Water deficit induces swainsonine of some locoweed taxa, but with no swainsonine-growth trade-off

Abstract

Environmental factors like nutrient availability (e.g., C:N) and water deficit can influence secondary metabolites levels in plants and fungal endophytes. Locoweeds (Astragalus and Oxytropis spp.) contain swainsonine (SWA), an alkaloid that causes chronic vertebrate toxicity. In two greenhouse experiments, we studied field transplants of three Astragalus mollissimus varieties that vary in SWA levels (barely detectable SWA in var. ‘thompsonae’, intermediate SWA in var. ‘bigelovii’, and high SWA in var. ‘mollissimus’), and intermediate SWA in Oxytropis sericea; our results confirmed these rankings. We tested whether SWA induction after water deficit (three 14d water-deficit periods separated by two 7d water recovery periods) was taxon-specific and whether degree of induction was influenced by taxon average SWA. We also looked for signs of a trade-off between primary physiology (photosynthesis) or growth (biomass, relative growth rate) with fungal endophyte SWA production; and if so, whether water stress magnified such a trade-off. Leaf photosynthetic activity decreased during the experiments and often more in water-deficit than control plants; leaf gs and Ci results suggest that stomatal closure reduced photosynthetic activity from mild water deficit. Yet, water-deficit affected plants in other ways: there were more dropped senesced leaves for all tested taxa; higher water-use efficiency for var. mollissimus, var. thompsonae, and O. sericea; higher root mass ratios for var. bigelovii, var. thompsonae, and O. sericea; and lower relative water content for var. thompsonae. Positive SWA increases of 1.6-fold for var. mollissimus and 4-fold for var. bigelovii occurred, so positive SWA induction was taxon-specific and limited to medium/high SWA A. mollissimus varieties. There was little evidence for a clear SWA trade-off with total (shoot + root) biomass, relative growth rate, or photosynthetic rate. Additional studies will need to test whether water deficit positively induces SWA in medium/high locoweed taxa besides A. mollissimus.