Carbon gain in a multispecies canopy: the role of specific leaf area and photosynthetic nitrogen-use efficiency in the tragedy of the commons

Models have been formulated for monospecific stands in which canopy photosynthesis is determined by the vertical distribution of leaf area, nitrogen and light. In such stands, resident plants can maximize canopy photosynthesis by distributing their nitrogen parallel to the light gradient, with high contents per unit leaf area at the top of the vegetation and low contents at the bottom. Using principles from game theory, we expanded these models by introducing a second species into the vegetation, with the same vertical distribution of biomass and nitrogen as the resident plants but with the ability to adjust its specific leaf area (SLA, leaf area:leaf mass). The rule of the game is that invaders replace the resident plants if they have a higher plant carbon gain than those of the resident plants. We showed that such invaders induce major changes in the vegetation. By increasing their SLA, invading plants could increase their light interception as well as their photosynthetic nitrogen-use efficiency (PNUE, the rate of photosynthesis per unit organic nitrogen). By comparison with stands in which canopy photosynthesis is maximized, those invaded by species of high SLA have the following characteristics: (1) the leaf area index is higher; (2) the vertical distribution of nitrogen is skewed less; (3) as a result of the supra-optimal leaf area index and the more uniform distribution of nitrogen, total canopy photosynthesis is lower. Thus, in dense canopies we face a classical tragedy of the commons: plants that have a strategy to maximize canopy carbon gain cannot compete with those that maximize their own carbon gain. However, because of this strategy, individual as well as total canopy carbon gain are eventually lower. We showed that it is an evolutionarily stable strategy to increase SLA up to the point where the PNUE of each leaf is maximized.     

Canopy structure and vertical patterns of photosynthesis and related leaf traits in a deciduous forest

Canopy structure and light interception were measured in an 18-m tall, closed canopy deciduous forest of sugar maple (Acer saccharum) in southwestern Wisconsin, USA, and related to leaf structural characteristics, N content, and leaf photosynthetic capacity. Light attenuation in the forest occurred primarily in the upper and middle portions of the canopy. Forest stand leaf area index (LAI) and its distribution with respect to canopy height were estimated from canopy transmittance values independently verified with a combined leaf litterfall and point-intersect method. Leaf mass, N and A _max per unit area (LMA, N/area and A _max/area, respectively) all decreased continuously by over two-fold from the upper to lower canopy, and these traits were strongly correlated with cumulative leaf area above the leaf position in the canopy. In contrast, neither N concentration nor A _max per unit mass varied significantly in relation to the vertical canopy gradient. Since leaf N concentration showed no consistent pattern with respect to canopy position, the observed vertical pattern in N/area is a direct consequence of vertical variation of LMA. N/area and LMA were strongly correlated with A _max/area among different canopy positions (r²=0.81 and r²=0.66, respectively), indicating that vertical variation in area-based photosynthetic capacity can also be attributed to variation in LMA. A model of whole-canopy photosynthesis was used to show that observed or hypothetical canopy mass distributions toward higher LMA (and hence higher N/area) in the upper portions of the canopy tended to increase integrated daily canopy photosynthesis over other LMA distribution patterns. Empirical relationships between leaf and canopy-level characteristics may help resolve problems associated with scaling gas exchange measurements made at the leaf level to the individual tree crown and forest canopy-level.     

Leaf senescence and decline of end-of-season gas exchange in five temperate deciduous tree species grown in elevated CO2 concentrations

We measured rates of leaf senescence and leaf level gas exchange during autumnal senescence for seedlings of five temperate forest tree species under current and elevated atmospheric CO₂ concentrations and low- and high-nutrient regimes. Relative indices of whole canopy carbon gain, water loss and water use efficiency through the senescent period were calculated based on a simple integrative model combining gas exchange per unit leaf area and standing canopy area per unit time. Seedlings grown under elevated [CO₂] generally had smaller canopies than their current [CO₂]-grown counterparts throughout most of the senescent period. This was a result of smaller pre-senescent canopies or accelerated rates of leaf drop. Leaf-level photosynthetic rates were higher under elevated [CO₂] for grey birch canopies and for low-nutrient red maple and high-nutrient ash canopies, but declined rapidly to values below those of their current [CO₂] counterparts by midway through the senescent period. CO₂ enrichment reduced photosynthetic rates for the remaining species throughout some or all of the senescent period. As a result of smaller canopy sizes and reduced photosynthetic rates, elevated [CO₂]-grown seedlings had lower indices of whole canopy end-of-season carbon gain with few exceptions. Leaf level transpiration rates were highly variable during autumnal senescence and neither [CO₂] nor nutrient regime had consistent effects on water loss per unit leaf area or integrated whole canopy water loss throughout the senescent period. Indices of whole canopy, end-of-season estimates of water use efficiency, however, were consistently lower under CO₂ enrichment, with few exceptions. These results suggest that whole canopy end-of-season gas exchange may be altered significantly in an elevated [CO₂] world, resulting in reduced carbon gain and water use efficiency for many temperate forest tree seedlings. Seedling growth and survivorship, and ultimately temperate forest regeneration, could be reduced in CO₂-enriched forests of the future.     

