Characterizing the causes and consequences of calcium oxalate crystal presence in Vitis riparia

Calcium oxalate biomineralization in plants is phylogenetically widespread and morphologically diverse, but the function of these inorganic crystals is an area of active debate. The variety of environmental conditions that produce the crystals, as well as the inconsistent evidence that they provide antiherbivore defense across plant and herbivore species, suggests that different crystal morphologies might have different functions.

Using Vitis riparia, or riverbank grape, we experimentally investigated the environmental influence of excess calcium and simulated herbivory on the formation of calcium oxalate druse and raphide crystals in leaves. We also investigated the putative defensive function of these crystals by using a no-choice herbivore bioassay manipulating herbivore diet composition to test for impacts of crystal shape on herbivore growth, both on its own and with plant chemistry.

We found that the addition of calcium to soil increased the density of both raphide and druse crystals in V. riparia leaves. Contrary to expectations, the herbivory treatment decreased the density of raphides in leaves, and V. riparia-derived crystals did not impact weight gain, time to pupation, or survival of moth larvae.

Our multifaceted test of the formation and function of calcium oxalate crystals in riverbank grape demonstrates that an abiotic factor (i.e., soil calcium) is a relatively stronger determinant of crystal production and that, contrary to hundreds of years of speculation on their function, these crystals do not seem to mediate plant-insect herbivory in all plant taxa. Instead, the alternative hypothesis of calcium regulation was supported by our experimental evidence.