ELEVATED CO2 CONCENTRATION HAS INDEPENDENT EFFECTS ON EXPANSION RATESAND THICKNESS OF SOYBEAN LEAVES ACROSS LIGHT AND NITROGEN GRADIENTS

The rate and extent of leaf thickness and development are important de terminants of plant photosynthetic capacity, The interactive effects o f photon flux density (PFD), nitrogen supply and CO2 concentration on leaf expansion rate were measured as well as final leaf size and thick ness of soybean, Leaf thickness and final area were not correlated wit h leaf relative expansion rate (RER) suggesting that these parameters are controlled by different mechanisms and that final leaf dimensions are determined by the duration rather than the rate of leaf expansion, Carbohydrate supply did not explain the variation ii? leaf RER since RER increased with increasing CO2 concentration, but decreased with in creasing PFD. Leaf thickness and final area were related to resource s upply but not in a simple fashion, Both positive and negative correlat ions between leaf thickness and carbohydrate and nitrogen concentratio ns were obtained depending on the environmental variable responsible f or the variation. In contrast, there was a simple proportional relatio nship between whole plant relative growth rate and a correlate of leaf thickness (leaf water content per unit area), suggesting that leaf th ickness responds to the balanced supply of all resources, in the same fashion as RGR, rather than to any individual resource.