Climatic Characters And Varietal Condition Of High-yielding Early,Middle And Late Season Rice In The Middle And Lower Reaches Of The Yangtze River

The Middle and Lower Reaches of the Yangtze River region is one of the most important rice production area in China. The rice productionin this area isvital to guarantee the food security of China. The main rice cropping systems in the Middle and Lower Reaches of the Yangtze River include middle and double(early and late) cropping seasons. On the basis of patterns of cropping systems, rice varieties are divided into early, middle and late seasonal ecotypes. Study in the 1970 s showed that the basic vegetative growth stage of early rice ecotypes is stable, and sensitive to radiation and temperature, while the basic vegetative growth stage of late rice ecotypes is unstable, and not sensitive to radiation and temperature. The stability of basic vegetative growth stage and the sensitivity to radiation and temperature of middle rice ecotypes were between that of early and late rice ecotypes. However, the “basic characteristics” and radiation and temperature coordination in early, late, middle rice ecotypes still need further explorations. There are significant differences in the seasonal radiation and temperature coordination in early, late, middle rice ecotypes. The cumulative temperature in early, late, middle rice cropping seasons experienced increase, increase to decrease, and decrease along with the growth duration, respectively. The radiation in early, late, middle rice cropping seasons along with the growth duration had not regularity, respectively. In the present study, early(Ezao18 and Liangyou287), middle(Huanghuazhan and YLiangyou1), late(Xiangwanxian13 and Tianyouhuazhan) rice ecotypes were planted in early, late, middle rice cropping seasons, respectively. Our study aimed to determine the suitable rice ecotypes through the comparison of yield differences of three rice ecotypes within single season, and determine the radiation and temperature coordination effects on grain yield through the comparison of yield differences of given rice ecotype across seasons. The results showed that:1. Growth period had significant variations across rice ecotypes and seasons. The whole growth duration was shortest in early rice ecotypes, and longest in early rice season. The whole growth duration were 100-130, 125-158, and 129-158 days in the early, middle and late rice ecotypes, were 124-158, 100-141, and 105-139 days in the early, middle and late rice seasons.2. There were significant yield variations in the early, middle and late rice seasons. The grain yield was highest in late rice season, and lowest in early rice season. The grain yield in the early, middle and late rice seasons were 8.45, 8.90 and 9.73 ha-1 in 2014, and 9.19, 9.36 and 9.91 ha-1 in 2015, respectively. The highest grain yield of late season was mainly due to its high grain filling rate, grain weight and harvest index. The lowest grain yield of early season was mainly due to its low grain weight and harvest index. Seasonal radiation and temperature coordination was best in late season and worst in early season. Along with the crop development, seasonal average temperature increased in the early rice cropping season, increased and then decreased in middle rice cropping season, decreased in late rice cropping season. There were significant variations in average temperature during different stages among three seasons. The average temperature rank of three seasons was Early<Middle<Late in vegetative stages, was Early=Late <Middle in reproductive stage, was Early>Middle>Late in grain filling stage, respectively. There was no significant change in the average daily radiation at different growth stages among the three seasons.3. Yield level showed significant difference among different rice ecotypes, were worst in early rice ecotypes. According to the rice grain yield level, the 6 rice ecotypes were divided into two groups: Low-yielding type(Ezao18, Liangyou287, Xiangwanxian13) and High-yielding type(Huanghuazhan, YLiangyou1, Tianyouhuazhan). High-yielding type displayed higher spikelets per unit area, among which Huanghuazhan, YLiangyou1 and Tianyouhuazhan had highest grain filling rate, spikelets per unit area, and grain weight, respectively.4. Yield stability showed significant difference among different rice ecotypes, were worst in early rice ecotypes.Coefficient variation of rice grain yield in 2014 and 2015 were 17.6% and 11.9%(Ezao18), 5.6% and 14.9%(Liangyou287), 2.1% and 8.7%(Huanghuazhan), 5.8% and 8.7%(YLiangyou1), 16.0% and 12.1%(Xiangwanxian13), 2.7% and 3.2%(Tianyouhuazhan), respectively. Yield stability was worst for Ezao18 and Liangyou287, and best for Tianyouhuazhan, respectively. The best yield stability of Tianyouhuazhan was attributed to the good coordination of yield components and harvest index.In summary, the limitation of further increase in rice grain yield was the low temperature during vegetative stage for early season, was the high temperature during grain filling stage for middle season, respectively. That is because that low temperature during vegetative stage in early season caused low biomass and small sink size, and high temperature during grain filling stage in middle season caused low grain filling and harvest index. Good radiation and temperature coordination in late season were beneficial to high grain filling, grain weight, and harvest index, thereby, was responsible for the high yield for late season. Yield level and stability showed significant difference among different rice ecotypes, were worst in early rice ecotypes. The necessary prerequisite of high rice grain yield was high biomass production, and the necessary prerequisite of stable rice grain yield was stable harvest index.In double rice cropping system, to increase the growth duration of early season and decrease the growth duration of late season is beneficial to the annual grain yield increasement.