Canopy Light Gradient Perception by Cytokinin

We have recently identified cytokinin as an important xylem-carried signal involved in the photosynthetic acclimation of plants to light gradients in dense canopies. Lower leaves become shaded in a dense canopy and consequently have reduced transpiration rates. our measurements have shown that this results in a reduced delivery of cytokinins carried in the transpiration stream to shaded leaves, as compared to light-exposed leaves. Cytokinins are involved in the regulation of photosynthetic acclimation to the light gradient by stimulating the expression of photosynthetic enzymes in light-exposed leaves. In shaded leaves, the low delivery rate of cytokinin leads to reduced photosynthetic capacity and ultimately senescence. We show evidence for this role of cytokinin, as part of a complex of signaling pathways where other regulatory mechanisms are also involved. A model is presented depicting the regulation of photosynthetic acclimation by cytokinin delivery to leaves dependent on the irradiance they receive.Key Words: canopy light gradient, transpiration, photosynthetic acclimation, cytokinin, nitrate, systemic signaling     

Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.     

Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.     

Photosynthesis and resource distribution through plant canopies

Plant canopies are characterized by dramatic gradients of light between canopy top and bottom, and interactions between light, temperature and water vapour deficits. This review summarizes current knowledge of potentials and limitations of acclimation of foliage photosynthetic capacity (A(max)) and light-harvesting efficiency to complex environmental gradients within the canopies. Acclimation of A(max) to high light availability involves accumulation of rate-limiting photosynthetic proteins per unit leaf area as the result of increases in leaf thickness in broad-leaved species and volume: total area ratio and mesophyll thickness in species with complex geometry of leaf cross-section. Enhancement of light-harvesting efficiency in low light occurs through increased chlorophyll production per unit dry mass, greater leaf area per unit dry mass investment in leaves and shoot architectural modifications that improve leaf exposure and reduce within-shoot shading. All these acclimation responses vary among species, resulting in species-specific use efficiencies of low and high light. In fast-growing canopies and in evergreen species, where foliage developed and acclimated to a certain light environment becomes shaded by newly developing foliage, leaf senescence, age-dependent changes in cell wall characteristics and limited foliage re-acclimation capacity can constrain adjustment of older leaves to modified light availabilities. The review further demonstrates that leaves in different canopy positions respond differently to dynamic fluctuations in light availability and to multiple environmental stresses. Foliage acclimated to high irradiance respond more plastically to rapid changes in leaf light environment, and is more resistant to co-occurring heat and water stress. However, in higher light, co-occurring stresses can more strongly curb the efficiency of foliage photosynthetic machinery through reductions in internal diffusion conductance to CO(2). This review demonstrates strong foliage potential for acclimation to within-canopy environmental gradients, but also highlights complex constraints on acclimation and foliage functioning resulting from light x foliage age interactions, multiple environmental stresses, dynamic light fluctuations and species-specific leaf and shoot structural constraints.     

RATP: a model for simulating the spatial distribution of radiation absorption, transpiration and photosynthesis within canopies: application to an isolated tree crown

The model RATP (radiation absorption, transpiration and photosynthesis) is presented. The model was designed to simulate the spatial distribution of radiation and leaf-gas exchanges within vegetation canopies as a function of canopy structure, canopy microclimate within the canopy and physical and physiological leaf properties. The model uses a three-dimensional (3D) representation of the canopy (i.e. an array of 3D cells, each characterized by a leaf area density). Radiation transfer is computed by a turbid medium analogy, transpiration by the leaf energy budget approach, and photosynthesis by the Farquhar model, each applied for sunlit and shaded leaves at the individual 3D cell-scale. The model typically operates at a 20–30 min time step. The RATP model was applied to an isolated, 20-year-old walnut tree grown in the field. The spatial distribution of wind speed, stomatal response to environmental variables, and light acclimation of leaf photosynthetic properties were taken into account. Model outputs were compared with data acquired in the field. The model was shown to simulate satisfactorily the intracrown distribution of radiation regime, transpiration and photosynthetic rates, at shoot or branch scales.     

