RESPONSES OF 7 DIVERSE RICE CULTIVARS TO WATER DEFICITS .3. ACCUMULATION OF ABSCISIC-ACID AND PROLINE IN RELATION TO LEAF WATER-POTENTIAL AND OSMOTIC ADJUSTMENT

Foliar content of free abscisic acid (ABA) and proline, leaf water potential (psi-1), and osmotic potential (psi-o) were determined in seven diverse rice (Oryza sativa L.) cultivars after these were subjected to a 10-day period of withholding irrigation or to well-watered conditions. For three of the cultivars, periodic determination of leaf ABA was performed during differential irrigation and recovery. Diurnal kinetics of leaf ABA content, along with observations on leaf bulk turgor (psi-t) and gas exchange, were established in two cultivars under well-watered conditions. Water-stress-induced proline and ABA accumulation differed among cultivars. Across cultivars, stress-induced proline accumulation was correlated negatively with midday psi-1 and positively with osmotic adjustment, but ABA accumulation was not. Proline accumulation therefore gave a similar discrimination among cultivars as did predawn turgid osmotic potential and psi-1. Diurnal oscillations of psi-o differed among cultivars but were not related to ABA accumulation or psi-1. Interpretation of stress-induced ABA accumulation kinetics was difficult due to large diurnal oscillations. The relative amplitude of diurnal ABA oscillation reflected diurnal changes in psi-1 and psi-t across cultivars. Diurnal ABA response to psi-t of a drought-resistant cultivar was less sensitive than that of a susceptible cultivar under well-watered conditions. Highest ABA concentrations were observed at 16:00 h. Diurnal ABA accumulation lagged 4 hours behind changes in leaf bulk turgor. The results suggest that high- and low-ABA-accumulating rice cultivars exist, but no clear linkage of this trait to behavior under water stress was observed.