Photosynthetic performance along a light gradient as related to leaf characteristics of a naturally occurring <Emphasis Type="Italic">Cypripedium flavum</Emphasis>

Photosynthesis, leaf structure, nitrogen content and nitrogen allocation in photosynthetic functions of Cypripedium flavum were studied in a naturally varying light regime. Light-saturated leaf net photosynthetic rate (A (max)) was strongly correlated with leaf dry mass per area (LMA), mesophyll conductance (g (m)) and area-based leaf nitrogen content (N(area)), with all variables increasing with increasing irradiance. Such coordinate variation of all these parameters illustrates the plastic response of leaf structure to high light (HL). Leaf N(area) was greater under HL than in low light (LL). The fractions of leaf nitrogen partitioning in carboxylation (P (R)) and bioenergetics (P (B)) were positively related to LMA. In contrast, P (R) and P (B) decreased with increasing mass-based leaf nitrogen content (N(mass)). However, no correlation was found between leaf nitrogen investment in light harvesting (P (L)) and either LMA or N(mass). Like maximum rate of carboxylation (V (cmax)) and electron transport (J (max)), the J (max)/V (cmax) ratio, which was strongly correlated to LMA, also increased significantly with irradiance. Under HL, leaf maximum photosynthetic nitrogen efficiency (ANUE) and intrinsic water use efficiency (WUE) were greater than in LL conditions, despite a small difference in WUE. This suggests that a functional balance in the photosynthetic machinery favors leaf photosynthetic plasticity of C. flavum in response to different light conditions. Given an ample soil nitrogen supply, C. flavum may offset its susceptibility to HL by efficient nitrogen use and higher stomatal and mesophyll conductance against photoinhibition so as to keep leaf photosynthesis positive.     

Photosynthetic acclimation to light in woody and herbaceous species: a comparison of leaf structure, pigment content and chlorophyll fluorescence characteristics measured in the field

Acclimation of foliage photosynthetic properties occurs with varying time kinetics, but structural, chemical and physiological factors controlling the kinetics of acclimation are poorly understood, especially in field environments. We measured chlorophyll fluorescence characteristics, leaf total carotenoid (Car), chlorophyll (Chl) and nitrogen (N) content and leaf dry mass per area (LMA) along vertical light gradients in natural canopies of the herb species, Inula salicina and Centaurea jacea, and tree species, Populus tremula and Tilia cordata, in the middle of the growing season. Presence of stress was assessed on the basis of night measurements of chlorophyll fluorescence. Our aim was to compare the light acclimation of leaf traits, which respond to light availability at long (LMA and N), medium (Chl a/b ratio, Car/Chl ratio) and short time scales (fluorescence characteristics). We found that light acclimation of nitrogen content per unit leaf area (N(area)), chlorophyll content per unit dry mass (Chl(mass)) and Chl/N ratio were related to modifications in LMA. The maximum PSII quantum yield (F(v) /F(m)) increased with increasing growth irradiance in I. salicina and P. tremula but decreased in T. cordata. Leaf growth irradiance, N content and plant species explained the majority of variability in chlorophyll fluorescence characteristics, up to 90% for steady-state fluorescence yield, while the contribution of leaf total carotenoid content was generally not significant. Chlorophyll fluorescence characteristics did not differ strongly between growth forms, but differed among species within a given growth form. These data highlight that foliage acclimation to light is driven by interactions between traits with varying time kinetics.     

A leaf gas exchange model that accounts for intra-canopy variability by considering leaf nitrogen content and local acclimation to radiation in grapevine (Vitis vinifera L.)

Understanding the distribution of gas exchange within a plant is a prerequisite for scaling up from leaves to canopies. We evaluated whether leaf traits were reliable predictors of the effects of leaf ageing and leaf irradiance on leaf photosynthetic capacity (V(cmax) , J(max) ) in field-grown vines (Vitis vinifera L). Simultaneously, we measured gas exchange, leaf mass per area (LMA) and nitrogen content (N(m) ) of leaves at different positions within the canopy and at different phenological stages. Daily mean leaf irradiance cumulated over 10 d (PPFD(10) ) was obtained by 3D modelling of the canopy structure. N(m) decreased over the season in parallel to leaf ageing while LMA was mainly affected by leaf position. PPFD(10) explained 66, 28 and 73% of the variation of LMA, N(m) and nitrogen content per area (N(a) ), respectively. Nitrogen content per unit area (N(a) = LMA × N(m) ) was the best predictor of the intra-canopy variability of leaf photosynthetic capacity. Finally, we developed a classical photosynthesis-stomatal conductance submodel and by introducing N(a) as an input, the model accurately simulated the daily pattern of gas exchange for leaves at different positions in the canopy and at different phenological stages during the season.     

