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The Influence of Available Nitrate Levels on Nitrogen Fixation in Three Cultivars of Cowpea¹

Soil N is known to affect N₂ fixation in legumes. Increased cowpea yields might be achieved by increasing N assimilation through greater symbiotic fixation, but identification of high-fixing genotypes requires an effective screening technique. The purpose of this investigation was to determine the optimum level of supplemental N for maximum N₂ fixation in cowpeas grown under greenhouse and field conditions, and the relative contributions of the measured variables on nitrogenase activity.

Three cowpea cultivars were tested for N₂ fixation, N₂ (C₂H₂), under greenhouse conditions with 0, 5, 10, and 20 ppm N applied as Ca(NO₃)₂, and under field conditions with 0, 11.2, 22.4, and 44.8 kg/ha N applied as Ca(NO₃)₂. The soil was Vertic Albaqualf, fine, Montmorillonitic thermic. Nitrogenase activity decreased with increasing rates of N. Path analysis indicated that nitrate effects on N₂ fixation activity were primarily on nodule mass, with the inhibition of nodule growth rather than nodule initiation. Nodule number was not a reliable predictor of N₂ fixation potential of cowpea genotypes and was negatively correlated with nitrogenase activity. The results clearly indicated no advantage of applied nitrate in promoting maximum genotypic differences for N₂ fixation.

Key Words: Vigna unguiculata • Vigna sinensis • Southernpea • Nitrogenase • Acetylene reduction • Nodulation • Nitrate inhibition

² Associate professor, Dep. of Horticultural Sciences, Texas A&M Univ., College Station, TX 77843.

³ Former graduate students. Present address: Graduate Research Fellow, Dep. of Vegetable Crops, Univ. California, Davis, CA 95616; plant breeder, Sun Seeds, Inc., Farmington, MN 55024; and assistant professor, Agricultural Research and Education Center, Homestead, FL 33031, respectively.

Received for publication September 5, 1980.