Quaking aspen (Populus tremuloides Michx.) expresses extrafloral nectaries (EFNs) on a subset of its leaves. The EFNs are visited by predatory arthropods, and the results of experimental predator exclusion indicate that EFNs function as a form of indirect defense in aspen. Many plant defenses are inducible by herbivory, which reduces costs of defense at low levels of herbivory. We aimed to test whether the rate of extrafloral nectar secretion in quaking aspen increases in response to defoliation, and, if so, whether drought stress modifies that induction response. We conducted a greenhouse experiment in which we subjected 32 aspen ramets representing four genotypes to defoliation and water treatments in a 2x2 factorial design. We removed 40% of total leaf area from defoliated ramets using scissors, while undefoliated ramets were undamaged. Drought-stressed ramets were allowed to reach soil moisture levels just above wilting point before receiving water, while well-watered ramets received abundant water throughout the study. We measured the rate of sugar secretion by collecting secretions from four leaves per ramet 2, 4, and 6 days after defoliation.
Results/Conclusions
The rate of extrafloral sugar secretion by defoliated ramets was significantly higher than that of undefoliated ramets 6 days post-defoliation. There was no observable effect of drought stress on the induction response. However, drought stress did reduce the overall sugar secretion rates of one of the four aspen genotypes, relative to the well-watered condition. These results suggest that the rate of extrafloral nectar secretion in aspen is induced by damage in aspen, potentially increasing levels of defense within a week. The observed difference between genotypes in response to drought stress indicates that some clonal stands may be at a disadvantage when faced with compound stresses of drought and herbivory. The results contribute to our understanding of defense in a widespread and ecologically-important tree species.