Genotypic variation in rice grown in low fertile soils and drought-prone, rainfed lowland environments

The grain yield of rainfed lowland rice (Oryza sativa L.) in many Asian cou ntries is reduced by drought and low soil fertility. This paper reviews rec ent work on the interactions of these effects on grain yield and also work on cultivar requirements for production under low soil fertility conditions , particularly in Thailand and Laos, Fertiliser application trials in the h eld and nutrient omission pot trials showed that low soil fertility resulte d from inadequate levels of many essential nutrients, but especially of nit rogen (N) and phosphorus (P), When standing water disappeared from the padd y, yield was reduced even if there was no apparent problem of water stress. Simulation modelling indicated that this was associated with reduced nutri ent availability. In multi-location trials involving a large number of genotypes, the varianc e component for grain yield was often greater for genotype by environment i nteraction than for genotype alone. While the genotype by fertiliser intera ction was often significant, its contribution to the genotype by environmen t interaction was rather small. These results indicate that the current pra ctice of direct selection for yield under rather high fertiliser applicatio n rate may be justified in breeding programs. The limited information from research conducted with irrigation and under r ainfed lowland conditions suggests that both uptake of N and P and their ut ilisation efficiency (grain yield/unit nutrient uptake) to produce grain yi eld are important for adaptation to low soil fertility environments. Genoty pic variation in utilisation efficiency appears to be consistent across env ironments, and the efficiency can be improved by the development of cultiva rs that require a low nutrient concentration in the plant and a higher nutr ient allocation to the grain. The amount of genetic variation for these spe cific traits, the ease with which these traits can be incorporated in an ap propriate genetic background and the extent of the specific nutritional pro blem will determine the usefulness of selection for a specific nutrient dis order, and they need to be identified in the future research. (C) 1999 Else vier Science B.V. All rights reserved.