Total Canopy Photosynthesis Research Articles

Vertical patterns of photosynthesis and related leaf traits in two contrasting agricultural crops.

To include within-canopy leaf acclimation responses to light and other resource gradients in photosynthesis modelling, it is imperative to understand the variation of leaf structural, biochemical and physiological traits from canopy top to bottom. In the present study, leaf photosynthetic traits for top and bottom canopy leaves, canopy structure and light profiles, were measured over one growing season for two contrasting crop types, winter barley (Hordeum vulgare L.) and rape seed (Brassica napus L.). With the exception of quantum yield, other traits such as maximum photosynthetic capacity (Amax), dark respiration, leaf nitrogen and chlorophyll contents, and leaf mass per area, showed consistently higher (P<0.05) values for top leaves throughout the growing season and for both crop types. Even though Amax was higher for top leaves, the bottom half of the canopy intercepted more light and thus contributed the most to total canopy photosynthesis up until senescence set in. Incorporating this knowledge into a simple top/bottom-leaf upscaling scheme, separating top and bottom leaves, resulted in a better match between estimated and measured total canopy photosynthesis, compared with a one-leaf upscaling scheme. Moreover, aggregating to daily and weekly temporal resolutions progressively increased the linearity of the leaf photosynthetic responses to light for top leaves.

Changes in leaf area, nitrogen content and canopy photosynthesis in soybean exposed to an ozone concentration gradient

Influences of ozone (O3) on light-saturated rates of photosynthesis in crop leaves have been well documented. To increase our understanding of O3 effects on individual- or stand level productivity, a mechanistic understanding of factors determining canopy photosynthesis is necessary. We used a canopy model to scale photosynthesis from leaf to canopy, and analyzed the importance of canopy structural and leaf ecophysiological characteristics in determining canopy photosynthesis in soybean stands exposed to 9 concentrations of [O3] (37–116 ppb; 9-h mean). Light intensity and N content peaked in upper canopy layers, and sharply decreased through the lower canopy. Plant leaf area decreased with increasing [O3] allowing for greater light intensity to reach lower canopy levels. At the leaf level, light-saturated photosynthesis decreased and dark respiration increased with increasing [O3]. These data were used to calculate daily net canopy photosynthesis (Pc). Pc decreased with increasing [O3] with an average decrease of 10% for an increase in [O3] of 10 ppb, and which was similar to changes in above-ground dry mass production of the stands. Absolute daily net photosynthesis of lower layers was very low and thus the decrease in photosynthesis in the lower canopy caused by elevated [O3] had only minor significance for total canopy photosynthesis. Sensitivity analyses revealed that the decrease in Pc was associated with changes in leaf ecophysiology but not with decrease in leaf area. The soybean stands were very crowded, the leaves were highly mutually shaded, and sufficient light for positive carbon balance did not penetrate to lower canopy leaves, even under elevated [O3].

Photosynthetic irradiance response, canopy photosynthesis and their variation with canopy strata in tall and dwarf × tall coconut cultivars (Cocos nucifera L.)

Estimation of the carbon balance of coconut (Cocos nucifera L.) requires characterization of its photosynthetic irradiance response. Accordingly, the objectives of this work were to determine an appropriate photosynthetic irradiance response function, estimate its parameters for different canopy strata and compute canopy photosynthetic rates of two contrasting coconut genotypes in Sri Lanka. Photosynthetic irradiance response was measured in the upper (1st to 10th fronds counted from top down), middle (11th to 20th fronds), and lower (21st to 30th fronds) on clear, sunny days under near optimal conditions in field-grown adult palms (30±2 years of age) of Sri Lankan Tall×Sri Lankan Tall (SLT, CRIC60) and Dwarf Green×Sri Lankan Tall (DGT, CRIC65) cultivars. Out of four models tested, the asymptotic exponential was selected as the best-fitting to quantify the photosynthetic irradiance of coconut. The two cultivars showed different patterns of variation across different canopy strata in irradiance-saturated photosynthetic rate (Pmax), photochemical efficiency (ϕ), irradiance compensation- (φ) and saturation points (Isat). In SLT, Pmax decreased from upper stratum to middle and lower strata while ϕ did not differ among strata. In contrast, Pmax of DGT decreased from upper and middle strata to the lower stratum while ϕ increased. SLT had higher φ and Isat than DGT but did not show significant variation between strata. In DGT, φ was higher and Isat lower in the lower stratum. The total canopy photosynthesis (Pc) of SLT was 23% greater than DGT, primarily because it had higher partial LAI in all strata. Leaf N% and chlorophyll content did not show significant variation among strata. The different variation patterns of Pmax and ϕ among canopy strata in the two cultivars reflected their adaptations to different patterns of radiation penetration in to their respective canopies. Accordingly, it is concluded that SLT is better adapted to coconut-growing environments in the humid tropics such as in Sri Lanka where irradiance can often be limiting due to cloudiness during rainy seasons. On the other hand, DGT showed evidence of within-canopy adaptations to decreasing irradiance in the lower strata by optimizing resource allocation between efficient capture of light energy and its maximum utilization. The absence of a positive correlation between Pc and yield of the two cultivars showed that allocation of assimilates to harvested nuts has a greater influence than assimilate production in the yield determination of coconut.

