Growth Response of Rice (Oryza sativa L.) to Drought. II. Varietal difference in transpiration under water stress and its related plant characteristics.

Transpiration of a rice plant under a low water potential of rooting medium and low air-humidity conditions was studied in order to evaluate varietal difference in response to water stress. Water stress in the rooting zone was induced by polyethylene glycol 6000, and the whole plant's transpiration was determined gravimetrically under different air-humidity conditions. Varietal difference in maintaining transpiration under water stress was observed, and the difference was significant under conditions of air humidity less than 45%. Transpiration measurement of plants exposed to low water potential of rooting medium under the low air-humidity was an effective method for evaluating varietal characteristics in water-use behavior. Based on the decrease of transpiration and leaf-water content (LWC), 13 rice varieties could be classified into three groups : tolerant, moderate and susceptible to water stress. Under water stress conditions, tolerant varieties had more characteristics for maintaining higher transpiration and LWC than susceptible varieties. Varietal difference may arise from the differences in sugar accumulation between plants, which may be a result of degradation of starch during water stress. Varieties in which sugar content increased more during water stress could maintain a higher transpiration and LWC than those which accumulated less sugar. An ability to convert starch to sugar during water stress might be a characteristic related to drought tolerance in rice.