# 15.3.1 Water use efﬁciency of crops

Water use efﬁciency is deﬁned here as yield of plant product (tonnes of wheat grain, *Y*) per unit of crop water use (megalitres of water lost by evapotranspiration, *ET*), and is important in all areas of plant production.

Water use efﬁciency (*Y*/*ET*) is the outcome of an entire suite of plant and environmental processes operating over the life of a crop to determine both *Y* and *ET*. Consequently, biomass production per unit *ET*, has been used extensively as an interim measure of water use efﬁciency.

*ET* comprises non-productive evaporation (*E*) of water from the soil surface and productive transpiration (*T*) of soil-stored water by the plant. Evaporation of free water from leaf surfaces adds to non-productive evaporation (interception evaporation). The basic equation describing *ET* distinguishes productive and non-productive evaporation:

In contrast to ratios of water use efﬁciency based on *ET*, the ratios *W*/*T* and *Y*/*T* (interim and ultimate transpirational water use efﬁciency, respectively) do not involve *E* (Equation 15.2) and so serve to focus attention on physiological aspects of water use efﬁciency. Note, however, that non-productive evaporation from leaf and soil surfaces in a wheat crop ac-counted for 49% of *ET* at Wagga Wagga (Leuning *et al*. 1994). Even higher values (60%) have been reported for a sparse crop of barley growing in Syria.

According to these deﬁnitions, maximum water use efﬁciency is achieved by maximising both *T* as a proportion of *ET* (because water lost as evaporation from soil is non-productive; Leuning *et al*. 1994) and transpirational water use (TWUE) (because maximal TWUE requires maximal yield per unit of water transpired; Tanner and Sinclair 1983; Richards 1991).

Interrelations between growth, yield and transpiration were studied extensively by Bierhuizen and Slatyer (1965). They concluded from assimilation and transpiration characteristics of cotton leaves that plant growth is directly proportional to transpirational water use, but inversely dependent on atmospheric vapour pressure deﬁcit. Their work was developed by Tanner (1981) and Tanner and Sinclair (1983), who described daily biomass production (*W*) in terms of daily transpiration (*T*) and mean daytime vapour pressure deﬁcit (*D*) where:

*k* accounts for several factors, including the biochemical pathway for photosynthesis, which varies between species (e.g. C_{3} v. C_{4}), and yield of hexose per unit of carbon assimilated. Production value, *PV*, accounts for differences in biomass production per unit of hexose, and also depends on the species. A consequence of the inverse relationship between growth rate and *D*, incorporated in Equation 15.3, is seen in Figure 15.10 where data of Warren and Lill (cited by Fischer 1979) demonstrate that transpiration efﬁciency (*W*/*T*) decreases sharply with increasing pan evaporation (where vapour pressure deﬁcit would feature prominently in evaporative conditions).

Wilson and Jamieson (1985) successfully applied Equation 15.3 to wheat grown in three seasons in New Zealand. The resultant linear relationship between biomass (*W*) and *D*-corrected transpiration (*T*/*D*) is shown in Figure 15.11. HI was ignored as a source of variation in *Y*. HI has attracted considerable attention as a means of crop improvement (Section 6.3). In determinate crops, HI increases in ac-cordance with water use (*T*) after flowering. Post-flowering water stress will increase use of pre-anthesis assimilate in seed yield (Hall *et al*. 1989; Sadras and Connor 1991; Section 15.3.3).

A simple rearrangement of Equation 15.3 shows that *PV*-adjusted growth, *W*/*PV*, is directly proportional to *D*-adjusted transpiration, *T*/*D*. That is:

Sadras and Connor (1991) related *PV*-adjusted HI of wheat and sunflower to the *D*-adjusted ratio, q, post-flowering *T*:(post-flowering *T*)/(pre- + post-flowering *T*). They demonstrated that variability in HI of sunflower is satisfactorily explained on the basis of the large decrease in *PV* after flowering (due to the large cost of synthesising high-energy, oil-rich seed), and a simple, linear decline in the use of pre-anthesis assimilate in yield, dependent on q. Sadras and Connor (1991) related *PV*-adjusted HI (HI_{PV}) to q by the relationship

on the basis that water-limited *PV*-adjusted yield comprises all post-flowering biomass production, supplemented by a quantity, D, of remobilised pre-flowering biomass, where D/*Y* = *a* - *b*q. Parameter *a* quantiﬁes the potential contribution of pre-anthesis assimilates to grain biomass, and 1/(1 – *a* + *b*) the potential HI. Both are genotype dependent, with *a* varying between 0.81 and 0.92, and *b* varying between 1.19 and 1.86. The resultant term 1/(1 – *a* + *b*) varied between 0.52 and 0.72 for three different cultivars of sunflower. The ultimate balance between *a* and *b*q therefore indicates the contribution of pre-anthesis assimilate to yield, *Y*.

Application of Equation 15.5 (Figure 15.12) demonstrates the ability of the Sadras–Connor approach to deﬁne the role of pre-anthesis assimilate in HI, yield and water use efﬁciency. An apparent drawback to its wider application is the need to estimate coefﬁcients *a* and *b* by statistical curve-ﬁtting procedures. Deﬁnition of pre- and post-flowering periods is an added problem in indeterminate species.

Relationships between *PV*-corrected biomass and *D*-cor-rected transpiration therefore provide essential information for estimation of yield. In terms of the framework adopted here (Monteith 1977), biomass production depends on total *D*-adjusted *T* (Equations 15.3, 15.4) and Figure 15.12 demonstrates that yield, at least in determinate crops, depends on the proportion of *D*-adjusted *T* realised in the pre- and post-flowering periods, but modiﬁed by contributions of pre-flowering assimilate.

Physiological and climatological features of water use efﬁciency outlined by Bierhuizen and Slatyer (1965) remain a cornerstone for present understanding of the TWUE component of water use efﬁciency. Many developments need sophisticated techniques that allow precise and direct measures of processes and rates in both ﬁeld and laboratory. Estimates of canopy conductance via aerodynamic methods and direct measures of transpiration via sap flux measurements (heat-pulse techniques) are thus ﬁnding widespread application in ﬁeld analysis of water use efﬁciency.