Improving tolerance to heat stress is a major challenge in many crops, especially C3 crops, given the threat of recent global warming. While significant achievements have been made on understanding the molecular and physiological mechanisms to short-term exposure to high temperature, the physiological mechanisms of summer tolerance in fields, which are characterized by prolonged exposure to high temperature stress, are not still well understood. In this study, six experiments were conducted to clarify the physiological mechanisms of tolerance to high temperature stress by comparing between genotypes, populations and different species including C3 and C4 photosynthetic types. Overall, although the complexity of heat tolerance was revealed by the involvement of various mechanisms such as suppression of ROS, scavenging of toxic ROS, and leaf functions, this study definitively showed that the oxidative stress resulting from the excessive production of ROS is the main cause of damages under heat stress.