Diurnal and seasonal responses of stomatal conductance for cowpea plants subjected to different levels of environmental drought.

Previously we reported that leaf conductance of cowpea (Vigna unguiculata) decreased with small changes in soil water status without associated changes in leaf water status. In these studies a larger range of soil water deficits was imposed in a rain-free environment by prolonged soil drying, and by weekly irrigation with different amounts of water. With progressive soil water deficits, leaf conductance and xylem pressure potential both declined, but in a manner which indicated that they were not related. Diurnal courses of leaf conductance usually indicated that stomatal opening occurred in the morning, and partial or complete stomatal closure occurred during midday and afternoon. This stomatal closure was associated with increases in air vapor pressure deficit. Day-to-day increases in leaf conductance, at times when radiation was not limiting stomatal opening, were associated with decreases in air vapor pressure deficits. However, maximum leaf conductances and their responses to vapor pressure deficit were generally smaller for plants subjected to greater depletion of soil water.