Athletic Field Paint Color Differentially Alters Light Spectral Quality and Bermudagrass Photosynthesis

ABSTRACTPainting of athletic fields is widespread throughout the world and can often cause declines in turfgrass health. Visible light and photosynthesis share the same wavelengths (400–700 nm), and it was hypothesized that alterations in visible light to produce specific colors would lead to reductions in photosynthetically active radiation (PAR) and total canopy photosynthesis (TCP). Lab experiments using a spectroradiometer and LICOR 1800‐12 integrating sphere examined the impacts of 10 colors of athletic field paint on PAR as well as wavelengths within PAR. These colors were then applied weekly for 5 wk to ‘Tifway’ bermudagrass [Cynodon dactylon (L.) Pers. × Cynodon transvaalensis Burtt Davy], and TCP was measured using a gas exchange system 24 h after each application. Spectroradiometry analyses revealed the significant effects of paint color (P ≤ 0.001) on reflection, transmission, and absorption of PAR. Lighter colors including white, yellow, orange, and red reflected 47 to 92% of PAR while darker colors including green, black, and dark blue absorbed 87 to 95% of PAR. Accompanying gas exchange measurements revealed that TCP was most negatively correlated with absorption of PAR (r = −0.959, P ≤ 0.001) and that darker colors negatively impact TCP more than lighter colors. The results clearly indicate that damage to turfgrasses with long‐term painting will be difficult to avoid, and this is particularly true with darker colors of paint.

Using the CSM‐CROPGRO‐Peanut Model to Simulate Late Leaf Spot Effects on Peanut Cultivars of Differing Resistance

Late leaf spot (LLS) caused by Cercosporidium personatum (Berk. and Curt.) Deighton leads to significant reductions in peanut (Arachis hypogaea L.) yield worldwide. This study was conducted to improve the mechanisms and methods by which LLS effects on defoliation and photosynthesis are linked to the CSM‐CROPGRO‐Peanut model for simulating growth and yield reductions in peanut cultivars. Field experiments were conducted in 2008 and 2009 to collect data on the effects of LLS on biomass accumulation and partitioning, leaf necrosis and defoliation, and total canopy photosynthesis (TCP) in peanut cultivars with more (York) and less (Carver) quantitative resistance to LLS. After incorporating LLS damage as defoliation percentage and necrotic area, the model accurately simulated crop growth and development for both cultivars despite different disease dynamics. Simulated TCP and leaf, total crop, and pod yield values were in good agreement with measured data. A modification in the model code to directly reduce leaf photosynthesis and quantum efficiency according to empirical observations resulted in improved simulations of LLS effects on growth and yield. Correlations among measured defoliation and necrotic area with disease ratings indicated that visual disease ratings could be successfully used to estimate necrosis and defoliation for model inputs. Results indicated that the CSM‐CROPGRO‐Peanut model has adequate capability to simulate LLS effects on growth and yield in peanut cultivars with differing levels of resistance to LLS when inputs on canopy necrotic area and defoliation are provided.

