GENETIC-VARIATION AND INHERITANCE CHARACTERISTICS FOR CARBON-ISOTOPE DISCRIMINATION IN ALFALFA

The negative correlation between carbon isotope discrimination (Delta) and water-use efficiency in C-3 species, including alfalfa (Medicago sativa L.), suggests that Delta might be useful in the selection of al falfa cultivars that use water more efficiently. We initiated field ex periments with alfalfa in northern Utah to determine genetic variation for Delta within representative breeding populations, the effect of d rought on Delta, magnitudes of heritability for Delta, genetic regulat ion of Delta, and how Delta differs among plant parts. In an experimen t conducted under a rainout shelter facility equipped with a line-sour ce sprinkler system, genetic variability for Delta was not detected in 15 clones each from the NC-83-1 germplasm and 'Spredor 2' cultivar. I n another experiment with 25 clones from the NC-83-1 germplasm, there was significant (P<0.01) genetic variation for Delta with a range of 1 .6 per mil (parts per thousand), and broad-sense heritabilities exceed ed 0.80. In a field trial with 78 cultivars and elite breeding lines, significant genetic variation for Delta was observed, although the ran ge for Delta was only 0.8 parts per thousand. We also detected signifi cant genetic variation for Delta in a diallel experiment with 196 cros ses from 14 parent clones from NC-83-1. Furthermore, general combining ability was significant, but specific combining ability and reciproca l effects were not, indicating that standard breeding techniques could be used to alter Delta response in alfalfa. Plant parts differed sign ificantly for Delta with stems having the lowest value (18.7 parts per thousand) followed by the entire shoot (19.0 parts per thousand), upp er leaves (19.4 parts per thousand), and bottom leaves (20.2 parts per thousand). The lack of significant statistical interactions among pla nt parts suggested that any plant part could be sampled to determine D elta. The results from these experiments indicated that promise exists for using Delta to improve water-use efficiency in alfalfa; however, use of more diverse germplasm may be necessary to expand opportunities for selection in North American alfalfa germplasm.