Abstract
Physiological responses to infection by strains of Rhizobium leguminosarum which differed in their capacity to reduce N2 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 N2 reduction rates, calculated from H2 evolution and C2H2-dependent C2H4 production measurements, were approximately 6-fold greater than the lowest. Higher N2 fixation rates were associated with greater CO2 exchange rates (R2 = 0.92) and carboxylation efficiency (R2 = 0.99). Increases in the apparent relative efficiency of N2 fixation [1-(H2 evolved in air/C2H2 reduced)] (acteroid efficiency) were associated with increases in whole-plant N2 fixation efficiency (N2/CO2 reduction ratio) (R2 = 0.95). Whole-plant dry weight and total N content were related by regression analysis (R2 = 0.98); both parameters were enhanced by increased N2 fixation in a manner analogous to previously reported increases caused by greater external applications of NH4+. These data reveal that photosynthetic parameters in genetically uniform host plants grown under identical environmental conditions are affected by N2 fixation characteristics of the rhizobial symbiont. The measured efficiencies of micro- and macrosymbiont are directly related under uniform environments.
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