Do the capacity and kinetics for modification of xanthophyll cycle pool size depend on growth irradiance in temperate trees?

The present study investigated the interaction of growth irradiance (Q_int) with leaf capacity for and kinetics of adjustment of the pool size of xanthophyll cycle carotenoids (sum of violaxanthin, antheraxanthin and zeaxanthin; VAZ) and photosynthetic electron transport rate (J_max) after changes in leaf light environment. Individual leaves of lower‐canopy/lower photosynthetic capacity species Tilia cordata Mill. and upper canopy/higher photosynthetic capacity species Populus tremula L. were either illuminated by additional light of 500–800 µmol m^?2 s^?1 for 12 h photoperiod or enclosed in shade bags. The extra irradiance increased the total amount of light intercepted by two‐fold for the upper and 10–15‐fold for the lower canopy leaves, whereas the shade bags transmitted 45% of incident irradiance. In control leaves, VAZ/area, VAZ/Chl and J_max were positively associated with leaf growth irradiance (Q_int). After 11 d extra illumination, VAZ/Chl increased in all cases due to a strong reduction in foliar chlorophyll, but VAZ/area increased in the upper canopy leaves of both species, and remained constant or decreased in the lower canopy leaves of T. cordata. The slope for VAZ/area changes with cumulative extra irradiance was positively associated with Q_int only in T. cordata, but not in P. tremula. Nevertheless, all leaves of P. tremula increased VAZ/area more than the most responsive leaves of T. cordata. Shading reduced VAZ content only in P. tremula, but not in T. cordata, again demonstrating that P. tremula is a more responsive species. Compatible with the hypothesis of the role of VAZ in photoprotection, the rates of photosynthetic electron transport declined less in P. tremula than in T. cordata after the extra irradiance treatment. However, foliar chlorophyll contents of the exposed leaves declined significantly more in the upper canopy of P. tremula, which is not consistent with the suggestion that the leaves with the highest VAZ content are more resistant to photoinhibition. This study demonstrates that previous leaf light environment may significantly affect the adaptation capacity of foliage to altered light environment, and also that species differences in photosynthetic capacity and acclimation potentials importantly alter this interaction.     

Shape of leaf photosynthetic electron transport versus temperature response curve is not constant along canopy light gradients in temperate deciduous trees

Responses of foliar light-saturated net assimilation rate (A_max), capacity for photosynthetic electron transport (J_max) and mitochondrial respiration rate (R_d) to long-term canopy light and temperature environment were investigated in a temperate deciduous canopy composed of Populus tremula L. in the upper (17–28 m) and of Tilia cordata Mill. in the lower canopy layer (4–17 m). Climatic measurements indicated that seasonal average daily maximum air temperature (T_max) was 5·5 °C (range 0·7–10·5 °C) higher in the top than in the bottom of the canopy, and strong positive correlations were observed between T_max and seasonal average integrated quantum flux density (Q_int), as well as between seasonal average daily mean temperature and Q_int. Because of changes in leaf dry mass and nitrogen per unit area, A_max, J_max, and R_d scaled positively with Q_int in both species at a common leaf temperature (T). According to J_max versus T response curves and dark chlorophyll fluorescence transients, photosynthetic electron transport was less heat resistant in P. tremula with optimum temperature of J_max, T_opt, of 33·5 ± 0·6 °C than in T. cordata with T_opt of 40·7 ± 0·6 °C. This difference was suggested to manifest evolutionary adaptation of photosynthetic electron transport to cooler environments in P. tremula, the range of which extends farther north than that in T. cordata. Possibly because of acclimation to long-term canopy temperature environment, T_opt was positively related to Q_int in P. tremula, foliage of which was also exposed to higher irradiances and temperatures, but not in T. cordata, in the canopy of which quantum flux densities and temperatures were lower, and gradients in the environmental factors less pronounced. Parallel to changes in T_opt, the activation energy for photosynthetic electron transport decreased with increasing Q_int in P. tremula, indicating that J_max of leaves acclimated to colder environment was more responsive to T in lower temperatures than that of high T acclimated leaves. Similar alterations in the activation energy for mitochondrial respiration rate were also observed, indicating that acclimation to temperature of mitochondrial and chloroplastic electron transport proceeds in a co-ordinated manner, and possibly involves long-term changes in membrane fluidity properties. We conclude that, because of correlations between temperature and light, the shapes of J_max versus T, and R_d versus T response curves vary within tree canopies, and this needs to be taken account in modelling whole canopy photosynthesis.     

