Optimal nitrogen distribution within a leaf canopy under direct and diffuse light

Nitrogen distribution within a leaf canopy is an important determinant of canopy carbon gain. Previous theoretical studies have predicted that canopy photosynthesis is maximized when the amount of photosynthetic nitrogen is proportionally allocated to the absorbed light. However, most of such studies used a simple Beer's law for light extinction to calculate optimal distribution, and it is not known whether this holds true when direct and diffuse light are considered together. Here, using an analytical solution and model simulations, optimal nitrogen distribution is shown to be very different between models using Beer's law and direct–diffuse light. The presented results demonstrate that optimal nitrogen distribution under direct–diffuse light is steeper than that under diffuse light only. The whole‐canopy carbon gain is considerably increased by optimizing nitrogen distribution compared with that in actual canopies in which nitrogen distribution is not optimized. This suggests that optimization of nitrogen distribution can be an effective target trait for improving plant productivity.     

Light-associated nitrogen distribution profile in flowering canopies of sunflower (Helianthus annuus L.) altered during grain growth

In vegetative canopies of many species, the vertical gradient of lamina nitrogen concentration (NW) parallels the profile of light distribution in such a way that the actual nitrogen partitioning approaches the optimum pattern for canopy photosynthesis. This paper evaluates the hypothesis that a strong sink for nitrogen, viz. growing grain, affects the pattern of lamina nitrogen distribution usually described for vegetative canopies. The light and NW profiles of sunflower (Helianthus annuus L.) crops were characterised from anthesis to physiological maturity. The factorial combination of two plant populations (2.4 and 4.8 plants m^–2) and two levels of nitrogen supply (0 and 5 g N m^–2) were the sources of variation for NW and light profiles. Before the onset of nitrogen accumulation in grain, the pattern of NW was similar to that described for other species and it was related to the distribution of light in the canopy. Important changes in the profile of NW occurred during grain filling that were unrelated to the light regime. Nitrogen was mobilised from leaves in all positions in the canopy and the rate of NW change was greater in leaves closer to the grain, which were also the leaves where nitrogen was more concentrated. It is concluded that the physiological mechanisms involved in determining the distribution of leaf nitrogen in vegetative canopies do not apply to sunflower during grain filling.     

The relative share of graminoid and forb life-forms in a natural gradient of herb layer productivity

The proportional share of graminoid and forb life-form in the herbaceous layer was investigated along a productivity gradient at Laelatu, western Estonia With an increase in the herbaceous layer standing crop from 43 5 to 723 7 g m⁻² the graminoid life-form became dominant in total above-ground mass and in species number Three hypotheses to better explain competitive ability of graminoids were tested 1) graminoids are able to form higher foliage, 2) they are able to distribute foliage nitrogen in a more beneficial way, 3) they have better nitrogen use efficiency 21 sample plots 50 × 50 cm were harvested All above-ground parts of vascular plants were removed by two canopy layers Vertical separation of layers were made according to the height of half light interception A species list was compiled, total and leaf masses and leaf nitrogen content of both life-forms were measured by layer ANOVA showed that there were no significant differences in vertical distribution of foliage or foliage nitrogen between life-forms in the productivity gradient, and hypotheses 1) and 2) are not supported by our data-set Hypothesis 3) is approved partly as the nitrogen concentration in graminoid foliage was 20% less than in forbs If one supposes that nitrogen retention time is equal in both life-forms then graminoids must have higher nitrogen use efficiency when compared to forbs Although the influence of life-form x productivity interaction on leaf nitrogen concentration was not significant, there was a tendency that difference in leaf mass to nitrogen ratio of the two life-forms increased with increasing incident light Thus, we can hypothesize that graminoid species dominate in high productive plots where the incident light intensity is also higher due to their better nitrogen use efficiency when compared to forb species     

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.     

Modelling canopy CO2 fluxes: are ‘big‐leaf’ simplifications justified?

