Development of a simple test for cool-weather tolerance at the flowering stage of rice plants [Oryza sativa] and evaluation of the genetic resources for the tolerance

The genetic difference in tolerance to cool weather that decreases the yield due to sterility consists of tolerance at the booting stage and at the flowering stage of rice plants. Cool-weather tolerance at the booting stage has been improved by breeding new varieties. However, tests for cool-weather tolerance at the flowering stage need much labor and equipment, such as an air-conditioned room, and only a few studies have been made that evaluated many lines. To develop programs to breed for cool-weather tolerance at the flowering stage. I devised a simple labor-saving test by using small pots. In a test with varieties having a long growing period (late varieties), such as varieties in the Tohoku district or further south in Japan and in Yunnan Province in China the number of days to heading was reduced by short-day treatment. By using this method, cool-weather tolerance at the flowering stage of varieties from Hokkaido, Tohoku district or further south in Japan and from Yunnan Province. China, was evaluated, and its relation with cool-weather tolerance at the booting stage was examined. I. Development of a simple test for cool-weather tolerance at the flowering stage of rice plants 1. A simple method to test for tolerance by using an air-conditioned room A simple method to test for tolerance to cool weather at the flowering stage in rice was developed. In square pots. 15 cm x 5 cm and 10 cm high, eight plants having only the main culms were planted and were grown in an air-conditioned room at 17.5 deg C for 15 days under 50% shading from 1996 to 1998. The tolerance to cool weather damage was estimated as the fertility percentage of spikelets. whose panicles had headed on the day the low temperature treatment started. At least 10 panicles in total between 3-4 pots for each variety were needed to classify the varieties into seven ranks (extremely high, high, a little high, intermediate, a little low, low, and extremely low tolerance), with a statistical error of +-0.8. After an 8-day treatment at 15t in this study, the lowest fertility percentage was observed in plants exposed to a low temperature from 0 to 3 days after heading; the number of days varied between the cultivars. In the 15-day at 17.5 deg C method, however, plants exposed to a low temperature from the day of heading showed the lowest fertility percentage in almost all varieties. Therefore, in this 15-day at 17.5 deg C method the fertility percentage was calculated using fewer panicles compared with the 8-day treatment at 15t, Data obtained by this 15-day at 17.5 deg C method showed a significant positive correlation (r=0.808**, n=14) between the different years and a significant positive correlation with data obtained by the 8-day treatment at 15 t in this study or a 6-day treatment at 12 deg C, in a previous study (r=0.750** and 0.802**, respectively, n=14). Plants examined by this 15-day at 17.5 deg C method were either very small plants or normal-sized plants cultivated in a 1/5000 a pot and were similar in size to plants grown in the paddy field: both plants showed a significant positive correlation (r=0.922**, n=8). 2. A labor-saving test by using short-day treatment to evaluate various genetic resources for tolerance The effect of short-day treatment on cool-weather tolerance at the flowering stage was examined for 14 cultivars of rice from the Tohoku district or further south in Japan, and 3 cultivars from Yunnan Province in China. The plants were exposed to 8-hour short days for 14 days from the 5.5 leaf stage (short-day plot SDP) or were grown under natural day length throughout the experimental period (natural-day plot: NDP) in 1999 and 2000. From the day of heading, the cultivars were exposed to 17.5 deg C for 15 days. The number of days to heading by the SDP cultivars was an average of 28 days less than for NDP cultlvars. NDP cultivars with a longer period to heading had a period that was reduced more markedly by the short-day treatment (r=0.926**, n=17). The variation in heading dates between SDP cultivars was smaller than for NDP cultivars. Differences in cool-weather tolerance at the flowering stage between SDP cultivars and NPP cultivars were larger for cultivars (n=16) with longer periods to heading by NDP cultivars (r=0.666**), with greater reduction in the number of days to heading by short -day treatment (0.536*), and are related to differences in duration of sunshine from 3 days before heading to 5 days after heading (0.741**), in daylength of 23 days before heading (0.570*) between SDP cultivars and NDP cultivars. However, the cool-weather tolerance between SDP and NDP showed a high. significant positive correlation (r=0.830**, n=16). Therefore, the short-day treatment in the test of cool-weather tolerance was labor-saving for growing materials for the