Growth of Actinorhizal Plants as Influenced by the Form of N with Special Reference to Myrica gale andAlnus incana

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Abstract
Growth of two actinorhizal species was studied in relation to the form of N supply in water culture. Non-nodulated bog myrtle (Myrica gale) and grey alder (Alnus incana) were grown with NH4+, NH4NO3− or NO3− (4 mol m−3 N). A nodulated series of bog myrtle was also cultivated in N-free medium. Relative growth rate (RGR), utilization rate of N, and shoot/root ratio were highest for the two species with the N completely supplied as NH4+. In both species, nitrate was largely reduced in the roots and the presence of NO3− in combined-N supply always affected the RGR and N utilization rate negatively. Both N2 fixation and complete NO3− nutrition represented conditions of relative N-deficiency resulting in relatively low tissue-N concentrations and a greater allocation of dry mass to the roots. The physiological N status of nodulated M. gale plants was comparatively good, as indicated by a normal nodule weight ratio and a relatively high N2-fixing rate per unit nodule mass. However, whole-plant N2-fixing capacity remained relatively low in comparison with acquisition rates of N in combined-N plants. The anion charge from the nitrate reduction was largely directly excreted as an OH− efflux. H + /N ratios generally agreed with the theory. In comparison with NH4+ nutrition, carboxylate concentrations were higher in N2-fixing M. gale plants and the H + /N ratio in nodulated plants was less than unity below the value for ammonium plants as previously found for other actinorhizal species. Therefore, NH4+ should be an energetically more attractive N source for actinorhizal plants than N2. The results agree with commonly accepted views on energetics of N uptake and assimilation in higher plants and support the concept of a basically similar physiological behaviour between non-legumes and legumes.

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