Relationships between leaf nitrogen and limitations of photosynthesis in canopies of Solidago altissima

Vertical distribution patterns of light, leaf nitrogen, and leaf gas exchange through canopies of the clonal perennial Solidago altissima were studied in response to mowing and fertilizer application in a field experiment. Consistent with the distribution of light, average leaf nitrogen content followed a `smooth' exponential decline along the fertilized stands both in control and mown plots. The nitrogen profile along the unfertilized stands in mown plots, however, was `disrupted' by high-nitrogen leaves at the top of shorter ramets that only reached intermediate strata of the canopies. Hence, in these stands leaf nitrogen was significantly increased in short ramets compared with tall ramets for a given light environment, suggesting suboptimal stand structure but not necessarily suboptimal single-ramet architecture. However, at least under the climatic conditions observed during measurements, such disrupture had no substantial effect on stand productivity: model calculations showed that vertical distribution patterns of leaf nitrogen along ramets only marginally influenced the photosynthetic performance of ramets and stands. This is explained by the observed photosynthesis-nitrogen relationship: the rate of photosynthesis per unit amount of leaf nitrogen did not increase with leaf nitrogen content even under saturating light levels indicating that leaf photosynthesis was not nitrogen limited during the measurement periods. Nevertheless, our study indicates that consideration of how architecture(s) of adjacent individual plants interact could be essential for a better understanding of the trade-offs between individual and canopy characteristics for maximizing carbon gain. Such trade-offs may end up in a suboptimal canopy structure, which could not be predicted and understood by classical canopy optimization models.     

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.     

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].     

棉花叶片氮含量的空间分布与光合特性

在棉花生长旺季,将冠层按高度分多层测定了田间叶片含氮量和叶片净光合速率对光合有效辐射通量密度的响应(光响应曲线,Pn-PPFD response curve)及相应的生物指标.结果表明,各层叶片氮含量与光合作用关系密切,各层平均值大小依次为上层＞中层＞下层,对应层叶片的最大净光合速率Pmax、表观暗呼吸速率Rd、光补偿点LCP及光饱和点LSP均从上到下依次递减,与氮含量分布一致,而表观光合量子效率AQY则略有不同;氮含量的指数衰减系数 kn =0.762(R2=0.593),根据观测结果,棉田叶片氮含量(N)空间分布可以用相对累积叶面积指数(Lc/Lt)为自变量的指数方程来模拟,从而为建立光合作用机理模型与进行生产力奠定基础.     

Leaf area index, canopy structure and photosynthesis of red clover (Trifolium pratense L.)

Abstract. The influence of leaf age, total leaf area and its dispersion in space on canopy photosynthesis were studied using microswards of red clover ( Trifolium pratense L.) which were established in the greenhouse. Two varieties, Renova (flowering) and Molstad (non-flowering), were sown in separate plastic boxes at densities of 225, 400 and 625 plants per m².
Vertical distribution of photosynthetically active radiation (PAR), leaf area, leaf age and ¹⁴CO₂-fixation were determined periodically. Net photosynthesis and dark respiration of canopies were measured. Maximum photosynthetic capacity of individual leaves was measured on plants taken from the intact canopy or from plants where shading of the growing leaves had been prevented.
Net photosynthetic rate of canopies increased linearly with leaf area index (LAI) up to an LAI of 3.5 and then declined at higher LAI, independent of variety and sowing density. Below the optimum LAI, net photosynthesis depended mainly on interception of PAR. Decrease in canopy photosynthesis above the optimum LAI was due to a higher proportion of old leaves with decreased photosynthetic capacity, and not to an increase in respiring plant parts. It is concluded that LAI and position of leaf age categories in the canopy are more important than vertical distribution of leaf area in determining canopy photosynthesis of red clover.     

