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Saline soils are characterized by an array of properties that are adverse to rice cultivation. The problem of salinity is compounded by mineral deficiencies (Zn, P) and toxicities (Fe, Al, organic acids), submergence, deep water and drought. These soil stresses vary in magnitude and interactions over time and place, making long-term adaptability of a cultivar dependent on its level of tolerance to all the stresses that occur in its growing environment. Thus, in breeding rice for saline environments, multiple stress tolerance traits must be considered. Some degree of cultivar tolerance for these stresses is in rice germplasm. However, understanding the physiological mechanisms of these traits, their biochemical basis, inheritance and efficient screening techniques are needed to hasten breeding progress. Recent progress in breeding for rice for saline environments includes the development of rapid and reliable techniques of screening for elongation ability and tolerance for salinity, submergence, iron toxicity, aluminum toxicity and phosphorus efficiency. Donor germplasm has been identified and improved, and its inheritance for tolerance for most soil-related stresses has been studied. Physiological mechanisms of some stresses are now fairly well understood. Rice lines developed with tolerance for multiple stresses have been successfully used as donor parents, and released as cultivars for salt-affected areas by national agricultural research and extension systems (NARES). Present studies are directed toward increasing the selection efficiency and accelerating pyramiding of various soil-related tolerance traits. F8 recombinant inbred line (RIL) mapping populations for various abiotic traits were developed and genotyped. Major genes and quantitative trait loci (QTLs) for elongating ability tolerance and for tolerance for salinity, submergence, P deficiency, and Al and Fe toxicities were mapped. Genotyping is in progress for other tolerance traits. Refined mapping using near-isogenic lines for the development of polymerase chain reaction (PCR)-based marker-assisted selection (MAS) is now possible and pursued. MAS techniques are being developed for elongation ability and tolerance of salinity, submergence, Al toxicity, P deficiency and Zn deficiency. |
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