Acclimation to high irradiance in temperate deciduous trees in the field: changes in xanthophyll cycle pool size and in photosynthetic capacity along a canopy light gradient

To test the hypothesis that in temperate deciduous trees acclimation to potentially damaging high irradiances occurs via long-term adjustments in foliar photosynthetic capacity, and short-term changes in xanthophyll cycle pool size in response to weather fluctuations, nitrogen concentration and pigment composition were examined along a canopy light gradient in three species –Betula pendula, Populus tremula and Tilia cordata (from most shade intolerant to tolerant), and foliage photosynthetic potentials in P. tremula and T. cordata. Integrated quantum flux density (Q_i) incident on leaves was estimated with a method combining hemispherical photography and light measurements with quantum sensors made over the growing season. Long- and short-term light indices – average total seasonal daily integrated quantum flux density (T_s, mol m^–2 d^–1) and that of the 3 d preceding foliage sampling (T_3d) – were calculated for each sampled leaf. In addition to total integrated quantum flux density, the part of Q_i attributable to direct flux was also computed. Strong linear relationships between the capacity for photosynthetic electron transport per area (J^a_max), estimated from in situ measurements of effective quantum yield of photosystem II (PS II), and Q_i averaged over the season and over the preceding 3 d were found for all studied species. However, the major determinant of J^a_max, the product of electron transport capacity per leaf dry mass (J^m_max) and leaf dry mass per area (M_A), was M_A rather than J^m_max, which was relatively constant along the light gradient. There was evidence that J^a_max is more tightly related to T_s, which characterizes the light climate during foliar development, than to short-term integrated light, possibly because there is little flexibility in adjustments in M_A after the completion of foliar growth. Leaf chlorophyll concentrations and the investment of leaf nitrogen in chlorophyll (Chl/N) were negatively related to Q_i– an investment pattern which improves light harvesting in low light. Xanthophyll cycle pool size (VAZ, violaxanthin + antheraxanthin + zeaxanthin) either expressed per unit chlorophyll (VAZ/Chl) or as a fraction of total carotenoids (VAZ/Car) increased with increasing Q_i in all species. However, contrary to J^a_max, it tended to correlate more strongly with short-term than with long-term average integrated light. There were few interspecific differences in J^a_max, Chl/N, VAZ/Chl and VAZ/Car when the variability in light level incident to the leaves was accounted for, indicating that the foliage of both shade-intolerant and -tolerant temperate tree species possesses considerable phenotypic flexibility. Collectively these results support the view that rapid adjustment of the xanthophyll cycle pool size provides an important means for acclimation to light fluctuations in a time scale of days, during which the potential for photosynthetic quenching of excitation energy is not likely to change appreciably.     

Photosynthetic parameters,dark respiration and leaf traits in the canopy of a Peruvian tropical montane cloud forest

Few data are available describing the photosynthetic parameters of the leaves of tropical montane cloud forests (TMCF). Here, we present a study of photosynthetic leaf traits (V _cmax and J _max), foliar dark respiration (R _d), foliar nitrogen (N) and phosphorus (P), and leaf mass per area (LMA) throughout the canopy for five different TMCF species at 3025 m a.s.l. in Andean Peru. All leaf traits showed a significant relationship with canopy height when expressed on an area basis, and V _cmax-area and J _max-area almost halved when descending through the TMCF canopy. When corrected to a common temperature, average V _cmax and J _max on a leaf area basis were similar to lowland tropical values, but lower when expressed on a mass basis, because of the higher TMCF LMA values. By contrast, R _d on an area basis was higher than found in tropical lowland forests at a common temperature, and similar to lowland forests on a mass basis. The TMCF J _max–V _cmax relationship was steeper than in other tropical biomes, and we propose that this can be explained by either the light conditions or the relatively low VPD in the studied TMCF. Furthermore, V _cmax had a significant—though relatively weak and shallow—relationship with N on an area basis, but not with P, which is consistent with the general hypothesis that TMCFs are N rather than P limited. Finally, the observed V _cmax–N relationship (i.e., maximum photosynthetic nitrogen use efficiency) was distinctly different from those in tropical and temperate regions, probably because the TMCF leaves compensate for reduced Rubisco activity in cool environments.     

Predicting biochemical acclimation of leaf photosynthesis in soybean under in-field canopy warming using hyperspectral reflectance

Traditional gas exchange measurements are cumbersome, which makes it difficult to capture variation in biochemical parameters, namely the maximum rate of carboxylation measured at a reference temperature (V_cmax25) and the maximum electron transport at a reference temperature (J_max25), in response to growth temperature over time from days to weeks. Hyperspectral reflectance provides reliable measures of V_cmax25 and J_max25; however, the capability of this method to capture biochemical acclimations of the two parameters to high growth temperature over time has not been demonstrated. In this study, V_cmax25 and J_max25 were measured over multiple growth stages during two growing seasons for field-grown soybeans using both gas exchange techniques and leaf spectral reflectance under ambient and four elevated canopy temperature treatments (ambient+1.5, +3, +4.5, and +6°C). Spectral vegetation indices and machine learning methods were used to build predictive models for V_cmax25 and J_max25, based on the leaf reflectance. Results showed that these models yielded an R² of 0.57–0.65 and 0.48–0.58 for V_cmax25 and J_max25, respectively. Hyperspectral reflectance captured biochemical acclimation of leaf photosynthesis to high temperature in the field, improving spatial and temporal resolution in the ability to assess the impact of future warming on crop productivity.     

