Water-use efficiency

Water-use efficiency (WUE) refers to the ratio of plant biomass to water lost by transpiration, can be defined either at the leaf, at the whole plant or a population/stand/field level:

• leaf level : photosynthetic water-use efficiency (also called instantaneous water-use efficiency WUE_inst), which is defined as the ratio of the rate of net CO₂ carbon assimilation (photosynthesis) to the rate of transpiration or stomatal conductance,^[1] then called intrinsic water-use efficiency^[2] (iWUE or W_i)
• plant level : water-use efficiency of productivity (also called integrated water-use efficiency or transpiration efficiency,TE), which is typically defined as the ratio of dry biomass produced to the rate of transpiration.^[3]
• field level : based on measurements of CO₂ and water fluxes over a field of a crop or a forest, using the eddy covariance technique^[4]

Research to improve the water-use efficiency of crop plants has been ongoing from the early 20th century, however with difficulties to actually achieve crops with increased water-use efficiency.^[5]

Intrinsic water-use efficiency W_i usually increases during soil drought, due to stomatal closure and a reduction in transpiration, and is therefore often linked to drought tolerance. Observatios from several authors^[3][6][7][8] have however suggested that WUE would rather be linked to different drought response strategies, where

• low WUE plants could either correspond to a drought tolerance strategy, for example by anatomical adaptations reducing vulnerability to xylem cavitation, or to a drought avoidance/water spender strategy through a wide soil exploration by roots or a drought escape strategy due to early flowering
• whereas high WUE plants could correspond to a drought avoidance/water saving strategy, through drought-sensitive, early closing stomata.

Increases in water-use efficiency are commonly cited as a response mechanism of plants to moderate to severe soil water deficits and have been the focus of many programs that seek to increase crop tolerance to drought.^[9] However, there is some question as to the benefit of increased water-use efficiency of plants in agricultural systems, as the processes of increased yield production and decreased water loss due to transpiration (that is, the main driver of increases in water-use efficiency) are fundamentally opposed.^[10][11] If there existed a situation where water deficit induced lower transpirational rates without simultaneously decreasing photosynthetic rates and biomass production, then water-use efficiency would be both greatly improved and the desired trait in crop production.

Water-use efficiency is also a much studied trait in Plant ecology, where it has been used already in the early 20th century to study the ecological requirements of Herbaceous plants^[12] or forest trees,^[13] and is still used today, for example related to a drought-induced limitation of tree growth^[14]