Genetic improvement of rice in aerobic systems: progress from yield to genes

Genetic improvement of rice for aerobic (non-flooded) environments has received less attention than breeding for lowland production systems. Aerobic rice has traditionally been grown in low-input systems, but as fresh water for irrigation becomes increasingly scarce, aerobic rice cultivation is expected to expand into regions with more intensive cropping. The primary yield constraints for the low-input aerobic crop include water deficit, acid and infertile soils, weed competition, and disease. Yield potential has been improved through traditional breeding approaches, and some improved upland rice cultivars show a similar pattern of interactions with environments as traditional cultivars. Critical environmental factors that interact with genotype are the distribution of rainfall during the season, the amount of solar radiation received in the period just prior to flowering, and disease pressure. In systems where adequate inputs are applied, aerobic rice tends to yield less than lowland rice, and yield reductions are dramatic when water deficit occurs. The poor adaptation of the lowland cultivar IR72 to aerobic soils is associated with reduced height and harvest index in aerobic conditions, but IR72 had similar biomass production by anthesis as better-adapted upland cultivars. The best-yielding upland lines had very stable pre-anthesis biomass production across five contrasting environments. The physiological and molecular dissection of aerobic rice yield is expected to identify opportunities to accelerate progress in the areas of aerobic adaptation, tolerance to water deficit, and improved weed competitiveness. QTLs have been reported for a number of traits potentially related to performance under water deficit, such as improved root morphology and osmotic adjustment. In an upland-by-lowland mapping population, alleles from the lowland cultivar contributed significantly to improved yield in aerobic environments. (C) 2002 Elsevier Science B.V. All rights reserved.