How It Works: Why Environmental Factors Change Transpiration Rate

How It Works: Why Environmental Factors Change Transpiration Rate

Each environmental factor affects transpiration through a specific physical or biological mechanism. Understanding the "why" — not just the "what" — is essential for exam explanations.

Temperature: Higher temperature gives water molecules more kinetic energy. This increases the rate at which water molecules evaporate from the wet surfaces of mesophyll cells. It also increases the concentration of water vapour inside the leaf (the internal air spaces become more saturated). This steepens the water vapour concentration gradient between the leaf interior and the outside air, driving faster diffusion of water vapour out through stomata.

Humidity: The leaf interior is effectively at 100% relative humidity (saturated). If the outside air is also humid, the concentration gradient between inside and outside is shallow — diffusion is slow. Dry air creates a steep gradient, so water vapour diffuses out rapidly. This is why plants wilt faster on hot, dry days than on warm, humid days.

Wind: Even on a dry day, water vapour accumulating just outside the stomata forms a thin humid boundary layer. This layer partially reduces the concentration gradient. Wind physically removes this boundary layer, restoring a steep gradient and maintaining rapid diffusion. Still air allows the boundary layer to build up, slowing transpiration.

Light: Light does not directly affect water movement — instead, it acts through stomata. Blue light receptors in guard cells trigger potassium ion pumps: K⁺ enters guard cells, water follows by osmosis, guard cells become turgid and bow outward, and the pore opens. With stomata open, water vapour can escape. In darkness, the reverse occurs: stomata close, blocking the main exit route for water vapour.