Osmotic Adjustment in Sorghum

Abstract

Osmotic adjustment, defined as a lowering of osmotic potential (*In) due to net solute accumulation in response to water stress, has been considered to be a beneficial drought tolerance mech- anism in some crop species. The objective of this experiment was to determine the relative contribution of passive versus active mechanisms involved in diurnal 'II changes in sorghum (Sorghum bicolor L. Moench) leaf tissue in response to water stress. A single sorghum hybrid (cv ATx623 x RTx430) was grown in the field under variable water supplies. Water potential, *I', and relative water content were measured diumally on expanding and the uppermost fully expanded leaves before flowering and on fully expanded leaves during the grain-filling period. Diurnal changes in total osmotic potential (A*In) in response to water stress was 1.1 megapascals before flowering and 1.4 megapas- cals during grain filling in comparison with 0.53 megapascal under well-watered conditions. Under water-stressed conditions, pas- sive concentration of solutes associated with dehydration ac- counted for 50% (0.55 megapascal) of the diumal AI,, before flowering and 47% (0.66 megapascal) of the change during grain filling. Net solute accumulation accounted for 42% (0.46 mega- pascal) of the diumal A*I' before flowering and 45% (0.63 mega- pascal) of the change during grain filling in water-stressed leaves. The relative contribution of changes in nonosmotic volume (de- creased turgid weight/dry weight) to diumal A*I' was less than 8% at either growth stages. Water stress did not affect leaf tissue elasticity or partitioning of water between the symplasm and apoplasm.