DIVERGENT SELECTION FOR CARBON-ISOTOPE DISCRIMINATION IN CRESTED WHEATGRASS

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 (DELTA) has been negatively associated with WUE, suggesting that selection for lowered DELTA would increase WUE. To determine the potential of altering DELTA through breeding, we selected nine clones from a crested wheatgrass breeding population based on their DELTA values, equally subdivided them into three groups (low, medium, and high DELTA), 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 DELTA were determined in 1990 and 1991; leaf gas exchange traits were determined for low and high DELTA classes in 1990, and for low, medium, and high DELTA classes in 1991. The previous ranking of DELTA classes was confirmed in the present studies. Combined across years, values for DELTA, leaf CO2 exchange rate (CER), and stomatal conductance (g(s)) were significantly (P < 0.05) lower in the low than high DELTA class. When data were combined across clones and progenies, DELTA was negatively correlated with leaf intrinsic water-use efficiency (WUE(i), expressed as the CER/g(s) ratio) in 1990 (r = -0.87**, df = 14) and in 1991 (r = -0.83**, df = 23). Forage yield and DELTA were not correlated, suggesting that the two traits may be under separate genetic control. Progenies from crosses among the low-DELTA clones had significantly lower DELTA values than progenies from either the medium-DELTA or high-DELTA clones during each year and when the data were combined across years. Divergent selection for low DELTA also was reflected by improved WUE(i) in the subsequent generation. Moreover, narrow-sense heritability values for DELTA were in excess of 0. 75 and correlations between progeny means for DELTA 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 DELTA.