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

Comprehensive studies on the processes involved in photosynthetic acclimation after a sudden change in light regime are scarce, particularly for trees. We tested (i) the ability of photosynthetic acclimation in the foliage of walnut trees growing outdoors after low‐to‐high and high‐to‐low light transfers made early or late in the vegetation cycle, and (ii) the relative importance of changes in total leaf nitrogen versus changes in the partitioning of leaf nitrogen between the different photosynthetic functions during a 2 month period after transfer. Changes in maximum carboxylation rate, light‐saturated electron transport rate, respiration rate, total leaf nitrogen, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) and total chlorophylls were surveyed before and after the change in light regime. Respiration rate acclimated fully within 1 week of transfer, and full acclimation was observed 1 month after transfer for the amount of Rubisco. In contrast, total nitrogen and photosynthetic capacity acclimated only partially during the 2 month period. Changes in photosynthetic capacity were driven by changes in both total leaf nitrogen and leaf nitrogen partitioning. The extent of acclimation also depended strongly on leaf age at the time of the change in light regime.     

Complex adjustments of photosynthetic potentials and internal diffusion conductance to current and previous light availabilities and leaf age in Mediterranean evergreen species Quercus ilex

Mature non-senescent leaves of evergreen species become gradually shaded as new foliage develops and canopy expands, but the interactive effects of integrated light during leaf formation (Q(int)G), current light (Q(int)C) and leaf age on foliage photosynthetic competence are poorly understood. In Quercus ilex L., we measured the responses of leaf structural and physiological variables to Q(int)C and Q(int)G for four leaf age classes. Leaf aging resulted in increases in leaf dry mass per unit area (M(A)), and leaf dry to fresh mass ratio (D(F)) and decreases in N content per dry mass (N(M)). N content per area (N(A)) was independent of age, indicating that decreases in N(M) reflected dilution of leaf N because of accumulation of dry mass (NA = N(M) M(A)). M(A), D(F) and N(A) scaled positively with irradiance, whereas these age-specific correlations were stronger with leaf growth light than with current leaf light. Area-based maximum ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylase activity (V(cmax)A), capacity for photosynthetic electron transport (J(max)A) and the rate of non-photorespiratory respiration in light (R(d)A) were also positively associated with irradiance. Differently from leaf structural characteristics, for all data pooled, these relationships were stronger with current light with little differences among leaves of different age. Acclimation to current leaf light environment was achieved by light-dependent partitioning of N in rate-limiting proteins. Mass-based physiological activities decreased with increasing leaf age, reflecting dilution of leaf N and a larger fraction of non-photosynthetic N in older leaves. This resulted in age-dependent modification of leaf photosynthetic potentials versus N relationships. Internal diffusion conductance (g(m)) per unit area (g(m)A) increased curvilinearly with increasing irradiance for two youngest leaf age classes and was independent of light for older leaves. In contrast, g(m) per dry mass (g(m)M) was negatively associated with light in current-year leaves. Greater photosynthetic potentials and moderate changes in diffusion conductance resulted in greater internal diffusion limitations of photosynthesis in higher light. Both area- and mass-based g(m) decreased with increasing leaf age. The decrease in diffusion conductance was larger than changes in photosynthetic potentials, leading to larger CO2 drawdown from leaf internal air space to chloroplasts (delta(c)) in older leaves. The increases in diffusion limitations in older leaves and at higher light scaled with age- and light-dependent increases in MA and D(F). Overall, our study demonstrates a large potential of foliage photosynthetic acclimation to changes in leaf light environment, but also highlights enhanced structural diffusion limitations in older leaves that result from leaf structural acclimation to previous rather than to current light environment and accumulation of structural compounds with leaf age.     

