Improvement of rice drought tolerance through backcross breeding: Evaluation of donors and selection in drought nurseries

A large-scale backcross breeding project has been undertaken to improve drought tolerance in rice. Over 160 donor cultivars from 25 countries have been used in this project, representing a significant proportion of the genetic variation in cultivated rice. These cultivars were evaluated in field experiments in the Philippines to assess their responses to drought in terms of plant height, heading date, and grain yield. Drought was imposed near heading stage, in experiments that were established either in lowland (anaerobic) fields or upland (aerobic) soil. Despite the poor adaptation of some cultivars to the tropics, it was possible to identify significant variation in plant response to drought treatments, and contrasting effects on flowering delay and growth. Subsequently, 325 BC2F2 bulk populations, developed by backcrossing many of these donors to one of three elite recurrent parents, were screened under drought in lowland or upland nurseries. Stress levels were managed to eliminate almost all seed set in recurrent parents, and those progeny that produced grain were selected as being putatively drought-tolerant. The selection intensity varied across years and in selection environments with more severe stress, higher selection intensity could be imposed. The number of plants selected within a population was not associated with the per se drought response of the donors in the direct evaluation, indicating the wide presence of cryptic genetic variation for drought tolerance in the apparently drought-susceptible cultivars. The genetic background of the recurrent parent affected the number of plants selected, as did the selection environment (upland versus lowland nurseries). These drought-selected introgression lines represent a useful genetic resource to develop improved cultivars for farmers in rainfed or water-scarce rice-growing regions, and also to improve our understanding of the genetic and molecular basis of drought tolerance in rice. Genetic analysis of the selected lines, reported elsewhere, indicated specific regions of high introgression. Yield evaluations of the selected lines are now underway across a range of drought scenarios.