Responses of cowpeas (Vigna unguiculata (L.) Walp.) to progressive soil drought

Greenhouse-grown cowpeas, Vigna unguiculata (L.) Walp., were subjected either to well-watered or to progressive soil drought conditions between 10-40 days after emergence. Stomatal closure was found to correlate with the progressive drying of soil while leaf water potentials were not very different from the well-watered plants. Reduction in leaf turgor resulted in a reduced rate of leaf extension but increased that of root. Stomatal conductance and transpiration rates of soil-drought plants were similar to well-watered plants in the morning, but were greatly reduced in the afternoon till evening. It is suggested that the maintenance of transpiration rates per unit leaf area of soil-drought cowpeas in the morning is due to the reduction in the leaf area per plant and possibly the hydration of the plants in the night through enhanced root growth.