Why does the rate of photosyntesis decreases when the temperature is high?
The dark reactions of photosynthesis being enzymatic are temperature controlled. At higher temperatures, the rate of photosynthesis decreases largely as a result of enzyme denaturation.
Also when the temperature is very high, the guard cells (stomata) close. Carbon dioxide used during photosynthesis first must pass through stomata into internal spaces within the leaf. It then diffuses into mesophyll cells where it becomes available for photosynthesis. When the stomata close, gaseous exchange will no longer be occurring so therefore CO2 levels drop rapidly within the leaf, inhibiting the light-independent reactions. This then causes photosynthesis to reduce or stop.