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Some plant species increase root allocation at the expense of reproduction in the presence of non-self and non-kin neighbors, indicating the capacity of neighbor-identity recognition at the root level. Yet in spite of the potential consequences of root identity recognition for the relationship between plant interactions and community structure and functioning, this phenomenon still remains poorly understood. We first critically assess the evidence for the existence of self/non-self and kin recognition at the root level in plants. While root identity recognition most likely exists to some degree, there remain valid points of criticism regarding experiments that have documented this, particularly concerning the effects of pot volume in self/non-self recognition experiments and the roles of size inequality and asymmetric competition in kin recognition studies. Subsequently we review and propose some plausible physiological mechanisms that may underlie these responses. Finally we briefly discuss the relation between under- and aboveground interactions and the potential consequences of root identity recognition for agriculture, and conclude with raising several questions for future studies., (Copyright © 2012 Elsevier Ireland Ltd. All rights reserved.)

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.

• Analysing plant phenotypic plasticity in response to wind is complicated as this factor entails not only mechanical stress but also affects leaf gas and heat exchange. • We exposed Plantago major plants to brushing (mechanical stress, MS) and wind (MS and air flow) and determined the effects on physiological, morphological and mechanical characteristics of leaf petioles and laminas as well as on growth and biomass allocation at the whole-plant level. • Both MS and wind similarly reduced growth but their effects on morphological and mechanical plant traits were different. MS induced the formation of leaves with more slender petioles, and more elliptic and thinner laminas, while wind tended to evoke the opposite response. These morphological and mechanical changes increased lamina and petiole flexibility in MS plants, thus reducing mechanical stress by reconfiguration of plant structure. Responses to wind, on the other hand, seemed to be more associated with reducing transpiration. • These results show that responses to mechanical stress and wind can be different and even in the opposite direction. Plant responses to wind in the field can therefore be variable depending on overall environmental conditions and plant characteristics., (© The Authors (2010). Journal compilation © New Phytologist Trust (2010).)

Different life history strategies among tropical rain forest species are generally related to inherent trade-offs in leaf and crown traits, with early-successional species having traits that facilitate high productivity but a relatively wasteful use of resources and shade-tolerant later-successional species exhibiting the opposite strategy. But the degree to which these trait differences contribute to short- and long-term carbon gain of different species that coexist in natural forest has not been quantitatively scaled. We applied a canopy model in combination with measurements of canopy structure, mass distribution, and leaf photosynthesis to determine whole-plant daily photosynthesis (P(pl)) of individuals of three short-lived pioneers (SLP), four later-successional species, and three lianas growing together in a 0.5-, 2-, and 3-yr-old secondary tropical forest stand. Whole-plant daily photosynthesis per unit leaf mass (P(lfm)) and aboveground mass (P(m)) were assumed to indicate daily returns on investment at the leaf and crown level. By integrating these calculations with measured leaf longevities, we determined the lifetime carbon gain per unit leaf mass. Vegetation height and leaf area index increased with stand age. Two of the SLPs, Trema and Ochroma, increasingly dominated the top of the vegetation. In the 0.5-yr-old stand, these species also had the highest P(m) and P(lfm) values. Whole-plant daily photosynthesis per unit leaf mass tended to decline with stand age but much more strongly so in the later-successional species than in the SLP. Leaf longevity was not significantly correlated with individual leaf traits (e.g., specific leaf area or leaf N content) but was strongly and negatively correlated with P(lfm) in the youngest stand. Later-successional species had considerably greater leaf longevities than SLP. Lifetime carbon gain per unit leaf mass, however, was relatively similar between the different species. Thus due to the strong negative correlation that exists between daily leaf productivity (P(lfm)) and longevity, short-lived pioneers and later-successional species achieve similar lifetime carbon gain per unit leaf mass in natural secondary forest. This could help explain why the slower-growing later-successional species are able to persist during the first years of succession.

By applying engineering theory, we found that in order to achieve a certain degree of stem mechanical stability, trees with low wood dry-mass density (rho(D)) need to produce thicker stems but invest less mass per unit stem length than those with high rho(D). Mechanical stability was expressed as the ability of the vertical stem to either support a plant's weight (i.e., the buckling safety factor) or resist wind forces without rupture. This contradicts the general notion that trees with low rho(D) are more prone to mechanical failure. Contrary to our results for stems, we predicted that high rho(D) can be more efficient than low rho(D) in terms of the mass needed to produce a branch of given length and resistance to rupture under its own weight. Such branches were also predicted to be more flexible. These predictions were generally in accordance with literature data for tropical tree species. This shows that differences in scaling rules associated with vertical self-loading, resistance to external forces, and the production of stable horizontal branches have important implications for the way in which different crown traits determine the balance between economy of crown design and mechanical stability.

