Answer: heat causing an excess of transpiration
Photosynthesis is a biochemical process by which green plants synthesize their food in the form of carbohydrates. Reactants like water, carbon dioxide in the presence of sunlight produces carbohydrates and oxygen. Transpiration is a process by which green plants loose excess water to their surrounding environment through tiny openings present in their leaves and stems. The excess of heat will fasten up the process of transpiration. This will affect the process of photosynthesis as if all the available amount of water get lost it will not be available for the process of photosynthesis.