Shade-tolerance as a predictor of responses to elevated CO2 in trees

Evidence from 10 studies comparing angiosperm trees and 5 studies comparing conifers or differing shade-tolerance was analysed. The number of intraphyletic comparisons in which the more shade-tolerant species showed the greater relative increase of biomass in elevated CO2 was significantly higher than would be expected by chance alone. It is suggested that more shade-tolerant species are inherently better disposed. in terms of plant architecture and partitioning of biomass and nitrogen, to utilise resources (light, water, nutrients) that are potentially limiting in elevated CO2 and that these traitu are responsible for the interaction between shade-tolerance and CO2 concentration. Compared with less shade-tolerant angiosperm trees, more shade-tolerant angiosperm species generally have a lower lear area ratio in ambient CO2 and show a smaller relative reduction in elevated CO2. Furthermore, leaf nitrogen content is usually lower in more shads-tolerant angiosperm species and tends to be more strongly reduced by elevated CO2 in those species. Within angiosperm trees, more shade-tolerant species showed a stronger stimulation of net leaf photosynthetic I ate in most experiments, but this trend was not significant

Cuticular water permeability and its physiological significance

Cuticles act as solution-diffusion membranes for water transport. Diffusion in pores does not contribute to cuticular transpiration. An extensive literature survey of cuticular permeances (P) and minimum leaf conductances (g(min)) to water is presented. The two variables cannot be distinguished with most experimental techniques. Results from different experiments are in good agreement with each other for some species, for example, Fagus sylvatica L., but not for others, such as Picea abies (L.) Karst. In a data set of 313 values of P or g(min) from 200 species, distributions of results obtained with different techniques were found to differ significantly. Likely reasons include water loss from incompletely closed or incompletely sealed stomata, and the dependence of P on moisture content of the cuticle and on storage time of isolated cuticles. Contrasting evidence for an interaction between cuticular transpiration and stomatal sensitivity to air humidity is presented. The occurrence of unusually high g(min) in trees growing at the alpine treeline and its physiological significance are discussed. It is shown that g(min) is of little value as a predictor for drought resistance of crops, with the possible exception of Sorghum bicolor L. Moench. Possible water uptake from fog or dew across cuticles is considered briefly

Meta-analysis of the interaction between shade-tolerance, light environment and growth response of woody species to elevated CO2

Growth responses of different tree species to elevated CO2 vary enormously, even when grown under the same basic experimental conditions (such as nutrient supply and light conditions). A test of the hypothesis that this variation is associated with the species' shade-tolerance is presented. A meta-analysis of 74 logarithm-transformed response ratios RR (total biomass in -700 mu mol.mol(-1) CO2/biomass in current ambient CO2), grouped according to the species' relative tolerance to shade, revealed that RR differed widely and significantly between seedlings or saplings of woody species of different shade-tolerance. Only comparative studies were included in the meta-analysis, and interference from differences in basic experimental conditions between experiments was minimised. The mean RR of species with very high shade-tolerance was 1.51. This was significantly greater than the value of 1.18 for species intolerant of shade. Smaller differences in mean RR were found between groups of species that differed less widely in shade-tolerance. When responses were categorised according to two aspects of light environment in the experiments, daily integrated or peak photosynthetically active photon flux density (PFD), the differences in RR between different shade-tolerance classes disappeared at daily integrated PFD below similar to 20 mol.m(-2).ad(-1) or peak PFD below similar to 250 mu mol.m(-2).s(-1). Growth stimulation in juvenile trees is likely to cause increased survivorship in subsequent years. Even if the observed differences in RR were restricted to sites experiencing no or only moderate shading, it appears likely that they will affect species composition of naturally regenerating forests in the long term. (C) 2001 Editions scientifiques et medicales Elsevier SAS

Signalling across the divide: A wider perspective of cuticular structure-function relationships

The cuticle is designed to keep water and solutes in, but to keep invaders out. Contrary to traditional thought, the cuticle's thickness and overall wax load do not control water transport across it, Why then are some cuticles very thick or made with apparently excessive amounts of waxes? A more comprehensive approach to understanding cuticular structure-function relationships is called for, taking into account the maintenance of barrier properties as well as a range of other functions, which include offering a medium for communication between potential invaders and the plant, impeding penetration by insects and fungi, and reducing water retention by leaf surfaces

In vivo manipulation of cuticular water permeance and its effect on stomatal response to air humidity

Cuticular water permeance was manipulated in Corylus avellana L., Hypericum androsaemum L. and Populus tremula L. by (1) long-term application of low doses of various systemic herbicides inhibiting biosynthesis of cuticular waxes, (2) very short-term application of organic solvents to the leaf surface, and (3) exposure to natural strong winds. Treatment effects were very variable, but increased the natural range of permeances by a factor of 10 or so in undamaged leaves. All species had hypostomatous leaves. Relative change of leaf conductance (g) in response to stepwise increases of leaf-to-air water vapour pressure difference (VPD) was measured for individual leaves (Corylus) or groups of leaves at the shoot or branch tip. Adaxial cuticular water permeance (P) was determined for the same leaves after measurement of the VPD-response. A proportional measure of relative change of g with VPD, d(log(e)g)/dVPD, was then plotted against P. No increase in the strength of the closing response to increasing VPD was found with increasing P, as would have been expected if water loss through the cuticle was involved in stomatal response to changes in VPD via a direct effect on guard cell turgor. By contrast, high P coincided, most clearly in Corylus, with a reduced strength of the stomatal closing response to increasing VPD, i.e. less negative d(log(e)g)/dVPD. As the responses were non-linear, the value of d(log(e)g)/dI/PD changed with VPD. With rising VPD, all three species and a fourth one previously studied showed a decline in the value of d[d(log(e)g)/dVPD]/d(log P), reaching negative values in one species. This is interpreted in terms of two independent and antagonistic effects of increased cuticular water permeance on guard cell response to VPD, one acting by reducing the backpressure exerted on guard cells by the epidermis, and the other one possibly causing greater depression of guard cell turgor through delivery of more chemical messengers (such as abscisic acid) to the guard cells with the cuticular transpiration stream

Water transport in plant cuticles: an update.

The scale, mechanism, and physiological importance of cuticular transpiration were last reviewed in this journal 5 and 10 years ago. Progress in our basic understanding of the underlying processes and their physiological and structural determinants has remained frustratingly slow ever since. There have been major advances in the quantification of cuticular water permeability of stomata-bearing leaf and fruit surfaces and its dependence on leaf temperature in astomatous surfaces, as well as in our understanding of the respective roles of epicuticular and intracuticular waxes and molecular-scale aqueous pores in its physical control. However, understanding the properties that determine the thousand-fold differences between permeabilities of different cuticles remains a huge challenge. Molecular biology offers unique opportunities to elucidate the relationships between cuticular permeability and structure and chemical composition of cuticles, provided care is taken to quantify the effects of genetic manipulation on cuticular permeability by reliable experimental approaches

Parameterization, comparison, and validation of models quantifying relative change of cuticular permeability with physicochemical properties of diffusants.

Predictions from two previously published models and a new model for the relative change in cuticular permeability with boiling point, octanol/air partition coefficient, and/or molar volume of a wide range of diffusants (not including ions and large hydrophilic compounds) are compared with each other and to experimental data sets not used for model parameterization. While the models work in a similar way for all cuticles for which data are available, it is not yet possible to predict in absolute terms the permeability of any cuticles for which no data are available—that is, while the slope of a plot representing the change in permeability with diffusant properties is predictable, the position of the linear relationship along the ordinate needs to be determined experimentally for each type of cuticle at or near the relevant temperature(s)