test of cool-weather tolerance. II. Estimation of genetic resources for cool-weather tolerance at the flowering stage of rice 1. Estimation of, cool-weather tolerance at the flowering stage in Hokkaido varieties The cool-weather tolerance at the flowering stage of 52 Hokkaido rice varieties was examined in 1998 and 1999 by using the simple test developed in this study. Among the tested varieties, Hayayuki and Hayakogane had high to extremely high tolerance. Akage, Fukuyuki, Uryuu, Hoshinoyurne, and Hatsushizuku had a high tolerance, and Fukoku, Wasenishiki, arid Shimahikari had low to extremely low tolerance. Although non-glutinous varieties of the present recommended varieties for Hokkaido farmers were a little high to high, glutinous varieties were a little low to intermediate. Varieties with low amylose content had low tolerance, and a "Sake" brewery variety had extremely low tolerance. The tolerances at the booting stage reported by previous studies and at the flowering stage in this study showed a significant positive correlation (r=0:541**), but the coefficient of correlation was not high. Although varieties bred more recently had a higher tolerance to cool weather at the booting stage (r= -0.366**), no relation was found between year of breeding and tolerance to cool weather at the flowering stage (r= -0.055ns). Thus to breed varieties highly tolerant at the flowering stage, direct selection for tolerance to cool weather is needed in breeding programs. In tests with 37 breeding lines that had extremely high tolerance at the booting stage, Eike 88223 and Hokuikumochi 87 had an Extremely high tolerance at the flowering stage. Thus these lines are suitable as parents for producing varieties tolerant to cool weather. 2. Estimation of cool-weather tolerance at the flowering stage of varieties in Tohoku district or further south in Japan, Cool-weather tolerance at the flowering stage of 54 rice varieties in Tohoku district or further south in Japan was examined in 1999 by using the. simple test developed in this study. The data on Hokkaido varieties reported in section II-1 were compared with the results on varieties in. Tohoku district or further south in Japan. Although the degree of cool tolerance at the flowering stage varied greatly within districts where varieties were bred, the higher the latitude of the breeding district, the higher was the tolerance (r=0.497**, n-106). Varieties with an early heading date had a higher tolerance (r= -0.576**, n=106). Among the tested varieties, Chuubo 42 had an extremely high tolerance, 6 varieties had high tolerance, and 16 varieties had extremely low tolerance. Koshihikari, Hitomebore and Akitakomachi, the currently leading varieties in Japan, had a little low tolerance at the flowering stage. Although the tolerance at the booting stage reported by previous studies and that at the flowering stage in this study showed a significant positive correlation (r=0.462**, n=51), the coefficient of correlation was not high. Among 24 varieties bred after 1945 in Tohoku district, varieties bred more recently had a higher tolerance to cool weather at the booting stage (r= -0.542**), and no relation was found between year of breeding and tolerance to cool weather at the flowering;stage (r=0.100ns). These results suggest that to breed a new rice variety with high tolerance to cool weather at the flowering stage, direct selection for tolerance is needed in breeding programs. 3. Estimation of cool-weather tolerance at the flowering stages in varieties of Yunnan Province, China This study was done in Yunnan Province of China from 1993 to 1994. as part of a joint research between the Japan International Research Center for Agricultural Sciences and the Yunnan Academy of Agricultural Sciences, to evaluate cool weather-tolerance of seven local varieties newly supplied to the joint research, and to compare them with four cool-weather tolerant local varieties already evaluated in the joint research. Most of these newly supplied local varieties were from an area at an altitude of about 2700 meters that was at the highest altitude among rice cultivation areas of Yunnan Province, and from an area with high rainfall during the flowering stage that induces sterility in rice. The newly examined local varieties had extremely high tolerance to cool weather at the booting stage, similar to other local varieties tolerant to cool weather already evaluated. The tolerance to cool weather at the flowering stage of four cool-weather tolerant local varieties in Yunnan Province was tested from 1998 to 2000 in Hokkaido, Japan. Although those varieties had extremely high tolerance at the booting stage, the estimated rankings were a little high or a little high to high, and were not higher than those of Japanese varieties. In addition to the local varieties tested, many local varieties tolerant to cool weather will be available in Yunnan Province, and these genetic resources need to be evaluated.