Specific leaf area and leaf nitrogen concentration in annual and perennial grass species growing in Mediterranean old-fields

 We evaluated the hypothesis that photosynthetic traits differ between leaves produced at the beginning (May) and the end (November–December) of the rainy season in the canopy of a seasonally dry forest in Panama. Leaves produced at the end of the wet season were predicted to have higher photosynthetic capacities and higher water-use efficiencies than leaves produced during the early rainy season. Such seasonal phenotypic differentiation may be adaptive, since leaves produced immediately preceding the dry season are likely to experience greater light availability during their lifetime due to reduced cloud cover during the dry season. We used a construction crane for access to the upper canopy and sampled 1- to 2-month-old leaves marked in monthly censuses for six common tree species with various ecological habits and leaf phenologies. Photosynthetic capacity was quantified as light- and CO₂-saturated oxygen evolution rates with a leaf-disk oxygen electrode in the laboratory (O_2max) and as light-saturated CO₂ assimilation rates of intact leaves under ambient CO₂ (A_max). In four species, pre-dry season leaves had significantly higher leaf mass per unit area. In these four species, O_2max and A_max per unit area and maximum stomatal conductances were significantly greater in pre-dry season leaves than in early wet season leaves. In two species, A_max for a given stomatal conductance was greater in pre-dry season leaves than in early wet season leaves, suggesting a higher photosynthetic water-use efficiency in the former. Photosynthetic capacity per unit mass was not significantly different between seasons of leaf production in any species. In both early wet season and pre-dry season leaves, mean photosynthetic capacity per unit mass was positively correlated with nitrogen content per unit mass both within and among species. Seasonal phenotypic differentiation observed in canopy tree species is achieved through changes in leaf mass per unit area and increased maximum stomatal conductance rather than by changes in nitrogen allocation patterns. Received: 7 March 1996 / Accepted: 1 August 1996     

Leaf internal diffusion conductance limits photosynthesis more strongly in older leaves of Mediterranean evergreen broad-leaved species

Leaf age-dependent changes in structure, nitrogen content, internal mesophyll diffusion conductance (g_m), the capacity for photosynthetic electron transport (J_max) and the maximum carboxylase activity of Rubisco (V_cmax) were investigated in mature non-senescent leaves of Laurus nobilis L., Olea europea L. and Quercus ilex L. to test the hypothesis that the relative significance of biochemical and diffusion limitations of photosynthesis changes with leaf age. The leaf life-span was up to 3 years in L. nobilis and O. europea and 6 years in Q. ilex. Increases in leaf age resulted in enhanced leaf dry mass per unit area (M_A), larger leaf dry to fresh mass ratio, and lower nitrogen contents per dry mass (N_M) in all species, and lower nitrogen contents per area (N_A) in L. nobilis and Q. ilex. Older leaves had lower g_m, J_max and V_cmax. Due to the age-dependent increase in M_A, mass-based g_m, J_max and V_cmax declined more strongly (7- to 10-fold) with age than area-based (5- to 7-fold) characteristics. Diffusion conductance was positively associated with foliage photosynthetic potentials. However, this correlation was curvilinear, leading to lower ratio of chloroplastic to internal CO₂ concentration (C_c/C_i) and larger drawdown of CO₂ from leaf internal air space to chloroplasts (Δ_C) in older leaves with lower g_m. Overall the age-dependent decreases in photosynthetic potentials were associated with decreases in N_M and in the fraction of N in photosynthetic proteins, whereas decreases in g_m were associated with increases in M_A and the fraction of cell walls. These age-dependent modifications altered the functional scaling of foliage photosynthetic potentials with M_A, N_M, and N_A. The species primarily differed in the rate of age-dependent modifications in foliage structural and functional characteristics, but also in the degree of age-dependent changes in various variables. Stomatal openness was weakly associated with leaf age, but due to species differences in stomatal openness, the distribution of total diffusion limitation between stomata and mesophyll varied among species. These data collectively demonstrate that in Mediterranean evergreens, structural limitations of photosynthesis strongly interact with biochemical limitations. Age-dependent changes in g_m and photosynthetic capacities do not occur in a co-ordinated manner in these species such that mesophyll diffusion constraints curb photosynthesis more in older than in younger leaves.     

