Transpiration has more than one purpose; it
- creates transpiration pull for absorption and transport of plants
- supplies water for photosynthesis
- transports minerals from the soil to all parts of the plant
- cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling
- maintains the shape and structure of the plants by keeping 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. The humidity of rainforests is largely due to this vast cycling of water from root to leaf to atmosphere and back to the soil.
The evolution of the C4 photosynthetic system is probably one of the strategies for maximising the availability of CO2 while minimising water loss. C4 plants are twice as efficient as C3 plants in terms of fixing carbon (making sugar).
However, a C4 plant loses only half as much water as a C3 plant for the same amount of CO2 fixed.