Canopy structure and nitrogen distribution in dominant and subordinate plants in a dense stand of Amaranthus dubius L. with a size hierarchy of individuals

The objective was to investigate how nitrogen allocation patterns in plants are affected by their vertical position in the vegetation (i.e. being either dominant or subordinate). A garden experiment was carried out with Amaranthus dubius L., grown from seed, in dense stands in which a size hierarchy of nearly equally aged individuals had developed. A small number of dominant plants had most of their leaf area in the highest layers of the canopy while a larger number of subordinate plants grew in the shade of their dominant neighbours. Canopy structure, vertical patterns of leaf nitrogen distribution and leaf photosynthetic characteristics were determined in both dominant and subordinate plants. The light distribution in the stands was also measured. Average N contents per unit leaf area (total canopy nitrogen divided by the total leaf area) were higher in the dominant than in the subordinate plants and this was explained by the higher average MPA (leaf dry mass per unit area) of the dominant plants. However, when expressed on a weight basis, average N contents (LNC_av; total canopy N divided by the total dry weight of leaves) were higher in the subordinate plants. It is possible that these higher LNC_av values reflect an imbalance between carbon and nitrogen assimilation with N uptake exceeding its metabolic requirement. Leaf N content per unit area decreased more strongly with decreasing relative photon flux density in the dominant than in the subordinate plants showing that this distribution pattern can be different for plants which occupy different positions in the light gradient in the canopy. The amount of N which is reallocated from the oldest to the younger, more illuminated leaves higher up in the vegetation may depend on the sink strength of the younger leaves for nitrogen. In the subordinate plants, constrained photosynthetic activity caused by shading might have reduced the sink intensity of these leaves.     

Photosynthetic parameters from two contrasting woody vegetation types in West Africa

Measurements of leaf gas exchange were made in contrasting wooded ecosystems in West Africa. Measurements were made on 10 species: seven from humid rain forest in Cameroon and three from the semi-arid Sahelian zone in Niger. For each species, two models of photosynthesis were fitted: the first based on a rectangular hyperbolic response to photosynthetic photon flux density (Q), and the second the biochemical model of Farquhar et al. (1980). In both communities, the species studied could be divided into those characteristic of early and late successional stages, but photosynthetic parameters were not closely related to successional stage. The data identified significant relationships between V _cmax and leaf nutrient (N and P) content when expressed on an area basis. Variation in leaf mass per unit area correlated with canopy exposure and dominated the leaf nutrient signal. Statistical analysis suggested weakly that leaf gas exchange was more limited by P than N at the rain forest site.     

Coping with branch excision when measuring leaf net photosynthetic rates in a lowland tropical forest

Measuring leaf gas exchange from canopy leaves is fundamental for our understanding of photosynthesis and for a realistic representation of carbon uptake in vegetation models. Since canopy leaves are often difficult to reach, especially in tropical forests with emergent trees up to 60 m at remote places, canopy access techniques such as canopy cranes or towers have facilitated photosynthetic measurements. These structures are expensive and therefore not very common. As an alternative, branches are often cut to enable leaf gas exchange measurements. The effect of branch excision on leaf gas exchange rates should be minimized and quantified to evaluate possible bias. We compared light-saturated leaf net photosynthetic rates measured on excised and intact branches. We selected branches positioned at three canopy positions, estimated relative to the top of the canopy: upper sunlit foliage, middle canopy foliage, and lower canopy foliage. We studied the variation of the effects of branch excision and transport among branches at these different heights in the canopy. After excision and transport, light-saturated leaf net photosynthetic rates were close to zero for most leaves due to stomatal closure. However, when the branch had acclimated to its new environmental conditions—which took on average 20 min—light-saturated leaf net photosynthetic rates did not significantly differ between the excised and intact branches. We therefore conclude that branch excision does not affect the measurement of light-saturated leaf net photosynthesis, provided that the branch is recut under water and is allowed sufficient time to acclimate to its new environmental conditions.     

