Divergent selection for carbon isotope discrimination in crested wheatgrass

Abstract

Because plant growth on semiarid rangelands is frequently water-limited, breeding for enhanced water-use efficiency (WUE, kg dry matter gained per kg water transpired) should improve forage production on these areas. In crested wheatgrass [Agropyron desertorum (Fischer ex Link) Schultes], variation for carbon isotope discrimination (Δ) has been negatively associated with WUE, suggesting that selection for lowered Δ would increase WUE. To determine the potential of altering Δ through breeding, we selected nine clones from a crested wheatgrass breeding population based on their Δ values, equally subdivided them into three groups (low, medium, and high Δ), and made a series of diallel crosses within each group. The parental clones and single-cross progenies were established in the field as replicated spaced plants on 1-m centers in spring 1989. Forage dry matter yield and Δ were determined in 1990 and 1991; leaf gas exchange traits were determined for low and high Δ classes in 1990, and for low, medium, and high Δ classes in 1991. The previous ranking of Δ classes was confirmed in the present studies. Combined across years, values for Δ, leaf CO2 exchange rate (CER), and stomatal conductance (gs) were significantly (P < 0.05) lower in the low than high Δ class. When data were combined across clones and progenies, Δ was negatively correlated with leaf intrinsic water-use efficiency (WUEi, expressed as the CER/gs ratio) in 1990 (r = −0.87**, df = 14) and in 1991 (r = −0.83**, df = 23). Forage yield and Δ were not correlated, suggesting that the two traits may be under separate genetic control. Progenies from crosses among the low-Δ clones had significantly lower Δ values than progenies from either the medium-Δ or high-Δ clones during each year and when the data were combined across years. Divergent selection for low Δ also was reflected by improved WUEi in the subsequent generation. Moreover, narrow-sense heritability values for Δ were in excess of 0.75 and correlations between progeny means for Δ and means of the corresponding midparents were significant in each of the three analyses (r = 0.87** in 1990, 0.91** in 1991, and 0.92** in the combined analysis). These results complement earlier findings and confirm that genetic improvement for WUE can be effectively achieved in crested wheatgrass through indirect selection for Δ. Key words: Agropyron desertorum, water-use efficiency, parent-progeny correlations, drought, carbon isotope ratio, grass breeding, forage yield