Interactive effects of soils, local environmental conditions and herbivores on secondary chemicals in tallow tree

Plants produce secondary chemicals that may vary along with latitude due to changing abiotic and biotic stress gradients and local environmental conditions. Teasing apart the individual and combined effects of these different abiotic, such as soil nutrients, and biotic factors, such as soil biota and herbivores, on secondary chemicals is critical for understanding of plant responses to changing environments. We conducted an experiment at different latitudes in China, using tallow tree (Triadica sebifera) seedlings sourced from a population at 31 °N. These seedlings were cultivated in gardens located at low, middle, and high latitudes, with either local soil or soil from the original seed collection site (“origin soil”). The seedlings were exposed to natural levels of aboveground herbivores or had them excluded. Plant secondary chemicals (both foliar and root), aboveground herbivores, and soil characteristics were measured. Results showed that most leaf and root secondary metabolites depended on the interaction of experimental site and soil type. Leaf and root phenolic and tannin concentrations were higher at the middle latitude site, especially in origin soil. Root and foliar flavonoid concentrations increased when aboveground herbivores were excluded. Microbial communities depended strongly on soil treatment. The different responses of tannins versus flavonoids suggest that these two chemical classes differ in their responses to the varying abiotic and biotic factors in these sites along latitudes. Together our results emphasize the importance of considering the interactive effects of local environmental conditions, soil properties and herbivory in regulating plant chemical defenses.