Spatial distribution of leaf water-use efficiency and carbon isotope discrimination within an isolated tree crown

The spatial variations in the stable carbon isotope composition (δ¹³C) of air and leaves (total matter and soluble sugars) were quantified within the crown of a well‐watered, 20‐year‐old walnut tree growing in a low‐density orchard. The observed leaf carbon isotope discrimination (Δ) was compared with that computed by a three‐dimensional model simulating the intracanopy distribution of irradiance, transpiration and photosynthesis (previously parameterized and tested for the same tree canopy) coupled to a biophysically based model of carbon isotope discrimination. The importance of discrimination associated with CO₂ gradients encountered from the substomatal sites to the carboxylation sites was evaluated. We also assessed by simulation the effect of current irradiance on leaf gas exchange and the effect of long‐term acclimation of photosynthetic capacity and stomatal and internal conductances to light regime on intracanopy gradients in Δ. The main conclusions of this study are: (i) leaf Δ can exhibit important variations (5 and 8‰ in total leaf material and soluble sugars, respectively) along light gradients within the foliage of an isolated tree; (ii) internal conductance must be taken into account to adequately predict leaf Δ, and (iii) the spatial variations in Δ and water‐use efficiency resulted from the short‐term response of leaf gas exchange to variations in local irradiance and, to a much lesser extent, from the long‐term acclimation of leaf characteristics to the local light regime.     

Increases of chlorophyll a/b ratios during acclimation of tropical woody seedlings to nitrogen limitation and high light

According to the theory of optimal nitrogen partitioning within a leaf, the chlorophyll (Chl) a/b ratio is expected to increase when leaf N content decreases. Here, we report the first empirical support for this prediction. The Chl a/b ratio increased while Chl content decreased in response to N limitation in photosynthetic cotyledons and leaves of seedlings of four tropical woody species in the Bignoniaceae. The responses of all four species were in the same direction, but differed in magnitude. For Tabebuia rosea, the species that exhibited the greatest increase in Chl a/b ratios (up to values of 5.9), detailed photosynthetic characteristics were also examined. Light and N availability were positively correlated with the light- and CO2-saturated photosynthetic O2 evolution rate, as well as with leaf carboxylation capacity (Vcmax) and electron transport rate (Vj). Severe N limitation and high light did not cause chronic photo-inhibition (i.e. no change in quantum yield or in dark-acclimated Fv/Fm). The observed change in the ratio of Vcmax to leaf N in response to N availability was consistent with likely functional reasons for change in the Chl a/b ratio. Adjustment of the Chl a/b ratio was apparently an integral feature of acclimation to high light conditions and low N availability.     

Flexible and reversible responses to different irradiance levels during photosynthetic acclimation of Cypripedium guttatum

Cypripedium guttatum can be found both in open and shady habitats. Photosynthetic acclimation of C. guttatum to different light availabilities was detected using measurements of chlorophyll fluorescence, photosynthesis and leaf traits. When growing under low light conditions, C. guttatum exhibited a greater efficiency in photochemical utilization of absorbed light energy, and a lower ability for non-photochemical dissipation of excess light energy, as compared to the plants growing under high light conditions. Under intermediate light conditions, C. guttatum exhibited higher photosynthetic capacity (A(max)) than those under both low light or high light conditions. The differences in A(max) among three light environments was linked to the differences in biochemical efficiency, leaf N content (LNC) and leaf dry mass per unit area (LMA), but not to the differences of chlorophyll content. However, there were no significant differences in the light compensation points (LCP) and light saturation points (LSP) for photosynthesis for the plants growing under the three light conditions. These results indicate that the photosynthetic capacity of C. guttatum leaves allows for flexible and reversible responses to different irradiance levels. Photosynthetic acclimation in C. guttatum was affected by biochemical changes, the changes in LMA and ratio of Chl a/b. Successful acclimation of C. guttatum to a broad range of light levels likely allows for its wide geographical distribution. A level of about 45% sunlight appears to be optimal for photosynthesis.     

Acclimation of plants to light gradients in leaf canopies: evidence for a possible role for cytokinins transported in the transpiration stream.