Acclimation of photosynthesis to lightflecks in tomato leaves: interaction with progressive shading in a growing canopy

Plants in natural environments are often exposed to fluctuations in light intensity, and leaf‐level acclimation to light may be affected by those fluctuations. Concurrently, leaves acclimated to a given light climate can become progressively shaded as new leaves emerge and grow above them. Acclimation to shade alters characteristics such as photosynthetic capacity. To investigate the interaction of fluctuating light and progressive shading, we exposed three‐week old tomato (Solanum lycopersicum ) plants to either lightflecks or constant light intensities. Lightflecks of 20 s length and 1000 μmol m^?2 s^?1 peak intensity were applied every 5 min for 16 h per day, for 3 weeks. Lightfleck and constant light treatments received identical daily light sums (15.2 mol m^?2 day^?1). Photosynthesis was monitored in leaves 2 and 4 (counting from the bottom) during canopy development throughout the experiment. Several dynamic and steady‐state characteristics of photosynthesis became enhanced by fluctuating light when leaves were partially shaded by the upper canopy, but much less so when they were fully exposed to lightflecks. This was the case for CO₂‐saturated photosynthesis rates in leaves 2 and 4 growing under lightflecks 14 days into the treatment period. Also, leaf 2 of plants in the lightfleck treatment showed significantly faster rates of photosynthetic induction when exposed to a stepwise change in light intensity on day 15. As the plants grew larger and these leaves became increasingly shaded, acclimation of leaf‐level photosynthesis to lightflecks disappeared. These results highlight continuous acclimation of leaf photosynthesis to changing light conditions inside developing canopies.     

Photosynthetic acclimation to elevated CO2 in a sunflower canopy

Sunflower canopies were grown in mesocosom gas exchange chambers at ambient and elevated CO2 concentrations (360 and 700 ppm) and leaf photosynthetic capacities measured at several depths within each canopy. Elevated [CO2] had little effect on whole-canopy photosynthetic capacity and total leaf area, but had marked effects on the distribution of photosynthetic capacity and leaf area within the canopy. Elevated [CO2] did not significantly reduce the photosynthetic capacities per unit leaf area of young leaves at the top of the canopy, but it did reduce the photosynthetic capacities of older leaves by as much as 40%. This effect was not dependent on the canopy light environment since elevated [CO2] also reduced the photosynthetic capacities of older leaves exposed to full sun on the south edge of the canopy. In addition to the effects on leaf photosynthetic capacity, elevated [CO2] shifted the distribution of leaf area within the canopy so that more leaf area was concentrated near the top of the canopy. This change resulted in as much as a 50% reduction in photon flux density in the upper portions of the elevated [CO2] canopy relative to the ambient [CO2] canopy, even though there was no significant difference in the total canopy leaf area. This reduction in PFD appeared to account for leaf carbohydrate contents that were actually lower for many of the shaded leaves in the elevated as opposed to the ambient [CO2] canopy. Photosynthetic capacities were not significantly correlated with any of the individual leaf carbohydrate contents. However, there was a strong negative correlation between photosynthetic capacity and the ratio of hexose sugars to sucrose, consistent with the hypothesis that sucrose cycling is a component of the biochemical signalling pathway controlling photosynthetic acclimation to elevated [CO2].     

Light and VPD gradients drive foliar nitrogen partitioning and photosynthesis in the canopy of European beech and silver fir

While foliar photosynthetic relationships with light, nitrogen, and water availability have been well described, environmental factors driving vertical gradients of foliar traits within forest canopies are still not well understood. We, therefore, examined how light availability and vapour pressure deficit (VPD) co-determine vertical gradients (between 12 and 42 m and in the understorey) of foliar photosynthetic capacity (Amax), 13C fractionation (∆), specific leaf area (SLA), chlorophyll (Chl), and nitrogen (N) concentrations in canopies of Fagus sylvatica and Abies alba growing in a mixed forest in Switzerland in spring and summer 2017. Both species showed lower Chl/N and lower SLA with higher light availability and VPD at the top canopy. Despite these biochemical and morphological acclimations, Amax during summer remained relatively constant and the photosynthetic N-use efficiency (PNUE) decreased with higher light availability for both species, suggesting suboptimal N allocation within the canopy. ∆ of both species were lower at the canopy top compared to the bottom, indicating high water-use efficiency (WUE). VPD gradients strongly co-determined the vertical distribution of Chl, N, and PNUE in F. sylvatica, suggesting stomatal limitation of photosynthesis in the top canopy, whereas these traits were only related to light availability in A. alba. Lower PNUE in F. sylvatica with higher WUE clearly indicated a trade-off in water vs. N use, limiting foliar acclimation to high light and VPD at the top canopy. Species-specific trade-offs in foliar acclimation to environmental canopy gradients may thus be considered for scaling photosynthesis from leaf to canopy to landscape levels.     