A model of dynamics of leaves and nitrogen in a plant canopy: an integration of canopy photosynthesis,leaf life span,and nitrogen use efficiency

A model of dynamics of leaves and nitrogen is developed to predict the effect of environmental and ecophysiological factors on the structure and photosynthesis of a plant canopy. In the model, leaf area in the canopy increases by the production of new leaves, which is proportional to the canopy photosynthetic rate, with canopy nitrogen increasing with uptake of nitrogen from soil. Then the optimal leaf area index (LAI; leaf area per ground area) that maximizes canopy photosynthesis is calculated. If leaf area is produced in excess, old leaves are eliminated with their nitrogen as dead leaves. Consequently, a new canopy having an optimal LAI and an optimal amount of nitrogen is obtained. Repeating these processes gives canopy growth. The model provides predictions of optimal LAI, canopy photosynthetic rates, leaf life span, nitrogen use efficiency, and also the responses of these factors to changes in nitrogen and light availability. Canopies are predicted to have a larger LAI and a higher canopy photosynthetic rate at a steady state under higher nutrient and/or light availabilities. Effects of species characteristics, such as photosynthetic nitrogen use efficiency and leaf mass per area, are also evaluated. The model predicts many empirically observed patterns for ecophysiological traits across species.     

沿林冠开度梯度的银杉幼树对光的适应性

用气体交换技术测定了四川省金佛山生长在一个典型林隙(林隙面积约为100 m²)不同位置的银杉(Cathaya argyrophylla)幼树叶片的部分光合生理生态指标,用改进的鱼眼镜头法测定了所测叶片的林冠开度(CO),并测定了这些银杉幼树个体的生长状况。研究了沿林冠开度梯度银杉幼树对光的适应性。银杉幼树在林隙边缘表现出较好的适应性,包括当年生树高生长速率(HG)、平均地茎生长速率(DG)和当年冠幅生长速率(CG)。随林冠开度的增加,单位叶面积的最大净光合速率(P_narea)、暗呼吸速率(R_darea)和单位叶面积的叶片干物质重(比叶重,SLM)明显增加,且相关显著。随林冠开度的增加,单位叶面积(LA)和单位叶干物质重的暗呼吸速率(R_dmass)略有降低,而单位叶片干物质重的最大净光合速率(P_nmass)和单位叶片干物质重(LM)略有升高,但相关不显著。沿林冠开度的银杉幼树的形态可塑性要略低于生理生态可塑性,但差异不显著。     

The phenology of leaf quality and its within‐canopy variation is essential for accurate modeling of photosynthesis in tropical evergreen forests

Leaf quantity (i.e., canopy leaf area index, LAI), quality (i.e., per‐area photosynthetic capacity), and longevity all influence the photosynthetic seasonality of tropical evergreen forests. However, these components of tropical leaf phenology are poorly represented in most terrestrial biosphere models (TBMs). Here, we explored alternative options for the representation of leaf phenology effects in TBMs that employ the Farquahar, von Caemmerer & Berry (FvCB) representation of CO₂ assimilation. We developed a two‐fraction leaf (sun and shade), two‐layer canopy (upper and lower) photosynthesis model to evaluate different modeling approaches and assessed three components of phenological variations (i.e., leaf quantity, quality, and within‐canopy variation in leaf longevity). Our model was driven by the prescribed seasonality of leaf quantity and quality derived from ground‐based measurements within an Amazonian evergreen forest. Modeled photosynthetic seasonality was not sensitive to leaf quantity, but was highly sensitive to leaf quality and its vertical distribution within the canopy, with markedly more sensitivity to upper canopy leaf quality. This is because light absorption in tropical canopies is near maximal for the entire year, implying that seasonal changes in LAI have little impact on total canopy light absorption; and because leaf quality has a greater effect on photosynthesis of sunlit leaves than light limited, shade leaves and sunlit foliage are more abundant in the upper canopy. Our two‐fraction leaf, two‐layer canopy model, which accounted for all three phenological components, was able to simulate photosynthetic seasonality, explaining ~90% of the average seasonal variation in eddy covariance‐derived CO₂ assimilation. This work identifies a parsimonious approach for representing tropical evergreen forest photosynthetic seasonality in TBMs that utilize the FvCB model of CO₂ assimilation and highlights the importance of incorporating more realistic phenological mechanisms in models that seek to improve the projection of future carbon dynamics in tropical evergreen forests.     

