The influence of water availability and defoliation on extrafloral nectar secretion in quaking aspen (Populus tremuloides)

Abstract

In recent years, water stress has led to widespread growth declines and dieback of several North American tree species. In addition to its direct effects on tree physiology, water stress may compromise anti-herbivore defenses. We tested whether extrafloral (EF) sugar secretion rate in Populus tremuloides Michx. (quaking aspen) increases in response to defoliation and whether water stress impairs constitutive and inducible EF sugar secretion. We subjected P. tremuloides ramets of four genotypes to water restriction and defoliation in a factorial design and measured EF sugar secretion rates 2, 4, and 6 days after defoliation. The sugar secretion rate of defoliated ramets was significantly higher than that of undefoliated ramets 6 days after defoliation. Low water availability reduced the sugar secretion rate of one of the four aspen genotypes but did not prevent induction. Populus tremuloides genotypes secreted EF sugar at different average rates, suggesting genetic variation for this trait. The results indicate that EF nectar secretion is inducible in P. tremuloides, which could increase the effectiveness of indirect defense following herbivory. Genotypic differences in the response of P. tremuloides to water stress suggest that some clonal stands may be at a disadvantage when faced with compound stresses of drought and herbivory.