Gas exchange acclimation to elevated CO2 in upper-sunlit and lower-shaded canopy leaves in relation to nitrogen acquisition and partitioning in wheat grown in field chambers

Growth at elevated CO₂ often decreases photosynthetic capacity (acclimation) and leaf N concentrations. Lower-shaded canopy leaves may undergo both CO₂ and shade acclimation. The relationship of acclimatory responses of flag and lower-shaded canopy leaves of wheat (Triticum aestivum L.) to the N content, and possible factors affecting N gain and distribution within the plant were investigated in a wheat crop growing in field chambers set at ambient (360 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂, and with two amounts of N fertilizer (none and 70 kg ha⁻¹ applied on 30 April). Photosynthesis, stomatal conductance and transpiration at a common measurement CO₂, chlorophyll and Rubisco levels of upper-sunlit (flag) and lower-shaded canopy leaves were significantly lower in elevated relative to ambient CO₂-grown plants. Both whole shoot N and leaf N per unit area decreased at elevated CO₂, and leaf N declined with canopy position. Acclimatory responses to elevated CO₂ were enhanced in N-deficient plants. With N supply, the acclimatory responses were less pronounced in lower canopy leaves relative to the flag leaf. Additional N did not increase the fraction of shoot N allocated to the flag and penultimate leaves. The decrease in photosynthetic capacity in both upper-sunlit and lower-shaded leaves in elevated CO₂ was associated with a decrease in N contents in above-ground organs and with lower N partitioning to leaves. A single relationship of N per unit leaf area to the transpiration rate accounted for a significant fraction of the variation among sun-lit and shaded leaves, growth CO₂ level and N supply. We conclude that reduced stomatal conductance and transpiration can decrease plant N, leading to acclimation to CO₂ enrichment.     

Comparative ecophysiology of leaf and canopy photosynthesis

Leaves and herbaceous leaf canopies photosynthesize efficiently although the distribution of light, the ultimate resource of photosynthesis, is very biased in these systems. As has been suggested in theoretical studies, if a photosynthetic system is organized such that every photosynthetic apparatus photosynthesizes in concert, the system as a whole has the sharpest light response curve and is most adaptive. This condition can be approached by (i) homogenization of the light environment and (ii) acclimation of the photosynthetic properties of leaves or chloroplasts to their local light environments. This review examines these two factors in the herbaceous leaf canopy and in the leaf. Changes in the inclination of leaves in the canopy and differentiation of mesophyll into palisade and spongy tissue contribute to the moderation of the light gradient. Leaf and chloroplast movements in the upper parts of these systems under high irradiances also moderate light gradients. Moreover, acclimation of leaves and chloroplasts to the local light environment is substantial. These factors increase the efficiency of photosynthesis considerably. However, the systems appear to be less efficient than the theoretical optimum. When the systems are optically dense, the light gradients may be too great for leaves or chloroplasts to acclimate. The loss of photosynthetic production attributed to the imperfect adjustment of photosynthetic apparatus to the local light environment is most apparent when the photosynthesis of the system is in the transition between the light-limited and light-saturated phases. Although acclimation of the photosynthetic apparatus and moderation of light gradients are imperfect, these markedly raise the efficiency of photosynthesis. Thus more mechanistic studies on these adaptive attributes are needed. The causes and consequences of imperfect adjustment should also be investigated.     

Photosynthetic responses to light in seedlings of selected Amazonian and Australian rainforest tree species

Summary Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO₂ partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO₂ partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.     

Effects of elevated [CO(2)] and nitrogen nutrition on cytokinins in the xylem sap and leaves of cotton

We measured the level of xylem-derived cytokinins (CKs) entering a cotton leaf, and the CK levels in the same leaf, thus enabling xylem sap and foliar CKs to be compared concurrently. Although zeatin was the dominant CK in xylem sap, zeatin, dihydrozeatin, and N(6)-(2-isopentenyl) adenine were present in approximately equimolar levels in leaves. Elevated [CO(2)] (EC) has an effect on the levels of cytokinins in sap and leaf tissues. This effect was modulated by the two levels of root nitrogen nutrition (2 and 12 mM nitrate). Growth enhancement (70%) in EC over plants in ambient [CO(2)] (AC) was observed for both nitrogen nutrition treatments. Low-nitrogen leaves growing in EC exhibited photosynthetic acclimation, whereas there was no sign of photosynthetic acclimation in high-nitrogen grown leaves. Under these prevailing conditions, xylem sap and leaf tissues were obtained for CK analysis. Higher nitrogen nutrition increased the delivery per unit leaf area of CKs to the leaf at AC. EC caused a greater increase in CK delivery to the leaf at low nitrogen conditions (106%) than at high nitrogen conditions (17%). EC induced a significant increase in CK content in low-nitrogen leaves, whereas CK content in leaf tissues was similar for high-nitrogen leaves growing in AC and EC.     