Photosynthetic capacity and temperature responses of photosynthesis of rubber trees (Hevea brasiliensis Müll. Arg.) acclimate to changes in ambient temperatures

The aim of this study was to assess the temperature response of photosynthesis in rubber trees (Hevea brasiliensis Müll. Arg.) to provide data for process-based growth modeling, and to test whether photosynthetic capacity and temperature response of photosynthesis acclimates to changes in ambient temperature. Net CO₂ assimilation rate (A) was measured in rubber saplings grown in a nursery or in growth chambers at 18 and 28°C. The temperature response of A was measured from 9 to 45°C and the data were fitted to an empirical model. Photosynthetic capacity (maximal carboxylation rate, V _cmax, and maximal light driven electron flux, J _max) of plants acclimated to 18 and 28°C were estimated by fitting a biochemical photosynthesis model to the CO₂ response curves (A–C _i curves) at six temperatures: 15, 22, 28, 32, 36 and 40°C. The optimal temperature for A (T _opt) was much lower in plants grown at 18°C compared to 28°C and nursery. Net CO₂ assimilation rate at optimal temperature (A _opt), V _cmax and J _max at a reference temperature of 25°C (V _cmax25 and J _max25) as well as activation energy of V _cmax and J _max (E _aV and E _aJ) decreased in individuals acclimated to 18°C. The optimal temperature for V _cmax and J _max could not be clearly defined from our response curves, as they always were above 36°C and not far from 40°C. The ratio J _max25/V _cmax25 was larger in plants acclimated to 18°C. Less nitrogen was present and photosynthetic nitrogen use efficiency (V _cmax25/N _a) was smaller in leaves acclimated to 18°C. These results indicate that rubber saplings acclimated their photosynthetic characteristics in response to growth temperature, and that higher temperatures resulted in an enhanced photosynthetic capacity in the leaves, as well as larger activation energy for photosynthesis.     

Canopy gradients in leaf intercellular CO2 mole fractions revisited: interactions between leaf irradiance and water stress need consideration

Intercellular CO₂ mole fractions (C_i) are lower in the upper canopy relative to the lower canopy leaves. This canopy gradient in C_i has been associated with enhanced rates of carbon assimilation at high light, and concomitant greater draw‐downs in C_i. However, increases in irradiance in the canopy are generally also associated with decreases in leaf water availability. Thus, stress effects on photosynthesis rates (A) and stomatal conductance (G), may provide a further explanation for the observed C_i gradients. To test the hypotheses of the sources of canopy variation in C_i, and quantitatively assess the influence of within‐canopy differences in stomatal regulation on A, the seasonal and diurnal variation in G was studied in relation to seasonal average daily integrated quantum flux density (Q_int) in tall shade‐intolerant Populus tremula L. trees. Daily time‐courses of A were simulated using the photosynthesis model of Farquhar et al. (Planta 149, 78–90, 1980). Stable carbon isotope composition of a leaf carbon fraction with rapid turnover rate was used to estimate canopy gradient in C_i during the simulations. Daily maximum G (G_max) consistently increased with increasing Q_int. However, canopy differences in G_max decreased as soil water availability became limiting during the season. In water‐stressed leaves, there were strong mid‐day decreases in G that were poorly associated with vapour pressure deficits between the leaf and atmosphere, and the magnitude of the mid‐day decreases in G occasionally interacted with long‐term leaf light environment. Simulations indicated that the percentage of carbon lost due to mid‐day stomatal closure was of the order of 5–10%, and seasonal water stress increased this percentage up to 20%. The percentage of carbon lost due to stomatal closure increased with increasing Q_int. Canopy differences in light environment resulted in a gradient of daily average C_i of approximately 20 µmol mol^?1. The canopy variation in seasonal and diurnal reductions in G led to a C_i gradient of approximately 100 µmol mol^?1, and the actual canopy C_i gradient was of the same magnitude according to leaf carbon isotope composition. This study demonstrates that stress effects influence C_i more strongly than within‐canopy light gradients, and also that leaves acclimated to different irradiance and water stress conditions may regulate water use largely independent of foliar photosynthetic potentials.     

Acclimation of photosynthetic capacity to irradiance in tree canopies in relation to leaf nitrogen concentration and leaf mass per unit area

