Influence of Water Deficits and Osmotic Adjustment on Leaf Elongation in Rice1

The influence of water deficits on leaf elongation of several cultivars of upland rice (Oryza sativa L.) was studied. Mild water deficits resulted in marked reductions in leaf elongation rates. Previous conditioning, by exposing plants to one or more moderate water stresses, decreased the sensitivity to subsequent stress allowing elongation to continue to more negative water potential. This reduced sensitivity in conditioned plants was accompanied by an enhanced capacity for tugor maintenance resulting from osmotic adjustment but was not entirely explained by this adjustment. The relation between leaf elongation rate and turgor potential was also altered in conditioned plants, indicating that growthinfluencing characters in addition to tugor maintenance capacity were altered. Control and conditioned plants appeared to have similar turgor thresholds for leaf elongation but, above this threshold, control plants required less turgor for similar rates of elongation. Under slower drying conditions, despite reductions of water potential, bulk leaf turgor was unchanged as a result of osmotic adjustment. However, despite this turgor maintenance, leaf expansion rates were markedly reduced. Although the sensitivity of leaf expansion to water deficits was similar among the varieties studied, this response provides a sensitive basis for scoring for the incidence of stress and provides a means for rapid screening for drought resistance in larger varietal comparisons.