An Ace in the Hole: Scanning Electron Microscopy Reveals that Stomata may Play a Role in Explosive Seed Discharge of Dwarf Mistletoe

Arceuthobium americanum, the lodgepole pine dwarf mistletoe, is a dioecious parasitic plant that infects the lodgepole pine in the Pacific Northwest. The infection causes stunted growth of the host tree and reduces lumber value and tensile strength. We conducted this research to gain a greater understanding of the development of female A. americanum through the second year of fruit maturation. The plant possesses a unique dispersal mechanism involving explosive seed discharge triggered by various processes. Of particular interest are stomata, which are small pores on the epidermis that function in providing gas exchange and a cooling mechanism through transpiration. The primary goals of the study are to observe changes in fruit development and morphology using environmental scanning electron microscopy and to measure several quantitative attributes: (1) fruit length (2) fruit diameter (3) flower tepal surface area and (4) stomatal density. We found that in all trees sampled, fruit diameter and length, and the tepal’s surface area all increased over the growing season. Conversely, stomatal density decreased in all trees. Developmentally, the fruit was found to swell and the floral organs persisted through the season. The decline in stomatal density could function in retaining water inside the fruit to facilitate discharge, or provide a heating mechanism through reduced transpiration. Future work should explore the influence of stomata in the explosive discharge directly