Elevated CO 2 concentration has independent effects on expansion rates and thickness of soybean leaves across light and nitrogen gradients

Introduction The rate and extent of leaf thickness and area Leaves which develop at elevated CO2 concentrations are development are important determinants of whole often thicker than those on ambient 2 plants. In a plant photosynthetic capacity. The interactive effects study of four Populus clones, Radoglou and Jarvis (1990) of photon flux density (PFD), nitrogen supply and CO 2 found increases in leaf thickness of 8% to 16% in response concentration on leaf expansion rate were measured to elevated CO2. Similar results were reported by Leadley as well as final leaf size and thickness of soybean. et al. (1987) for soybeans where they found a 10% Leaf thickness and final area were not correlated with increase in leaf thickness for elevated CO2-grown plants. leaf relative expansion rate (RER) suggesting that Growth at high photon flux density (PFD) also increases these parameters are controlled by different mechanleaf thickness (Boardman, 1977; Björkman, 1981). The isms and that final leaf dimensions are determined by increase in leaf thickness with high PFD is often closely the duration rather than the rate of leaf expansion. correlated with increases in photosynthetic capacity per Carbohydrate supply did not explain the variation in unit leaf area, suggesting that the high photosynthetic leaf RER since RER increased with increasing CO 2 capacities of sun leaves result from increased volume of concentration, but decreased with increasing PFD. mesophyll tissue, and thus increased quantities of photoLeaf thickness and final area were related to resource synthetic enzymes, without a change in photosynthetic supply but not in a simple fashion. Both positive and capacity per unit volume of tissue (Louwerse and negative correlations between leaf thickness and carZweerde, 1977; Patterson et al., 1977, 1978; Sims and bohydrate and nitrogen concentrations were obtained Pearcy, 1992). depending on the environmental variable responsible Whereas PFD acclimation generally has little effect on for the variation. In contrast, there was a simple prophotosynthetic capacity per unit leaf thickness, elevated portional relationship between whole plant relative 2 appears to decrease photosynthetic capacity per unit growth rate and a correlate of leaf thickness (leaf thickness since thickness increases while photosynthetic water content per unit area), suggesting that leaf thickcapacity per unit area generally decreases (Gunderson ness responds to the balanced supply of all resources, and Wullschleger, 1994). However, elevated CO2 does in the same fashion as RGR, rather than to any indinot always decrease photosynthetic capacity per unit area vidual resource. and in some cases even increases it, at least in soybean (Campbell et al., 1988). This could result from the combination of increased leaf thickness and decreased