Soil drying and its effect on leaf conductance and CO2 assimilation of Vigna unguiculata (L.) Walp. I. The response to climatic factors and to the rate of soil drying in young plants

Well watered plants of Vigna unguiculata (L.) Walp cv. California Blackeye No. 5 had maximum photosynthetic rates of 16 μmol m-2s-1(at ambient CO₂ concentration and environmental parameters optimal for high CO₂ uptake). Leaf conductance declined with increasing water vapour concentration difference between leaf and air (Δw), but it increased with increasing leaf temperature at a constant small Δw. When light was varied, CO₂ assimilation and leaf conductance were correlated linearly. We tested the hypothesis that g was controlled by photosynthesis via intercellular CO₂ concentration (ci). No unique relationship between (1) ci, (2) the difference between ambient CO₂ concentration (ca) and ci, namely$c_{a}-c_{i}$, or (3) the$c_{i}/c_{a}$ratio and g was found. g and A appeared to respond to environmental factors fairly independently of each other. The effects of different rates of soil drying on leaf gas exchange were studied. At unchanged air humidity, different rates of soil drying were produced by using (a) different soils, (b) different irrigation schemes and (c) different soil volumes per plant. Although the soil dried to wilting point the relative leaf water content was little affected. Different soil drying rates always resulted in the same response of photosynthetic capacity ($A_{\text{max}}$) and corresponding leaf conductance ($g_{(\text{Amax})}$) when plotted against percent relative plant-extractable soil water content ($W_{e}$%) but the relationship with relative soil water content (Ws) was less clear. Above a range of$W_{e}$of 15%-25%, Amaxand$g_{(\text{Amax})}$were both high and responded little to decreasing$W_{e}$. As soon as$W_{e}$fell below this range,$A_{\text{max}}$and$g_{(\text{Amax})}$declined. The data suggest root-to-leaf communication not mediated via relative leaf water content. However,$g_{(\text{Amax})}$was initially more affected than$A_{\text{max}}$.