Effects on transpiration

Number of leaves: More leaves (or spines, or other photosynthesizing organs) means a bigger surface area and more stomata for gaseous exchange. This will result in greater water loss.
Number of stomata: More stomata will provide more pores for transpiration.
Size of the leaf: A leaf with a bigger surface area will transpire faster than a leaf with a smaller surface area.
Presence of plant cuticle: A waxy cuticle is relatively impermeable to water and water vapour and reduces evaporation from the plant surface except via the stomata. A reflective cuticle will reduce solar heating and temperature rise of the leaf, helping to reduce the rate of evaporation. Tiny hair-like structures called trichomes on the surface of leaves also can inhibit water loss by creating a high humidity environment at the surface of leaves. These are some examples of the adaptations of plants for conservation of water that may be found on many xerophytes.
Light supply: The rate of transpiration is controlled by stomatal aperture, and these small pores open especially for photosynthesis. While there are exceptions to this (such as night or "CAM photosynthesis"), in general a light supply will encourage open stomata.
Temperature: Temperature affects the rate in two ways; An increased rate of evaporation due to a temperature rise will hasten the loss of water. And decreased relative humidity outside the leaf will increase the water potential gradient.
Relative humidity: Drier surroundings gives a steeper water potential gradient, and so increases the rate of transpiration.
Wind: In still air, water lost due to transpiration can accumulate in the form of vapor close to the leaf surface. This will reduce the rate of water loss, as the water potential gradient from inside to outside of the leaf is then slightly less. Wind blows away much of this water vapor near the leaf surface, making the potential gradient steeper and speeding up the diffusion of water molecules into the surrounding air. Even in wind, though, there may be some accumulation of water vapor in a thin boundary layer of slower moving air next to the leaf surface. The stronger the wind, the thinner this layer will tend to be, and the steeper the water potential gradient.
Water supply: Water stress caused by restricted water supply from the soil may result in stomatal closure and reduce the rates of transpiration.

	Created by Emily Rowland-Rawson about 6 years ago

These are notes for chapter 9, transport in plants, of the OCR A-level biology specification.