Responses of Vigna unguiculata (L.) Walp. To Atmospheric and Soil Drought

Abstract

Growth responses, water relations, leaf conductance, and gas exchange of V.unguiculata were studied under conditions of drought in both the aerial and the soil environment. Dry air caused a significant reduction in whole plant biomass and all its components despite a plant water status which was not different from plants in humid air. Stomata closed in dry air and water loss per plant was lower in dry than in humid air because of both lower leaf conductances and a smaller total leaf area.

Dry soil also caused decreases in biomass growth, but leaf growth continued at the soil water status at which stomata began to close (45% depletion of plant available water). Relative leaf expansion rate started to decline below the rate of control plants when 60% (humid air) or 65% (dry air) of plant available water was depleted. Therefore, in V. unguiculata, stomata appear to be more sensitive to soil drought than growth of leaves. Leaf relative water content and osmotic potential remained at the level of control plants when stomata started to close and decreased only slightly below the level of control plants even under severe water stress. A trend of decreasing osmotic potentials at 55-60% depletion of plant available water was observed, but leaf growth ceased only when 90% of plant available water was depleted. Leaf conductance decreased more rapidly than CO₂ assimilation during development of soil drought.