Nitrogen Stress and Apparent Photosynthesis in Symbiotically Grown Pisum sativum L

Pea plants (Pisum sativum L. cv. Alaska) were inoculated individually with one of 15 Rhizobium leguminosarum strains and grown under uniform environmental conditions in the absence of combined N. Differences in effectiveness of the Rhizobium strains produced plants with differing rates of whole plant apparent N₂ fixation and total N content at the same morphological stage of development. Plants were analyzed to determine interactions between N₂ fixation, N allocation, apparent photosynthesis, and growth. Total leaf N increased linearly with total N₂ fixation (R² = 0.994). The proportion of total N allocated to leaves, the per cent N content of individual leaves, and the photosynthetic efficiency of individual leaves showed a curvilinear response with increasing plant N content. Differences in allocation patterns of leaf N between plants with low and high N content resulted in differences in the relationship between total N content and plant dry weight. Results from this study show that N₂ fixation interacts with leaf photosynthetic efficiency and plant growth in a manner that is dependent upon the allocation of symbiotically fixed N.