NITRATE ENTRY AND NITRITE FORMATION IN THE INFECTED REGION OF SOYBEANNODULES

The entry of nitrate into the infected region of soybean nodules and t he possibility of a subsequent nitrite accumulation was studied. Nitra te was observed to gain access to the infected region in the short-ter m and significant amounts could be measured within 2 d of nitrate supp ly. The availability of nitrate in the bacteroid-containing region did not cause free-nitrite accumulation for at least 8 d, However, to avo id the artefactual production of nitrite during extraction it was nece ssary to disrupt nodules in the presence of zinc acetate and ethanol, to prevent bacteroid nitrate reductase activity, Nitrite rapidly accum ulated if nodules were extracted without prior enzyme-inactivation, or if bacteroids were allowed access to nitrate, or, more significantly, if nodules were not extracted immediately following detachment, Nitra te accumulation in detached nodules was mediated by oxygen concentrati on within the nodule; in the presence of pure N-2 gas, nitrite accumul ation was three times greater than in air and, conversely, it was prev ented by exposure to pure O-2. Furthermore, nitrite produced in detach ed nodules under atmospheric conditions was scavenged by transfering t hese nodules into 100% oxygen. However, measurements of apparent funct ional leghaemoglobin, using a nodule oximeter, suggested that after 8 d nitrate exposure up to 83% of Lb activity was lost, possibly due to interactions with nitrite produced in the nodule interior leading to t he formation of nitrosylleghaemoglobin.