Effects of irradiance and leaf water deficit on net carbon dioxide assimilation and mesophyll and transport resistances

Rate of net CO₂ assimilation by soil-grown soybean plants were studied over a range of relative leaf water contents at each of four levels of irradiance. There was a large interaction between light level and leaf water deficit on the rate of CO₂ assimilation. The effect of leaf water deficit on assimilation became larger as irradiance increased. Both stomatal resistance to CO₂ transport and mesophyll resistance to CO₂ assimilation increased as leaf-water deficit increased. The increase in both resistance with changing leaf-water content was largest at high irradiance and became smaller as irradiance decreased. Relief of soil-moisture stress by watering induced large oscillations of CO₂ assimilation, stomatal resistance, and mesophyll resistance. The oscillation of the mesophyll resistance occurred in the absence of changes in relative water content and appeared to be related to oscillations in leaf temperature. The observed increase in mesophyll resistance with decreasing leaf-water content under nonoscillative conditions may be caused by changes in leaf temperature rather than leaf water content.