DROUGHT RESISTANCE OF SORGHUM BICOLOR. 1. DROUGHT AVOIDANCE MECHANISMS RELATED TO LEAF WATER STATUS

Abstract

Leaf ψ s (osmotic potential) of two Sorghum bicolor (L.) Moench cultivars M35 and NK300 given an irrigation treatment decreased with increasing plant age. Following 3 wk without irrigation, leaf ψ s of plants of both cultivars was lower than that in irrigated controls. Following 9 wk without irrigation, leaf ψ s of M35 plants was still significantly lower than that in irrigated controls; but in NK300 the difference in leaf ψ s between irrigated and non-irrigated plants was no longer significant due to an ageing effect changing leaf ψ s less in non-irrigated than in irrigated plants. Evidence indicated that leaf ψ s decreased due to osmoregulation, a drought avoidance mechanism, resulting in formation of more osmotically active cellular solutes. Greater leaf senescence of non-irrigated plants is a second drought avoidance mechanism used by sorghum to decrease transpiration requirements. Leaf diffusive resistance measurements indicated open stomata in both irrigated and non-irrigated plants so that under the imposed water stress conditions stomatal closure was not affecting the transpiration requirement.