ABSTRACT
The term ‘‘plant water use’’ is commonly used, but it is unfortunate as it suggests that plants ‘‘consume’’ water in some biochemical processes.[1] However, less than 2% of the water that is taken up by plants is actually transformed during biochemical reactions, the rest (98% or more) is simply ‘‘lost’’ during transpiration, the process of evaporation from inside plants. Understanding and quantifying this transpiration is of critical importance in many applied and scientific disciplines, in particular in hydrology, crop science, forestry, ecology, meteorology, and climatology. Transpiration arises as an inevitable consequence of the need for plants to expose the surfaces of their photosynthetic cells to the air, to take up CO2 during photosynthesis and therefore provide carbohydrate for growth. Aerial parts of terrestrial plants (and of emergent aquatic plants) are covered in impermeable materials; for leaves and photosynthetic stems this is the cuticle, a hydrophobic layer made up of lipids and waxes secreted to the outside of the epidermal cell layer that reduces the diffusional loss of water to the atmosphere to a very low rate in normal conditions. However, the cuticle is also impermeable to the diffusion of CO2, so plants have ‘‘pores’’ in the cuticle with variable apertures in order to control CO2 uptake and H2O loss. These pores, termed stomata (single: stoma), are formed by a pair of specialized epidermal cells, the guard cells, which have both unusual anatomy and physiology. As well as determining CO2 and H2O exchange, stomata also influence the atmosphere-plant exchange of other gases, such as the phytotoxic pollutant, O3. Stomatal apertures change over periods of minutes, and their size and shape vary between species and in different conditions.
