The rate of photosynthesis in relation to temperature forms a bell curve.
At low temperatures the enzymes responsible for photosynthesis have very little energy so the rate of photosynthesis is very slow. if it is cold enough for water to freeze it can be very harmful to a plant.
as the tempearture increases, the enzymes get more energy so the rate of photosynthesis inreases.
If it gets too hot the enzymes begin to lose their shape (denature). this means they are unable to function properly and the rate of photosynthesis decreases again. also at higher temperatures the stomata close to prevent water loss. this also stops gas exchange which slows photosynthesis even further. if it is hot enough for water to boil this can be very harmful.