| Abstract | To study the genetic variability of N₂(C₂H₂) fixation rates in peas, 85 genotypes were screened in the field. Genotype means differed (P < 0.01), and ranged from 1.5 to 12.5 μmoles C₂H₂∙h⁻¹∙plant⁻¹ and from 10 to 49 μmoles C₂H₂∙h⁻¹∙(g nodulated root)⁻¹. Under environmentally controlled conditions the rankings of six genotypes, selected to represent the full genotypic range of fixation, were significantly correlated to those in the field. A diallel mating design among the six genotypes showed that heterosis occurred in fixation per plant, probably partly due to nonallelic interaction. In fixation per gram nodulated root, however, nonadditive effects were small. Heterosis occurred in total shoot N and in shoot weight, probably due in the latter case to both nonallelic interaction and overdominance. Total dominance, associated with decreased expression, was evident for percent shoot N. Study of the genes controlling fixation may best be done by examining N₂(C₂H₂) reduction on a per-gram nodulated root basis because results are less affected by the genetics of correlated whole plant characters. Fixation expressed in this way might be improved by simple selection. |
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