The observation of acclimation in leaf photosynthetic capacity to differences in growth irradiance has been widely used as support for a hypothesis that enables a simplification of some soil‐vegetation‐atmosphere transfer (SVAT) photosynthesis models. The acclimation hypothesis requires that relative leaf nitrogen concentration declines with relative irradiance from the top of a canopy to the bottom, in 1 : 1 proportion. In combination with a light transmission model it enables a simple estimate of the vertical profile in leaf nitrogen concentration (which is assumed to determine maximum carboxylation capacity), and in combination with estimates of the fraction of absorbed radiation it also leads to simple ‘big‐leaf’ analytical solutions for canopy photosynthesis. We tested how forests deviate from this condition in five tree canopies, including four broadleaf stands, and one needle‐leaf stand: a mixed‐species tropical rain forest, oak (Quercus petraea (Matt.) Liebl), birch (Betula pendula Roth), beech (Fagus sylvatica L.) and Sitka spruce (Picea sitchensis (Bong.) Carr). Each canopy was studied when fully developed (mid‐to‐late summer for temperate stands). Irradiance (Q, µmol m^?2 s^?1) was measured for 20 d using quantum sensors placed throughout the vertical canopy profile. Measurements were made to obtain parameters from leaves adjacent to the radiation sensors: maximum carboxylation and electron transfer capacity (V_a, J_a, µmol m^?2 s^?1), day respiration (R_da, µmol m^?2 s^?1), leaf nitrogen concentration (N_m, mg g^?1) and leaf mass per unit area (L_a, g m^?2). Relative to upper‐canopy values, V_a declined linearly in 1 : 1 proportion with N_a. Relative V_a also declined linearly with relative Q, but with a significant intercept at zero irradiance (P < 0·01). This intercept was strongly related to L_a of the lowest leaves in each canopy (P < 0·01, r² = 0·98, n= 5). For each canopy, daily lnQ was also linearly related with lnV_a(P < 0·05), and the intercept was correlated with the value for photosynthetic capacity per unit nitrogen (PUN: V_a/N_a, µmol g^?1 s^?1) of the lowest leaves in each canopy (P < 0·05). V_a was linearly related with L_a and N_a(P < 0·01), but the slope of the V_a : N_a relationship varied widely among sites. Hence, whilst there was a unique V_a : N_a ratio in each stand, acclimation in V_a to Q varied predictably with L_a of the lowest leaves in each canopy. The specific leaf area, L_m(cm² g^?1), of the canopy‐bottom foliage was also found to predict carboxylation capacity (expressed on a mass basis; V_m, µmol g^?1 s^?1) at all sites (P < 0·01). These results invalidate the hypothesis of full acclimation to irradiance, but suggest that L_a and L_m of the most light‐limited leaves in a canopy are widely applicable indicators of the distribution of photosynthetic capacity with height in forests.     

Leaf Biomechanics and Biomass Investment in Support in Relation to Long-Term Irradiance in Fagus

Abstract: We investigated biomass investment in support and assimilative leaf biomass in Fagus orientalis Lipsky and F. sylvatica L., and foliar biomechanical characteristics in F. orientalis to gain mechanistic insight into the determinants of leaf inclination in Fagus along the canopy light gradient. Because the leaf laminas of Fagus are elliptical, with petioles comprising only ca. 8 % of total leaf length, a leaf was approximated as a continuous sine load. Lamina load increased with increasing seasonal integrated quantum flux density in the canopy (Q_int), but leaf length was independent of irradiance. Despite greater load, leaf deflection under leaf own weight was lower for leaves at higher Q_int, indicating that foliage flexural stiffness (EI), that is a variable characterizing the resistance of beam‐like structures to bending, scaled positively with irradiance. The components of EI ‐ the leaf apparent Young's modulus of elasticity (E), which is a measure of leaf material properties, and lamina second moment of area (I), which characterizes the distribution of mass around the axis of bending ‐ were also related to irradiance, with E decreasing, but I increasing with Q_int. The positive scaling of I with Q_int was associated with increases in leaf thickness and, in particular, with increases in the degree of leaf rolling, allowing the distribution of leaf mass further away from the neutral axis. Decreases in E were correlated with decreased leaf biomass investments in the midrib at higher irradiance. Both lamina and midrib nitrogen concentrations decreased with increasing Q_int, suggesting that foliage dry mass based physiological activity was lower at higher irradiance, possibly because of an interaction of Q_int with water stress in the canopy. Given that the veins also provide a pathway for water and nutrient transport to the leaf cells, as well as for carbon translocation from the leaf, lower leaf physiological activity in high light may provide an explanation for the lower biomass investment in major veins in high light. We conclude that foliage biomechanical characteristics and leaf inclination in the canopy are significantly affected by irradiance, and that the light effects may be modified by the reverse correlation between light and water availabilities in the canopy.     

Do photosynthetic limitations of evergreen Quercus ilex leaves change with long‐term increased drought severity?

Seasonal drought can severely impact leaf photosynthetic capacity. This is particularly important for Mediterranean forests, where precipitation is expected to decrease as a consequence of climate change. Impacts of increased drought on the photosynthetic capacity of the evergreen Quercus ilex were studied for two years in a mature forest submitted to long‐term throughfall exclusion. Gas exchange and chlorophyll fluorescence were measured on two successive leaf cohorts in a control and a dry plot. Exclusion significantly reduced leaf water potential in the dry treatment. In both treatments, light‐saturated net assimilation rate (A_max), stomatal conductance (g_s), maximum carboxylation rate (V_cmax), maximum rate of electron transport (J_max), mesophyll conductance to CO₂ (g_m) and nitrogen investment in photosynthesis decreased markedly with soil water limitation during summer. The relationships between leaf photosynthetic parameters and leaf water potential remained identical in the two treatments. Leaf and canopy acclimation to progressive, long‐term drought occurred through changes in leaf area index, leaf mass per area and leaf chemical composition, but not through modifications of physiological parameters.     

