The amount of light reaching the leaves in seagrass (Zostera marina) meadows

Seagrass meadows, and other submerged vegetated habitats, support a wide range of essential ecological services, but the true extents of these services are in many ways still not quantified. One important tool needed to assess and model many of these services is accurate estimations of the systems´ primary productivity. Such productivity estimations require an understanding of the underwater light field, especially regarding the amount of light that actually reaches the plants' photosynthetic tissue. In this study, we tested a simple practical approach to estimate leaf light exposure, relative to incoming light at the canopy, by attaching light sensitive film at different positions on leaves of Zostera marina, eelgrass, in four seagrass meadows composed of different shoot density and at two different depths. We found that the light reaching the leaves decreased linearly down through the canopy. While the upper parts of the leaves received approximately the same level of light (photosynthetic photon flux density, PPFD) as recorded with a PAR meter at the canopy top, the average light that the seagrass leaves were exposed to varied between 40 and 60% of the light on top of the canopy, with an overall average of 48%. We recommend that actual light interception is measured when assessing or modelling light depending processes in submerged vegetation, but if this is not achievable a rough estimation for vegetation similar to Z. marina would be to use a correction factor of 0.5 to compensate for the reduced light due to leaf orientation and internal shading.

Dividing the pie: A quantitative review on plant density responses

Plant 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.

Leaf shedding increases the photosynthetic rate of the canopy in N2-fixing and non-N2-fixing woody species

It has long been hypothesized that timing of leaf shedding is critical for plant fitness but there is little experimental evidence to support the hypothesis. According to an optimality theory, shedding of old leaves increases canopy photosynthesis despite some nitrogen (N) being lost as litterfall, when the ratio of daily photosynthesis to leaf N (N-use efficiency, ε) in old leaves, expressed as a fraction of ε in new leaves, becomes lower than the fraction of leaf N that is resorbed before shedding (RN). This was shown to be true for N-poor plants but not for N-rich plants in a pot experiment; however, the use of planting pots imposes a variety of physical, chemical and biological constraints that could change the experimental results. Here we conducted a 3-year field survey in a cool temperate deciduous forest to examine whether Alnus sieboldiana Matsum. (N2-fixing) and Carpinus tschonoskii Maxim. (non-N2-fixing) shed their leaves to increase canopy photosynthesis in accord with the above criterion. These species often grow sympatrically and were chosen as representatives of N-rich and N-poor plants, respectively. Overall, daily photosynthesis decreased with leaf age, accompanied by small changes in leaf N, resulting in a decrease in ε. In both species, ε of leaves at shedding expressed as a fraction of ε in new leaves was nearly equal to RN in all years, implying that the old leaves were shed to increase canopy photosynthesis. Our results, together with those of previous field surveys, suggested that the timing of leaf shedding is explained by N use in maximizing canopy photosynthesis across broad groups of species.

The role of biomass allocation between lamina and petioles in a game of light competition in a dense stand of an annual plant

BACKGROUND AND AIMS

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.

METHODS

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.

KEY RESULTS

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.

CONCLUSION

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.

Optimality of nitrogen distribution among leaves in plant canopies

The vertical gradient of the leaf nitrogen content in a plant canopy is one of the determinants of vegetation productivity. The ecological significance of the nitrogen distribution in plant canopies has been discussed in relation to its optimality; nitrogen distribution in actual plant canopies is close to but always less steep than the optimal distribution that maximizes canopy photosynthesis. In this paper, I review the optimality of nitrogen distribution within canopies focusing on recent advancements. Although the optimal nitrogen distribution has been believed to be proportional to the light gradient in the canopy, this rule holds only when diffuse light is considered; the optimal distribution is steeper when the direct light is considered. A recent meta-analysis has shown that the nitrogen gradient is similar between herbaceous and tree canopies when it is expressed as the function of the light gradient. Various hypotheses have been proposed to explain why nitrogen distribution is suboptimal. However, hypotheses explain patterns observed in some specific stands but not in others; there seems to be no general hypothesis that can explain the nitrogen distributions under different conditions. Therefore, how the nitrogen distribution in canopies is determined remains open for future studies; its understanding should contribute to the correct prediction and improvement of plant productivity under changing environments.

Effects of controlled-release fertilizer on leaf area index and fruit yield in high-density soilless tomato culture using low node-order pinching

To further development of a simplified fertigation system using controlled-release fertilizers (CRF), we investigated the effects of differing levels of fertilizers and plant density on leaf area index (LAI), fruit yields, and nutrient use in soilless tomato cultures with low node-order pinching and high plant density during spring-summer (SS), summer-fall (SF), and fall-winter (FW) seasons. Plants were treated with 1 of 3 levels of CRF in a closed system, or with liquid fertilizer (LF) with constant electrical conductivity (EC) in a drip-draining system. Two plant densities were examined for each fertilizer treatment. In CRF treatments, LAI at pinching increased linearly with increasing nutrient supply for all cropping seasons. In SS, both light interception by plant canopy at pinching and total marketable fruit yield increased linearly with increasing LAI up to 6 m(2) · m(-2); the maximization point was not reached for any of the treatments. In FW, both light interception and yield were maximized at an LAI of approximately 4. These results suggest that maximizing the LAI in SS and FW to the saturation point for light interception is important for increasing yield. In SF, however, the yield maximized at an LAI of approximately 3, although the light interception linearly increased with increasing LAI, up to 4.5. According to our results, the optimal LAI at pinching may be 6 in SS, 3 in SF, and 4 in FW. In comparing LAI values with similar fruit yield, we found that nutrient supply was 32-46% lower with the CRF method than with LF. In conclusion, CRF application in a closed system enables growers to achieve a desirable LAI to maximize fruit yield with a regulated amount of nutrient supply per unit area. Further, the CRF method greatly reduced nutrient use without decreasing fruit yield at similar LAIs, as compared to the LF method.

