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Variation for Nitrogen Fixation Among Progenies of a Virginia x Spanish Peanut Cross¹

One method of increasing symbiotic N₂ fixation is to improve the N₂-fixing potential of the host plant. Peanut (Arachis hypogaea L.) cultivars of the Virginia (ssp. hypogaea var. hypogaea) and Spanish (ssp. fastigiata Waldron var. vulgaris Harz.) types differ in their ability to fix N₂. This study's objective was to determine the variation and relationship of traits indicative of N₂ fixed for late generation progenies from the cross of a Virginia and Spanish cultivar. The variability for traits indicative of N₂ fixation activity was investigated for 30 F₂ families in the F₅ and F₆ generation from the cross of the Virginia cv., NC 6, and the Spanish breeding line, 922. Nodule number, nodule weight, nitrogenase activity measured by acetylene reduction, specific nitrogenase activity, and shoot weight were measured at three sampling dates at two locations for 2 yrs for the parental lines and F₂ families. Fruit weight was determined on the final sampling date for all entries. Cultivar NC 6 was superior to 922 for all measured traits except specific nitrogenase activity. Significant variation was observed among families for most traits. Broad sense heritability estimates for nodule number, nodule weight, nitrogenase activity, shoot weight and fruit weight were moderate to high, indicating that superior genotypes within this population could be readily identified. Nodule number and weight were positively correlated with each other and with nitrogenase activity, suggesting that only nitrogenase activity was required to identify superior families. Phenotypic and genotypic correlations of nitrogenase activity with shoot weight were significant with genotypic correlations ranging from 0.66 to 0.89 over sampling dates and environments. However, correlations of nitrogenase activity and yield were lower (0.36 for F₅, 0.57 for F₆ and 0.47 over generations) suggesting that nitrogenase activity has less effect on fruit weight than on shoot weight. Selection for families with greater N₂-fixing activity should be possible and should result in indirect selection for yield.

Key Words: Arachis hypogaea L. • Acetylene reduction • Broad sense heritability • Rhizobium

² Graduate assistant and Crop Science professor; professor of Microbiology, North Carolina State Univ., Raleigh, NC 27650 and assistant professor of Crop and Soil Sciences, Michigan State Univ., East Lansing, M1 48824, respectively.

Received for publication August 24, 1984.