Nitrate entry and nitrite formation in the infected region of soybean nodules |
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Authors: | Arrese-Igor C; Gordon A; Minchin F; Denison R |
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Institution: | Departamento Ciencias del Medio Natural, Universidad Publica de Navarra, Campus de Arrosadia, E-31006 Pamplona, Spain; Institute for Grassland and Environmental Research, Plas Gogerddan, Aberystwyth, Ceredigion SY23 3EB, UK; Agronomy and Range Science Department, University of California, Davis, CA 95616, USA; Corresponding author e-mail: cesarai@upna.es |
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Abstract: | The entry of nitrate into the infected region of soybean nodules and the
possibility of a subsequent nitrite accumulation was studied. Nitrate was
observed to gain access to the infected region in the short-term and
significant amounts could be measured within 2 d of nitrate supply. The
availability of nitrate in the bacteroid-containing region did not cause
free-nitrite accumulation for at least 8d. However, to avoid the
artefactual production of nitrite during extraction it was necessary to
disrupt nodules in the presence of zinc acetate and ethanol, to prevent
bacteroid nitrate reductase activity. Nitrite rapidly accumulated 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. Nitrate accumulation in
detached nodules was mediated by oxygen concentration within the nodule; in
the presence of pure N2 gas, nitrite accumulation was three times greater
than in air and, conversely, it was prevented by exposure to pure O2.
Furthermore, nitrite produced in detached nodules under atmospheric
conditions was scavenged by transferring these nodules into 100% oxygen.
However, measurements of apparent functional 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 the formation of
nitrosylleghaemoglobin.Key words: Glycine max, cortex,
infected region, leghaemoglobin, nitrate, nitrite, nodules, soybean
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