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 countries is reduced by drought and low soil fertility. This paper reviews recent 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 held and nutrient omission pot trials showed that low soil fertility resulted from inadequate levels of many essential nutrients, but especially of nitrogen (N) and phosphorus (P), When standing water disappeared from the paddy, yield was reduced even if there was no apparent problem of water stress. Simulation modelling indicated that this was associated with reduced nutrient availability. In multi-location trials involving a large number of genotypes, the variance component for grain yield was often greater for genotype by environment interaction than for genotype alone. While the genotype by fertiliser interaction was often significant, its contribution to the genotype by environment interaction was rather small. These results indicate that the current practice of direct selection for yield under rather high fertiliser application rate may be justified in breeding programs. The limited information from research conducted with irrigation and under rainfed lowland conditions suggests that both uptake of N and P and their utilisation efficiency (grain yield/unit nutrient uptake) to produce grain yield are important for adaptation to low soil fertility environments. Genotypic variation in utilisation efficiency appears to be consistent across environments, and the efficiency can be improved by the development of cultivars that require a low nutrient concentration in the plant and a higher nutrient allocation to the grain. The amount of genetic variation for these specific traits, the ease with which these traits can be incorporated in an appropriate genetic background and the extent of the specific nutritional problem will determine the usefulness of selection for a specific nutrient disorder, and they need to be identified in the future research. (C) 1999 Elsevier Science B.V. All rights reserved