The mechanism of response of plants to vertical light intensity gradients in leaf canopies was investigated. Since shaded leaves transpire less than leaves in high light, it was hypothesized that cytokinins (CKs) carried by mass transport in the transpiration stream would be distributed over the leaf area of partially shaded plants parallel to the gradient in light intensity. It was also hypothesized that this causes the distribution of leaf growth, leaf N and photosynthetic capacity, and possibly chloroplast acclimation as observed in plants growing in leaf canopies. In a field experiment, the distribution of Ca, N and CKs in a bean leaf canopy of a dense and an open stand supported the concept of a role for CKs in the response of N allocation to the light gradient when a decreasing sensitivity for CKs with increasing leaf age is assumed. Both shading of one leaf of the pair of primary bean leaves and independent reduction of its transpiration rate in a growth cabinet experiment caused lower dry mass, N and Ca per unit leaf area in comparison to the opposite not treated leaf. Shading caused a parallel reduction in CK concentration, which supports the hypothesis, but independent reduction of transpiration rate failed to do the same. Application of benzylaminopurine (BA) counteracted the reduction caused by shade of leaf N, photosynthetic capacity and leaf area growth. The experiments show an important role for the transpiration stream in the response of plants to light gradients. Evidence is presented here that CKs carried in the transpiration stream may be important mediators for the acclimation of plants to leaf canopy density.     

Diaheliotropic leaf movement enhances leaf photosynthetic capacity and photosynthetic light and nitrogen use efficiency via optimising nitrogen partitioning among photosynthetic components in cotton (Gossypium hirsutum L.)

• Phototropic leaf movement of plants is an effective mechanism for adapting to light conditions. Light is the major driver of plant photosynthesis. Leaf N is also an important limiting factor on leaf photosynthetic potential. Cotton (Gossypium hirsutum L.) exhibits diaheliotropic leaf movement. Here, we compared the long‐term photosynthetic acclimation of fixed leaves (restrained) and free leaves (allowed free movement) in cotton.
• The fixed leaves and free leaves were used for determination of PAR, leaf chlorophyll concentration, leaf N content and leaf gas exchange. The measurements were conducted under clear sky conditions at 0, 7, 15 and 30 days after treatment (DAT).
• The results showed that leaf N allocation and partitioning among different components of the photosynthetic apparatus were significantly affected by diaheliotropic leaf movement. Diaheliotropic leaf movement significantly increased light interception per unit leaf area, which in turn affected leaf mass per area (LMA), leaf N content (N_A) and leaf N allocation to photosynthesis (N_P). In addition, cotton leaves optimised leaf N allocation to the photosynthetic apparatus by adjusting leaf mass per area and N_A in response to optimal light interception.
• In the presence of diaheliotropic leaf movement, cotton leaves optimised their structural tissue and photosynthetic characteristics, such as LMA, N_A and leaf N allocation to photosynthesis, so that leaf photosynthetic capacity was maximised to improve the photosynthetic use efficiency of light and N under high light conditions.

     

不同光照条件下喜阴植物谢君魔芋对稳态和动态光源的响应特征

谢君魔芋(Amorphophallus xiei)是起源于云南西南地区热带雨林的典型喜阴植物,近年来得到了广泛种植和推广,在种植过程中,谢君魔芋需要采用遮荫栽培模式。为了揭示谢君魔芋对光照强度的适应策略,该研究探讨了生长在不同光照强度下(透光率为50%、29%、17%、7%)谢君魔芋叶片的光合作用特征、光合诱导特征、光合色素含量以及叶片氮素(N)含量和N分配。结果表明:随着生长环境光照强度的降低,单位叶面积和单位叶质量最大净光合速率、光合色素含量、最大羧化速率、最大电子传递速率及比叶面积均增大,而暗呼吸和光补偿点均减小。在光合诱导过程中,生长在透光率为17%光环境中的谢君魔芋完成50%光合诱导所需的时间最短,约为81.4 s;在光诱导进行10 min时,诱导状态最高,为87.3%。完成50%和90%光合诱导所需的时间与低光下初始气孔导度呈负相关关系。随着生长光照强度降低,叶片中的N分配到羧化组分和生物能转化组分中的比例先增大后减小,在透光率为17%的光环境下具有最大值;而叶片中的N分配到捕光色素组分中的比例随着生长环境光照强度降低而增加。该研究结果表明,喜阴植物谢君魔芋通过加强对低光和动态光源的利用能力及有效的N资源分配策略来适应低光照环境。     