The interacting effects of different environmental cues in determining a plant's phenotype and performance are poorly understood aspects of phenotypic plasticity. We examined the interacting effects of shading and mechanical stress (MS) on growth, reproduction, and mechanical stability. We subjected 10 grassland genotypes and 10 forest genotypes of Impatiens capensis to two levels of spectral shading and two levels of MS. Shade induced the production of taller, thinner internodes, but this response was inhibited by MS. This interactive effect was stronger in the grassland genotypes than in the forest genotypes, indicating that shade avoidance can be suppressed by MS and that the degree of this suppression differs between habitats. Among genotypes, greater plasticity in shade-induced internode elongation resulted in a larger reduction in the ability of plants to carry their own weight. This suggests that the occurrence of mechanical constraints may strongly contribute to the selection against shade-induced elongation responses in forest understory genotypes. Our results show that combined responses to different environmental cues can result in a fine-tuning of phenotypic expression by plants by maintaining the potential to strongly respond to single environmental cues but also by preventing potential future costs resulting from poor adaptation to other stresses.

Background and Aims: While within-species competition for light is generally found to be asymmetric - larger plants absorbing more than proportional amounts of light - between-species competition tends to be more symmetric. Here, the light capture was analysed in a 5-year-old competition experiment that started with ten genotypes of the clonal plant Potentilla reptans. The following hypotheses were tested: (a) if different genotypes would do better in different layers of the canopy, thereby promoting coexistence, and (b) if leaves and genotypes with higher total mass captured more than proportional amounts of light, possibly explaining the observed dominance of the abundant genotypes., Methods: In eight plots, 100 leaves were harvested at various depths in the canopy and their genotype determined to test for differences in leaf biomass allocation, leaf characteristics and the resulting light capture, calculated through a canopy model using the actual vertical light and leaf area profiles. Light capture was related to biomass to determine whether light competition between genotypes was asymmetric., Key Results: All genotypes could reach the top of the canopy. The genotypes differed in morphology, but did not differ significantly in light capture per unit mass (Phi(mass)) for leaves with the laminae placed at the same light levels. Light capture did increase disproportionately with leaf mass for all genotypes. However, the more abundant genotypes did not capture disproportionately more light relative to their mass than less-abundant genotypes., Conclusions: Vertical niche differentiation in light acquisition does not appear to be a factor that could promote coexistence between these genotypes. Contrary to what is generally assumed, light competition among genetic individuals of the same species was size-symmetric, even if taller individual leaves did capture disproportionately more light. The observed shifts in genotype frequency cannot therefore be explained by asymmetric competition for light.

Background and Aims: Leaf mechanical properties, which are important to protect leaves against physical stresses, are thought to change with light and nutrient availabilities. This study aims to understand phenotypic changes of leaf mechanical properties with respect to dry mass allocation and anatomy., Methods: Leaf lamina strength (maximum force per unit area to fracture), toughness (work to fracture) and stiffness (resistance against deformation) were measured by punch-and-die tests, and anatomical and physiological traits were determined in Plantago major plants grown at different light and nutrient availabilities. A conceptual approach was developed by which punch strength and related carbon costs can be quantitatively related to the underlying anatomical and morphological traits: leaf thickness, dry-mass allocation to cell walls and cell-wall-specific strength., Key Results: Leaf lamina strength, toughness and stiffness (all expressed on a punch area basis) increased with light availability. By contrast, nutrient availability did not change strength or toughness, but stiffness was higher in low-nutrient plants. Punch strength (maximum force per unit punch area, F(max)/area) was analysed as the product of leaf mass per area (LMA) and F(max)/leaf mass (= punch strength/LMA, indicating mass-use efficiency for strength). The greater strength of sun leaves was mainly explained by their higher LMA. Shade leaves, by contrast, had a higher F(max)/leaf mass. This greater efficiency in shade leaves was caused by a greater fraction of leaf mass in cell walls and by a greater specific strength of cell walls. These differences are probably because epidermis cells constitute a relatively large fraction of the leaf cross-section in shaded leaves. Although a larger percentage of intercellular spaces were found in shade leaves, this in itself did not reduce 'material' strength (punch strength/thickness); it might, however, be important for increasing distance between upper and lower epidermis per unit mass and thus maintaining flexural stiffness at minimal costs., Conclusions: The consequences of a reduced LMA for punch strength in shaded leaves was partially compensated for by a mechanically more efficient design, which, it is suggested, contributes importantly to resisting mechanical stress under carbon-limited conditions.

