Locoweed dose responses to nitrogen: Positive for biomass and primary physiology, but inconsistent for an alkaloid

• Premise of the Study: Plant communities may be influenced by toxic secondary metabolites or enhanced plant growth from plant–symbiont interactions. The C:N hypothesis predicts that carbon or nitrogen constrains plant secondary metabolite production, but it does not consider compounds produced by plant symbionts. Locoweeds are legumes that can have fungal endophyte alkaloid (swainsonine [SWA]) production, which causes livestock poisoning. We studied four locoweed taxa to test whether average SWA concentrations influenced SWA positive dose responses to N fertilizer. • Methods: We measured locoweed leaf SWA, pigment concentrations and photosynthetic activity, and plant biomass dose responses to N supplementation for 3 mo in two greenhouse experiments. • Key Results: Leaf photosynthesis, leaf pigment concentrations, and plant biomass had positive, unsaturated dose responses across tested N doses. Although N enhanced primary growth, two moderate‐SWA taxa ( Astragalus mollissimus var. bigelovii and Oxytropis sericea ) had negative SWA dose responses to increasing N, the high‐SWA taxon ( A. moll . var. mollissimus ) had no SWA change, and the very low‐SWA taxon ( A. moll . var. matthewsii ) had a transient positive dose response. • Conclusions: Supplemented N led to positive dose responses for plant biomass and leaf photosynthesis and pigments, but SWA dose responses differed across locoweed taxa and time. At N levels that enhanced plant growth and reduced antioxidant protective systems, fungal endophyte alkaloid production was not strongly influenced. Production of SWA may be more strongly influenced by factors other than C:N supply (e.g., seasonality, plant age) in the locoweed–endophyte– Rhizobium complex.