Photosynthetic Characteristics,Dark Respiration,and Leaf Mass Per Unit Area in Seedlings of Four Tropical Tree Species Grown Under Three Irradiances

We investigated the effect of growth irradiance (I) on photon-saturated photosynthetic rate (P _max), dark respiration rate (R _D), carboxylation efficiency (CE), and leaf mass per unit area (LMA) in seedlings of the following four tropical tree species with contrasting shade-tolerance. Anthocephalus chinensis (Rubiaceae) and Linociera insignis (Oleaceae) are light-demanding, Barringtonia macrostachya (Lecythidaceae) and Calophyllum polyanthum (Clusiaceae) are shade-tolerant. Their seedlings were pot-planted under shading nets with 8, 25, and 50 % daylight for five months. With increase of I, all species displayed the trends of increases of LMA, photosynthetic saturation irradiance, and chlorophyll-based P _max, and decreases of chlorophyll (Chl) content on both area and mass bases, and mass-based P _max, R _D, and CE. The area-based P _max and CE increased with I for the light-demanders only. Three of the four species significantly increased Chl-based CE with I. This indicated the increase of nitrogen (N) allocation to carboxylation enzyme relative to Chl with I. Compared to the two shade-tolerants, under the same I, the two light-demanders had greater area- and Chl-based P _max, photosynthetic saturation irradiance, lower Chl content per unit area, and greater plasticity in LMA and area- or Chl-based P _max. Our results support the hypothesis that light-demanding species is more plastic in leaf morphology and physiology than shade-tolerant species, and acclimation to I of tropical seedlings is more associated with leaf morphological adjustment relative to physiology. Leaf nitrogen partitioning between photosynthetic enzymes and Chl also play a role in the acclimation to I.     

Nitrogen partitioning in the photosynthetic apparatus of Plantago asiatica leaves grown under different temperature and light conditions: similarities and differences between temperature and light acclimation

Effects of growth temperature and irradiance on nitrogen partitioning among photosynthetic components were studied. Plantago asiatica was grown under different temperature and light conditions. Growth conditions were regulated such that the Chl a/b ratio in leaves grown at a low temperature with a low irradiance was similar to that in leaves grown at a high temperature with a high irradiance, suggesting that the balance between acquisition and utilization of light energy in the photosynthetic apparatus was similar between the two growth conditions. When plotted against the leaf nitrogen content, the RuBP (ribulose-1,5-bisphosphate) carboxylase content did not significantly differ depending on growth conditions. Both high irradiance and low temperature decreased nitrogen partitioning to Chl-protein complexes. Low temperature increased nitrogen allocation to stroma FBPase (fructose-1,6-phosphatase) irrespective of growth irradiance. Gas exchange measurement indicated that the ratio of the electron transport (J(max)) to the maximum carboxylation rate (V(cmax)) was not affected by growth irradiance but by growth temperature. It is concluded that nitrogen partitioning between acquisition and utilization of light energy responds to both growth temperature and irradiance, while nitrogen partitioning between carboxylation and regeneration of RuBP responds only to growth temperature.     

The dynamics of photosynthetic acclimation to changes in light quanlity and quality in three Australian rainforest tree species

Photosynthetic acclimation was studied in seedlings of three subtropical rainforest species representing early (Omalanthus populifolius), middle (Duboisia myoporoides) and late (Acmena ingens) successional stages in forest development. Changes in the photosynthetic characteristics of pre-existing leaves were observed following the transfer of plants between deep shade (1–5% of photosynthetically active radiation (PAR), selectively filtered to produce a red/far-red (R/FR) ratio of 0.1) and open glasshouse (60% PAR and a R/FR ratio of 1.1–1.2), and vice versa. The extent and rate of response of the photosynthetic characteristics of each species to changes in light environment were recorded in this simulation of gap formation and canopy closure/overtopping. The light regimes to which plants were exposed produced significant levels of acclimation in all the photosynthetic parameters examined. Following transfer from high to low light, the light-saturated rate of photosynthesis was maintained near pre-transfer levels for 7 days, after which it decreased to levels which closely approximated those in leaves which had developed in low light. The decrease in photosynthetic capacity was associated with lower apparent quantum yields and stomatal conductances. Dark respiration was the parameter most sensitive to changes in light environment, and responded significantly during the first 4–7 days after transfer. Acclimation of photosynthetic capacity to increases in irradiance was significant in two of the three species studied, but was clearly limited in comparison with that of new leaves produced in the high light conditions. This limitation was most pronounced in the early-successional-stage species, O. populifolius. It is likely that structural characteristics of the leaves, imposed at the time of leaf expansion, are largely responsible for the limitations in photosynthetic acclimation to increases in irradiance.     

