Photosynthesis : It is the process of manufacture of food from C02 and water by the green parts of plant.
It is the transpiration which creates a transpiration pull for absorption of water and minerals The water is raw material for photosynthesis. If water availability is limited then photosynthesis also becomes limited. Actively photosynthetic plant needs proper supply of water. C4 plants were evolved only to use minimum amount of water and maximum use of C02 and they are double efficient to C3 plants.
C4 plant loses half the amount of water as compared to C3 plants for fraction of same amount of C02.
Thus it appears to be a compromise between transpiration and photosynthesis.
