Exploring variation in water use efficiency in barley (Hordeum vulgare L.) among wild and circadian mutant varieties, and its impact on responses to drought

Climate change will drive substantial changes in water resources and uses across the world, while population growth, changing dietary preferences and biofuel use will require greater agricultural output. Yet farmland is not likely to increase, therefore yields must rise. Episodes of drought arising from an increasingly-unstable climate will also become more frequent, severe and persistent. As a result, water use in agricultural production must be managed both where water availability appears satisfactory and where water stress exists. Finding novel phenotypes which blend high yield and parsimonious water use could help water-efficient production, and two approaches were considered for the major cereal crop, barley. Circadian clock mutants introgressed into the elite variety, Bowman, were interrogated for their stomatal anatomical and physiological performance in relation to water use. In the short-term, Bowman A & gs peaked later, and WUEi earlier in the day than mutants under steady-state light, although dynamic responses to fluctuating light were not affected. Steady-state performance did not arise from anatomical causes, and were more likely to have a functional cause, where Bowman was sink-limited while clock mutants were source-limited. Drought of increasing severity was introduced to the clock mutants, and a mechanistic explanation of patterns of water use was sought in the production of the chloroplast-nuclear signalling molecule, phospho-adenosine phosphate (PAP). PAP concentration tends to be elevated in clock mutants compared to Bowman, and is elicited by drought stress. Clock mutants have better WUEi under drought than Bowman, and are more responsive to fluctuations in light, leading to improved relative yield. The water-use efficiency of a selection of wild barleys and landraces were also characterised relative to elite varieties. Stomatal anatomy was linked to gs, A and WUEi. Wild barleys and landraces sometimes produced better yield components (such as tillering) than elite varieties despite lacking targeted breeding. Meanwhile, phenotypic responses of the wild barleys were compared with the drought-tolerant variety, Bowman, under water stress conditions at germination and at tillering. Drought is an extreme form of WUEi risk, and water stress initiated a range behaviours in wild barleys that ranged between conservative and non-conservative but were seldom clear-cut in their overall phenotype. Drought affects stomatal density and size when initiated early in development. Physiological responses were more-important at tillering. Tiller number, leaf area and growth stage effectively differentiated between varieties, and all are valuable yield components that could be targeted by breeders.