Plasticity of morphological and physiological traits in response to different levels of irradiance in seedlings of silver fir (Abies alba Mill)

The ability of silver fir ( Abies alba Mill.) to acclimate to different levels of irradiance was tested with 3-year-old seedlings, grown for 2 years in a nursery close to Nancy (eastern France) under 100, 48, 18 and 8% of incident irradiance (neutral shade nets). Growth, total nutrients in needles, maximal carboxylation rate ( V _cmax), maximal light driven electron flow ( J _max) and the relative amount of nitrogen allocated to photosynthetic processes (carboxylation, bioenergetics, light harvesting) were investigated. The sensitivity to drought stress was assessed among the phenotypes resulting from light acclimation. Leader-shoot and branch elongation were greatest under 18% irradiance. Total seedling biomass, root-to-total biomass ratio, total leaf area, leaf mass-to-area ratio and needle-area based nitrogen content responded positively to increasing irradiance while leaf area ratio decreased. Both V _cmax and J _max increased by a factor of 1.6 and 1.8, respectively, from the lowest to the highest irradiance but the ratio J _max/ V _cmax remained stable. All these parameters, expressed on a projected needle area basis, remained within the lower range of values measured for broadleaved trees. Relative allocation of needle N to the different components of the photosynthetic apparatus was very low: 12, 3 and 7% of total nitrogen were invested in carboxylation, bioenergetics and light harvesting, respectively. The relative allocation of nitrogen to carboxylation and bioenergetics remained stable while that to light harvesting decreased with increasing irradiance. During drought, seedlings pre-acclimated to shade closed their stomata at higher predawn needle water potential than those which were grown under higher irradiance. Critical temperature for PSII photochemistry in needles was unaffected by irradiance and was close to 47°C. Drought significantly increased the critical temperature up to 51°C. In general, the amplitude of responses of silver fir to changing irradiance (phenotypic plasticity) was smaller than that recorded in broadleaved species.     

植物叶片最大羧化速率及其对环境因子响应的研究进展

植物叶片最大羧化速率对环境因子的响应关系是陆地生态系统生产力与碳收支研究的重要方面。论文从测定方法、影响因子与模拟模型3方面综述了植物叶片最大羧化速率及其对环境因子响应研究的最新进展,指出现有的植物叶片最大羧化速率对单个环境因子的响应研究严重制约着陆地生态系统生产力的准确评估。为弄清植物叶片最大羧化速率对环境因子的综合响应关系,迫切需要加强以下研究:(1)植物叶片最大羧化速率的生物与环境控制机制研究;(2)生物与环境因子协同作用下的植物叶片最大羧化速率定量模拟及其尺度化研究;(3)植物叶片最大羧化速率的环境因子阈值研究。     

Light- and seasonal-induced plasticity in leaf morphology, N partitioning and photosynthetic capacity of two temperate deciduous species

Processes involved in leaf photosynthetic acclimation to light and throughout the growing season were investigated in two hardwood species (Acer saccharum and Betula alleghaniensis), which differed in their level of shade-tolerance. For both species, variation in traits related to (i) leaf morphology (LMA, leaf mass:area ratio), (ii) leaf N content (N_A, leaf nitrogen content on an area basis and N_M, N concentration in leaf dry mass), (iii) leaf N partitioning among photosynthetic functions (P_r, N allocated to Rubisco, and P_b, N allocated to bioenergetics), and (iv) leaf photosynthetic capacity (V_cmax, maximal carboxylation rates, and J_max, maximal light-driven electron flow) were assessed at three different times during the growing season (early, mid- and late summer) and under four contrasting light regimes (40, 17, 6 and 2% of full sunlight). For both species, light-driven variation in most traits was greater than their seasonally driven variation. Furthermore, results showed for both species the pre-eminence of LMA changes in the light-driven acclimation of N_A. Importance of N_M to variation in N_A was restricted to seasonal acclimation, especially for the less shade-tolerant species, B. alleghaniensis. Similarly, for both species, light-driven acclimation of leaf photosynthetic capacities was tightly related to variation in N_A, which was related to LMA changes. However, variation in P_r and P_b better explained seasonally driven variation in V_cmax and J_max, specifically under lower light levels, where N_A was low. Thus, the great variability observed for leaf activity in response to contrasting light environments was related to efficient morphological adjustments, regardless of species level of shade-tolerance. Finally, physiological adjustments were mainly involved in fine-scale changes observed during seasonally driven acclimation of leaves, when LMA was constrained to a slight range of variation.     

