RESUMO
Although the inhibitory effects of high concentrations of mineral N (> 1.0 mM) on nodule development and function have often been studied, the effects of low, static concentrations of NH4+ (< 1.0 mM) on nodulation are unknown. In the present experiments we examine the effects of static concentrations of NH4+ at 0, 0.1 and 0.5 mM in flowing, hydroponic culture on nodule establishment and nitrogenase activity in field peas [Pisum sativum L. cv. Express (Svalöf AB)] for the initial 28 days after planting (DAP). Peas grown in the presence of low concentrations of NH4+ had significantly greater nodule numbers (up to 4-fold) than plants grown without NH4+. Nodule dry weight per plant was significantly higher at 14, 21 and 28 DAP in plants grown in the presence of NH4+, but individual nodule mass was lower than in plants grown without NH4+. The nodulation pattern of the plants supplied with NH4+ was similar to that often reported for supernodulating mutants, however the plants did not express other growth habits associated with supernodulation. Estimates of N2 fixation indicate that the plus-NH4+ peas fixed as much or more N2 than the plants supplied with minus-NH4+ nutrient solution. There were no significant differences in nodule numbers, nodule mass or NH4+ uptake between the plants grown at the two concentrations of NH4+. Nodulation appeared to autoregulate by 14 DAP in the minus-NH4+ treatment. Plant growth and N accumulation in the minus-NH4+ plants lagged behind those of the plus-NH4+ treatments prior to N2 fixation becoming well established in the final week of the experiment. The plus-NH4+ treatments appeared not to elicit autoregulation and plants continued to initiate nodules throughout the experiment.
