Osmotic adjustment to water stress in pearl millet ( Pennisetum americanum [L.] Leeke) under field conditions

. Osmotic adjustment, a mechanism whereby plants maintain positive turgor despite low water potential (ψ), was investigated in pearl millet ( Pennisetum americanum [L.] Leeke) in three types of field experiment at Hyderabad, India:1 Osmotic adjustment during the growing season was evaluated by comparing solute potential (ψ s ) of leaves taken at midday from irrigated and droughted plots and allowed to rehydrate in the laboratory. The degree of seasonal adjustment was also estimated by comparing observed values of ψ s in the field with those expected if ψ s decreased solely in proportion to water loss. Both types of assessment indicated the maximum seasonal adjustment to be about 0.2 MPa. The cultivars BJ 104 and Serere 39 differed in their capacity to adjust osmotically over the season; Serere 39 was least able to osmoregulate. 2 Measurements of diurnal variations in ψ and ψ s in BJ 104 revealed osmotic adjustment during the afternoon hours. At a given value of ψ, turgor (ψ p ) was about 0.1 MPa higher in irrigated, and over 0.2 MPa higher in droughted plants, in the afternoon, than in the morning. 3 Osmotic adjustment of different leaves within the canopy was investigated. Upper leaves had lower ψ than basal leaves. Differences in ψ were matched by gradients in ψ s , so that turgor was similar for all leaf layers.