Strong modulation of nutrient distribution in Alnus glutinosa as a function of the actinorhizal symbiosis

Abstract

Micro- and macro-nutrient acquisition by plants and microorganisms is a cornerstone for their survival and has a direct impact on biogeochemical cycling. In this study, we investigated, in controlled conditions, how the availability of exogenous nitrate impacted nutrient acquisition and distribution in black alder (Alnus glutinosa (L.) Gaertn.) in the presence, or absence, of its nitrogen-fixing bacterial symbiont (Frankia sp.). Our findings show that alder physiology and distribution of nutrients between aerial and root tissues were strongly influenced by the presence of the symbiont. In both nodulated and non-nodulated alders, root allocation and total plant biomass were positively correlated, except when nodulated alders were subjected to low nitrate conditions (≤15 ppm). Alders receiving 45 ppm exogenous nitrate had a less developed actinorhizal symbiosis. These findings reflect the importance of root exploration in relation to plant dependence to exogenous nitrate. Nutrient composition of alder aerial tissues, in particular molybdenum, was significantly altered in the presence of Frankia. In the context of plant leaf-litter mutualism involving metals and N exchange, our findings of high Mo and P translocation to shoots of non-nodulated alders underscores how the state of the symbiosis in actinorhizal plants can influence the biogeochemical cycling of elements.