Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/1/2004
Publication Date: 9/13/2004
Citation: Rutger, J.N. 2004. Trends in temperate rice germplasm improvement. Proceedings International Rice Science Conference. p. 16. Interpretive Summary:
Technical Abstract: Germplasm improvement trends in the US have progressed from complete reliance on japonica germplasm, to backcrossing of single genes from indicas into japonicas and to the recent release of improved cultivars from indica by japonica crossing. However most improved cultivars are still heavily japonica based. A recent base-broadening program, relying on indicas, shows promise for creating indica germplasm that has both agronomic adaptation and satisfactory grain quality for US markets. Thus, US germplasm has been very narrow, either temperate japonica as in short and medium grains such as those grown I California and the southern US, or tropical japonica as in the long grains grown principally in the southern US. Only one indica, Jasmine 85, is grown. The narrowness of the US germplasm base is a consequence of the need for adaptation to high latitudes plus the exacting grin quality standards for US markets. Even for introgression of single genes from indica sources, it usually has been necessary to do intensive backcrossing to the japonica parent in order to recover suitable adaptation and grain quality. For example, when IR 8 was used as the sd1 semidwarfism source in California, three backcrosses to California medium grains were required for development of the popular semidwarf cultivar M9. Meanwhile, the first semidwarf in California actually came from induced mutation of the sd1 gene in local germplasm. The induced mutant gene was rapidly incorporated into several additional cultivars through crossbreeding. The indica source of sd1 and the induced mutant source of sd1 were soon intercrossed, producing additional semidwarf cultivars. In the southern US, a similar pattern of backcrossing to the local cultivars was required to introduce sd1 from indicas. The key source of semidwarfism was from the cross Bluebelle *6/TN 1, which was further crossed to southern long grains to produce the first widely grown semidwarf long grain cultivar, Lemont. Disease resistance genes also have been introgressed from indicas into japonica, by the same process of backcrossing to local germplasm. In the 1980s and 1990s comparisons of indicas and japonicas in the southern US demonstrated yield superiority of indicas, albeit at the price of unsatisfactory grain quality. The availabilit7 of very early maturing indicas from China, and of IRRI germplasm that closely approaches US long grain quality has presented new opportunities for indica utilization in the US. A base-broadening program, initiated in the late 1990s, has resulted in the 2004 release of nine improved indica germplasm for US breeders. Other improved indica germplasms are in the pipeline.