Using a new experimental set up, the way in which height growth of stoloniferous plants is adjusted to that of their neighbours, as well as differences between genotypes in their ability to keep up with neighbour height growth were tested. Five Potentilla reptans genotypes inherently differing in petiole length were subjected to three experimental light gradients, involving light intensity and red : far-red ratio. Each plant was placed in a vertically adjustable cylinder of green foil, and the treatments differed in the speed of cylinder height increase and final height. Total weight of plants decreased from the 'Slow' to the 'Fast' treatment, while petiole length increased. Leaves reaching the top of the cylinder stopped petiole elongation, resulting in similar final heights for all genotypes in the 'Slow' treatment. In the 'Fast' treatment only the fastest-growing genotype maintained its position in the top of the cylinder and genotypes differed strongly in final height within the cylinders. Plants adjust their height growth to that of the surrounding vegetation, leading to height convergence in short light gradients that slowly increase. These adjustments and genotypic differences in ability to keep up with fast-growing neighbours can influence the outcome of competition for light.

Background and Aims: Because plants protect each other from wind, stand density affects both the light climate and the amount of mechanical stress experienced by plants. But the potential interactive effects of mechanical stress and canopy shading on plant growth have rarely been investigated and never in stoloniferous plants which, due to their creeping growth form, can be expected to respond differently to these factors than erect plants., Methods: Plants of ten genotypes of the stoloniferous species Potentilla reptans were subjected to two levels of mechanical stress (0 or 40 daily flexures) and two levels of spectral shading (15 % of daylight with a red:far red ratio of 0.3 vs. 50 % daylight and a red:far red ratio of 1.2)., Key Results: Mechanically stressed plants produced more leaves with shorter more flexible petioles, more roots, and more but less massive stolons. Responses to spectral shading were mostly in the opposite direction to thigmomorphogenesis, including the production of thinner, taller petioles made of more rigid tissue. The degree of thigmomorphogenesis was either independent of light climate or stimulated by spectral shading. At the genotypic level there were no clear correlations between responses to shade and mechanical stress., Conclusions: These results suggest that in stoloniferous plants mechanical stress results in clones with a more compact, shorter shoot structure and more roots. This response does not appear to be suppressed by canopy shading, which suggests that wind shielding (reduced mechanical stress) by neighbours in dense vegetation serves as a cue that induces shade avoidance responses such as increased petiole elongation.

We have studied the interactive effects of salinity and light on Avicennia germinans mangrove seedlings in greenhouse and field experiments. We hypothesized that net photosynthesis, growth, and survivorship rates should increase more with an increase in light availability for plants growing at low salinity than for those growing at high salinity. This hypothesis was supported by our results for net photosynthesis and growth. Net daily photosynthesis did increase more with increasing light for low-salinity plants than for high-salinity plants. Stomatal conductance, leaf-level transpiration, and internal CO(2) concentrations were lower at high than at low salinity. At high light, the ratio of leaf respiration to assimilation was 2.5 times greater at high than at low salinity. Stomatal limitations and increased respiratory costs may explain why, at high salinity, seedlings did not respond to increased light availability with increased net photosynthesis. Seedling mass and growth rates increased more with increasing light availability at low than at high salinity. Ratios of root mass to leaf mass were higher at high salinity, suggesting that either water or nutrient limitations may have limited seedling growth at high salinity in response to increasing light. The interactive effects of salinity and light on seedling size and growth rates observed in the greenhouse were robust in the field, despite the presence of other factors in the field--such as inundation, nutrient gradients, and herbivory. In the field, seedling survivorship was higher at low than at high salinity and increased with light availability. Interestingly, the positive effect of light on seedling survivorship was stronger at high salinity, indicating that growth and survivorship rates are decoupled. In general, this study demonstrates that environmental effects at the leaf-level also influence whole plant growth in mangroves.

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