• Energy Research

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.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.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.

The rising atmospheric CO2 concentration ([CO2]) is a ubiquitous selective force that may strongly impact species distribution and vegetation functioning. Plant-plant interactions could mediate the trajectory of vegetation responses to elevated [CO2], because some plants may benefit more from [CO2] elevation than others. The relative contribution of plastic (within the plant's lifetime) and genotypic (over several generations) responses to elevated [CO2] on plant performance was investigated and how these patterns are modified by plant-plant interactions was analysed.Plantago asiatica seeds originating from natural CO2 springs and from ambient [CO2] sites were grown in mono stands of each one of the two origins as well as mixtures of both origins. In total, 1944 plants were grown in [CO2]-controlled walk-in climate rooms, under a [CO2] of 270, 450 and 750 ppm. A model was used for upscaling from leaf to whole-plant photosynthesis and for quantifying the influence of plastic and genotypic responses.It was shown that changes in canopy photosynthesis, specific leaf area (SLA) and stomatal conductance in response to changes in growth [CO2] were mainly determined by plastic and not by genotypic responses. We further found that plants originating from high [CO2] habitats performed better in terms of whole-plant photosynthesis, biomass and leaf area, than those from ambient [CO2] habitats at elevated [CO2] only when both genotypes competed. Similarly, plants from ambient [CO2] habitats performed better at low [CO2], also only when both genotypes competed. No difference in performance was found in mono stands.The results indicate that natural selection under increasing [CO2] will be mainly driven by competitive interactions. This supports the notion that plant-plant interactions have an important influence on future vegetation functioning and species distribution. Furthermore, plant performance was mainly driven by plastic and not by genotypic responses to changes in atmospheric [CO2].

Models of plant three-dimensional (3-D) architecture have been used to find optimal morphological characteristics for light capture or carbon assimilation of a solitary plant. However, optimality theory is not necessarily useful to predict the advantageous strategy of an individual in dense stands, where light capture of an individual is influenced not only by its architecture but also by the architecture of its neighbours. Here, we analysed optimal and evolutionarily stable biomass allocation between the lamina and petiole (evolutionarily stable strategy; ESS) under various neighbour conditions using a 3-D simulation model based on the game theory.We obtained 3-D information of every leaf of actual Xanthium canadense plants grown in a dense stand using a ruler and a protractor. We calculated light capture and carbon assimilation of an individual plant when it stands alone and when it is surrounded by neighbours in the stand. We considered three trade-offs in petiole length and lamina area: biomass allocation, biomechanical constraints and photosynthesis. Optimal and evolutionarily stable biomass allocation between petiole and lamina were calculated under various neighbour conditions.Optimal petiole length varied depending on the presence of neighbours and on the architecture of neighbours. The evolutionarily stable petiole length of plants in the stand tended to be longer than the optimal length of solitary plants. The mean of evolutionarily stable petiole length in the stand was similar to the real one. Trade-offs of biomechanical constraint and photosynthesis had minor effects on optimal and evolutionarily stable petiole length.Actual plants realize evolutionarily stable architecture in dense stands. Interestingly, there were multiple evolutionarily stable petiole lengths even in one stand, suggesting that plants with different architectures can coexist across plant communities.

It is well known that leaf photosynthesis per unit dry mass (A(mass)) is positively correlated with nitrogen concentration (N(mass)) across naturally growing plants. In this article we show that this relationship is paradoxical because, if other traits are identical among species, plants with a higher A(mass) should have a lower N(mass), because of dilution by the assimilated carbon. To find a factor to overcome the dilution effect, we analyze the N(mass)-A(mass) relationship using simple mathematical models and literature data. We propose two equations derived from plant-growth models. Model prediction is compared with the data set of leaf trait spectrum obtained on a global scale. The model predicts that plants with a higher A(mass) should have a higher specific nitrogen absorption rate in roots (SAR), less biomass allocation to leaves, and/or greater nitrogen allocation to leaves. From the literature survey, SAR is suggested as the most likely factor. If SAR is the sole factor maintaining the positive relationship between N(mass) and A(mass), the variation in SAR is predicted to be much greater than that in A(mass); given that A(mass) varies 130-fold, SAR may vary more than 2000-fold. We predict that there is coordination between leaf and root activities among species on a global scale.