Photosynthetic acclimation to simultaneous and interacting environmental stresses along natural light gradients: optimality and constraints

There is a strong natural light gradient from the top to the bottom in plant canopies and along gap-understorey continua. Leaf structure and photosynthetic capacities change close to proportionally along these gradients, leading to maximisation of whole canopy photosynthesis. However, other environmental factors also vary within the light gradients in a correlative manner. Specifically, the leaves exposed to higher irradiance suffer from more severe heat, water, and photoinhibition stresses. Research in tree canopies and across gap-understorey gradients demonstrates that plants have a large potential to acclimate to interacting environmental limitations. The optimum temperature for photosynthetic electron transport increases with increasing growth irradiance in the canopy, improving the resistance of photosynthetic apparatus to heat stress. Stomatal constraints on photosynthesis are also larger at higher irradiance because the leaves at greater evaporative demands regulate water use more efficiently. Furthermore, upper canopy leaves are more rigid and have lower leaf osmotic potentials to improve water extraction from drying soil. The current review highlights that such an array of complex interactions significantly modifies the potential and realized whole canopy photosynthetic productivity, but also that the interactive effects cannot be simply predicted as composites of additive partial environmental stresses. We hypothesize that plant photosynthetic capacities deviate from the theoretical optimum values because of the interacting stresses in plant canopies and evolutionary trade-offs between leaf- and canopy-level plastic adjustments in light capture and use.     

Improved growth and water use efficiency of cherry saplings under reduced light intensity

Cherry (Prunus avium L.) saplings were grown under natural sunlight (controls) or moderate shading (up to 30%, depending on the incident light intensity and the hour of the day). Reduced light intensity increased the dry mass of each of the plant components studied. Consequently, the total dry mass of shaded plants was significantly greater than that of controls at the end of the growing season. However, the diurnal trend in the level of photosynthesis (per unit of leaf area) of shaded plants was similar to the controls in August, but lower in September. As the growing season proceeded, reduced photosynthetic rates, thinner mesophyll and larger specific leaf area in the shaded plants indicated that leaf development had adapted to shaded conditions throughout the growing season. It is suggested that increased growth of shaded plants was caused by a higher initial relative growth rate and a greater whole-plant photosynthesis. Shading consistently reduced transpiration over the season, therefore improving water use efficiency of shaded leaves. Our results suggest that a moderate reduction in light intensity can be a useful method for improving growth and saving water in hot and dry environments.     

Species-specific variation in the importance of the spectral quality gradient in canopies as a signal for photosynthetic resource partitioning

BACKGROUND AND AIMS: Plants adjust the distribution of photosynthetic capacity and chlorophyll to canopy density. The importance of the gradient in the red : far-red ratio (R : FR) relative to the irradiance gradient was studied for its perception with respect to this partitioning of photosynthetic resources. Whether the relative importance of these two signals varied between six species of different growth habit (Phaseolus vulgaris, Lysimachia vulgaris, Hedera helix, Ficus benjamina, Carex acutiformis and Brachypodium pinnatum) was investigated further. METHODS: Single leaves of plants were shaded in daylight by a spectrally neutral filter or a leaf. In another experiment, leaves were treated with supplemental FR. In most cases, treatment effects were evaluated after 2 weeks. KEY RESULTS: Nitrogen and photosynthetic capacity (Amax) per leaf area, parameters pertaining to between-leaf resource partitioning, were strongly reduced in neutral shade but not additionally by spectral leaf shade. Supplemental FR reduced these parameters also, except in Carex. Acceleration of induction of senescence was observed in spectral leaf shade in primary bean leaves. Amax per unit chlorophyll, a parameter pertaining to within-leaf resource partitioning, was reduced in neutral shade, but not in spectral leaf shade or supplemental FR. CONCLUSIONS: Signalling mechanisms associated with perception of the R : FR gradient in canopies were less important than those associated with the irradiance gradient for between-leaf and within-leaf partitioning of photosynthetic resources. The relative importance of the signals differed between species because Carex was the only species for which no indications were found for an involvement of the spectral gradient in perception of canopy density.     

