Physiological evaluation of responses of rice ( Oryza sativa L.) to water deficit

Abstract

In view of the need to increase rice yields in rainfed areas, an experiment was done to (1) determine physiological traits that contribute to tolerance for water deficit at the seedling stage, (2) identify quick but reliable indices for selection for tolerance for water deficit, and (3) characterize rice cultivars popularly used for water-deficit studies in rainfed ecosystems. The study used 27 cultivars, which include those often used for drought studies in rainfed lowland and upland ecosystems. Mild water-deficit conditions were generated by adding polyethylene glycol 1500 to nutrient solution to provide an osmotic stress of −0.5 MPa to 3-week-old seedlings. Water deficit caused a larger reduction in leaf area than in shoot dry matter, demonstrating the greater sensitivity of leaf enlargement to water stress than dry matter accumulation. Visual scoring to assess damage was found to be a reliable measure of tolerance for water deficit. Cultivars tolerant of mild water stress had a high relative transpiration (transpiration under stress compared with that under non-stressed conditions), low initial leaf area (leaf area before the onset of water stress), high carbon isotope discrimination in the leaf, and low specific leaf weight. These factors enabled tolerant cultivars to maintain high moisture in the leaf and to have high values of leaf area, shoot dry matter, and sugar and starch in tissues in stressed plants relative to the control. Mild water deficit increased water use efficiency in stressed plants, caused more degradation of starch than sugar in the leaf blade, and resulted in more accumulation of these carbohydrates in the leaf sheath.