• 1 The ‘big‐leaf’ approach to calculating the carbon balance of plant canopies is evaluated for inclusion in the ETEMA model framework. This approach assumes that canopy carbon fluxes have the same relative responses to the environment as any single leaf, and that the scaling from leaf to canopy is therefore linear.
• 2 A series of model simulations was performed with two models of leaf photosynthesis, three distributions of canopy nitrogen, and two levels of canopy radiation detail. Leaf‐ and canopy‐level responses to light and nitrogen, both as instantaneous rates and daily integrals, are presented.
• 3 Observed leaf nitrogen contents of unshaded leaves are over 40% lower than the big‐leaf approach requires. Scaling from these leaves to the canopy using the big‐leaf approach may underestimate canopy photosynthesis by ~20%. A leaf photosynthesis model that treats within‐leaf light extinction displays characteristics that contradict the big‐leaf theory. Observed distributions of canopy nitrogen are closer to those required to optimize this model than the homogeneous model used in the big‐leaf approach.
• 4 It is theoretically consistent to use the big‐leaf approach with the homogeneous photosynthesis model to estimate canopy carbon fluxes if canopy nitrogen and leaf area are known and if the distribution of nitrogen is assumed optimal. However, real nitrogen profiles are not optimal for this photosynthesis model, and caution is necessary in using the big‐leaf approach to scale satellite estimates of leaf physiology to canopies. Accurate prediction of canopy carbon fluxes requires canopy nitrogen, leaf area, declining nitrogen with canopy depth, the heterogeneous model of leaf photosynthesis and the separation of sunlit and shaded leaves. The exact nitrogen profile is not critical, but realistic distributions can be predicted using a simple model of canopy nitrogen allocation.

     

Leaf nitrogen distribution and whole canopy photosynthetic carbon gain in herbaceous stands

The amount of photosynthetically-active photon flux density incident upon a leaf and the nitrogen content of that leaf strongly affect the photosynthetic carbon gain of that leaf. Therefore, the canopy structure of a stand, affecting the light climate in the canopy, and the leaf nitrogen distribution pattern in the canopy, affect the carbon gain of the whole canopy. This review discusses the results of studies directed to this problem and obtained so far in open and in dense canopies of stands of herbaceous, monocotyledonous or dicotyledonous, plants in their growing or flowering stages. It is found that the leaf nitrogen distribution pattern in the canopy is vertically non-uniform, and in dense stands more strongly so than in open stands. The leaf nitrogen distribution pattern in most canopies closely approaches an optimal pattern in that it maximizes whole canopy potential carbon gain as calculated for the actual total leaf nitrogen content and leaf area index of the stand. The resulting increase in potential carbon gain as compared to a uniform leaf nitrogen distribution pattern is considerable and it is larger in dense stands than in open stands. For at least some dense stands simulation studies show that with the available total leaf nitrogen content, whole canopy carbon gains could still be considerable higher had a lower leaf area index been developed.     

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.     

Effects of leaf age,nitrogen nutrition and photon flux density on the distribution of nitrogen among leaves of a vine (Ipomoea tricolor Cav.) grown horizontally to avoid mutual shading of leaves

Effects of leaf age, nitrogen nutrition and photon flux density (PFD) on the distribution of nitrogen among leaves were investigated in a vine, Ipomoea tricolor Cav., which had been grown horizontally so as to avoid mutual shading of leaves. The nitrogen content was highest in newly developed young leaves and decreased with age of leaves in plants grown at low nitrate concentrations and with all leaves exposed to full sunlight. Thus, a distinct gradient of leaf nitrogen content was formed along the gradient of leaf age. However, no gradient of leaf nitrogen content was formed in plants grown at a high nitrate concentration. Effects of PFD on the distribution of nitrogen were examined by shading leaves in a manner that simulated changes in the light gradient of an erect herbaceous canopy (i.e., where old leaves were placed under increasingly darker conditions with growth of the canopy). This canopy-type shading steepened the gradient of leaf nitrogen content in plants grown at a low nitrogen supply, and created a gradient in plants grown at high concentrations of nitrate. The steeper the gradient of PFD, the larger the gradient of leaf nitrogen that was formed. When the gradient of shading was inverted, that is, younger leaves were subjected to increasingly heavier shade, while keeping the oldest leaves exposed to full sunlight, an inverted gradient of leaf nitrogen content was formed at high nitrate concentrations. The gradient of leaf nitrogen content generated either by advance of leaf age at low nitrogen availability, or by canopy-type shading, was comparable to those reported for the canopies of erect herbaceous plants. It is concluded that both leaf age and PFD have potential to cause the non-uniform distribution of leaf nitrogen. It is also shown that the contribution of leaf age increases with the decrease in nitrogen nutrition level.     

