Water relations

Abstract

In order to absorb carbon dioxide (CO₂) for photosynthesis, plants expose wet surfaces to a dry atmosphere and in consequence suffer evaporative water loss. The resultant cooling, nevertheless, often accounts for a considerable proportion of heat dissipation by leaves and is probably essential for maintaining equable temperatures for photosynthesis. Too much water loss would result in dehydration. Plants have therefore evolved leaves with an epidermis composed of a relatively impermeable cuticle and turgor-operated valves, called stomata. The epidermis not only reduces rates of CO₂ and water-vapour exchange, but it also provides a means of controlling assimilation and transpiration through the size of the stomatal pores. Thus stomata play a pivotal role in controlling the balance between water loss and carbon gain, i.e. biomass production. Measurements of the size of the stomatal opening (stomatal aperture) or of the resistance to CO₂ and water-vapour (H₂O) transfer between the atmosphere and the internal tissue of the leaf imposed by the stomata (stomatal resistance) are important in many studies of biomass production. This is particularly the case in cropping situations where it is important to maximize water-use efficiency which we define as the mass of CO₂ assimilated (or dry weight gained) per unit mass of water transpired.