THE EFFECT OF CO2 ENRICHMENT ON ROOT NODULE DEVELOPMENT AND SYMBIOTIC N2 REDUCTION IN PISUM SATIVUM L.

A very significant increase in N₂(C₂H₂) reduction by Visum sativum L. infected with Rhizobium leguminosarum occurred when plants were grown in the light with 6 hr of CO₂ enrichment (0.00120 atm). Plants grown for 4 wk under 0.00120 atm CO₂ showed significant increases over control plants at 0.00032 atm CO₂ in plant dry weight, N content, root nodule mass, number of nodules, and mean nodule dry weight. Acetylene-reduction assays, however, revealed no reproducible increase in nitrogenase activity/mg nodule in plants subjected to long-term CO₂ enrichment. Both control and CO₂-enriched plants optimized the sink/source ratio between the mass of nodules and the extranodular plant mass. The optimum ratio for N₂ reduction by 4-week-old peas was 0.05. Long-term CO₂ enrichment did not promote root nodule formation to a greater degree than total plant development, and increases in N content were directly proportional to increases in nodule mass. Morphological data revealed significantly greater deposits of starch in root nodules of plants grown under CO₂-enriched conditions. The results are interpreted as showing that short-term increases in CO₂ levels promote N₂ reduction by affecting root nodule functioning, whereas long-term CO₂ enrichment promotes N₂ reduction by increasing total plant and root nodule development.