Identification method of high temperature resistance of hybrid rice based on flowering rate

Using cultivars with high temperature resistance is an effectively approach to reduce yield loss induced by extreme high temperature. The objective of this study was to improve the identification method for high temperature resistance of hybrid rice varieties in both pot and field experiments. To that end, a pot experiment was conducted in an intelligent artificial climate chamber to determine the effects of extreme high temperature (38℃) on percentage grain filling at grain-filling stage in 2013 and 2014. Four cultivars were planted and exposed to extreme high temperature for 5 hours at 5.0 hours before and during flowering stage, and 1.0 hour, 1.5 hours and 2 hours after flowering stage. Also concurrently, hybrid rice cultivars were grown under normal background conditions (daily maximum temperature of panicle layer of approximately 31.2-32.8℃) as control (CK). A field experiment was conducted using 40 hybrid rice cultivars to study the relationship between high temperature resistance and flowering rate of hybrid rice at different times of the days based on 5 sowing dates (March 5, March 25, April 20, May 4 and May 24) in 2015-2016. In each year, 20 cultivars with same heading date were selected from 5 sowing dates to observe flowering rate at different times of the day. The results showed a significantly positive linear correlation between high temperature resistance index and flowering rate of hybrid rice before 11:30 AM under high temperature condition or before 12:00 noon under normal temperature condition. The increase in seed setting rate of high temperature resistant variety under high temperature condition was indirectly related with high temperature resistance index, while its' seed setting rate under high temperature condition was directly related with high temperature resistance index. Flowering of hybrid rice occurred earlier under high temperature condition, the cultivar flowering was also earlier under normal temperature condition. Flowering of hybrid rice was mainly concentrated around 9:30 AM to 12:30 PM. However, flowering rate differed with cultivar at different times of the day. Seed setting rate was not significantly affected by high temperature after 2 hours of flowering. When panicle layer temperature was 34℃ and 35℃, and their frequency reached 80% at 13:30 PM and at 15:30 PM, respectively, choosing cultivars that flowered 2 hours before high temperature (daily maximum temperature greater than 34℃ or at about 35℃) could effectively resist high temperature injury. Regression equation was established based on flowering rate before 11:30 AM under high temperature and before 12:00 noon under normal temperature condition and seed setting rate under high temperature condition. The equation provided a new identification method of high temperature resistance of hybrid rice cultivars at flowering stage at a percent accuracy (estimated value/measured value) of up to 92.29%-102.73%.