Elevated CO2 levels can cause heat stress in two types of Japanese rice plants by reducing their ability to release water through transpiration. This study found that when these rice plants were exposed to higher CO2 levels, their photosynthesis and overall plant mass increased, but their grain yield and quality suffered. The plants showed signs of heat stress, with lower spikelet fertility and poor grain appearance. Even under normal CO2 levels, the plants struggled due to high air temperatures. The reduction in transpiration during flowering led to increased canopy temperatures, potentially causing heat stress in the rice plants.