Variation Among Alfalfa Genotypes for Rate of Acetylene Reduction1

Abstract

Alfalfa (Medicago sativa L) cultivars with increased symbiotic N fixation potential would be useful in many agricultural systems for supplying N for grain and pasture crop production. This paper describes a greenhouse procedure for evaluating alfalfa for rates of acetylene reduction and characterizes genetic variation of traits that may influence nitrogen fixation in alfalfa. Two thousand plants of six alfalfa cultivars were scored for nodulation when inoculated with a mixture of Rhizobium meliloti strains. Seventy‐seven plants, representing extremes in nodulation, were selected and vegetatively propagated (cloned). Clones differed significantly for nodulation, root type, and acetylene reduction in replicated tests. Crosses were made in all possible combinations among six clones differing in acetylene reduction potential. Progenies were established in sand benches, allowed to grow to flowering, and then cut off. When the regrowth was at the early bud stage, plants were assayed for acetylene reduction and evaluated for nodulation and root morphology. Crosses between clones with high acetylene reduction values produced progenies with acetylene reduction rates more than twice those of progenies from low ✕ low crosses. Crosses between clones with high acetylene reduction values and clones with low acetylene reduction values produced progenies with intermediate values. Morphological traits associated with high levels of acetylene reduction were: many fibrous roots, many nodules, and high top and root weights. These greenhouse results indicated that significant variation exists among alfalfa genotypes for factors that may be associated with N fixation.