Progress in breeding for improved water-use efficiency of rain-fed wheat is reviewed …

• Abstract
• Introduction
• Water-use efficiency as a breeding target
• Breeding for greater leaf-level water-use efficiency
• Greater early vigour to improve agronomic water-use efficiency
• Conclusion
• Acknowledgements
• References
• Figures
• Table

Biology Articles » Agriculture » Breeding for high water-use efficiency » Water-use efficiency as a breeding target

Water-use efficiency as a breeding target

- Breeding for high water-use efficiency

Breeding to address a specific objective implies first, thatthe objective has been well defined and, second, that heritabletraits have been identified that can come some way towards achievingthe breeding objective. ‘Water-use efficiency’ asa breeding target could be defined in many ways, depending onthe scale of measurement and the units of exchange being considered.All potential definitions will have some measure of water beingexchanged for some unit of production. For physiologists, thebasic unit of production could be moles of carbon gained inphotosynthesis (A) in exchange for water used in transpiration(T). Thus a physiological definition might equate, at its mostbasic level, to the instantaneous water-use efficiency of leafgas exchange (A/T). For farmers and agronomists, the unit ofproduction is much more likely to be the yield of harvestedproduct achieved from the water made available to the crop throughprecipitation and/or irrigation, i.e. a farmer's definitionis one of agronomic water-use efficiency.

While agronomic water-use efficiency will be taken to be theultimate breeding target, a major thrust of this article willbe to place the physiological definition of water-use efficiencyin the context of the farmer's definition. To do this it isuseful to consider crop yield as being constructed from a frameworkof relatively simple components (equation 1).

(1)

In this framework, grain yield is described as being a functionof the amount of water used by the crop (evapotranspiration,ET), the proportion of that water actually transpired by thecrop (T/ET), the transpiration efficiency of biomass production(W), i.e. how much biomass is produced per millimetre of watertranspired, and, lastly, how effectively the achieved biomassis partitioned into the harvested product, i.e. the ratio ofgrain yield to standing biomass termed the harvest index (HI).This framework is not based on the notion of ‘droughtresistance’, but rather on the broad processes by whichcrops actually achieve yield in water-limited environments (Passioura,1977; Condon and Richards, 1993; Richards et al., 2002). Noneof the components of this yield framework is truly independentof the others (Condon and Richards, 1993), but each can be considereda target for genetic improvement. Leaf-level water-use efficiency,A/T, is directly related to only one of these components, W,the transpiration efficiency of biomass production. However,as will be discussed in following sections, A/T also has thepotential to influence each of the other three components inthe yield framework.