Final answer:
Plants wilt on a hot, sunny day due to dehydration caused by excessive water loss through transpiration. This leads to a decrease in photosynthesis as the stomata close to conserve water.
Step-by-step explanation:
On a hot, sunny day, plants tend to wilt due to dehydration caused by the excessive loss of water through transpiration. Transpiration is the process by which plants release water vapor through their stomata. When the temperature is high and the plants close their stomata to conserve water, this leads to a decrease in the rate of photosynthesis, as the plants are unable to take in carbon dioxide needed for the process.