The pollen fertility of photo-sensitive genic male-sterile (PGMS) lines is regulated interactively by photoperiod and temperature. There is a temperature range within which the pollen fertility alteration of PGMS is regulated by photoperiod, being sterile at long days and fertile at short days. The critical values of day length and temperature vary as the PGMS gene(s) is (are) transferred into different genic backgrounds. According to the temperature reactions in response to fertility alterations, the current sterile lines fall into four photothermal types. The climatic adaptation of these is different. Lines of the high-low (a high critical fertile temperature with a low critical sterile temperature point) group have both stable sterile and fertile periods in the low-altitude subtropical rice-growing areas, which are the major rice-production areas of the world. We could exploit them in hybrid rice production there by a two-line method, because there is no need for both male-sterile and maintainer lines. At high altitudes of the subtropical and tropical rice-growing areas, lines of this group are unstable in their sterility. The low-low group is stable in sterility in long-day conditions at all temperatures, and at high temperatures despite day length. They could be used for hybrid seed production from the tropical through to temperate rice-growing conditions. However, their pollen fertility, and hence, seed multiplication, is easily lost in short days if high temperatures occur. The high-high group of sterile lines can be conveniently multiplied in short days, but their sterility is not stable in long days if temperature decreases in the critical fertility alteration period. The low-high group is stable at higher temperature irrespective of photoperiod, and this could be used in the tropics, but their sterilities are not stable in long days if lower temperatures are encountered. Their multiplication in short-day conditions may also be affected by high temperature. © 1994.
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