Athletic Field Paint Impacts Light Spectral Quality and Turfgrass Photosynthesis

ABSTRACTAthletic field paints are applied to turf surfaces with little or no acute injury. However, field managers notice chronic declines in turfgrass health after repeated applications. This study examines athletic field paint impacts on spectral quality and associated turfgrass photosynthesis. Growth chamber experiments evaluated effects of red and white athletic field paint as well as one, two, three, and four repeated weekly applications on total canopy photosynthesis (TCP) of perennial ryegrass (Lolium perenne L.). Paint treatments were applied weekly for 6 wk with TCP recorded 24 h after each application using a gas exchange system. Spectroradiometry experiments evaluated reflection, absorption, and transmission of light at various wavelengths based on paint color, dilution, and thickness. Over a six week period all treatments reduced TCP based on color (P ≤ 0.0001) and dilution (P ≤ 0.0001). Red no‐dilution paint produced a 75% reduction in TCP over 6 wk while white 1:1 diluted paint only produced a 19% reduction. Spectroradiometry data suggests this is likely due to reductions in photosynthetically active radiation (PAR) with red paint absorbing 51% of incident PAR while transmitting and reflecting 6 and 43%, respectively. White paint transmitted 5% of PAR while reflecting 95%. Alterations in light spectral quality resulting from athletic field paint applications can impact PAR, which may result in reduced turfgrass health.

Late Leaf Spot Effects on Growth, Photosynthesis, and Yield in Peanut Cultivars of Differing Resistance

Cercosporidium personatum (Berk. &amp; Curt.) Deighton causes late leaf spot (LLS) in peanut (Arachis hypogaea L.), which leads to necrotic lesions, early leaf senescence and yield losses. Detailed physiological analyses can contribute to an improved understanding of peanut‐disease interactions and cultivar improvement. A study was conducted evaluating two peanut cultivars with more (York) and less (Carver) quantitative resistance to C. personatum grown under fungicide‐sprayed and nonsprayed conditions in the field at Citra, FL over 2 yr. Data were collected on disease severity using the Florida 1 to 10 visual rating scale and by direct measurement of percent canopy lesion area. Leaf lifespan, total canopy photosynthesis (TCP), plant growth, and pod yield were also measured. Disease severity based on canopy lesion area was reduced by 30% in York compared to Carver. No additive effects of combining the resistant cultivar with fungicide were seen, as fungicide use increased yield by 364 kg ha−1 for both cultivars. Yield was more strongly related to disease severity based on canopy lesion area than to the Florida scale. Yield improvement with York was not as closely related to disease severity with only a 6% gain in pod yield in York compared to Carver. In addition, reduction in TCP was greater in York compared to Carver given their respective disease severity. These results indicated that combining resistance with the maintenance of physiological function during LLS infection could result in improved peanut yields under diseased conditions.

Simulation Model for Photosynthetic Production in Oilseed Rape

Photosynthetic production is a major determinant of final yield in crop plants. A simulation model was developed for canopy photosynthesis and dry matter accumulation in oilseed rape ( Brassica napus L.) based on the ecophysiological processes and using a three-layer radiation balance scheme for calculating the radiation interception and absorption by the layers of flowers, pods, and leaves within the canopy. Gaussian integration method was used to calculate photosynthesis of the pod and leaf layers, and the daily total canopy photosynthesis was determined by the sum of photosynthesis from the two layers of green organs. The effects of physiological age, temperature, nitrogen, and water deficit on maximum photosynthetic rate were quantified. Maintenance and growth respiration were estimated to determine net photosynthetic production. Partition index of the shoot in relation to physiological development time was used to calculate shoot dry matter from plant biomass and shoot biomass loss because of freezing was quantified by temperature effectiveness. Testing of the model for dynamic dry matter accumulation through field experiments of different genotypes, sowing dates, and nitrogen levels showed good fit between the observed and simulated data, with an average root mean square error of 10.9% for shoot dry matter. Thus, the present model appears to be reliable for the prediction of photosynthetic production in oilseed rape.

Short-term impacts of nutrient manipulations on leaf gas exchange and biomass partitioning in contrasting 2-year-old Pinus taeda clones during seedling establishment

We conducted a 1-year greenhouse experiment to assess the impact of nutrient manipulations on seedling growth, biomass partitioning, and leaf gas exchange between two fast growing Pinus taeda clones that differed in growth efficiency. After 1 year we observed significant treatment and treatment by clone effects on growth, biomass partitioning, and gas exchange parameters. Fertilization increased total seedling biomass 18% primarily through an increase in foliage and coarse-roots. Clones did not differ in total seedling biomass, however, clone 85 produced more stem than clone 93 leading to 37% greater stem:leaf, while clone 93 maintained more branch biomass. The logging residue treatment increased stem:leaf by 30%, but had no effect on total biomass or partitioning. Differences in leaf morphology resulted in significantly greater canopy leaf area in clone 93 than clone 85. Increased foliar N concentration from fertilization had only minor effects on specific photosynthesis under saturating light ( A Sat), but lowered stomatal conductance ( g s), transpiration ( E), and internal to external CO 2 concentration ratio ( C i/ C a) as well as improved water use efficiency ( WUE) independently of genotype. When gas exchange data was scaled to the canopy level both genotypes achieved similar canopy level CO 2 assimilation rates, but our data suggests they did this by different means. Although we did see a small effect of nutrient limitations in total canopy photosynthesis under saturating light ( A Canopy), A Sat, and total leaf area ( TLA), our foliar N concentration ([N]) indicated that our level of logging residue incorporation did not cause [N] to decrease below sufficiency limits. From a practical standpoint, a better understanding of strategies for capturing and partition C may lead to better selection of clonal material, thereby, optimizing productivity.

