Salicaceae, the willow and poplar family, consists of shrubs and trees with unisexual female and male inflorescences (catkins) on separate plants (dioecious). Salix purpurea, or purple willow, has a wide variety of visitors, including potential pollinators and herbivores. However, the chemical machinery that promotes pollination and deterrence of herbivores between the sexes has been scarcely studied. The aims of this work are 1) to identify the compounds emitted by catkins and foliage of female and male S. purpurea individuals and determine their role in attracting floral visitors or herbivores, 2) to elucidate the defensive compounds in the catkins and leaves of these individuals that may affect herbivory, and 3) to determine the contribution of wind or insect pollination to reproduction in this species. To answer the aims, we will analyze volatiles from catkins and foliage using dynamic headspace methods and gas chromatography-mass spectrometry (GC-MS) and also determine the defensive compounds from both of these tissues using liquid chromatography-mass spectrometry (LC-MS). We will also conduct pollination experiments, including floral visitor observations, measurement of floral traits, and pollinator exclusions and correlate these characteristics with the pollinator community and pollination efficiency. Finally, we will perform herbivore bioassays using female and male foliage.
Results/Conclusions
Initial results from the collection and analysis of volatiles and defensive compounds from S. purpurea catkins showed qualitative and quantitative differences in chemical composition between sexes. In general, the catkins of male plants emitted more terpenoids and more total compounds and females more benzenoids. Male catkins produced more compounds (phenolic glycosides) known to be deterrent to herbivores and females contained more chemicals (flavonoids) active against pathogens. Given these results, we predict to see differential patterns of floral visitation and herbivory between females and males. Pollinator exclusion and pollen shape analysis will help determine the relative importance of either wind or insect pollution in this species. These results will offer insights into how common and separate chemical traits between sexes drive interactions between S. purpurea and its pollinators and herbivores. A better understanding of these ecological processes may help answer deeper questions about the roles of pollination and herbivory in the evolution of dioecy and transitions between insect and wind pollination.