Osmotic Adjustment to Water Stress in Pearl Millet (Pennisetum americanum(L. ) Leeke) in a Controlled Environment

Abstract

A pressure-volume (P-V) and an expressed sap (cryoscopic) technique were compared for assessing osmotic adjustment to water stress by pearl millet (Pennisetum americanum (L. ) Leeke) plants grown in a controlled environment cabinet. For leaf water potentials (Ψ ) above the point of zero turgor, there was good agreement between estimates of solute potential (Ψ _s)and turgor (ψ _p) obtained by the two methods. Reductions in pre-dawn leaf Ψ to −1.8 MPa over 5–6 d resulted in net solute accumulation as indicated by a fall in Ψ s at full hydration of about 0.3 MPa. The degree of osmotic adjustment increased linearly with the decrease in pre-dawn Ψ. Adjustment in cv. BJ 104 was significantly (P < 0.05) less during a second drought than during a first, and cv. Serere 39 was significantly (P < 0.05) less able to adjust osmotically than BJ 104. Adjustment was greater in leaves which were undergoing extension growth during the drought than in leaves already fully extended before drought started. Much of the adjustment was lost within 24 h following rewatering, the loss being most complete in the older, fully extended leaves.