Do Variations in Local Leaf Irradiance Explain Changes to Leaf Nitrogen within Row Maize Canopies?

Numerous studies have dealt with the relationship between leafnitrogen content and leaf irradiance. However, most of themrefer to dense stands presenting reduced horizontal heterogeneityof foliage distribution. Both gradients of leaf nitrogen andleaf irradiance related to canopy depth are significant undersuch conditions, and modelling radiative exchange using a turbid-mediumanalogy and dividing the canopy into vegetation layers is sufficient.Conversely, row crops such as maize are characterized by stronghorizontal heterogeneity of foliage distribution and the one-dimensional(1D) approach may be unsuitable. We thus modelled the three-dimensional(3D) geometry of maize canopies with varying densities and atdifferent developmental stages using plant digitizing underfield conditions. The nitrogen content per unit area of eachleaf part was obtained subsequently by nitrogen analysis. Wenext calculated radiative exchange using a 3D volume-based approachwithin the canopies in order to estimate local leaf irradianceon a daily integration scale. Vertical gradients in leaf nitrogencontent per unit area observed in dense stands during the vegetativephase corresponded largely to those reported in the literature.We also identified significant gradients in nitrogen contentalong the leaves, which had not before been clearly demonstrated.Our study shows that local light climate during plant developmentplays a major role in leaf nitrogen distribution and remobilization.Moreover, brutal plant thinning involves rapid changes in leafnitrogen partitioning. It is concluded that taking account ofthe 3D heterogeneity of nitrogen and irradiance distributionmay have implications for modelling crop photosynthesis andproduction. Copyright 1999 Annals of Botany Company 3D plant architecture, horizontal gradients in leaf nitrogen, leaf irradiance, leaf nitrogen content per unit area, maize, nitrogen partitioning, nitrogen remobilization, virtual plant, Zea mays L.     

Effects of Prosopis flexuosa on soil properties and the spatial pattern of understorey species in arid Argentina

Abstract. In arid zones dominant woody plants are capable of causing changes in microclimate and soil properties likely to affect species composition, as well as the establishment and spatial distribution of plant species. In North American and European deserts species richness appears to be higher under the canopy of shrubs and trees, in contrast with Chilean deserts where it seems to be lower. Since Prosopis flexuosa (Fabaceae, Mimosoideae) is the most conspicuous tree in the central Monte desert, Argentina, we analysed the effect of this species on the composition and abundance of the shrub and herbaceous layers and on soil properties. We considered two mesohabitats: ‘under P. flexuosa canopy’ and ‘intercanopy areas’. In addition, we analysed the differences between two microhabitats under canopies: ‘northern part of the canopy’ and ‘southern part of the canopy’. Results indicate that species composition and soil properties are affected by both mesohabitats and microhabitats. We found a higher number of shrubs under canopies, whereas that of grasses and perennial forbs increased in intercanopy areas. Concentrations of organic matter, nitrogen, potassium and phosphorus, factors limiting biological productivity in Monte desert soils, were significantly higher under than outside P. flexuosa canopies. Electrical conductivity and concentrations of Na⁺, Ca⁺⁺, Mg⁺⁺ were higher in the northern than in the southern microhabitats. No differences in species richness, evenness or diversity were found between mesohabitats or between microhabitats. We conclude that P. flexuosa modifies the spatial pattern of plant species in the shrub and herbaceous layers and the chemical conditions of the soil, generating spatial heterogeneity on different scales.     

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.     

Profiles of Leaf Nitrogen and Light in Reproductive Canopies of Cotton (Gossypium hirsutum)

During vegetative growth, the vertical profile of leaf nitrogen(N) often parallels the profile of light distribution withinthe canopy. This is more advantageous in terms of canopy photosynthesisthan a uniform distribution of leaf N. We investigated the influenceof both reproductive growth and N supply on the profiles ofN and light in canopies of irrigated cotton crops (Gossypiumhirsutum L.). Regular samplings were made from soon after theonset of reproductive growth until crop maturity. Every 2 weeks,a 1 m²sample of the canopy was cut in four successive verticallayers of equal thickness. Leaf area and N concentration (%)in each layer were measured. The vertical N gradient becamemore marked with ongoing reproductive development. It is hypothesizedthat because of the high rate of growth after the onset of reproductivedevelopment and the long duration of this phase compared toother species, the whole canopy photosynthetic benefit thatwould accrue from maintaining the N gradient is likely to beaccentuated. The rate of decline in leaf N concentration ina layer was not related to either the initial concentrationin the leaves nor the boll load within the layer.Copyright 2001Annals of Botany Company Gossypium hirsutum, leaf nitrogen, light profile, nitrogen, nitrogen distribution, remobilization, reproductive growth     

