Phosphorus and the regulation of nodulation in the actinorhizal symbiosis between Discaria trinervis (Rhamnaceae) and Frankia BCU110501

Summary After nitrogen (N), phosphorus (P) is the nutrient that most limits plant productivity. The role of P on growth and root nodulation was studied in the actinorhizal symbiosis between Discaria trinervis and Frankia, an intercellular infected N2–fixing association. Growth, nodulation and nutrient content (N and P) were analysed in symbiotic plants receiving different supplies of P in nutrient solutions. The relative requirement of P for nodulation was analysed in P‐deficient plants. Nodule initiation was less impaired than general plant growth by low P. However, low P impaired nodule growth to a greater extent than plant growth. The proportion of nodule biomass, although not the number of nodules per plant, was stimulated by P supply. Autoregulation of nodulation was not affected by P. Use of N was limited by availability of P. Reserves of P in seeds were enough for the seedling to establish nodules. However nodule (and plant) growth was limited in the absence of exogenous P. It is possible that P interacts with the feedback control of nodule growth that is associated with the plant demand for N. Leaf N : P ratio is negatively correlated with the proportion of nodule tissue.