The Effect of Source—Sink Manipulations on Nitrogen Fixation in Peas

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Abstract
The effect on nitrogen fixation of excising leaves or pods in pea (Pisum sativum L. cv. Alaska) was determined over a 60‐day period. Flower buds or their subtending leaves were removed, and C2H2 reduction, H2 evolution and N accumulation were measured at weekly intervals. Highest percentage nitrogen content in all treatments coincided with time of maximal C2H2‐reduction rates. Nitrogen fixation, calculated from C2H2 reduction and H2‐ evolution data, was significantly lower in the partially defoliated and generally higher in the depodded plants than in the controls. Total N accumulation was greatest in the depodded plants and least in the defoliated ones. Percentage nitrogen content and N2‐fixation rates in the depodded plants were maximized approximately 10 days later than in the defoliated or control plants. The absolute rates of C2H2 reduction and H2 evolution were significantly altered by plant organ removal, but the relative rates were proportional. As a result the ratios of H2/C2H4 production and the related relative efficiency of N2 fixation in the treatments were not significantly different from the controls.