EFFECT OF TIMING AND SEVERITY OF WATER-DEFICIT ON 4 DIVERSE RICE CULTIVARS .3. PHENOLOGICAL DEVELOPMENT, CROP GROWTH AND GRAIN-YIELD

Water deficit is known to retard phenological development and reduce g rowth of rice. We investigated whether the ability of a cultivar to ex tract soil water and to tolerate water deficit affected its growth dur ing and following water deficit, and consequently its grain yield. Fou r contrasting rice (Oryza sativa L.) cultivars (CPIC8, Lemont, Rikuto- Norin 12, and Todoroki-Wase) were subjected to water deficit during ei ther vegetative or reproductive growth stages. We examined the contrib ution of genotypic differences in water-extraction ability and drought tolerance to phenological development, crop growth rate and grain yie ld. Biomass production during water deficit was less than 56% of that in irrigated conditions, depending on severity of stress and cultivar. Following the relief of water stress, crop growth rate depended on gr owth stage and severity of stress imposed. The cultivars differed in r ate of recovery from water deficit and this was partly attributed to d ifferences in dehydration tolerance. Severe water deficit suspended ap ical development until rewatering occurred, while mild water deficit r educed the rate of apical development. Water deficit during vegetative growth did not significantly reduce grain yield, whereas water defici t imposed during the reproductive period reduced grain yield to 20-70% of the irrigated control. A small growth rate during panicle developm ent reduced grain number and potential grain size, while cultivars whi ch recovered quickly after water deficit had a relatively larger grain yield. Greater ability to extract water and greater dehydration toler ance during water deficit, increased growth rate during recovery. Sele cting for greater water extraction capacity and dehydration tolerance may be useful in improving adaptation of cultivars to water-limited en vironments.