Screening for drought resistance in rainfed lowland rice

In this review, reasons for the slow progress in development of drought res istant cultivars for rainfed lowland rice are considered first. Recent adva nces in screening methods for development of drought resistant cultivars fr om mostly our research in Thailand in the 1990s, are then described for dro ught that develops early or late in the wet-season. There now appears to be a good prospect for developing drought resistant cultivars that produce hi gher yield than existing cultivars when drought develops late in the wet-se ason. Appropriate phenology to escape late-season drought and high potentia l yield under well-watered conditions are important characters for cultivar s adapted to rainfed lowland conditions. In addition, ability to maintain h igher leaf water potential when drought develops late in the season is anot her desirable character. Maintenance of leaf water potential just prior to flowering is associated with higher panicle water potential, reduced delay in flowering time, and reduced spikelet sterility, and hence contributes to higher yield. Genotypes that are adapted to areas of late-season drought s hould also have high harvest index, intermediate height and rather small to tal dry matter compared to existing traditional cultivars, under well-water ed conditions. This combination of characters would ensure high potential y ield under favourable conditions and also contribute to resistance against late-season drought. Screening against early-season drought that develops d uring the vegetative stage is more difficult, as the genotype's ability to recover from the stress appears more important than drought tolerance durin g the stress period. Other than appropriate phenology, high potential yield and ability to maint ain high leaf water potential, no specific physiological and morphological characters appear to contribute directly to higher yield under drought cond itions in rainfed lowland rice in Thailand, where drought develops rather r apidly due to the prevailing coarse textured soils. It is thus appropriate to develop a breeding program that is primarily based on selection for grai n yield. There are large genotype by environment interactions for yield in rainfed lowland rice and hence it is important that genotypes are selected for yield under appropriate target environments. Addition of a drought scre ening program that is conducted in the field in the wet-season to the overa ll breeding program would enhance the opportunity to select for drought res istance within the breeding materials and increase the chance of developing high yielding cultivars adapted to the drought-prone rainfed lowland envir onments. (C) 1999 Elsevier Science B.V. All rights reserved.