Functional significance of shade-induced leaf senescence in dense canopies: an experimental test using transgenic tobacco

Canopy photosynthesis models have predicted an optimal leaf area index (LAI; leaf area per unit surface area) and leaf nitrogen distribution at which whole-plant carbon gain per unit N is maximized. In this study we experimentally tested these models, using transgenic P(SAG12)-IPT tobacco (SAG; Nicotiana tabacum L.) plants with delayed leaf senescence and therefore a greater LAI and more uniform N distribution than the wild type (WT). In a competition experiment, the increased density of surrounding WT plants caused a greater reduction in dry mass of mature SAG target plants than in that of WT target plants, indicating negative effects of delayed leaf senescence on performance at high canopy density. Vegetative SAG plants achieved a lower calculated daily carbon gain than competing WT plants because the former retained leaves with a negative carbon gain in the shaded, lower part of the canopy. Sensitivity analyses showed that the carbon gain of SAG plants would increase if these lower leaves were shed and the N reallocated from these leaves were used to form additional leaf area at the canopy top. This strategy, which is adopted by the WT, is most advantageous because it results in the shading of competing neighbors.     

Photosynthesis-Nitrogen Relationships in Pioneer Plants of Disturbed Tropical Montane Forest Sites

Tropical forest disturbances lead to the establishment of secondary successional plant communities constituted by light demanding species with high relative growth rate that conduct to rapid canopy closure. Two main strategies for N nutrition are: (a) mineral N acquisition in the form of NH4 and NO3, and (b) symbiotic atmospheric N2 fixation. Given the high N requirement for maximization of leaf area and radiant energy absorption, we hypothesize that contrasting strategies of N nutrition in these environments are reflected in leaf photosynthetic characteristics. We compared the N-photosynthesis relationships and carbon balance parameters per unit leaf area as they vary with age in two species with contrasting N acquisition strategies: a N2-fixer Crotalaria anagyroides HBK (Papilionoideae), and a mineral-N user Verbesina turbacensis HBK (Asteraceae). N2 fixation capacity was associated to higher specific leaf area (SLA), higher photosynthetic capacity (Pmax) per unit leaf area and leaf mass, and higher N content per unit leaf mass. The N2-fixer species showed higher slope in the relationship Pmax-N per unit leaf mass and area when compared to the leaves of non-fixer species. Moreover, the intrinsic photosynthetic N use efficiency (Pmax/N) was higher in the N2 fixer than in leaves of the non-fixer species. Changes in N due to leaf age resulted in larger changes in CO2 flux density at the leaf level in the N2-fixer species. The higher photosynthetic capacity of the N2-fixer species was mechanistically related to higher stomatal conductance, internal CO2 concentration (ci) values closer to atmospheric CO2 concentration (ca), and lower intrinsic water use efficiency than the mineral N-user species. Despite their higher Pmax per unit leaf area, total non-structural saccharides concentration was lower in mature leaves of the N2-fixer plant as compared to the non-fixer counterpart. This might be caused by the presence of a larger root sink (symbionts) stimulating saccharides export and higher diurnal respiration rates.     

Light-exposed shoots of seven coexisting deciduous species show common photosynthetic responses to tree height

Functional traits of light-exposed leaves have been reported to show tree height-dependent change. However, it remains unknown how plastic response of leaf traits to tree height is linked with shoot-level carbon gain. To answer this question, we examined the photosynthetic properties of fully lit current-year shoots in crown tops with various heights for seven deciduous broad-leaved species dominated in a cool–temperate forest in northern Japan. We measured leaf mass, stomatal conductance, nitrogen content, light-saturated net photosynthetic rate (all per leaf lamina area), foliar stable carbon isotope ratio, and shoot mass allocation to leaf laminae. We employed hierarchical Bayesian models to simultaneously quantify inter-trait relationships for all species. We found that leaf and shoot traits were co-varied in association with height, and that there was no quantitative inter-specific difference in leaf- and shoot-level plastic responses to height. Nitrogen content increased and stomatal conductance decreased with height. Reflecting these antagonistic responses to height, photosynthetic rate was almost unchanged with height. Photosynthetic rate divided by stomatal conductance as a proxy of photosynthetic water use efficiency sufficiently explained the variation of foliar carbon isotope ratio. The increase in mass allocation to leaves in a shoot compensated for the height-dependent decline in photosynthetic rate per leaf lamina mass. Consequently, photosynthetic gain at the scale of current-year shoot mass was kept unchanged with tree height. We suggest that the convergent responses of shoot functional traits across species reflect common requirements for trees coexisting in a forest.     

