Water-use efficiency#Water use efficiency

{{about|water use in plant physiology|water use efficiency by humans|Water 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,{{cite journal |last1=Farquhar|first1=G.D. |last2=Rashke |first2=K. |year=1978 |title=On the resistance to transpiration of the sites of evaporation within the leaf |journal=Plant Physiology |volume=61 |issue=6 |pages=1000–1005 |doi=10.1104/pp.61.6.1000 |pmid=16660404 |pmc=1092028 }}

then called intrinsic water-use efficiency{{cite journal | vauthors=((Meinzer, F. C.)), ((Ingamells, J. L.)), ((Crisosto, C.)) | journal=HortScience | title=Carbon Isotope Discrimination correlates with bean yield of diverse coffee seedling populations | volume=26 | issue=11 | pages=1413–1414 | date= 1991| doi=10.21273/HORTSCI.26.11.1413 }}

(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.{{cite book | vauthors=((Maximov, N. A.)) | date= 1929 | title=The plant in relation to water | publisher=George Allen & Unwin LTD London}}

• field level : based on measurements of CO₂ and water fluxes over a field of a crop or a forest, using the eddy covariance technique{{cite journal |last1=Tallec|first1=T. |last2=Béziat |first2=P. |last3=Jarosz |first3=N. |last4=Rivalland |first4=V. |last5=Ceschia |first5=E. |year=2013 |title=Crops' water use efficiencies in temperate climate: Comparison of stand, ecosystem and agronomical approaches |journal=Agricultural and Forest Meteorology |volume=168 |pages=69–81 |doi=10.1016/j.agrformet.2012.07.008 |bibcode=2013AgFM..168...69T }}

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.{{cite journal |last1=Vadez|first1=V. |last2=Kholova |first2=J. |last3=Medina |first3=S. |last4=Kakkera |first4=A. |last5=Anderberg |first5=H. |year=2014 |title=Transpiration efficiency: new insights into an old story |journal=Journal of Experimental Botany |volume=65 |issue=21 |pages=6141–6153 |doi=10.1093/jxb/eru040 |pmid=24600020 }}

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{{cite journal | vauthors=((Ehleringer, J. R.)) | journal=Oecologia | title=Variation in Leaf Carbon-Isotope Discrimination in Encelia farinosa : Implications for Growth Competition and Drought Survival | volume=95 | pages=340–346 | date= 1993 | issue=3 | issn=0029-8549 | doi=10.1007/BF00320986| pmid=28314008 | bibcode=1993Oecol..95..340E }}

{{cite journal | vauthors=((Kenney, A. M.)), ((McKay, J. K.)), ((Richards, J. H.)), ((Juenger, T. E.)) | journal=Ecology and Evolution | title=Direct and indirect selection on flowering time, water-use efficiency (WUE, δ13C), and WUE plasticity to drought in Arabidopsis thaliana | volume=4 | issue=23 | pages=4505–4521 | date= 2014 | issn=2045-7758 | doi=10.1002/ece3.1270| pmid=25512847 | pmc=4264900 | bibcode=2014EcoEv...4.4505K }}

{{cite journal | vauthors=((Campitelli, B. E.)), ((Des Marais, D. L.)), ((Juenger, T. E.)) | journal=Ecology Letters | title=Ecological interactions and the fitness effect of water-use efficiency: Competition and drought alter the impact of natural MPK12 alleles in Arabidopsis | volume=19 | issue=4 | pages=424–434 | date= February 2016 | issn=1461-023X | doi=10.1111/ele.12575| pmid=26868103 | bibcode=2016EcolL..19..424C }}

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.{{cite journal | vauthors=((Condon, A. G.)), ((Richards, R. A.)), ((Rebetzke, G. J.)), ((Farquhar, G. D.)) | journal=Journal of Experimental Botany | title=Breeding for high water-use efficiency | volume=55 | pages=2447–2460 | date= 2004 | issue=407 | issn=0022-0957 | doi=10.1093/jxb/erh277| pmid=15475373 }} 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.Bacon, M. Water Use Efficiency in Plant Biology. Oxford: Blackwell Publishing Ltd., 2004. {{ISBN|1-4051-1434-7}}. Print.{{cite journal |last=Blum|first=A. |year=2009 |title=Effective use of water (EUW) and not water-use efficiency (WUE) is the target of crop yield improvement under drought stress |journal=Field Crops Research |volume=112 |issue=2–3 |pages=119–123 |doi=10.1016/j.fcr.2009.03.009 |bibcode=2009FCrRe.112..119B }}

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{{cite journal |last=Iljin|first=V. |year=1916 |title=Relation of transpiration to assimilation in steppe plants. |journal=Journal of Ecology |volume=4|issue=2 |pages=65–82 |doi=10.2307/2255326 |jstor=2255326 |bibcode=1916JEcol...4...65I }}

or forest trees,{{cite journal |last=Bates|first=C.G. |year=1923 |title=Physiological requirements of Rocky Mountain trees. |journal=Journal of Agricultural Research |volume=24|pages=97–164 }}[https://books.google.com/books?id=B8NFAQAAIAAJ&dq=bates+1923+physiological+requirements+of+mointain+trees+journal+of+agricultural+research&pg=PA97] and is still used today, for example related to a drought-induced limitation of tree growth{{cite journal |last1=Linares|first1=J. C. |last2=Camarero |first2=J.J. |year=2012 |title=From pattern to process: linking intrinsic water-use efficiency to drought-induced forest decline |journal=Global Change Biology |volume=18 |issue=3 |pages=1000–1015 |doi=10.1111/j.1365-2486.2011.02566.x |bibcode=2012GCBio..18.1000L }}