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

Photosynthetic efficiency, primary productivity, and N(2) reduction were determined in peas (Pisum sativum L. var. 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 N(2), computed from measured C(2)H(2) reduction and H(2) evolution rates, also increased with growth light intensity, while the previously proposed relative efficiency of N(2) fixation, based on these same parameters, declined. The plot of N/C ratios (total nitrogen content/plant dry weight) increased hyperbolically with light intensity, and the plot of N(2)/CO(2) uptake ratios (N(2) uptake rate/net CO(2) uptake rate) increased linearly. Both plots extrapolated to the light compensation point. The data indicate that the relative efficiency of N(2) fixation is not necessarily correlated with maximum plant productivity and that evaluation of a plant's capacity to reduce N(2) is related directly to concurrent CO(2) reduction. A measure of whole plant N(2) fixation efficiency based on the N(2)/CO(2) uptake ratio is proposed.