SIMULATING SEASONAL PATTERNS OF INCREASED GREENHOUSE CROP PRODUCTION BY CONVERSION OF DIRECT RADIATION INTO DIFFUSE RADIATION

ISHS III International Symposium on Models for Plant Growth, Environmental Control and Farm Management in Protected Cultivation (HortiModel 2006) SIMULATING SEASONAL PATTERNS OF INCREASED GREENHOUSE CROP PRODUCTION BY CONVERSION OF DIRECT RADIATION INTO DIFFUSE RADIATION

Shoot structure and photosynthetic efficiency along the light gradient in a Scots pine canopy.

We examined the effects of structural and physiological acclimation on the photosynthetic efficiency of Scots pine (Pinus sylvestris L.) shoots. We estimated daily light interception (DLI) and photosynthesis (DPHOT) of a number of sample shoots situated at different positions in the canopy. Photosynthetic efficiency (epsilon) was defined as the ratio of DPHOT to the potential daily light interception (DLI(ref)) defined as the photosynthetically active radiation (PAR) intercepted per unit area of a sphere at the shoot location. To calculate DLI(ref), DLI and DPHOT, the radiation field surrounding a shoot in the canopy was first modeled using simulated directional distributions of incoming PAR on a clear and an overcast day, and estimates of canopy gap fraction in different directions provided by hemispherical photographs. A model of shoot geometry and measured data on shoot structure and photosynthetic parameters were used to simulate the distribution of PAR irradiance on the needle surface area of the shoot. Photosynthetic efficiency (epsilon) was separated into light-interception efficiency (epsilon(I) = DLI/DLI(ref)) and conversion efficiency (epsilon(PHOT) = DPHOT/DLI). This allowed us to quantify separately the effect of structural acclimation on the efficiency of photosynthetic light capture (epsilon(l)), and the effect of physiological acclimation on conversion efficiency (epsilon(PHOT)). The value of epsilon increased from the top to the bottom of the canopy. The increase was largely explained by structural acclimation (higher epsilon(I)) of the shade shoots. The value of epsilon(PHOT) of shade foliage was similar to that of sun foliage. Given these efficiencies, the clear-day value of DPHOT for a sun shoot transferred to shade was only half that of a shade shoot at its original position. The method presented here provides a tool for quantitatively estimating the role of acclimation in total canopy photosynthesis.

Photosynthesis of Daughter-Tuber Leaves in Eddoe Population and Its Contribution to Canopy Photosynthesis and Tuber Yield.

サトイモの個体群光合成に対する子イモ葉身光合成の寄与率を,閉鎖系同化箱と13CO2の供与を併用することによって推定し,子イモ葉身の光合成と塊茎収量との関係について考察した.子イモ用品種の烏播と石川早生,親子イモ兼用品種の赤芽および親イモ用品種の台湾芋を水田転換畑で栽培し,分球イモ葉(ここでは子イモ葉)を全て切除した切除区と,これを行わなかった対照区を設けた.10月中旬に烏播の個体群光合成速度を閉鎖系同化箱で測定するとともに,これに13CO2を供与した.対照区,切除区の個体群光合成速度はそれぞれ24.6,17.5μmol m-2S-1で,対照区の個体群光合成のうち子イモ葉身は29.1%を占めた.塊茎収量は,全品種を込みにしたとき,対照区が総収量では16%,上イモ収量では34%勝った.子イモ葉身の光合成産物が,各塊茎の肥大を促進することによって個体当たり塊茎重の増大に寄与したと考えられた.

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.