Leaf nitrogen, photosynthesis, conductance and transpiration: scaling from leaves to canopies

A model is presented which solves simultaneously for leaf-scale stomatal conductance, CO₂ assimilation and the energy balance as a function of leaf position within canopies of well-watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen- and light-use efficiency), and to set bounds on the magnitudes of the parameters. For canopies low in total N, daily assimilation rates are ～10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, G_c should be calculated as G_c=Aσ(f_slg_sl+f_shg_sh), where Δ is leaf area index, f_si and f_sh are the fractions of sunlit and shaded leaves at each level, and g_si and g_sh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big-leaf’ version of the Penman-Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation. Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen- and radiation-use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2- to 3-fold variation in these efficiencies.     

美国黑核桃实生苗生态生理过程对环境因素响应的数值模拟(Ⅲ):植株冠层光合作用数理模型

以1-2年生北加州黑核桃为试材,建立了具有较高分辨能力的植株群体结构、光分布模型和冠层光合作用模型．将植株冠层按叶面积指数划分为若干层次。上下层之间水平面上太阳辐照度按Monsi＆Saeki所提出的指数递减规律分布．冠层内太阳散射光的消光系数由冠层结构决定,而直射光的消光系数则决定于冠层结构与太阳在天空的位置．在同一层次。将叶片的叶倾角划分为6个等级。将叶片的水平位置划分为8个方位．设同一层次中水平面上的太阳辐照度相同。某一方位角和叶倾角的叶面的直接辐射由太阳视运动方程决定．以此为基础,分别计算“光斑区”和“遮荫区”内叶片的光合速率,并通过数值积分计算整个冠层的光合速率及光合日总量．用田间实测资料验证了冠层内太阳辐射分布模型和冠层光合作用模型．敏感性试验分析表明。模型对环境因子和生物学因素有良好的响应．     

Optimal photosynthetic use of light by tropical tree crowns achieved by adjustment of individual leaf angles and nitrogen content

Background and Aims

Theory for optimal allocation of foliar nitrogen (ONA) predicts that both nitrogen concentration and photosynthetic capacity will scale linearly with gradients of insolation within plant canopies. ONA is expected to allow plants to efficiently use both light and nitrogen. However, empirical data generally do not exhibit perfect ONA, and light-use optimization per se is little explored. The aim was to examine to what degree partitioning of nitrogen or light is optimized in the crowns of three tropical canopy tree species.

Methods

Instantaneous photosynthetic photon flux density (PPFD) incident on the adaxial surface of individual leaves was measured along vertical PPFD gradients in tree canopies at a frequency of 0·5 Hz over 9–17 d, and summed to obtain the average daily integral of PPFD for each leaf to characterize its insolation regime. Also measured were leaf N per area (N_area), leaf mass per area (LMA), the cosine of leaf inclination and the parameters of the photosynthetic light response curve [photosynthetic capacity (A_max), dark respiration (R_d), apparent quantum yield (ϕ) and curvature (θ)]. The instantaneous PPFD measurements and light response curves were used to estimate leaf daily photosynthesis (A_daily) for each leaf.

Key Results

Leaf N_area and A_max changed as a hyperbolic asymptotic function of the PPFD regime, not the linear relationship predicted by ONA. Despite this suboptimal nitrogen partitioning among leaves, A_daily did increase linearly with PPFD regime through co-ordinated adjustments in both leaf angle and physiology along canopy gradients in insolation, exhibiting a strong convergence among the three species.

Conclusions

The results suggest that canopy tree leaves in this tropical forest optimize photosynthetic use of PPFD rather than N per se. Tropical tree canopies then can be considered simple ‘big-leaves’ in which all constituent ‘small leaves’ use PPFD with the same photosynthetic efficiency.Key words: Optimal resource allocation, nitrogen, photosynthetic capacity, leaf mass per area, tropical trees, radiation use efficiency, scaling, leaf angle, canopy architecture, big leaf model     

林下与全光下地枫皮叶片形态和光合特性的比较

测量了林下与全光下地枫皮的叶片形态和光合-光响应曲线,探讨光强对地枫皮的形态和生理特性的影响。结果表明：林下与全光下地枫皮叶片净光合速率（Pn）、气孔导度（Gs）、蒸腾速率（Tr）和水分利用效率（WUE）对光强的响应趋势均基本一致,但全光下的Pn、Gs和Tr值较高,林下WUE值较高。全光下地枫皮的最大净光合速率、光饱和点和光补偿点均极显著高于林下,但弱光下的量子效率无显著差异;林下地枫皮的叶长、叶宽、干物质重、叶面积和比叶面积等叶片形态参数均极显著大于全光。推断地枫皮为耐阴性较弱的阳生植物,其光合能力和光饱和点较低,是对干旱环境的适应性反应;全光下地枫皮叶片狭小降低了吸光面积,有利于避免过高光强对叶光合器官的损伤。