An ace in the hole: Explosive seed discharge by Arceuthobium americanum (lodgepole pine dwarf mistletoe) may be facilitated by declining stomatal density

Arceuthobium americanum, the lodgepole pine dwarf mistletoe, is a dioecious parasitic flowering plant that infects lodgepole pine (Pinus contorta var. latifolia) in the Pacific Northwest. The infection causes stunted growth of the host tree while also compromising timber value. The plant utilizes a unique dispersal strategy involving explosive seed discharge, and thus understanding its reproductive biology is an integral step toward managing its spread. A. americanum’s complete life cycle occurs over five to six years, and the fruit matures over two consecutive growing seasons. Of particular interest are stomata (small pores in the plant’s epidermis), which not only permit gas exchange, but also allow for cooling through transpiration. The primary goal of this study was to observe changes in fruit development and morphology using environmental scanning electron microscopy (SEM). Developmental changes in second- year fruits were assessed to gain a better understanding of the underlying physiological processes and changes that precede explosive discharge. The length and diameter of second-year fruits were found to significantly increase over the growing season (April-September), whereas stomatal density significantly decreases over the same time period. Developmentally, the fruit was observed to swell, and the floral organs persisted through the season. The decline in stomatal density may be simply a consequence of the expansion of surface area, but could function in retaining water inside the fruit to facilitate discharge and/or provide a heating mechanism through reduced transpiration. Future work should explore the influence of stomata in the explosive discharge directly, by measuring discrete transpiration levels