NONINVASIVE MEASUREMENT OF INTERNAL OXYGEN CONCENTRATION OF FIELD-GROWN SOYBEAN NODULES

Recent research suggests that decreased N2 fixation with drought, defoliation, or nitrate exposure is due to decreased nodule permeability to O2. Decreased O2 permeability would result in lower O2 concentration in the nodule interior and a decreased rate of bacteroid respiration in support of N2 fixation. To test this hypothesis under field conditions, a noninvasive method (based on spectrophotometry of leghemoglobin) was used to measure O2 concentration within intact, attached nodules of soybean [Glycine max (L.) Merr.]. A detopping (shoot removal) treatment, which interrupted phloem supply to nodules, resulted in lower internal O2 concentration relative to controls, suggesting that nodule respiration was directly limited by O2 rather than carbohydrate. When the gas stream around a nodule was switched from pure N2 to pure O2, internal O2 concentration increased more rapidly in control nodules than in nodules attached to detopped plants. This difference in reoxygenation rate is also consistent with reduced nodule gas permeability after detopping. The method described here, with modifications, should be useful for a variety of field studies.