栲树冠层光合生理特性的空间异质性

森林冠层在能量传输、光合有效辐射和微气象等方面的差异可导致冠层光合生产力空间分布的变化.叶片光合生理特性在空间上的差异对精确估算森林冠层的初级生产力十分重要.本文以亚热带常绿阔叶林优势种---栲树(Castanopsis fargesii)为对象,研究叶片光合生理特性在冠层空间上的变化.结果表明:1)在垂直方向上,冠层北向叶的饱和光合速率(Amax)、光饱和点(LSP)和CO2羧化效率(CCE)均表现为上部>中部>底部,且依次平均降低19.4%、18.1%和37.1%;光补偿点(LCP)、光下暗呼吸(Rd)以及冠层南向叶的饱和光合速率、光饱和点和CO2羧化效率均表现为上部>底部>中部,上部比中部和底部高出12.3%~71.4%;表观量子效率(AQY)表现为底部>上部>中部,底部分别是顶部和中部叶的1.2和1.3倍;2)在水平方向上,冠层上部和底部南向叶的饱和光合速率、光饱和点和CO2羧化效率比北向叶高0.9%~31.5%;冠层中部北向叶的饱和光合速率等6个参数比南向叶高9.6%~63.2%.因此,在冠层水平上模拟和估算植物生产力时,必须考虑冠层光合生理特性的空间差异.     

The vertical distribution of nitrogen and photosynthetic activity at different plant densities in Carex acutiformis

Shoots of the monocotyledonous perennial Carex acutiformis were grown in open (28 shoots m⁻²) and dense stands (280 shoots m⁻²). For fully grown stands the distribution of relative PPFD and leaf nitrogen per unit leaf area over canopy depth was determined. Light response of photosynthesis was measured on leaf segments sampled at various heights in the stands. Relations between parameters of these curves and leaf nitrogen were investigated. Simulations showed that in the open stand daily canopy photosynthesis was not affected by nitrogen redistribution in the canopy. For the dense stand however, a uniform nitrogen distribution would lead to only 73% of the maximum net carbon gain by the stand under optimal nitrogen distribution. The actual canopy photosynthesis was only 7% less than this theoretical maximum; the actual nitrogen distribution of the dense stand clearly tended to the optimal distribution. The vertical pattern of the nitrogen distribution was to a large extent determined by the minimum leaf nitrogen content. The relatively high minimum leaf nitrogen content found for Carex leaves may perhaps be necessary to maintain the physiological function of the basal parts of the leaves.     

A Vertical Gradient of the Chloroplast Abundance among Leaves of Chenopodium album

The abundances of chloroplasts in leaves on the main stems ofChenopodium album at different height levels were investigatedin relation to the photosynthetic capacity and light environmentof the leaves. (1) The number of chloroplasts per mesophyllcell decreased with descending position of leaves, except foryoung developing leaves at the top of plants that had smallerchloroplast numbers per cell than matured leaves beneath them.Contents of chlorophyll and ribulose-1,5-bisphosphate carboxylase/oxygenaseper leaf area that were highest in the topmost young leavesand decreased with decreasing height level indicate that thereis a vertical gradient of chloroplast abundance per leaf areadecreasing from the top of the leaf canopy with depth. (2) Light-saturatingrate of photosynthetic oxygen evolution per leaf area of maturedleaves decreased more steeply with decreasing leaf positionthan the chloroplast number per cell. Gradients of chlorophylland the enzyme protein contents were also steeper than thatof the chloroplast number. Loss of photosynthesis in lower leavesis, therefore, ascribed partly to loss of whole chloroplastsand partly to reduced photosynthetic capacities of the remainingchloroplasts. (3) The chloroplast number per cell in newly expandedsecond leaves was comparable to those in leaves that have developedat later stages of the plant growth but decreased graduallyduring leaf senescence both in the dark and light. The formationof the vertical gradient of chloroplast abundance is, therefore,ascribed to loss of whole chloroplasts during senescence ofleaves. (4) Irradiance a leaf receives decreased sharply fromthe top of the canopy with depth. The physiological or ecophysiologicalsignificance of the vertical distribution of chloroplasts amongleaves was discussed taking light environments of leaves intoconsideration. (Received July 31, 1995; Accepted October 20, 1995)