Responses of Fourteen Vietnamese Rice (Oryza sativa L.) Cultivars to High Temperatures during Grain Filling Period under Field Conditions

High temperatures significantly affect rice grain yield and quality. However, little information is known about the response of indica cultivars, especially Vietnamese cultivars, to high temperature. In this study, field experiments were conducted in 2015 and 2016 to evaluate the response of Vietnamese cultivars under high temperatures during the grain filling period. The high temperature was applied after the first cultivar started anthesis, by opening two sides of a plastic chamber that housed the cultivar when the temperature reached above 36 °C under field conditions. The difference in the maximum temperature between the control and the high temperature treatment was about 1.3 °C to 10.1 °C in 2015, and 0.73 °C to 10.2 °C in 2016. Decreases in crop growth rate (CGR) and yield were correlated with increased temperature conditions during the grain filling period. The grain yield of 14 Vietnamese cultivars fell to 81.5 and 79.4% of the control in 2015 and 2016, respectively. The variable with the greatest impact on grain yield was spikelet sterility induced by high temperature. Under high temperature conditions during the grain filling period, the percentage of grain chalkiness in the high temperature-treatment group increased compared to the control. Our study showed that Vietnamese rice yield and quality were significantly affected by high temperature.