and IV (Table 2). Although air VPD in treatment IV was 30% lower than in treatment III and resulted in significantly lower transpiration, leaf characteristics were not significantly different between the two treatments (Table 2).
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Relationships between transpiration, carbon gain and local irradiance
A close non-linear relationship was observed between daily mean carbon gain and the simulated mean leaf irradiance over the last 10 d of the treatment period, without any significant effect of VPD and R/FR (Fig. 3). A linear relationship was observed between transpiration rate and mean leaf irradiance (Fig. 3), although the effect of VPD as covariate was significant.
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Variations in TNC-free LMA, leaf TNCa and Na with PPFDi and R/FR
A close linear relationship was observed between TNC-free LMA (r2=0.85) or Na (r2=0.83) and PPFDi (Fig. 4). In addition, a significant effect of R/FR as covariate was detected, with lower TNC-free LMA and Na observed under low R/RF. TNCa was strongly correlated to PPFDi (r2=0.78) without any significant effect of R/FR. No significant effect of VPD as covariate was detected for the three variables.
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Variation in photosynthetic capacity with PPFDi and R/FR
Linear increases in maximum carboxylation rate (from 38 to 54 µmol CO2 m–2 s–1) and light-saturated electron transport rate (from 80 to 118 µmol m–2 s–1) were observed with increasing PPFDi (r2=0.5 and 0.64 for Vcmax, and Jmax, respectively) (Fig. 5). In addition, significant effects of R/FR as covariate were detected, with lower photosynthetic capacity observed under low R/FR.
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Variation in leaf N partitioning with PPFDi and R/FR
Leaf N partitioning between carboxylation (Pc) and bioenergetics (Pb) weakly but significantly decreased with increasing PPFDi (r2=0.35 and 0.22 for Pc, and Pb, respectively) (Fig. 6). A significant effect of R/FR as covariate was detected for Pc but not for Pb.
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Discussion |
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Importance of local transpiration rate for leaf photosynthetic acclimation
Branches experiencing reduced VPD exhibited lower transpiration rates but similar leaf characteristics than those under ambient VPD. This lack of effect of transpiration flux through leaves on their photosynthetic capacity contrasts with the findings of Pons and Bergkotte (1996) and Pons et al. (2001), who observed a 20–30% decrease in Na with a 70–90% decrease in E for bean leaves treated over 1 week. However, the latter study cannot be compared easily with this study because E was very low in bean leaves and changes in E could thus result in marked changes in fluxes of N and cytokinins to the leaves. In young walnut trees (as observed within mature walnut tree crowns: Daudet et al., 1999, Sinoquet et al., 2001), E was always substantial, even for shade leaves, and changes in E thus probably induced lower changes in fluxes of N and cytokinins. This study’s findings, therefore, do not support the hypothesis that relative rates of import of xylem sap into leaves of a plant play a key role in acclimation of photosynthetic characteristics. This hypothesis assumed that compounds present in the xylem, such as cytokinins, may serve as mediators to drive photosynthetic acclimation to local environmental conditions (Pons and Bergkotte, 1996; Pons et al., 2001). Measurements of cytokinin concentration in xylem sap and/or amounts of cytokinins per unit leaf area would be necessary to test this hypothesis fully in walnut.
The present results also contrast with the close relationships observed between leaf characteristics and local E in tree canopies under natural conditions (r2=0.96 according to results on walnut from Sinoquet et al., 2001) where the range of local E values is also weak. This suggests that correlations between Na and E observed under natural conditions mainly result from the correlation between Na and PPFDi on the one hand, and the correlation between E and PPFDi on the other hand.
Importance of light quality and light level for leaf photosynthetic acclimation
Close positive relationships were observed between LMA, Na, Vcmax or Jmax and leaf irradiance (PPFDi). Similar relationships have been reported for Na, LMA, Vcmax, and Jmax in mature walnut trees (Le Roux et al., 1999a, b) and a range of tree species (DeJong and Doyle, 1985; Kull and Niinemets, 1998; Niinemets et al., 1998; Le Roux et al., 2001a). Furthermore, leaf N investment into carboxylation (Pc) and bioenergetics (Pb) significantly decreased with increasing PPFDi. Such a trend was already observed in walnut by Le Roux et al. (1999a). However, the effect of PPFDi on Pc and Pb is largely species-dependent (see Le Roux et al. (1999a), and references therein).
The effect of low red/far-red ratio was also tested for acclimation to shade. The red/far-red ratios applied to individual walnut branches ranged from 1.25 to 0.22. These values were consistent with those (ranging from 0.1 to 1.2) measured in sunlit and shaded locations within an individual 20-year-old walnut tree crown (Combes et al., 2000). In this study, LMA, Na, and photosynthetic capacity were significantly lower under low R/FR as compared to neutral shade. With six Australian rainforest tree species, Turnbull (1991) detected a species-dependent impact of light quality on photosynthetic characteristics. In two species, low R/FR (0.2) induced lower maximum assimilation rates as compared to neutral shade (R/FR=1.2) at around 15% incident irradiance, whereas no effect was observed in the other species. A decrease in leaf mass to area ratio and maximum net assimilation rate under low R/FR was observed in the rainforest tree species Terminalia ivorensis, but not in Khaya senegalensis (Kwesiga and Grace, 1986). By contrast, several studies reported no effect of low R/FR on leaf characteristics in birch seedlings (Aphalo and Lehto, 1997), and in tropical tree seedlings (Tinocoojanguren and Pearcy, 1995). The R/FR ratio was also reported to have no effect on photosynthetic capacity per unit area in herbaceous species like tobacco (Kasperbauer and Peaslee, 1973; Kasperbauer, 1988) and white clover (Heraut-Bron et al., 1999).
The R/FR ratio may influence other photosynthetic characteristics such as thylakoid morphology and chlorophyll a/b ratio (Glick et al., 1985; Chow et al., 1990; Buisson and Lee, 1993). In this study, leaf N investment into carboxylation (Pc) was significantly lower under low R/FR, but leaf N investment into bioenergetics (Pb) was not significantly influenced by R/FR.
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Conclusion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionReferences |
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It is concluded that local transpiration rate is not the major driving variable for photosynthetic acclimation within the foliage of walnut trees. Photosynthetic acclimation is mainly driven by local light level and light quality. The major effect of light level agrees with recently proposed mechanistic models that assume there are direct effects of leaf irradiance (Thornley, 1998) or the leaf C labile pool (Dewar et al., 1998; Kull and Kruijt, 1999) on photosynthetic acclimation and leaf N. The relative importance of leaf local irradiance versus local carbon gain for the spatial distribution of N and photosynthetic acclimation within plant canopies still has to be tested, but uncoupling local carbon gain and local leaf irradiance still represents a major challenge.
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Acknowledgements |
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The authors thank N Dones and S Ploquin (INRA PIAF, Clermont Ferrand) for help during shoot digitizing, P Chaleil (INRA PIAF, Clermont Ferrand) for plant management and C de Berranger (INRA Lusignan) for light quality analysis. The first two authors also thank Thijs L Pons (Utrecht University, The Netherlands) for helpful discussions. This work was funded as part of the twinning agreement between INRA (Institut National de Recherche Agronomique) and the Macaulay Institute. The Macaulay Institute received grant-in-aid funding from the Scottish Executive Environment, Agriculture and Rural Affair Department. EF, XLR, and PM acknowledge financial support from the Alliance programme 99-120. The PhD grant of EF was funded by INRA and the Auvergne region.
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References |
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