Submitted to: Acta Oecologica
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/27/2012
Publication Date: 7/27/2012
Publication URL: http://handle.nal.usda.gov/10113/57114
Citation: Vallotton, A.D., Murray, L.W., Delaney, K.J., Sterling, T.M. 2012. Water deficit induces swainsonine of some locoweed taxa, but with no swainsonine-growth trade-off. Acta Oecologica. 43: 140-149. Available: http://dx.doi.org/10.1016/j.actao.2012.06.006. Interpretive Summary: Plant investment into secondary compounds like alkaloids can potentially lead to trade-offs with photosynthesis and subsequent plant growth, and this trade-off might be magnified when a plant is under stress like drought. We studied whether white locoweed and three woolly locoweed varieties varying in constitutive levels of the alkaloid ‘swainsonine’ had reduced photosynthetic activity or biomass accumulation in the higher vs. lower swainsonine taxa, whether such reductions would be further magnified in plants under water deficit, and whether water deficit would induce higher swainsonine levels in locoweeds. We found no clear indication of photosynthesis or growth trade-offs between lower and higher swainsonine taxa. Water deficit induced higher swainsonine levels in two woolly locoweed varieties at the end of our study, but caused no change in swainsonine levels in white locoweed and one woolly locoweed variety to suggest taxon-specific locoweed swainsonine responses to water deficit. Yet, multiple assays indicated that our water deficit treatment affected plants. We also observed that swainsonine levels steadily increased over time independent of water stress in two woolly locoweed varieties, suggesting a possible seasonal effect on locoweed swainsonine production. Our results showed that an important type of abiotic stress, water deficit, did not cause large variation in swainsonine levels across out test locoweeds. However, genetic differences in swainsonine levels across locoweed taxa were conserved in plants with and without water-deficit. Our study relates to understanding the importance of environmental and genetic factors on locoweed swainsonine levels, since livestock poisoning from locoweed consumption has been a problem in the western U.S. for several decades.
Technical Abstract: Locoweeds (Astragalus and Oxytropis spp.) contain swainsonine (SWA), an alkaloid toxic to vertebrates. In two common-garden experiments, we studied three A. mollissimus varieties that vary in SWA levels (barely detectable SWA var. ‘thompsonae’, intermediate SWA var. ‘bigelovii’, and high SWA var. ‘mollissimus’), and intermediate SWA O. sericea; our results confirmed these rankings. We tested whether higher SWA production had a trade-off with primary physiology (photosynthesis) and growth (biomass production), and whether water stress magnified a trade-off. Only after all three water-deficit treatments did SWA increase 1.5-fold for var. mollissimus and 4-fold for var. bigelovii compared to control plants, but no change in SWA for var. thompsonae or O. sericea. Our results did not support the hypothesis that water-deficit causes SWA increases in all tested locoweed taxa. 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 caused reduced photosynthetic activity. Water-deficit plants had higher water-use efficiency for var. mollissimus, var. thompsonae, and O. sericea; higher root mass ratios for var. bigelovii, var. thompsonae, and O. sericea; lower relative water content for var. thompsonae but higher for var. bigelovii, and more dropped senesced leaves for all tested taxa. There was little evidence for a SWA growth trade-off since high SWA-producing taxon ranked third for total dry biomass, and a barely detectable SWA-producing taxon was ranked only second for biomass. In addition, relative growth rate and photosynthetic rate results had no clear negative association with SWA and thus no trade-off was suggested with these plant parameters. Overall, no clear SWA production trade-off with plant growth or primary physiological activity suggests that locoweed SWA production is not physiologically expensive.