Comparison of Water Relations and Osmotic Adjustment in Sorghum and Cowpea Under Field Conditions

Abstract

Seasonal and diurnal patterns of xylem pressure potential, Ψ_x, and osmotic potential, Ψ_s, were compared using contrasting genotypes of sorghum [Sorghum bicolor (L.) Moench] and cowpea [Vigna unguiculata (L.) Walp.] under frequently imgated 'wet' and water-limited 'dry' field conditions. Generally, differences in Ψ_x and Ψ_s among genotypes within each species were small compared with differences between the two species. Sorghum exhibited a larger diurnal range and larger differences between wet and dry treatments for Ψ_x, Ψ_s, and estimates of bulk leaf turgor than did cowpea. Seasonal and drought- induced osmotic adjustment occurred in sorghum, but not in cowpea. Diurnal patterns of leaf conductance indicated that, under the dry treatment, cowpea avoided the occurrence of large plant water deficits by substantial reductions in leaf conductance, especially at midday, whereas sorghum maintained moderate values of leaf conductance throughout the day. Cowpea exhibited a larger diurnal range of leaf conductance and larger differences between wet and dry treatments than did sorghum. Differences were not observed between the species in overall soil water depletion or in root activity as estimated from profiles of soil water depletion until senescence occurred in cowpea. Sorghum genotypes that had been selected on the basis of differences in Ψ_s did exhibit significant differences in average values of leaf conductance and total soil water depletion, but the genotype that had the lowest Ψ_s (M35-1) also had the lowest leaf conductance and the least soil water depletion.