Osmotic adjustment in sorghum: I. Mechanisms of diurnal osmotic potential changes

Osmotic adjustment, defined as a lowering of osmotic potential (ψ_π) due to net solute accumulation in response to water stress, has been considered to be a beneficial drought tolerance mechanism in some crop species. The objective of this experiment was to determine the relative contribution of passive versus active mechanisms involved in diurnal ψ_π changes in sorghum (Sorghum bicolor L. Moench) leaf tissue in response to water stress. A single sorghum hybrid (cv AT×623 × RT×430) was grown in the field under variable water supplies. Water potential, ψ_π, and relative water content were measured diurnally 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 (Δψ_π) in response to water stress was 1.1 megapascals before flowering and 1.4 megapascals during grain filling in comparison with 0.53 megapascal under well-watered conditions. Under water-stressed conditions, passive concentration of solutes associated with dehydration accounted for 50% (0.55 megapascal) of the diurnal Δψ_π before flowering and 47% (0.66 megapascal) of the change during grain filling. Net solute accumulation accounted for 42% (0.46 megapascal) of the diurnal Δψ_π before flowering and 45% (0.63 megapascal) of the change during grain filling in water-stressed leaves. The relative contribution of changes in nonosmotic volume (decreased turgid weight/dry weight) to diurnal Δψ_π 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.