Modelling Canopy Production. I. Optimal Distribution of Photosynthetic Resources

On the basis of detailed numerical simulations, Field (1983. Oecologia 56, 341-347) stated that total canopy photosynthesis will be a maximum for a fixed total canopy leaf nitrogen provided the derivative δA/δN, where A is photosynthetic rate and N is leaf nitrogen concentration, has the same value throughout the canopy. This paper uses the calculus of variations to formally prove Field's assertion. It shows that if the single-leaf light response is a first-degree homogeneous function of both light-saturated photosynthetic rate Amax and intensity I of photosynthetically active radiation and if Amax is linearly related to N, then the optimal distribution of leaf nitrogen is linearly related to the decline in I with canopy depth, and Amax is proportional to this decline. The nature of photosynthetic gains due to optimisation of canopy nitrogen distribution is illustrated numerically for a simple model canopy. It is found that, for canopies with fixed mean leaf nitrogen, canopy photosynthesis is approximately proportional to canopy leaf area index (LAI), and the gain due to canopy optimisation compared with a uniform canopy is small for shallow canopies but pronounced for deep canopies. It is also found that, for canopies with fixed total leaf nitrogen, there is a canopy LAI which maximises canopy photosynthesis, and that this LAI and the corresponding canopy photosynthesis are approximately proportional to total canopy nitrogen.

Maximization of Daily Canopy Photosynthesis: Effects of Herbivory on Optimal Nitrogen Distribution

Recent models have shown that higher leaf nitrogen concentration per unit area maximizes nitrogen use efficiency with increasing light intensity. As a result, total canopy photosynthesis is maximized when nitrogen concentrations are higher towards the top of a canopy. Expanding upon this previous work, a model of daily canopy photosynthesis was constructed based on distributions of light, leaf nitrogen, and folivory. The model indicates that the optimal distribution of nitrogen depends significantly upon both the severity and location of folivory. Relative to nitrogen distributions that maximized daily canopy photosynthesis without herbivory, the optimal nitrogen distribution shifted towards either more uniform or skewed distributions when herbivores fed on high nitrogen foliage at the top of the canopy or on low nitrogen foliage towards the bottom of the canopy, respectively. These results suggest that, because foliar losses are balanced by increased irradiance of remaining leaves, plants' nitrogen allocation patterns should depend on how severe defoliation is and whether damage is concentrated towards the top or bottom of a canopy. Moreover, the critical importance of nitrogen distribution to photosynthesis implies that plants should not necessarily minimize loss of leaf area to folivores, but should protect the ratio of total nitrogen to leaf area and the distribution of nitrogen within the canopy. As a corollary to the nutrient stress hypothesis of plant defense theory, the model suggests that plants may need to translocate nutrients to maintain an optimal distribution in the canopy following herbivory. The model reinforces the point that leaf area loss alone is a poor indicator of loss of photosynthetic capacity when nitrogen is non-uniformly distributed among leaves. To accurately assess damage to a plant, one must consider not only what resources have been removed, but what resources remain.

Effects of plant canopy structure on light interception and photosynthesis

A set of different plant canopies with vertically different and horizontally clumped leaf areal density was generated on a mainframe computer. Three-dimensional transfer of direct light in these canopies was numerically calculated and the irradiance of light on inclined leaf surfaces was predicted. Using this information the daily gross photosynthesis of the canopy was estimated. The effects of spatial distribution of leaf areal density and of leaf orientation on light interception and photosynthesis were analyzed. The distribution of leaf areal density affects the spatial distribution of sunflecked leaf area, while leaf orientation affects both the spatial distribution of sunflecked leaf area and the distribution of light irradiance on leaf surfaces. The results show that the influence of spatial distribution of leaf areal density is only about 1–8% of daily total canopy photosynthesis, and that leaf orientation has much greater effects on canopy photosynthesis. At summer solstice and lower latitudes, erectophile leaves could produce up to 25% higher daily photosynthesis in the canopy than planophile leaves.

