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

Abstract

A very significant increase in N 2 (C 2 H 2 ) reduction by Visum sativum L. infected with Rhizobium leguminosarum occurred when plants were grown in the light with 6 hr of CO 2 enrichment (0.00120 atm). Plants grown for 4 wk under 0.00120 atm CO 2 showed significant increases over control plants at 0.00032 atm CO 2 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 CO 2 enrichment. Both control and CO 2 ‐enriched plants optimized the sink/source ratio between the mass of nodules and the extranodular plant mass. The optimum ratio for N 2 reduction by 4‐week‐old peas was 0.05. Long‐term CO 2 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 CO 2 ‐enriched conditions. The results are interpreted as showing that short‐term increases in CO 2 levels promote N 2 reduction by affecting root nodule functioning, whereas long‐term CO 2 enrichment promotes N 2 reduction by increasing total plant and root nodule development.