Transpiration and Photosynthesis – a Compromise

Transpiration and Photosynthesis — a Necessary Compromise

Far from being a wasteful side-effect, transpiration serves several genuinely useful purposes at once:

• Generates the transpirational pull that drives water absorption and long-distance transport.
• Supplies the water needed for photosynthesis.
• Carries dissolved minerals from the soil to every part of the plant.
• Cools leaf surfaces, sometimes by as much as 10–15°C, through evaporative cooling.
• Keeps cells turgid, maintaining the plant's overall shape and structural rigidity.

Because of this, a photosynthesising plant has an almost constant, high demand for water, and the rate of photosynthesis can quickly become limited if water is being lost through transpiration faster than it can be replaced. The high humidity typical of rainforests is largely a product of this same large-scale water cycling — root to leaf to atmosphere and back to soil. …