Changes in leaf photosynthetic parameters with leaf position and nitrogen content within a rose plant canopy (Rosa hybrida)

This paper deals with changes in leaf photosynthetic capacity with depth in a rose (Rosa hybrida cv. Sonia) plant canopy. Measurements of leaf net CO₂ assimilation (A_l) and total nitrogen content (N_l) were performed in autumn under greenhouse conditions on mature leaves located at different layers within the plant canopy, including the flower stems and the main shoots. These leaves were subjected (i) to contrasting levels of CO₂ partial pressure (p_a) at saturating photosynthetic photon flux density (I about 1000 μ mol m ^{? 2} s ^{? 1}) and (ii) to saturating CO₂ partial pressure (p_a about 100 Pa) and varying I, while conditions of temperature were those prevailing in the greenhouse (20–38 °C). A biochemical model of leaf photosynthesis relating A_l to intercellular CO₂ partial pressure (p_i) was parameterized for each layer of leaves, supplying corresponding values of the photosynthetic Rubisco capacity (V_lm) and the maximum rate of electron transport (J_m). The results indicated that rose leaves growing at the top of the canopy had higher values of J_m and V_lm, which resulted from a higher allocation of nitrogen to the uppermost leaves. Mean values of total leaf nitrogen, N_l, decreased about 35% from the uppermost leaves of flower stem to leaves growing at the bottom of the plant. The derived values of non‐photosynthetic nitrogen, N_b, varied from 76 mmol_N m ^{? 2}_leaf (layer 1) to 60 mmol_N m ^{? 2}_leaf (layer 4), representing a large fraction of N_l (50 and 60% in layer 1 and 4, respectively). Comparison of leaf photosynthetic nitrogen (N_p = N_l–N_b) and I profiles supports the hypothesis that rose leaves acclimate to the time‐integrated absorbed I. The relationships between I and N_p, obtained during autumn, spring and summer, indicate that rose leaves seem also to acclimate their photosynthetic capacity seasonally, by allocating more photosynthetic nitrogen to leaves in autumn and spring than in summer.     

Surprising lack of sensitivity of biochemical limitation of photosynthesis of nine tree species to open‐air experimental warming and reduced rainfall in a southern boreal forest

Photosynthetic biochemical limitation parameters (i.e., V_cmax, J_max and J_max:V_cmax ratio) are sensitive to temperature and water availability, but whether these parameters in cold climate species at biome ecotones are positively or negatively influenced by projected changes in global temperature and water availability remains uncertain. Prior exploration of this question has largely involved greenhouse based short‐term manipulative studies with mixed results in terms of direction and magnitude of responses. To address this question in a more realistic context, we examined the effects of increased temperature and rainfall reduction on the biochemical limitations of photosynthesis using a long‐term chamber‐less manipulative experiment located in northern Minnesota, USA. Nine tree species from the boreal‐temperate ecotone were grown in natural neighborhoods under ambient and elevated (+3.4°C) growing season temperatures and ambient or reduced (≈40% of rainfall removed) summer rainfall. Apparent rubisco carboxylation and RuBP regeneration standardized to 25°C (V_cmax25°C and J_max25°C, respectively) were estimated based on AC_i curves measured in situ over three growing seasons. Our primary objective was to test whether species would downregulate V_cmax25°C and J_max25°C in response to warming and reduced rainfall, with such responses expected to be greatest in species with the coldest and most humid native ranges, respectively. These hypotheses were not supported, as there were no overall main treatment effects on V_cmax25°C or J_max25°C (p > .14). However, J_max:V_cmax ratio decreased significantly with warming (p = .0178), whereas interactions between warming and rainfall reduction on the J_max25°C to V_cmax25°C ratio were not significant. The insensitivity of photosynthetic parameters to warming contrasts with many prior studies done under larger temperature differentials and often fixed daytime temperatures. In sum, plants growing in relatively realistic conditions under naturally varying temperatures and soil moisture levels were remarkably insensitive in terms of their J_max25°C and V_cmax25°C when grown at elevated temperatures, reduced rainfall, or both combined.     

岩溶与非岩溶区典型植物最适光合模型和光合特征研究

为探讨不同生境典型植物光合特征对主要环境因子的响应,选择Ethier&Livingston、Ellsworth和Sharkey等3种光合生化模型拟合我国西南岩溶与非岩溶区8种典型植物的A-Ci曲线.结果表明,用Ethier&Livingston模型拟合的A-Ci曲线要优于Ellsworth模型和Sharkey模型,拟合...     

