D3156: Chemical defenses of the plant horsenetttle (Solanum carolinense) in relation to herbivore stress and resource availability

Insect herbivores cause substantial damage to plants, potentially reducing growth, reproductive output, and population abundance. In response to these damaging effects of herbivores, plants have evolved a diverse suite of defenses to protect themselves from herbivory. Because plants have limited resources, allocating to defenses usually comes at a cost to growth. While this growth-defense tradeoff is predicted by plant defense theory, empirical evidence for tradeoffs within species is mixed. Testing new hypotheses, therefore, may provide new insights into understanding the substantial variation in plant defenses within species. The herbivore pressure hypothesis predicts that plants growing in high-resource environments should be more defended because herbivore activity is also expected to be high, whereas the resource availability hypothesis predicts that plants growing in low-resource environments should be more defended because damage is less tolerated. We tested these hypotheses by measuring herbivory and collecting leaves of Solanum carolinense (Solanaceae) from 12 southern USA (high-resource) and 15 northern USA (low-resource) sites. Chemical assays were performed on leaves for glycoalkaloids, which are toxic to insect herbivores. As expected, herbivory damage was greater in the southern compared to northern sites. We found that S. carolinense in northern populations had slightly higher concentration of glycoalkaloids compared to plants growing in southern populations. These results support the resource availability hypothesis because of the negative correlation between herbivore pressure and glycoalkaloid concentration. Therefore, S. carolinense populations are primarily adapting to variation in resource availability, rather than herbivore pressure, across its geographic range.