Use of Interactive Computer Graphics for Simulation of Radiation Interception and Photosynthesis for Canopies of Kiwifruit Vines with Heterogeneous surface Shape and Leaf Area Distribution

A method incorporating interactive computer graphics to simulate spatially variable interception and canopy photosynthesis is described. The method presents a graphical interface to a conventional model of radiation interception and canopy photosynthesis. Included is the capacity to consider a large number of positions within the canopy, thus providing a rapid and convenient representation of the dynamics of photosynthesis while also overcoming limitations of one-dimensional models applied to complex plant canopies. The method was applied to examine spatial variability of photosynthesis within canopies of kiwifruit (Actinidia deliciosa) vines growing on two trellis types. The diurnal integral of simulated canopy photosynthesis, assuming sunny conditions, for a vine trained on a horizontal 'Pergola' trellis was 14% higher than that for a vine with similar leaf area distribution trained on a 'T-bar' trellis with inclined surfaces. Simulations of photosynthesis for vines on a T-bar trellis, assuming spatially variable leaf area distributions as measured under filed conditions, indicated disproportionate contributions from different regions of the canopy. Canopy regions inclined to the east or the west were usually the major sites for photosynthesis immediately after sunrise and before sunset respectively, while regions near the cordon were the most important overall. For any day, the maximum simulated photosynthetic rate generally declined with distance from the cordon and, at any distance from the cordon, increased with leaf index. For a vine with an average leaf area index of 2·7, diurnal integrals of photosynthesis on a sunny day in late summer ranged from 1·0 mol CO2 m-2 near the cordon to 0·5 mol CO2 m-2 at 1·5 m from the cordon. Within-canopy shading was more important on sunny days than on cloudy days, while the spatial distribution of leaf area was especially important on cloudy days. Comparison of simulations with direct measurements of canopy photosynthesis indicated that a numerical integral of simulated photosynthesis, based on a large number of canopy positions, provided a reasonable estimate of total canopy photosynthesis.

Variation in Interception of the Direct Solar Beam by Top Canopy Layers

Leaves that show paraheliotropic movement are inclined at a shallow angle with respect to the horizontal near dawn and dusk, and at a steep angle near solar noon. However, the mean inclination angle is only a first approximation to the distribution of leaf angles, and, as such, is not a good measure of the complexity of the underlying distribution. In order to ascertain the accuracies of statistical summaries of angular data used to measure the orientation of plant leaves for intercepting light, I used cellulation plots to analyze the distribution of leaf angles or cosines of the angle of incidence. Examination of these plots shows that inclinations of leaves in the top canopy layer of individual heliotropic plants are distributed over a wide range of angles during a period of several hours before and after solar noon, but that there is a sharp lower limit to this distribution of angles. Experimental observations show that near midday leaves are inclined at least at some minimum angle with respect to the horizontal. In order to interpret these data, I used a simple model of plant canopy photosynthesis and optimized the distribution of leaf angles to maximize total canopy photosynthesis. The results of this model suggest that the existence of a limit angle may be a function primarily of stress factors, and that the distribution of leaf angles more vertical than the limit angle may be important primarily for maximization of whole—plant photosynthesis.

Canopy structure and leaf nitrogen distribution in a stand of Lysimachia vulgaris L. as influenced by stand density.

A hypothesis that a dense stand should develop a less uniform distribution of leaf nitrogen through the canopy than an open stand to increase total canopy photosynthesis was tested with experimentally established stands of Lysimachia vulgaris L. The effect of stand density on spatial variation of photon flux density, leaf nitrogen and specific leaf weight within the canopy was examined. Stand density had little effect on the value of the light extinction coefficient, but strongly affected the distribution of leaf nitrogen per unit area within a canopy. The open stand had more uniform distribution of leaf nitrogen than the dense stand. However, different light climates between stands explained only part of the variation of leaf nitrogen in the canopy. The specific leaf weight in the canopy increased with increasing relative photon flux density and with decreasing nitrogen concentration.

Maximizing daily canopy photosynthesis with respect to the leaf nitrogen allocation pattern in the canopy.

A model of daily canopy photosynthesis was constructed taking light and leaf nitrogen distribution in the canopy into consideration. It was applied to a canopy of Solidago altissima. Both irradiance and nitrogen concentration per unit leaf area decreased exponentially with increasing cumulative leaf area from the top of the canopy. The photosynthetic capacity of a single leaf was evaluated in relation to irradiance and nitrogen concentration. By integration, daily canopy photosynthesis was calculated for various canopy architectures and nitrogen allocation patterns. The optimal pattern of nitrogen distribution that maximizes the canopy photosynthesis was determined. Actual distribution of leaf nitrogen in the canopy was more uniform than the optimal one, but it realized over 20% more photosynthesis than that under uniform distribution and 4.7% less photosynthesis than that under the optimal distribution. Redeployment of leaf nitrogen to the top of the canopy with ageing should be more effective in increasing total canopy photosynthesis in a stand with a dense canopy than in a stand with an open canopy.