Light capture efficiency decreases with increasing tree age and size in the southern hemisphere gymnosperm Agathis australis

We investigated leaf and shoot architecture in relation to growth irradiance (Q_int) in young and mature trees of a New Zealand native gymnosperm Agathis australis (D. Don) Lindl. to determine tree size-dependent and age-dependent controls on light interception efficiency. A binomial 3-D turbid medium model was constructed to distinguish between differences in shoot light interception efficiency due to variations in leaf area density, angular distribution and leaf aggregation. Because of the positive effect of light on leaf dry mass per area (M_A), nitrogen content per area (N_A) increased with increasing irradiance in both young and mature trees. At a common irradiance, N_A, M_A and the components of M_A, density and thickness, were larger in mature trees, indicating a greater accumulation of photosynthetic biomass per unit area, but also a larger fraction of support biomass in older trees. In both young and mature trees, shoot inclination angle relative to horizontal, and leaf number per unit stem length decreased, and silhouette to total leaf area ratio (S_S) increased with decreasing irradiance, demonstrating more efficient light harvesting in low light. The shoots of young trees were more horizontal and less densely leafed with a larger S_S than those of mature trees, signifying greater light interception efficiency in young plants. Superior light harvesting in young trees resulted from more planar leaf arrangement and less clumped foliage. These results suggest that the age-dependent and/or size-dependent decreases in stand productivity may partly result from reduced light interception efficiency in larger mature relative to smaller and younger plants.     

Leaf age dependent changes in within-canopy variation in leaf functional traits: a meta-analysis

Within-canopy variation in leaf structural and photosynthetic characteristics is a major means by which whole canopy photosynthesis is maximized at given total canopy nitrogen. As key acclimatory modifications, leaf nitrogen content (N _A) and photosynthetic capacity (A _A) per unit area increase with increasing light availability in the canopy and these increases are associated with increases in leaf dry mass per unit area (M _A) and/or nitrogen content per dry mass and/or allocation. However, leaf functional characteristics change with increasing leaf age during leaf development and aging, but the importance of these alterations for within-canopy trait gradients is unknown. I conducted a meta-analysis based on 71 canopies that were sampled at different time periods or, in evergreens, included measurements for different-aged leaves to understand how within-canopy variations in leaf traits (trait plasticity) depend on leaf age. The analysis demonstrated that in evergreen woody species, M _A and N _A plasticity decreased with increasing leaf age, but the change in A _A plasticity was less suggesting a certain re-acclimation of A _A to altered light. In deciduous woody species, M _A and N _A gradients in flush-type species increased during leaf development and were almost invariable through the rest of the season, while in continuously leaf-forming species, the trait gradients increased constantly with increasing leaf age. In forbs, N _A plasticity increased, while in grasses, N _A plasticity decreased with increasing leaf age, reflecting life form differences in age-dependent changes in light availability and in nitrogen resorption for growth of generative organs. Although more work is needed to improve the coverage of age-dependent plasticity changes in some plant life forms, I argue that the age-dependent variation in trait plasticity uncovered in this study is large enough to warrant incorporation in simulations of canopy photosynthesis through the growing period.     

Disentangling the contributions of ontogeny and water stress to photosynthetic limitations in almond trees

Very few studies have attempted to disentangle the respective role of ontogeny and water stress on leaf photosynthetic attributes. The relative significance of both effects on photosynthetic attributes has been investigated in leaves of field‐grown almond trees [Prunus dulcis (Mill.) D. A. Webb] during four growth cycles. Leaf ontogeny resulted in enhanced leaf dry weight per unit area (W_a), greater leaf dry‐to‐fresh weight ratio and lower N content per unit of leaf dry weight (N_w). Concomitantly, area‐based maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max), mesophyll conductance to CO₂ diffusion (gm)′ and light‐saturated net photosynthesis (A_max) declined in both well‐watered and water‐stressed almond leaves. Although g_m and stomatal conductance (g_s) seemed to be co‐ordinated, a much stronger coordination in response to ontogeny and prolonged water stress was observed between g_m and the leaf photosynthetic capacity. Under unrestricted water supply, the leaf age‐related decline of A_max was equally driven by diffusional and biochemical limitations. Under restricted soil water availability, A_max was mainly limited by g_s and, to a lesser extent, by photosynthetic capacity and g_m. When both ontogeny and water stress effects were combined, diffusional limitations was the main determinant of photosynthesis limitation, while stomatal and biochemical limitations contributed similarly.     

