During photosynthesis

Explanation

Transpiration has more than one purpose, it
1) Creates transpiration pull for absorption and transportation in plants
2) Supplies water for photosynthesis
3) Transports minerals from the soil to all parts of the plant
4) Cools leaf surface
5) Maintains the shape and structure of the plants by keeping the cells turgid. An actively photosynthesising plant has an insatiable need for water. Photosynthesis is limited by available water which can be swiftly depleted by transpiration. `C_4` photo-synthetic system is one of the strategies for maximising the availability of `CO_2` and minimizing water loss. `C_4` plants are twice efficient than `C_3` plants in fixing carbon and also water loss.