Physiological bases of tolerance of abiotic stresses in rice and mechanisms of adaptation

Rice is grown over a wide range of climatic and edaphic conditions. The large gap in the productivity of rainfed ecosystems compared with irrigated ecosystem is largely caused by abiotic stresses such as drought, floods, and poor soils, coupled with the lack of well-adapted high-yielding varieties. Substantial progress has been made in unravelling the physiological and molecular bases of tolerance, however, progress in developing high yielding, tolerant varieties has been slow because of the undesirable phenotypes of the tolerant donors and the complexity of the tolerance mechanisms. Flash-floods normally affect rice during vegetative stage, however, in areas where direct seeding is practiced, heavy rains immediately after seeding cause severe damage to germinating seeds, resulting in poor crop stand. Complete submergence annually affects about 16 million ha of rice in South and Southeast Asia, and about one third of the total rice growing area in Africa. Salinity is particularly troubling in coastal regions in the tropics because of marine influence and build up of salt in some inlands as a consequence of excessive irrigation with improper drainage. Mechanism of adaptation of rice to flooded condition at different stages of crop growth and adaptation to salt stress have been reviewed in the present paper.