Drying conditions alter the defensive function of seed mucilage against granivores

Abstract

Environmental conditions alter the function of many plant traits that drive species interactions, producing context-dependency in the outcomes of those interactions. Seed mucilage is a common, convergently evolved trait found in thousands of plant species. When wetted, the seed coat swells into a viscid mass; when dried, the mucilage strands strongly cement the seed to whatever it is in contact with. This binding to the ground has been previously shown to protect seeds from granivory. Previous research found both that mucilage volume—and the correlated attachment strength—are higher in species from hot, dry, areas suggesting an environmental component of this trait's function. Here, we (1) quantified the effect of temperature on attachment across many species in a lab setting, (2) tested the potential mechanism behind this correlation by accelerating desiccation speed without changing temperature, and (3) tested whether these relationships introduce context dependency of the defensive function of mucilage in the field, using field trials with harvester ants. We found that (1) increasing temperature during mucilage drying strongly reduced the force needed to dislodge seeds for most species, (2) drying time was likely the driving mechanism behind the loss of attachment strength at higher temperatures, not temperature per se, (3) seeds attached to substrate during higher temperatures or under accelerated drying conditions were far more susceptible to granivory. Synthesis. These results show not only the mechanism behind an abiotic modification of a functional trait of seeds, but that this change majorly alters a key interaction contributing to seed survival. These results add to a small, but growing, literature on the importance of seed mucilage in seed survival and demonstrate strong and largely predictable context-dependency in this trait's defensive function.