Morphology and stomatal density of developing Arceuthobium americanum (lodgepole pine dwarf mistletoe) fruit: a qualitative and quantitative analysis using environmental scanning electron microscopy

Arceuthobium americanum Nutt. ex Engelm., the lodgepole pine dwarf mistletoe, is a dioecious parasitic flowering plant infecting lodgepole pine (Pinus contorta subsp. latifolia (Engelm.) Critchfield) in the Pacific Northwest, compromising timber value by stunting the host’s growth. The plant disperses its seeds by explosive discharge, and thus understanding its reproduction is an integral step toward managing its spread. The life cycle of Arceuthobium americanum occurs over five to six years, and the fruit matures over two consecutive growing seasons, dispersing in the second. Using low-vacuum environmental scanning electron microscopy (eSEM), we examined branching architecture as well as the morphology and anatomy of fruits during their second year of development up to explosive discharge. We found that branching patterns shifted from opposite-decussate to verticillate after several years’ growth. In addition, the perianth parts (sepals), style, and stigma persisted through the season, and the fruit’s abscission zone developed slightly distal to the end of the pedicel. Stomata, found only on the sepals, were located in crypts surrounded by two subsidiary cells, and stomatal density significantly decreased during development. The decline in stomatal density along with the thick pericarp cuticle may function in retaining water inside the fruit to facilitate discharge and (or) provide a heating mechanism through reduced transpiration.