Effect of Light Intensity on Efficiency of Carbon Dioxide and Nitrogen Reduction in Pisum sativum L.

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Abstract
Photosynthetic efficiency, primary productivity and N2 reduction were determined in peas (P. sativum L. variety Alaska) grown at light intensities ranging from severely limiting to saturating. Plants grown under higher light intensities showed greater carboxylation and light capture potential and higher rates of net C exchange. Uptake of N2, computed from measured C2H2 reduction and H2 evolution rates, increased with growth light intensity, while the previously proposed relative efficiency of N2 fixation, based on these same parameters, declined. The plot of N/C ratios (total N content/plant dry weight) increased hyperbolically with light intensity, and the plot of N2/CO2 uptake ratios (N2 uptake rate/net CO2 uptake rate) increased linearly. Both plots extrapolated to the light compensation point. Apparently the relative efficiency of N2 fixation is not necessarily correlated with maximum plant productivity and evaluation of a plant''s capacity to reduce N2 is related directly to concurrent CO2 reduction. A measure of whole plant N2 fixation efficiency based on the N2/CO2 uptake ratio is proposed.