NITRATE EFFECTS ON NODULATION AND NITROGENASE ACTIVITY OF ACTINORHIZAL CASUARINA STUDIED IN SPLIT-ROOT SYSTEMS

Nodulation and nitrogenase activity of inoculated Casuarina cunninghamiana Miq. were evaluated after exposing parts of root systems of intact plants to varied concentrations of potassium nitrate using split-root, solution culture systems. In a first experiment, plants were supplied with either: low concentrations of nitrate (0.05 mm) on both sides of the root system (L:L), high concentrations of nitrate (3.00 mm) on both sides of the root system (H:H), or low concentrations of nitrate on one side and high concentrations on the other side of the root system (L:H). In a second experiment, L:L and H:H treatments were retested using an apparatus which continuously supplied nutrient solutions to each side of the split-root system. In this experiment, the L:H treatment was substituted by a L:VH (6.00 mm) treatment to evaluate the effects of very high nitrate concentrations. In both experiments, all plants were exposed to nitrate treatments 2 weeks prior to inoculation with Frankia strain CcI3 (HFP 020203). In the first experiment, root hair development was suppressed and no nodules formed on root systems directly exposed to 3.00 mm nitrate, but root hairs and nodules were observed on all root systems exposed to 0.05 mm nitrate. The number of nodules and nodule biomass on the low side of L:H plants were not significantly different from those observed on either side of L:L plants. In the second experiment, root hair development was not suppressed by any of the nitrate treatments (6.00, 3.00 and 0.05 mm), however, the number of nodules formed per side and their biomass were generally inversely related to the nitrate concentration on that side. Specific nitrogenase activities measured on the low side of L:VH plants were significantly lower than those measured on either side of L:L plants. The results of both experiments showed that: (1) nitrate inhibited nodule initiation strictly locally, (2) nodule development appeared to be inhibited locally, and (3) specific nitrogenase activity was inhibited systemically.