Heritabilities and interrelationships of water-use efficiency and agronomic traits in irrigated alfalfa

Improvements in alfalfa (Medicago sativa L.) water-use efficiency (WUE), the amount of forage and root biomass produced per unit of water transpired, could benefit irrigated production environments. Correlations between important agronomic traits and key physiological traits associated with WUE must be determined if benefits from improved WUE are to be realized. This study characterized genetic correlations among carbon isotope discrimination (delta), canopy temperature, ash concentration, specific leaf mass (SLM), dry matter yield, forage maturity, and leaf-to-stem ratio (LSR) in irrigated alfalfa. Heritabilities of the traits on a progeny mean basis were also determined. Twenty-eight half-sib families possessing moderate fall dormancy were evaluated in seeded plots that were irrigated every 14 d during 2 yr near Las Cruces, NM. An increase in dry matter yield was associated with higher delta (r = 0.39; P < or = 0.05) and reduced LSR (r = 0.75; P < or = 0.01). Carbon isotope discrimination was negatively correlated with specific leaf mass (r = -0.43; P < or = 0.05) but was not correlated with canopy temperature or ash concentration. Ash concentration, delta, LSR, and yield were moderately heritable (h(2) = 0.44 to 0.61) indicating that these traits could be altered through breeding and selection. A positive relationship between delta and forage yield indicated that both traits should be evaluated when characterizing alfalfa for high WUE to minimize potential yield reductions that may result from selection based only on delta. The magnitude of the positive correlation between delta and yield indicated that opportunities may exist to simultaneously improve both traits in irrigated environments. Moderate to low correlations between delta and SLM, canopy temperature, and ash concentration indicated that none of these traits provided highly reliable estimates of delta in irrigated alfalfa.