Chemical defenses in leaves can deter leaf herbivores. However, leaf chemical defenses are not the only defenses plants have against antagonists. Chemical defenses can also be found in flowers, and some plants rely on indirect defenses such as extrafloral nectaries (EFNs) to attract predators of herbivores. Each of these types of defenses can effectively prevent herbivory under some conditions, but they also each have physiological and/or ecological costs. It is therefore likely that there are trade-offs between defenses that will change which defense type is most efficient under varying environmental conditions. To test this, we completed two experiments using Chamaecrista fasciculata, which has condensed tannins in both leaves and flowers and uses EFNs to attract ant protectors. In our first experiment, we completed a survey across a wide range of latitudes to look for patterns in plant defense. In our second experiment, we grew plants in a greenhouse with varying water and nutrient levels, then measured defenses.
Results/Conclusions
In our geographic survey, we found higher concentrations of leaf defenses at low latitudes, while EFNs were larger at high latitudes, suggesting a trade-off between chemical and indirect leaf defenses. Flower defenses were stable across all latitudes, perhaps because the reproductive importance of flowers necessitates their consistent defense. In our greenhouse experiment manipulating abiotic factors, EFNs were larger and produced more nectar as a reward to ant protectors under high water and high nutrient conditions. Chemical defenses were also affected by water and nutrient levels, but in a less consistent manner. Taken together, the results of our two experiments suggest that plant defenses of all types are resource-limited, but that the geographic patterns seen in the field are driven by more than just resource availability. There may be strong trade-offs between defense types dependent on local genotypes or based on other environmental characteristics, such as the severity of herbivory or presence of ants.