A comparative analysis of agronomic water-use efficiency and its proxy measures as derived from key morpho-physiological and supportive quantitative genetics attributes of perennial ryegrass under imposed drought.

Water-use efficiency (WUE) is an under-researched but very important drought tolerance trait in forage breeding. This research estimated quantitative genetic parameters of morpho-physiological traits linked to agronomic water-use efficiency (WUE_A) and its proxy measures based on δ¹³C (WUE_i) or gas exchange (evapotranspiration, WUE_AET, or stomatal conductance WUE_ASC) of genotypes from half-sib families of Lolium perenne L. (PRG) in a simulated summer drought cycle. Principal component analysis (PCA) of trait data distinguished a group of PRG genotypes where high WUE_A and dry matter yield was associated with deep rooting, leaf hydration at more negative leaf osmotic and water potential, and reduced soil moisture depletion. Plants with this trait association sustained net assimilation and postdefoliation regrowth in drought. However, WUEi, WUE_ASC, and WUE_AET were poorly correlated with most traits of interest at p < .05. Another PCA revealed a weak association between WUE_A and its proxy measures under conditions tested. Quantitative genetic parameters including high estimates of narrow-sense heritability ($h_n^2>0.7;p<.05$) of WUE_A and related traits emphasized the genetic potential of the key trait combination for selecting PRG for improved drought tolerance. Research findings highlight the relative importance of WUE_A and its proxy measures in the broad definition of PRG drought tolerance for breeding purposes.