A model of canopy photosynthesis incorporating protein distribution through the canopy and its acclimation to light, temperature and CO2

Background and Aims

The distribution of photosynthetic enzymes, or nitrogen, through the canopy affects canopy photosynthesis, as well as plant quality and nitrogen demand. Most canopy photosynthesis models assume an exponential distribution of nitrogen, or protein, through the canopy, although this is rarely consistent with experimental observation. Previous optimization schemes to derive the nitrogen distribution through the canopy generally focus on the distribution of a fixed amount of total nitrogen, which fails to account for the variation in both the actual quantity of nitrogen in response to environmental conditions and the interaction of photosynthesis and respiration at similar levels of complexity.

Model

A model of canopy photosynthesis is presented for C₃ and C₄ canopies that considers a balanced approach between photosynthesis and respiration as well as plant carbon partitioning. Protein distribution is related to irradiance in the canopy by a flexible equation for which the exponential distribution is a special case. The model is designed to be simple to parameterize for crop, pasture and ecosystem studies. The amount and distribution of protein that maximizes canopy net photosynthesis is calculated.

Key Results

The optimum protein distribution is not exponential, but is quite linear near the top of the canopy, which is consistent with experimental observations. The overall concentration within the canopy is dependent on environmental conditions, including the distribution of direct and diffuse components of irradiance.

Conclusions

The widely used exponential distribution of nitrogen or protein through the canopy is generally inappropriate. The model derives the optimum distribution with characteristics that are consistent with observation, so overcoming limitations of using the exponential distribution. Although canopies may not always operate at an optimum, optimization analysis provides valuable insight into plant acclimation to environmental conditions. Protein distribution has implications for the prediction of carbon assimilation, plant quality and nitrogen demand.     

Plant light interception can be explained via computed tomography scanning: demonstration with pyramidal cedar (Thuja occidentalis, Fastigiata)

BACKGROUND AND AIMS: Light interception by the leaf canopy is a key aspect of plant photosynthesis, which helps mitigate the greenhouse effect via atmospheric CO(2) recycling. The relationship between plant light interception and leaf area was traditionally modelled with the Beer-Lambert law, until the spatial distribution of leaves was incorporated through the fractal dimension of leafless plant structure photographed from the side allowing maximum appearance of branches and petioles. However, photographs of leafless plants are two-dimensional projections of three-dimensional structures, and sampled plants were cut at the stem base before leaf blades were detached manually, so canopy development could not be followed for individual plants. Therefore, a new measurement and modelling approach were developed to explain plant light interception more completely and precisely, based on appropriate processing of computed tomography (CT) scanning data collected for developing canopies. METHODS: Three-dimensional images of canopies were constructed from CT scanning data. Leaf volumes (LV) were evaluated from complete canopy images, and fractal dimensions (FD) were estimated from skeletonized leafless images. The experimental plant species is pyramidal cedar (Thuja occidentalis, Fastigiata). KEY RESULTS: The three-dimensional version of the Beer-Lambert law based on FD alone provided a much better explanation of plant light interception (R(2) = 0.858) than those using the product LV*FD (0.589) or LV alone (0.548). While values of all three regressors were found to increase over time, FD in the Beer-Lambert law followed the increase in light interception the most closely. The delayed increase of LV reflected the appearance of new leaves only after branches had lengthened and ramified. CONCLUSIONS: The very strong correlation obtained with FD demonstrates that CT scanning data contain fundamental information about the canopy architecture geometry. The model can be used to identify crops and plantation trees with improved light interception and productivity.     

