ALTERATIONS IN APOPLASTIC AND TOTAL SOLUTE CONCENTRATIONS IN SOYBEAN NODULES RESULTING FROM TREATMENTS KNOWN TO AFFECT GAS-DIFFUSION

Ureide concentration in the cortical apoplast of soybean (Glycine max (L.) Merr.) nodules increases rapidly in response to nodule excision. The objective here was to determine if changes in apoplastic ureide may be related to the control of resistance to gas diffusion which is thought to be localized in the nodule cortex. Following decapitation of shoots, nitrogenase activity (acetylene reduction) and ureide concentration in total nodule extracts declined over a period of several hours. Apoplastic ureide concentration relative to total nodule ureide was elevated under these conditions, but the treatment effect was small compared to non-decapitated controls. Decapitation also caused a significant decline in the concentrations of sucrose, glucose, and D-pinitol in nodules. However, the decline in carbohydrates was similar in the nodule cortex and the nodule as a whole, suggesting that the carbohydrate changes are not related to a cortex-localized mechanism. Non-invasive treatments involving increases or decreases in oxygen concentration supplied to nodulated roots caused rapid decreases in respiration of nodulated roots and in ureide concentration in total nodule extracts, but did not cause major changes in apoplastic ureide concentrations. The combined results indicate that apoplastic ureide is probably not involved in the regulation of resistance to gas diffusion. The rapid decline in nodule ureide concentrations in response to changing oxygen supply documents the sensitivity of ureide synthesis and/or transport to alterations in nodule respiration and/or nitrogenase activity.