Abstract
BackgroundThe 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.ResultsDuring 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.ConclusionsThe 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.
Highlights
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
Evaluating the high‐temperature thresholds of rice lethality and establishing high temperature treatment conditions To determine the survival rates and lethal temperatures of the freshly germinated rice seedlings, various temperature (i.e., 44, 45, 46, and 48 °C) and time conditions were separately employed in this study to evaluate the high-temperature lethal thresholds of the rice seedlings
The results indicated that the survival rate of the seedlings gradually decreased with the increase of temperature and treatment time (Table 2)
Summary
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. High-temperature stress severely influences early rice growth and causes yield losses. 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. In the century, the average global temperature is predicted to increase again by 1.1– 6.4 °C (IPCC 2012). Rice exposed to a temperature of 33 °C for 4–6 h on flowering day results in more than 50% sterile rice grains, thereby severely influencing its production (Satake 1995). In addition to the severe reduction in rice production, the increase in the proportion of chalky and fissured kernels under high temperature has seriously affected rice quality in Japan (Kobata et al 2004; Nagata et al 2004). The breeding of highly thermotolerant rice varieties is crucial for adapting to future global warming
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