THE USE OF SEEDLING LEAF DEATH SCORE FOR EVALUATION OF DROUGHT RESISTANCE OF RICE

Leaf death (drought score) of seedlings has been extensively used in r ice breeding programs as a selection index for drought resistance. Thi s paper investigates the factors that affect the drought score of rice cultivars exposed to water stress during the vegetative stage and the relationship between drought score and yield. Two upland experiments consisting of 21 and 8 cultivars, respectively, were subjected to wate r deficit during vegetative growth. In the first experiment, the plant s were grown under three stress conditions and pre-dawn leaf water pot ential, epidermal conductance, canopy light interception and drought s core were determined during the stress period. In the second experimen t leaf area, water use, dry matter growth and grain yield were also ex amined. Large genotypic variation in plant size existed in both experi ments and this affected development of water stress and drought score. The cultivars with large plant size tended to have high drought score . A significant positive correlation (r = 0.74 *, n = 63) existed bet ween drought score and Light interception. However, analysis of covari ance indicated that genotypic variation in drought score existed after the effect of plant size was taken into account. Significant genotypi c variation in leaf water potential existed and this was negatively co rrelated with drought score (r = 0.75 *, n = 42). Cultivars with larg e leaf area tended to lose more green leaves during the stress period, but this did not result in reduced dry matter growth during or after the stress period. Green leaf area remaining after the stress period, and not amount of dead leaf (drought score) determined subsequent dry matter growth (r = 0.45 *, n = 32), and there was no correlation betw een drought score of seedlings and grain yield. Thus for use in a vege tative screening program for drought resistance, estimates of drought score should be accompanied by estimates of plant size. (C) 1998 Elsev ier Science B.V.