Cyanobacterial soil crusts and woody shrub canopies in Kalahari rangelands

Intensive grazing of Kalahari rangelands has led to woody plant encroachment, notably of Acacia mellifera and Grewia flava. The mechanisms causing this process, and the ecological stability of woody plant encroached ecosystems, remain uncertain. Past studies suggest that canopy–soil relations may enhance woody plant competitive dominance. This study aims to investigate one element of this ecological change by examining the spatial distribution of cyanobacterial soil crusts in two vegetation sub‐habitats at sites of different disturbance. Crust burial by litter was also assessed to analyse the dynamics of canopy–crust relations. Our results show there is enhanced cyanobacterial crust cover under A. mellifera canopies and that unlike G. flava canopies, the crust cover remains under A. mellifera even at highly disturbed sites. This canopy–crust association suggests A. mellifera encroachment exhibits intrinsic resilience because of the crusts ability to stabilize the soil surface and increase nutrient retention. Crust burial by litter that accumulates under larger woody plants restricts crust development under canopies. Disturbance restricts crust development in plant interspaces and under G. flava. These two mechanisms combine to restrict crust development to an observed 40% threshold, with nonlinear models required to explain spatial patterns of crust dynamics.     

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.     

Patterns of light and nitrogen distribution in relation to whole canopy carbon gain in C3 and C4 mono- and dicotyledonous species

An analytical model was used to describe the optimal nitrogen distribution. From this model, it was hypothesized that the non-uniformity of the nitrogen distribution increases with the canopy extinction rate for light and the total amount of free nitrogen in the canopy, and that it is independent of the slope of the relation between light saturated photosynthesis (P_m) and leaf nitrogen content (n_L). These hypotheses were tested experimentally for plants with inherently different architectures and different photosynthetic modes. A garden experiment was carried out with a C₃ monocot [rice, Oryza sativa (L.)], a C₃ dicot [soybean, Glycine max (L.) Merr] a C₄ monocot [sorghum, Sorghum bicolor (L.) Moensch] and a C₄ dicot [amarantus, Amaranthus cruentus (L.)]. Leaf photosynthetic characteristics as well as light and nitrogen distribution in the canopies of dense stands of these species were measured. The dicot stands were found to have higher extinction coefficients for light than the monocot stands. Dicots also had more non-uniform N distribution patterns. The main difference between the C₃ and C₄ species was that the C₄ species were found to have a greater slope value of the leaf-level P_m—n_L relation. Patterns of N distribution were similar in stands of the C₃ and C₄ species. In general, these experimental results were in accordance with the model predictions, in that the pattern of nitrogen allocation in the canopy is mainly determined by the extinction coefficient for light and the total amount of free nitrogen.     

Leaf canopy as a dynamic system: ecophysiology and optimality in leaf turnover

BACKGROUND AND AIMS: In a leaf canopy, there is a turnover of leaves; i.e. they are produced, senesce and fall. These processes determine the amount of leaf area in the canopy, which in turn determines canopy photosynthesis. The turnover rate of leaves is affected by environmental factors and is different among species. This mini-review discusses factors responsible for leaf dynamics in plant canopies, focusing on the role of nitrogen. SCOPE: Leaf production is supported by canopy photosynthesis that is determined by distribution of light and leaf nitrogen. Leaf nitrogen determines photosynthetic capacity. Nitrogen taken up from roots is allocated to new leaves. When leaves age or their light availability is lowered, part of the leaf nitrogen is resorbed. Resorbed nitrogen is re-utilized in new organs and the rest is lost with dead leaves. The sink-source balance is important in the regulation of leaf senescence. Several models have been proposed to predict response to environmental changes. A mathematical model that incorporated nitrogen use for photosynthesis explained well the variations in leaf lifespan within and between species. CONCLUSION: When leaf turnover is at a steady state, the ratio of biomass production to nitrogen uptake is equal to the ratio of litter fall to nitrogen loss, which is an inverse of the nitrogen concentration in dead leaves. Thus nitrogen concentration in dead leaves (nitrogen resorption proficiency) and nitrogen availability in the soil determine the rate of photosynthesis in the canopy. Dynamics of leaves are regulated so as to maximize carbon gain and resource-use efficiency of the plant.     

群体中叶片光合能力的分布及其对群体光合作用的影响

利用数学变分原理分析了群体中叶片光合能力对环境适应和有限氮资源利用的最优分布。叶片光合能力呈现与光强相同的负指数衰减分布时,“群体的光合速率和对氮的利用率最高;叶片对环境光强适应的优越性随群体消光系数和叶面积指数增加而增加。由此推导了叶片光合能力最优分布下的群体光合模型。