Herbivory and calcium concentrations affect calcium oxalate crystal formation in leaves of Sida (Malvaceae)

Calcium oxalate crystals have potential roles in plants as part of a defence mechanism against herbivores and/or in accumulating excess calcium. To date, these potential roles have been studied independently. In this experimental study the effects of calcium levels and herbivory on the production of calcium oxalate crystals (i.e. druse, spherical crystal aggregates) were examined in seedlings or Sida rhombifolia. Seedlings were subjected to three calcium levels (low, normal or high) and an artificial herbivory treatment. Calcium levels and herbivory both affected density of crystals in leaves. Leaves from seedlings grown in low calcium had a greater crystal density than those grown in high calcium. Leaves from seedlings subjected to herbivory had a greater crystal density than those from seedlings not subjected to herbivory. This study provides additional evidence that calcium oxalate crystal production depends not only on calcium levels but can also be influenced by external pressures such as herbivory. In addition to their physiological role in plants, these results suggest that calcium oxalate crystals can also act as a defence mechanism against herbivores.