Vegetative and floral development in the pistillate plant of Arceuthobium americanum (lodgepole pine dwarf mistletoe): an environmental scanning electron microscopy study of its phenology and shoot organization

Abstract

Dwarf mistletoes (DM), genus Arceuthobium, are dioecious parasitic flowering plants having prolonged life cycles lasting six years, culminating with explosive discharge of the single seed from the fruit. Arceuthobium americanum Nutt. ex Engelm., the lodgepole pine dwarf mistletoe, infects lodgepole pine (Pinus contorta subsp. latifolia (Engelm.) Critchfield) in western North America, compromising the forest economy. Highly reduced flowers appear no later than two years following shoot development, with female flowers appearing and persisting for over two years. Development of the pistillate plant, including initiation of floral growth, has not been fully explored. Here, we used environmental scanning electron microscopy to demark phenological waypoints throughout the pistillate plant’s development. As successive crops of female flowers emerged every year, up to three generations of flowers/fruit could be found on a single shoot in late summer; we used these three generations to delineate specific developmental stages. Vegetative shoots could initiate growth at any time within the growing season, could assume a terminal position, could also adopt sympodial branching, but were never whorled or adventitious. Floral branches, however, could initiate adventitiously from older nodes in a whorled pattern, and could house flowers/fruits of any generation. Vegetative and floral units were structurally homogeneous, suggesting shared developmental pathways.