| Two-line hybrid rice technology, which is originally created by China, has been widely used in commercial production and made a worldwide acknowledged achievement. However, the instability in male sterility of the existing photo- and thermo-sensitive genie male sterile rice (PTGMSR) has perplexed the units of production and application thus greatly limit the rapid development of two-line hybrid rice, so it is the key problem to be solved in two-line hybrid research and application. To understand better the genetic mechanism for instability in male sterility expression in PTGMSR and to search for the approach to genetically overcoming the drift of critical sterility- inducing temperature (CSIT) of PTGMS lines for the production of two-line hybrid rice, the characteristic of fertility expression and the effect of selection on fertility purification and sterility stabilization were studied under controlled low temperatures of 22.0-23.0 (daily mean temperature) in phytotron, using the original line for producing core seeds and its selected progenies of Pei'ai 64S, the most widely used PTGMS line in China, as the materials. The main results are:1) The initial day of sterility reduction varied with lines and low temperatures after 10 days of 22.0-23.0'C treatment. However, it was at least 16 days, generally 17-20 days, after exposure to low temperature. The maximal sterility reduction stage occurred mostly between 21 -24 days after exposure to low temperature.2) After 10 days of low temperature treatment, the dynamics of male sterility was from completely or highly sterile, to partially sterile, again to highly sterile, which is a gradually changing process. The low temperature effect was soon disappeared with ending of low temperature treatment. Different lines and different low temperatures showed the same dynamics of male sterility. However, the extent of male sterility reduction was different among different lines, indicating the different responses to low temperature among them. The extent of male sterility reduction also varied with different temperatures. When temperature was lower,not only the sterility reduction was greater, but also the sterility-affecting stage including the initial day of sterility reduction and the maximal sterility reduction stage occurred earlier. Therefore, it is inferred that low temperature (intensity X duration) within the sterility thermo-sensitive stage may have a cumulative effect on sterility of PTGMSR.3) All of the 2-10 days of 23.0C treatments showed some influence on male sterility of the original line of Pei'ai 64S, but 2 days of low temperature had not resulted in a distinct effect on decrease of pollen sterility in population, while 4 days of low temperature significantly decreased pollen sterility, and the longer the low temperature treatment lasted, the lower was the pollen sterility.4) Lower temperature affected more greatly on either pollen sterility or selfed seed set. The pollen sterility decreased or the selfed seed set increased with the treated temperature lowered down.5) According to the distinct difference in male sterility expression under lower temperatures (22.5C, 22.0C) among different lines, which were multiplied from the completely male sterile single plants selected in the same population (the core seeds of Pei'ai 64S produced in 1996) by 10 days of 23.0 treatments in 1997, it was indicated that the plant for producing core seeds of Pei'ai 64S was still heterozygous in some micro-genes controlling CSIT. It reflects, on the one hand, the fact that the genetic basis of CSIT controlling fertility expression has not been homogenized even in high-generation PTGMS lines, while such genetic heterogeneity is the root resulting in the instability of sterility in PTGMSR. In addition, it also indicates that it is necessary to further implement selection and purification on male sterility of Pei'ai 64S.6) The analysis on CV of pollen sterility in the large-population lines (more than 100 plants) treated with low tempe...
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