Causes and consequences of variable extrafloral nectar secretions in quaking aspen (Populous tremuloides Michx.)

Abstract

Extrafloral (EF) nectaries mediate a defensive mutualism in many plant populations, wherein plants attract predatory arthropods by providing nectar rewards. The primary objectives of this study were to identify abiotic and biotic factors that may affect secretion by EF nectaries in quaking aspen (Populus tremuloides Michx.) and to determine how variation in secretion rate affects attractiveness of P. tremuloides ramets to predatory arthropods such as ants. I investigated the effects of water stress, defoliation, and genotype on extrafloral sugar secretions in P. tremuloides and tested how variations in EF sugar quantity affect ant visitation to P. tremuloides ramets in interior Alaska. Additionally, I analyzed P. tremuloides sugar composition from three genotypes. Extrafloral sugar secretions were inducible by defoliation, and the induction response was not inhibited by water stress. Irrespective of defoliation, water stress had a variable effect on sugar secretion rates between genotypes, with one out of four genotypes exhibiting a reduction in secretion rate in response to low water availability. Genotypes differed in secretion rates overall, which could potentially influence defensive levels among clonal stands. Ant visitation to ramets with experimentally increased sugar availability was increased for one of three genotypes in early summer, though in mid-summer ants did not respond to nectar supplementation. There was no effect of nectar reduction on ant visitation in either early or mid-summer trials. Genotypes attracted different average numbers of ants, which may have been a result of intrinsic variation in volatile organic compound emission, EF nectar secretion rates, or nectar composition. Analysis of EF sugar secretions of P. tremuloides using nuclear magnetic resonance spectroscopy revealed that EF nectar tends to be dominated by sucrose over glucose and fructose. This composition may increase attractiveness to mutualistic ant species, which tend to favor sucrose dominated nectar blends. This study expands our knowledge of the sources of variation in EF nectar secretion and their impact in a widespread, ecologically important tree species.