Is analysing the nitrogen use at the plant canopy level a matter of choosing the right optimization criterion?

Optimization theory in combination with canopy modeling is potentially a powerful tool for evaluating the adaptive significance of photosynthesis-related plant traits. Yet its successful application has been hampered by a lack of agreement on the appropriate optimization criterion. Here we review how models based on different types of optimization criteria have been used to analyze traits-particularly N reallocation and leaf area indices-that determine photosynthetic nitrogen-use efficiency at the canopy level. By far the most commonly used approach is static-plant simple optimization (SSO). Static-plant simple optimization makes two assumptions: (1) plant traits are considered to be optimal when they maximize whole-stand daily photosynthesis, ignoring competitive interactions between individuals; (2) it assumes static plants, ignoring canopy dynamics (production and loss of leaves, and the reallocation and uptake of nitrogen) and the respiration of nonphotosynthetic tissue. Recent studies have addressed either the former problem through the application of evolutionary game theory (EGT) or the latter by applying dynamic-plant simple optimization (DSO), and have made considerable progress in our understanding of plant photosynthetic traits. However, we argue that future model studies should focus on combining these two approaches. We also point out that field observations can fit predictions from two models based on very different optimization criteria. In order to enhance our understanding of the adaptive significance of photosynthesis-related plant traits, there is thus an urgent need for experiments that test underlying optimization criteria and competing hypotheses about underlying mechanisms of optimization.

Why does Viola hondoensis (Violaceae) shed its winter leaves in spring?

PREMISE OF THE STUDY

Viola hondoensis is a perennial herb that inhabits the understory of temperate, deciduous forests. It is an evergreen plant with a leaf life span that is shorter than a year. Its summer leaves are produced in spring and shed in autumn; winter leaves are produced in autumn and shed in spring. Here we asked why the plant sheds its winter leaves in spring, though climate conditions improve from spring to summer. We proposed four hypotheses for the cause of shedding: (1) changes in seasonal environment such as day length or air temperature, (2) shading by canopy deciduous trees, (3) self-shading by taller summer leaves, and (4) competition for nutrients between summer and winter leaves. •

METHODS

To test these hypotheses, we manipulated the environment of winter leaves: (1) plants were transplanted to the open site where there was no shading by canopy trees. (2) Petioles of summer leaves were anchored to the soil surface to avoid shading of winter leaves. (3) Sink organs were removed to eliminate nutrient competition. •

KEY RESULTS

Longevity of winter leaves was extended when shading by summer leaves was eliminated and when sink organs were removed, but not when plants were transplanted to the open site. •

CONCLUSION

We conclude that the relative difference in light availability between summer and winter leaves is a critical factor for regulation of leaf shedding, consistent with the theory of maximization of the whole-plant photosynthesis.

Evolutionarily stable leaf area production in plant populations

Using an analytical model, it was shown that for a given amount of nitrogen in the canopy of a stand (N(T)), there exists an evolutionarily stable leaf area index (ES-LAI), and therefore an evolutionarily stable average leaf nitrogen content (n(ES)(av);n(ES)(av) =N(T)/ES-LAI), at which no individual plant in the stand can increase its photosynthesis by changing its leaf area. It was also shown that this ES-LAI is always greater than the optimal LAI that maximizes photosynthesis per unit N(T) of the stand. This illustrates that the canopy structure that maximizes photosynthesis of a population is not the same as the canopy structure that maximizes photosynthesis of individuals within a population. It was further derived that the ES-LAI at given N(T) increases with the ratio between the light-saturated photosynthesis and the N content per unit leaf area (leaf-PPNUE) and that it decreases with the canopy extinction coefficient for light (K(L)), the light availability and the apparent quantum yield (phi). These hypotheses were tested by comparing calculated ES-LAI and n(ES)(av) values to actual LAIs and leaf N contents measured for stands of a large variety of herbaceous plants. There was a close correspondence between the calculated and measured values. As predicted by the model, plants with high leaf-PPNUEs produced more leaf area per unit nitrogen than those with low leaf-PPNUEs while plants with horizontal leaves, forming stands with higher K(L) values, produced less leaf area than those with more vertically inclined leaves. These results suggest that maximization of individual plant photosynthesis per unit of nitrogen plays an important role in determining leaf area production of plants and the resulting canopy structure of stands of vegetation. They further suggest this optimization to be a mechanism by which leaf traits such as leaf-PPNUE and leaf inclination angle are causally related to structural characteristics of the population, i.e. the leaf area index of the stand.