Interdependence of Nitrogen Nutrition and Photosynthesis in Pisum sativum L

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Abstract
Photosynthesis, primary productivity, N content, and N2 fixation were determined as a function of applied NH4+ in peas (Pisum sativum L. cv. Alaska) which were inoculated or not inoculated with Rhizobium leguminosarum. Cabon dioxide exchange rate (CER) increased 10-fold, total N content 7-fold, and total dry weight 3-fold in 26-day-old uninoculated plants as applied NH4+ was increased from 0 to 16 millimolar. In inoculated plants of the same age CER and dry weight were maximal at 2 millimolar NH4+, and total N content increased between 0 and 2 millimolar NH4+ but did not change significantly with higher NH4+ applications. Per cent N content of uninoculated plants was significantly lower than that of inoculated plants except at the highest NH4+ concentration (16 millimolar). Symbiotic N2 fixation by inoculated plants was maximal in peas grown with 2 millimolar NH4+; and apparent relative efficiency of N2 fixation, calculated from C2H2 reduction and H2 evolution, was maximal in the 2 to 4 millimolar NH4+ concentration range. The capacity to fix N2 through the Rhizobium-legume symbiosis significantly enhanced the rate and efficiency of photosynthesis and plant N content when NH4+ concentration in the nutrient solution was below 8 millimolar. Above 8 millimolar NH4+ concentration uninoculated plants had greater CER, N content, and dry weight.
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