Question

How both transpiration and light affect the rate of photosynthesis?

Answer 1

For plants to perform photosynthesis, they must keep their stomata open so that they can take in carbon dioxide. But leaves must also retain a certain amount of water internally, to maintain turgor pressure and to provide water for photosynthesis. The result is that, on a sunny day when the plant is photosynthesizing, the plant is losing lots and lots of water, as water vapor is released through the open stomata. The plant can close its stomata to conserve water - but then photosynthesis would stop for lack of carbon dioxide.
Therefore, for photosynthesis to takes place plant has to maintain a balance between transpiration and light absorption.

Rate of photosynthesis will affect negatively if too much transpiration takes place.