Isolation, characterization, and field evaluation of rice (Oryza sativa L., Var. IR64) mutants with altered responses to salt stress

Salinity is one of the most serious constraints to rice production worldwide. Rice is salt-sensitive and its growth and yield can drastically be reduced by salt stress. Salt-tolerant mutants are promising varieties for salt-affected soils and they constitute unique genetic stocks for gene discovery. Out of the 60,000 IR64 mutant lines generated at IRRI, 5000 mutants were screened for salt-stress tolerance at the seedling stage using hydroponics. Mutants that are either more tolerant or more sensitive than IR64 were identified and advanced through single seeds after phenotyping for salt tolerance at each generation. Furthermore, the genetic background of the selected mutants was confirmed using SSR markers. Tolerant mutants absorbed less Na+ and more K+, resulting in lower Na+:K+ ratio in their shoots, and produced more shoot biomass through faster growth that can further dilute the absorbed Na+. Contrastingly, the sensitive mutants had higher Na+ and Na+:K+ ratio and lower survival and produced less biomass under salt stress. Four tolerant and three sensitive mutants were selected and, together with tolerant and sensitive checks, were evaluated in a series of field trials under non-saline field conditions to compare their agronomic performance with that of IR64, and in saline fields to confirm their responses relative to IR64 and to assess the impacts of these mutations on growth and yield in salt-affected fields. When evaluated under control conditions in the field, the sensitive and tolerant mutants were similar to IR64 in the majority of growth and yield traits. However, in saline fields, the tolerant mutants had lower Standard Evaluation System (SES) scores and better survival and growth than either IR64 or the sensitive mutants. They also produced an average of 0.8–1.0 t ha−1 higher yield than IR64 and 1.2–1.47 t ha−1 than the sensitive lines. This yield enhancement was associated with more productive tillers and heavier panicles with more fertile spikelets in the tolerant mutants. IR64 was intermediate for most traits. These salt-tolerant mutants are potential varieties for salt-affected coastal areas provided that other quality traits of IR64 have not been significantly altered to ensure acceptance by farmers. Mapping and cloning of these mutations can help understand their genetic and physiological bases and identify new genes for use in breeding, which is currently being pursued.