Growth, nodulation and nitrate reductase activity in the aquatic legume Neptunia plena (L.) benth. At different external nitrate concentrations

Abstract

The effects of different external nitrate concentrations (0 (control), 1, 50, 100, 500, 1000 and 20 000 mmol m −3) on growth, nodulation and nitrate-reductase activity (NRA) of inoculated Neptunia plena (L.) Benth. were examined. Plants given 500 and 1000 mmol m −3 nitrate had greater ( P < 0.05) shoot length, leaf, stem and root dry mass, and carbon and nitrogen contents than the controls and plants given 20 000 mmol m −3 nitrate. Nodule number was not significantly affected by nitrate concentration up to 50 mmol m −3, but 100 mmol m −3 nitrate reduced nodulation by 68% and concentrations above 100 mmol m −3 completely inhibited nodule development. Plants given 100–20000 mmol m −3 nitrate had a greater nitrate content per g leaf, stem and root dry mass (DM) than controls. Nitrate per g root DM did not increase with external nitrate concentration above 500 mmol m −3, but levels in leaf and stem were greater at 20 000 mmol m −3 nitrate than at all other concentrations. NRA per g leaf, stem and root fresh mass (FM) was greater for plants given 500–20000 mmol m −3 than for controls, but there was no significant increase with nitrate concentration above 500 mmol m −3. Substantial proportions of total plant nitrate and NRA were found in both root and shoot over the entire range of external nitrate concentrations given. Findings for N. plena are compared with data obtained previously for terrestrial legumes.