Interdependence of Nitrogen Nutrition and Photosynthesis in Pisum sativum L

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.