Drought Effects on Transpiration and Leaf Water Status of Cowpea in Controlled Environments

Abstract

The factors responsible for the extreme drought avoidance of cowpeas [Vigna unguiculata (L.) Walp.] previously observed in several field environments were investigated. Drought avoidance is defined as the extent to which high plant water potentials are maintained in the presence of environmental drought. Cowpeas were grown in controlled environments and subjected to either frequent irrigation or progressive drought. Plants in the drought treatment were grown for up to 2 months on 2-2.5 litres per plant of available water supplied 5 days after emergence. Transpiration of whole plants was determined by pot weighing, and leaf water status was determined with a pressure chamber. Well watered cowpeas avoided drought by two mechanisms: by avoiding water loss, because at constant air temperature transpiration increased only slightly with increases in vapour pressure deficit from 20 to 40 mbar; and due to an efficient water transport system which resulted in only small decreases in pressure chamber values with increases in transpiration at high rates of transpiration. Non-irrigated cowpeas avoided drought by maintaining low transpiration rates due to reduced leaf area and reduced transpiration per unit leaf area compared with irrigated plants.