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Genetic Variation for Acetylene Reduction Rate and Other Characters in Alfalfa

The objective of this study was to determine the general (GCA) and specific (SCA) combining abilities of alfalfa (Medicago sativa L.) for rate of acetylene reduction, plant dry weight, N content (%), total plant N and nodule score. The parental clones were selected from the commercial cultivars ‘Beaver,’ ‘Roamer,’ ‘Saranac,’ and ‘Rambler.’ Five strains of Rhizobium meliloti Dang., including four single strains and one commercial inoculant ‘Balsac’ were used. The most effective strain of Rhizobium was NRG61, while RM110 was the poorest. Diallel analysis of the six-parent half diallel cross revealed that mean squares of both GCA and SCA were highly significant for all characters except the SCA mean square for nodule score. The highly significant interactions of GCA and SCA with Rhizobium strains suggested differential expressions of gene effects for alfalfa genotypes with different strains of Rhizobium. The GCA variance components were higher than SCA for total plant dry weight, total plant N and nodule score. This suggested that the major portion of the total genetic variation was additive. Recurrent selection for these characters should be possible. However, the additive and non-additive gene effects were equally important for acetylene reduction rate with Rhizobium strains NRG43, NRG61, and NRG118. Reciprocal recurrent selection method might be necessary to improve this character with these single Rhizobium strains. Combining ability analysis for each of the parental clones indicated that Clone 4 had consistently high and positive GCA effects over all Rhizobium treatments. Clone 2 was on the other extreme with negative GCA effects in all cases. None of the other parental clones produced consistently positive GCA effects across all the Rhizobium strains

Key Words: Rhizobium meliloti • Nodulation • Nitrogen fixation • Diallel • General combining ability • Specific combining ability • Medicago sativa L.

Received for publication February 12, 1980.