Changes in needle nitrogen partitioning and photosynthesis during 80 years of tree ontogeny in Picea abies

Needle nitrogen partitioning and photosynthesis of Norway spruce were studied in a forest chronosequence in Järvselja Experimental Forest, Estonia. Current- and previous-year shoots were sampled from upper and lower canopy positions in four stands, ranging in age from 13 to 82 years. A/c _i curves were determined to obtain maximum carboxylation rate (V _cmax) and maximum rate of electron transport (J _max), whereas needle nitrogen partitioning into carboxylation (P _R), bioenergetics associated with electron transport (P _B) and thylakoid light harvesting components (P _L) was calculated from the values of V _cmax, J _max and leaf chlorophyll concentration. The greatest changes in studied needle characteristics took place between tree ages of 13 and 26 years, and this pattern was independent of needle age and canopy position. Needle mass per projected area (LMA) was lowest in the 13-year-old stand and mass-based nitrogen concentration (N^M) was generally highest in that stand. The values of LMA were significantly higher and those of N^M lower in the 26-year-old stand. Mass-based V _cmax and J _max were highest in the 13-year-old stand. Area-based photosynthetic capacity was independent of tree age. The proportion of photosynthetic nitrogen (P _R, P _B and P _L) was highest and that of non-photosynthetic nitrogen lowest in the 13-year-old stand. Current-year needles had lower LMA and P _L, but higher photosynthetic capacity compared to 1-year-old foliage. Needles from lower canopy positions exhibited lower LMA, area-based nitrogen concentration and photosynthetic capacity than needles from upper canopy. The period of substantial reductions in needle photosynthetic capacity and changes in nitrogen partitioning coincides with the onset of reproductive phase during tree ontogeny.     

A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade-tolerant species Acer saccharum

A process-based leaf gas exchange model for C₃ plants was developed which specifically describes the effects observed along light gradients of shifting nitrogen investment in carboxylation and bioenergetics and modified leaf thickness due to altered stacking of photosynthetic units. The model was parametrized for the late-successional, shade-tolerant deciduous species Acer saccharum Marsh. The specific activity of ribulose-1,5-bisphosphate carboxylase (Rubisco) and the maximum photosynthetic electron transport rate per unit cytochrome f (cyt f) were used as indices that vary proportionally with nitrogen investment in the capacities for carboxylation and electron transport. Rubisco and cyt f per unit leaf area are related in the model to leaf dry mass per area (M_A), leaf nitrogen content per unit leaf dry mass (N_m), and partitioning coefficients for leaf nitrogen in Rubisco (P_R) and in bioenergetics (P_B). These partitioning coefficients are estimated from characteristic response curves of photosynthesis along with information on lear structure and composition. While P_R and P_B determine the light-saturated value of photosynthesis, the fraction of leaf nitrogen in thylakoid light-harvesting components (P_L) and the ratio of leaf chlorophyll to leaf nitrogen invested in light harvesting (C_B), which is dependent on thylakoid stoichiometry, determine the initial photosynthetic light utilization efficiency in the model. Carbon loss due to mitochondrial respiration, which also changes along light gradients, was considered to vary in proportion with carboxylation capacity. Key model parameters - N_m, P_R, P_B, P_LC_B and stomatal sensitivity with respect to changes in net photosynthesis (G_r) – were examined as a function of M_A, which is linearly related to irradiance during growth of the leaves. The results of the analysis applied to A. saccharum indicate that P_B and P_R increase, and G_f, P_L and C_B decrease with increasing M_A. As a result of these effects of irradiaiice on nitrogen partitioning, the slope of the light-saturated net photosynthesis rate per unit leaf dry mass (A^m_max) versus N_m relationship increased with increasing growth irradiance in mid-season. Furthermore, the nitrogen partitioning coefficients as well as the slopes of A^m_max versus N_m were independent of season, except during development of the leaf photosynthetic apparatus. Simulations revealed that the acclimation to high light increased A^m_max by 40% with respect to the low light regime. However, light-saturated photosynthesis per leaf area (A^a_max) varied 3-fold between these habitats, suggesting that the acclimation to high light was dominated by adjustments in leaf anatomy (A^a_max=A^m_maxM_A) rather than in foliar biochemistry. This differed from adaptation to low light, where the alterations in foliar biochemistry were predicted to be at least as important as anatomical modifications. Due to the light-related accumulation of photosynthetic mass per unit area, A^a_max depended on M_A and leaf nitrogen per unit area (N_a). However, N_a conceals the variation in both M_A and N_m (N_a=N_mM_A), and prevents clear separation of anatomical adjustments in foliage structure and biochemical modifications in foliar composition. Given the large seasonal and site nutrient availability-related variation in N_m, and the influences of growth irradiance on nitrogen partitioning, the relationship between A^a_max and N_a is universal neither in time nor in space and in natural canopies at mid-season is mostly driven by variability in M_A. Thus, we conclude that analyses of the effects of nitrogen investments on potential carbon acquisition should use mass-based rather than area-based expressions.