Explain the effect of decreasing water potential on the rate of photosynthesis. Explain the effect of decreasing water potential on the rate of photosynthesis. Solution Water potential is the term used for the potential of water to move upward from roots to the leaves. Since photosynthesis is occured in the leaves and in the presence of water, so water potential have important role in the photosynthesis. Decrease in water potential will lead to the decrease in the rate of photosynthesis..