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 efficiecy 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]

References

^ Farquhar, G.D.; Rashke, K. (1978). "On the resistance to transpiration of the sites of evaporation within the leaf". Plant Physiology. 61: 1000–1005. doi:10.1104/pp.61.6.1000. PMC 1092028.
^ Meinzer, F. C., Ingamells, J. L., Crisosto, C. (1991). "Carbon Isotope Discrimination correlates with bean yield of diverse coffee seedling populations". HortScience. 26 (11): 1413–1414.
^ ^a ^b Maximov, N. A. (1929). The plant in relation to water. George Allen & Unwin LTD London.
^ Tallec, T.; Béziat, P.; Jarosz, N.; Rivalland, V.; Ceschia, E. (2013). "Crops' water use efﬁciencies in temperate climate: Comparison of stand, ecosystem and agronomical approaches". Agricultural and Forest Meteorology. 168: 69–81. doi:10.1016/j.agrformet.2012.07.008.
^ Vadez, V.; Kholova, J.; Medina, S.; Kakkera, A.; Anderberg, H. (2014). "Transpiration efficiency: new insights into an old story". Journal of Experimental Botany. 65 (21): 6141–6153. doi:10.1093/jxb/eru040.
^ Ehleringer, J. R. (1993). "Variation in Leaf Carbon-Isotope Discrimination in Encelia farinosa : Implications for Growth Competition and Drought Survival". Oecologia. 95: 340–346. doi:10.1007/BF00320986. ISSN 0029-8549.
^ Kenney, A. M., McKay, J. K., Richards, J. H., Juenger, T. E. (2014). "Direct and indirect selection on flowering time, water-use efficiency (WUE, δ13C), and WUE plasticity to drought in Arabidopsis thaliana". Ecology and Evolution. 4 (23): 4505–4521. doi:10.1002/ece3.1270. ISSN 2045-7758. PMC 4264900.
^ Campitelli, B. E., Des Marais, D. L., Juenger, T. E. (February 2016). "Ecological interactions and the fitness effect of water-use efficiency: Competition and drought alter the impact of natural MPK12 alleles in Arabidopsis". Ecology Letters. 19 (4): 424–434. doi:10.1111/ele.12575. ISSN 1461-023X.
^ Condon, A. G., Richards, R. A., Rebetzke, G. J., Farquhar, G. D. (2004). "Breeding for high water-use efficiency". Journal of Experimental Botany. 55: 2447–2460. doi:10.1093/jxb/erh277. ISSN 0022-0957.
^ Bacon, M. Water Use Efficiency in Plant Biology. Oxford: Blackwell Publishing Ltd., 2004. ISBN 1-4051-1434-7. Print.
^ Blum, A. (2009). "Effective use of water (EUW) and not water-use efﬁciency (WUE) is the target of crop yield improvement under drought stress". Field Crops Research. 112: 119–123. doi:10.1016/j.fcr.2009.03.009.
^ Iljin, V. (1916). "Relation of transpiration to assimilation in steppe plants". Journal of Ecology. 4: 65–82. doi:10.2307/2255326.
^ Bates, C.G. (1923). "Physiological requirements of Rocky Mountain trees". Journal of Agricultural Research. 24: 97–164.[1]
^ Linares, J. C.; Camarero, J.J. (2012). "From pattern to process: linking intrinsic water-use efﬁciency to drought-induced forest decline". Global Change Biology. 18: 1000–1015. doi:10.1111/j.1365-2486.2011.02566.x.

References

Further reading