Research tool uses of rice mutants for increasing crop productivity

Induced mutants are being used in a three-phase programme to develop higher yielding models of the rice plant. Phase 1 is to use induced mutant genes, together with existing genes, to quickly develop near-isogenic comparisons. Phase 2 is to use these near-isogenics to test agronomical and physiological hypotheses about the bases for increased yield. Phase 3 is to use information developed from phase 2 to design more productive models of the rice plant for the future. Completed phase 1 and 2 studies show the following: Semidwarf plant height (90cm) increases grain yield about 15% over the tall height (120cm) in California's high yield environment. An induced mutant semidwarfing gene, sd1, does not reduce seedling vigour in water-sown rice. Semidwarfism has a beneficial effect in minimizing cold-induced sterility in warm water-cool night air situations. Doubledwarf recombinants from crosses between non-allelic semidwarfs generally are too short (80cm) for present cultural practices. Early maturing lines are higher yielding than late lines in cool environments, and yield as much as the late varieties even in warm environments. Studies on source/sink relationships and carbohydrate partitioning suggest that sink size (panicle size) is limiting rice yield in California. Therefore, genotypes are being developed with increased numbers of kernels/panicle, and/or larger kernel size. Thus, phase 1 and 2 results to date indicate that critical elements of phase-3-developed lines must include semidwarfism and early maturity. It is believed that further additions to the high yield model will include larger sink size, either through larger kernels or more kernels/panicle, or a combination of these two yield components. (author)