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Variability Among Plants in Dinitrogen Fixation (Acetylene Reduction) Rates by Field-Grown Soybean¹

High variability in acetylene reduction rates within cultivars and treatments has been a problem in evaluating treatment and cultivar effects. This study examined the sources of such variability, using field-grown soybean [Glycinee max (L.) Merr.] plants. Acetylene reduction rate per plant was measured in situ for thinned and unthinned ‘Chippewa 64’ soybean near Ithaca, NY for well-watered and drought-stressed ‘Biloxi’ soybean in Gainesville, FL and for ‘Guelph’ soybean subject to extended photoperiods in Gainesville, FL. A number of morphological traits, including leaf weight per plant and nodule weight per plant, were measured for each plant assayed. In addition, the nodule gas conductance (permeability x surface area) was computed for each plant. Correlations between these characteristics and acetylene reduction rate per plant were used to assess their relative importance. Acetylene reduction rate was consistently more closely correlated with nodule weight than with shoot characteristics. The highest correlation with acetylene reduction rate was, however, with nodule gas conductance. This correlation was highly significant across all treatments, cultivars, and maturity stage. Nodule gas conductance was concluded to play an important role in determining acetylene reduction rates in field-grown plants.

Key Words: Glycine max (L.) Merr. • Nodule mass • Morphological traits

² Postdoctoral research agronomist, Dep. of Agronomy and Range Sci., Univ. of California, Davis CA 95616; graduate research assistant, Agronomy Dep., Univ. of Florida; and plant physiologist, USDA-ARS, and adjunct professor, Agronomy Dep., Univ. of Florida, Gainesville, FL 32611, respectively.

Received for publication March 13, 1985.