Understanding and measuring plant water use

Plant uptake of water is controlled by two interconnected balances at the land surface: the partitioning of energy arriving as solar radiation and the partitioning of water arriving as precipitation. Evapotranspiration (ET) is the critical link between the two, as it requires large amounts of energy to change the state of water and return it to the atmosphere as vapour. Plants are both the regulators and the beneficiaries of this complex process. As water flows from underground stores through a plant to the atmosphere, it provides structure to the plant’s tissues, to act as a reagent in biochemical reactions, transport dissolved constituents such as gases and critical nutrients and regulate its temperature. This flow of water along what is known as the soil-plant-atmosphere continuum (SPAC) is typically regarded as ‘the dominant process in plant water relations’ (Kramer and Boyer, 1995).