Some shade leaves increase their photosynthetic capacity (P (max)) when exposed to a higher irradiance. The increase in P (max) is associated with an increase in chloroplast size or number. To accommodate those chloroplasts, plants need to make thick leaves in advance. We studied the cost and benefit of photosynthetic acclimation in mature leaves of a tree species, Kalopanax pictus Nakai, in a cool-temperate deciduous forest. Costs were evaluated as the additional investment in biomass required to make thick leaves, while the benefit was evaluated as an increase in photosynthetic carbon gain. We created gaps by felling canopy trees and examined the photosynthetic responses of mature leaves of the understorey seedlings. In the shade, leaves of K. pictus had vacant spaces that were not filled by chloroplasts in the mesophyll cells facing the intercellular space. When those leaves were exposed to higher irradiance after gap formation, the area of the mesophyll surface covered by chloroplasts increased by 17% and P (max) by 27%. This increase in P (max) led to an 11% increase in daily carbon gain, which was greater than the amount of biomass additionally invested to construct thicker leaves. We conclude that the capacity of a plant to acclimate to light (photosynthetic acclimation) would contribute to rapid growth in response to gap formation.

In crowded stands, height is often similar among dominant plants, as plants adjust their height to that of their neighbours (height convergence). We investigated which of the factors, light quality, light quantity and mechanical stimuli, is primarily responsible for stem elongation and height convergence in crowded stands. We established stands of potted Chenopodium album plants. In one stand, target plants were surrounded by artificial plants that were painted black to ensure that the light quality was not modified by their neighbours. In a second stand, target plants were surrounded by real plants. In both stands, one-half of the target plants were anchored to stakes to prevent flexing by wind. The target plants were lifted or lowered by 10 cm to test whether height convergence was affected by the different treatments. Stem length was affected by being surrounded by artificial plants, anchoring and pot elevation, indicating that light quality, light quantity and mechanical stimuli all influenced stem elongation. Height convergence did not occur in the stand with artificial plants or in anchored plants. We conclude that light quality and mechanical stimuli are important factors for the regulation of stem growth and height convergence in crowded stands.

Both nutrient availability and defoliation affect the carbon-nutrient balance in plants, which in turn influences biomass allocation (e.g. shoot-to-root ratio) and leaf chemical composition (concentration of nitrogen and secondary compounds). In this study it is questioned whether defoliation alters biomass allocation and chemical defence in a similar fashion to the response to nutrient deficiency.Current-year seedlings of Quercus serrata were grown with or without removal of all leaves at three levels of nutrient availability.Plant nitrogen concentration (PNC), a measure of the carbon-nutrient balance in the plant, significantly decreased immediately after defoliation because leaves had higher nitrogen concentrations than stems and roots. However, PNC recovered to levels similar to or higher than that of control plants in 3 or 6 weeks after the defoliation. Nitrogen concentration of leaves produced after defoliation was significantly higher than leaf nitrogen concentration of control leaves. Leaf mass per plant mass (leaf mass ratio, LMR) was positively correlated with PNC but the relationship was significantly different between defoliated and control plants. When compared at the same PNC, defoliated plants had a lower LMR. However, the ratio of the leaf to root tissues that were newly produced after defoliation as a function of PNC did not differ between defoliated and control plants. Defoliated plants had a significantly lower concentration of total phenolics and condensed tannins. Across defoliated and control plants, the leaf tannin concentration was negatively correlated with the leaf nitrogen concentration, suggesting that the amount of carbon-based defensive compounds was controlled by the carbon-nutrient balance at the leaf level.Defoliation alters biomass allocation and chemical defence through the carbon-nutrient balance at the plant and at the leaf level, respectively.

AbstractPlant population density is an important variable in agronomy and forestry and offers an experimental way to better understand plant–plant competition. We made a meta‐analysis of responses of even‐aged mono‐specific stands to population density by quantifying for 3 stand and 33 individual plant variables in 334 experiments how much both plant biomass and phenotypic traits change with a doubling in density. Increasing density increases standing crop per area, but decreases the mean size of its individuals, mostly through reduced tillering and branching. Among the phenotypic traits, stem diameter is negatively affected, but plant height remains remarkably similar, partly due to an increased stem length‐to‐mass ratio and partly by increased allocation to stems. The reduction in biomass is caused by a lower photosynthetic rate, mainly due to shading of part of the foliage. Total seed mass per plant is also strongly reduced, marginally by lower mass per seed, but mainly because of lower seed numbers. Plants generally have fewer shoot‐born roots, but their overall rooting depth seems hardly affected. The phenotypic plasticity responses to high densities correlate strongly with those to low light, and less with those to low nutrients, suggesting that at high density, shading affects plants more than nutrient depletion.