Welti, Ellen; Kaspari, Michael
Humans, caterpillars, and fungal pathogens agree, food tastes better with a pinch of salt. While humans are omnivores and can obtain sodium when eating animal products like meat and dairy, herbivores and detritivores have fewer salty menu items. This is why porcupines will eat backpackers’ sweaty hiking boots, deer come to salt licks, and sweat bees seek human perspiration. Unlike animals, most plants do not require sodium to live, but they do passively take up sodium. One study found 100,000 fold variation in leaf sodium content across 200 plant species and 26 grasslands. Plants that take up more salt are thus potentially more delicious. Why then is there not evolutionary selection against plant sodium uptake? One hypothesis is that sodium-rich plants also attract beneficial partners like pollinators, seed dispersers, and/or mycorrhiza. If this hypothesis is true, plants may preferentially store sodium in tissues attractive to their mutualists, like their flowers. To test for the effects of increased sodium content on the cost to plants from their consumers, we “salted the earth” in a large field experiment, replicated across four grasslands in the central North American Great Plains. Salty water, about the same concentration as bison urine, was applied monthly across the growing season to treatment plots, while only plain water was applied to control plots. In each plot, we scored the damage done to the site’s dominant grass species by chewing herbivores (like grasshoppers and caterpillars), leaf miners (often moth or fly larvae which live inside leaves), and leaf fungal pathogens . We did the same for herbaceous perennials (forbs) whenever they were in bloom at a site, collecting both leaf and flower tissue to quantify plant chemistry. Adding salty water increased plant sodium content and the amount of damage done by insect herbivores and fungal pathogens to plant leaves. However, we found no widespread evidence that plants concentrate sodium in flowers rather than leaves. Salting winter roads, irrigating crops with increasingly saline water, and climate change-induced rising of sea levels are all increasing the saltiness of terrestrial ecosystems, with potentially increasing costs for plants.