Thermotolerance evaluation of Taiwan Japonica type rice cultivars at the seedling stage

Background: The subtropical rice varieties grown in Taiwan are mainly Japonica-type rice varieties, which are grown in the southernmost- and lowest-latitude Japonica type rice cultivation area in the world. In Taiwan, seedlings that are planted either by transplanting or direct seeding in the second crop will face the season with the highest temperatures during the year. High-temperature stress severely influences early rice growth and causes yield losses. With global warming deteriorating, this problem is becoming increasingly severe. This study attempted to establish a high-efficiency and time-saving screening tool for rice varieties that exhibit thermotolerance during the early growth stages and further identify good donors with better tolerance for high temperature stress from Taiwan Japonica type rice germplasm. Results: During the initial germination stage, there were significantly different responses to heat stress among the different rice varieties. After the temperature induction response technique (TIR) treatment, the seedling survival rate and relative growth rate of the rice varieties under high temperature stress were significantly improved. In addition, the seedlings of the thermotolerant varieties demonstrated greater thermotolerance performance in the pot experiment as well as cell membrane stability (CMS) and cell activity (2,3,5-triphenyl-tetrazolium chloride; TTC) test results. However, the correlation between the thermotolerance of the seedlings and seeds was low. A phylogenetic dendrogram was plotted and revealed that thermotolerant genes did not concentrate in specific clusters. Furthermore, there was a non-significant correlation between the thermotolerance of the varieties and the years in which they were released. Conclusions: The temperature induction screening tool established by this study could determine the potential of each variety to adapt to high temperature stress. Additionally, thermotolerance during different growth stages (i.e., the germination, seedling, and grain maturation stages) exhibited low correlations. In this study, the varieties obtained through preliminary screening (i.e., TK14, HC56, TT30, TNG70, and TK8) exhibited outstanding thermotolerance. The screen tools and thermotolerance varieties could be valuable resources for the countries that grow Japonica type rice to apply when breeding thermotolerant varieties in the future.