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C₃ species grown in photon irradiances of 200 and 1000 µmol m^?2 s^?1. Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high‐light‐grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light‐saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low‐light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low‐light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.     

Effect of fruit load and girdling on leaf photosynthesis in Mangifera indica L

Leaf nitrogen concentration (Nm), mass-to-area ratio, amount of nitrogen per unit leaf area (Na), non-structural carbohydrate concentration (TNCa), maximal rate of carboxylation (Vcmax), light-saturated rate of photosynthetic electron transport (Jmax), dark respiration (Rd), net photosynthetic assimilation (Anet), quantum yield of photosystem II (PhiPSII), and intercellular CO2 concentration (Ci) were measured in Mangifera indica L. leaves on three types of fruit-bearing branches (non-girdled, NG; girdled with high (HFL) and low (LFL) fruit load), experiencing similar light exposure. TNCa, Vcmax/Na, Jmax/Na, Rd/Na, Ci, and the initial quantum yield of photosynthetic electron transport (alpha) were similar in both HFL and NG treatments, but Nm, Na, and photosynthetic capacity parameters (Vcmax and Jmax) were lower in the HFL than in the NG treatment. The strong depressing effect of girdling on leaf nitrogen concentration cannot therefore be attributed to a change in TNCa. By contrast, Na and TNCa were lower and higher, respectively, in the LFL than in the HFL treatment, suggesting that carbohydrate content may become the driving force behind photosynthetic acclimation to changing source-sink relationships, like the ones resulting from the presence of developing fruits. Vcmax/Na and Jmax/Na were lower in the LFL than in the HFL treatment, while Rd/Na, Ci, and alpha were not affected by fruit load. It is concluded that girdling and high fruit load affect photosynthesis permanently by decreasing and increasing, respectively, leaf nitrogen concentration. Fruit load, moreover, may have an additional effect on photosynthetic capacity by affecting the relationship between Vcmax and Jmax, and Na.     

Steady-state and dynamic photosynthetic performance and nitrogen partitioning in the shade-demanding plant Panax notoginseng under different levels of growth irradiance

In this study, we examined steady-state and dynamic photosynthetic performance and leaf nitrogen (N) partitioning in the typical shade-demanding herb Panax notoginseng grown along a light gradient. Gas exchange on a leaf area basis was significantly reduced under low irradiance, with gas exchange on a leaf mass basis reaching a maximum value and then decreasing along the light gradient. Specific leaf area significantly increased with decreasing irradiance levels (P < 0.001), whereas carboxylation efficiency was decreased (P < 0.001). In addition, decreasing growth irradiance levels led to declines in maximum carboxylation rate (V _cmax) and maximum electron transport rate (J _max), although V _cmax/mass and J _max/mass were relatively less affected than V _cmax/area and J _max/area. Slow photosynthetic response to simulated sunflecks was observed under low levels of growth irradiance, with stomatal limitations only detected in leaves grown under low-light conditions. Chlorophyll content increased significantly with decreasing irradiance levels. N content on a leaf mass basis apparently increased, while N content on a leaf area basis markedly decreased. The fraction of leaf N allocated to light-harvesting components increased significantly with decreasing growth irradiance levels, whereas the fraction allocated to carboxylation and bioenergetics was significantly reduced. As an adaptation strategy to growth irradiance, we conclude that adjustments in specific leaf area may be more important than changes in leaf physiology and biochemistry in typical shade-demanding species such as P. notoginseng.