Relationships between leaf water status and transpiration of cowpea with progressive soil drying

It was previously reported that leaf water status of cowpea (Vigna unguiculata) exhibited only small changes with progressive soil drying over extended periods of time. In these studies, under field conditions, it was demonstrated that when soil water was not limiting, xylem pressure potential (ψ _x ) exhibited a linear relationship with transpirational flux density with no obvious hysteresis. With progressive soil drying, values of ψ _x and transpiration rate fell below the regression line established for nonlimiting soil conditions. It is proposed that paired measurements of ψ _x and transpirational flux density made between midday and sunset can provide a measure of the extent to which soil conditions are limiting water uptake. The relation between ψ _x and transpiration exhibits a family of curves, with more negative slopes and decreases in maximum transpiration rate indicating progressive limitation due to soil drying.