THE PHYSIOLOGY AND NITROGEN-FIXING CAPABILITY OF AQUATICALLY AND TERRESTRIALLY GROWN NEPTUNIA-PLENA - THE IMPORTANCE OF NODULE OXYGEN-SUPPLY

The aquatic legume Neptunia plena (L.) Benth. was grown in non-aerated water culture or vermiculite. Growth, nodulation, nitrogen fixation and nodule physiology were investigated. Over an 80-d period, plants grew and fixed nitrogen and carbon equally well in both rooting media, although distribution of growth between plant parts varied. Total nodule dry weights and volumes were similar but vermiculite-grown plants had three times as many (smaller) nodules than those grown in water. Oxygen diffusion resistance of nodules exposed to 21% oxygen and 10% acetylene did not differ significantly. Both treatments showed similar declines in root respiration and acetylene reduction activity (approx. 10%) when root systems were exposed to stepped decreases and increases in rhizosphere oxygen concentration. However, nitrogenase activity of aquatically grown plants was irreversibly inhibited by rapid exposure of nodules to ambient air, whereas vermiculite-grown plants were unaffected. Aeration of water-cultured N. plena reduced stem length (but not mass) and number of nodules per plant. The concentration of nitrogen fixation by 163%. Possible O2 transport pathways from the shoot atmosphere to roots and nodules are discussed. © 1992 Annals of Botany Company.