IDENTIFICATION OF THERMOSENSITIVE GENIC MALE-STERILE LINES WITH LOW CRITICAL STERILITY POINT FOR HYBRID RICE BREEDING

Abstract

The purity of hybrid rice seeds utilizing the 2-line system is increased when the thermosensitive genic male-sterile (TGMS) line possesses a low critical sterility point (CSP), which minimizes self-fertility induced by temperature fluctuations during hybrid rice seed production. To identify such TGMS lines at IRRI, Norin PL12, ID24, IR32364 (TGMS) and six IRRI-bred TGMS lines derived from Norin PL12 were sown in 3-week intervals from June to December 1999. Heading date, pollen fertility and spikelet fertility were obtained from each line in every planting interval. Correlation analyses between spikelet fertility and maximum and daily mean temperatures at 1-30 days before heading were done to determine the critical stage where temperature influences the sterility/fertility expression of the TGMS lines. The CSP values of the TGMS lines were identified through regression analyses between spikelet fertility and maximum and daily mean temperatures at the critical stage. The data showed that the critical stage for most TGMS lines occurs at the developmental stages between the differentiation of secondary branch primordium and the filling stage of pollen, or approximately 24 to 5 days before heading. Norin PL12, ID24, IR32364 (TGMS), IR72093 and IR72096 were identified to have low CSP (Tmax<32°C; Tmean<27°C). The data indicates that evaluating for sterility in the wet season is more effective than in the dry season in identifying TGMS lines with low CSP. TGMS lines developed using this new strategy were found to be more stable in terms of sterility expression. Preliminary evaluation of 2-line hybrids from these low-CSP TGMS lines showed a high frequency of heterotic combinations (77%).