Distribution of calcium oxalate crystals in floral organs of Araceae in relation to pollination strategy

 Premise of the study: Many fl owers are pollinated by potentially hungry insects, yet fl owers also contain gametes and embryos which must be protected from predation. Microscopic calcium oxalate crystals in plant tissues have been proposed to protect against herbivory. Aroids, which have an unusual diversity of such crystals, also exhibit diverse pollination strategies. Many species have pollinators that do not feed while visiting the fl owers, while other species, especially those pollinated by beetles, offer sterile staminodia as food rewards. We examined fl owers of 21 aroid species with various pollination strategies to test the hypothesis that crystals protect vital gametes and embryos while allowing consumption of food bribes.  Methods: Aroid infl orescences collected from the fi eld or from greenhouse material were sectioned, cleared, and examined by bright fi eld and polarization microscopy.  Key results: All species examined, regardless of pollination strategy, arrayed crystals around unshed pollen and ovules. Less vital tissues, such as odoriferous appendages, had few crystals. Staminodia offered as food to beetle pollinators, however, differed greatly between species in their crystal contents. Some had minimal crystals; some had crystals in patterns suggesting they limit beetle feeding; still others had abundant crystals in no obvious pattern.  Conclusions: The results are consistent with crystals protecting against insect predation of gametes and embryos. However, the role of crystals in food-bribe staminodia is unclear. They may limit and direct feeding by beetles in some species, while in others they might have no protective role.