Interdependence of Nitrogen Nutrition and Photosynthesis in Pisum sativum L: II. Host Plant Response to Nitrogen Fixation by Rhizobium Strains.

Physiological responses to infection by strains of Rhizobium leguminosarum which differed in their capacity to reduce N(2) were determined in 26-day-old pea plants (Pisum sativum L. cv. Alaska) grown under uniform environmental conditions in the absence of combined N. The highest N(2) reduction rates, calculated from H(2) evolution and C(2)H(2)-dependent C(2)H(4) production measurements, were approximately 6-fold greater than the lowest. Higher N(2) fixation rates were associated with greater CO(2) exchange rates (R(2) = 0.92) and carboxylation efficiency (R(2) = 0.99). Increases in the apparent relative efficiency of N(2) fixation [1-(H(2) evolved in air/C(2)H(2) reduced)] (acteroid efficiency) were associated with increases in whole-plant N(2) fixation efficiency (N(2)/CO(2) reduction ratio) (R(2) = 0.95). Whole-plant dry weight and total N content were related by regression analysis (R(2) = 0.98); both parameters were enhanced by increased N(2) fixation in a manner analogous to previously reported increases caused by greater external applications of NH(4) (+). These data reveal that photosynthetic parameters in genetically uniform host plants grown under identical environmental conditions are affected by N(2) fixation characteristics of the rhizobial symbiont. The measured efficiencies of micro- and macrosymbiont are directly related under uniform environments.