Water-use efficiency (WUE) refers to the

ratio In mathematics, a ratio () shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...

to water lost by

transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. It is a passive process that requires no energy expense by the plant. Transpiration also cools plants, c ...

, can be defined either at the

leaf A leaf (: leaves) is a principal appendage of the plant stem, stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leav ...

, at the whole plant or a population/stand/field level: *leaf level :

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

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

or stomatal conductance, then called intrinsic water-use efficiency (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

produced to the rate of transpiration. * field level : based on measurements of CO₂ and water fluxes over a field of a

crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. In other words, a crop is a plant or plant product that is grown for a specific purpose such as food, Fiber, fibre, or fuel. When plants of the same spe ...

or a forest, using the

eddy covariance The eddy covariance (also known as eddy correlation and eddy flux) is a key atmospheric measurement technique to measure and calculate vertical turbulent fluxes within planetary boundary layer, atmospheric boundary layers. The method analyses hig ...

technique 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. 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 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 Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

water deficits and have been the focus of many programs that seek to increase

tolerance to

drought A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, ...

. However, there is some question as to the benefit of increased water-use efficiency of plants in

agricultural systems Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created food ...

, 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. 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 Plant ecology is a subdiscipline of ecology that studies the distribution and abundance (ecology), abundance of plants, the effects of environmental factors upon the abundance of plants, and the interactions among plants and between plants and ...

, where it has been used already in the early 20th century to study the ecological requirements of

Herbaceous plants Herbaceous plants are vascular plants that have no persistent wood, woody stems above ground. This broad category of plants includes many perennial plant, perennials, and nearly all Annual plant, annuals and Biennial plant, biennials. Definition ...

or forest trees, and is still used today, for example related to a drought-induced limitation of tree growth

References

Further reading