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: 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.
Observatios from several authors[3][6][7][8] have however suggested that WUE would rather be linked to different drought response strategies, where 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]