The comparative xylem structure and function of petioles and twigs of mistletoe Loranthus europaeus and its host Quercus pubescence

Abstract

The xylem structure of the petiole and twig revealed in this study increase our knowledge about mistletoe and host function and their interaction. Although transpiration and photosynthesis relationships between mistletoes and their hosts have been studied extensively, only a few studies have been performed on their comparative anatomical structures. To our knowledge, no studies have focused on comparisons between the petiole and twig xylem structures of mistletoe and its host, although xylem structure is closely connected to tree physiology. In this study, the xylem structure and hydraulic conductivity of petioles and current-year twigs of the mistletoe Loranthus europaeus Jacq. and its host Quercus pubescens Willd. were measured. It was found that the petiole conduit lumen area, xylem area, and xylem-to-cross-sectional area ratio were, respectively, 1.4, 6.2, and 4.4 times higher in Quercus compared to Loranthus. The anatomical differences in xylem between both species were more pronounced in twigs than in petioles. Although the studied petiole conduit traits differed between Quercus and Loranthus, specific hydraulic conductivities were similar in both species. The similar specific hydraulic conductivities for both water competing species indicate that Loranthus and Quercus reach the same transpiration rates under good water supply conditions and under similar leaf water potential. The narrower conduits in Loranthus could make the xylem of this hemiparasite well adapted to withstand a higher water potential gradient without cavitation. Nevertheless, further studies are needed to define the anatomical and morphological traits involved in xylem cavitation in mistletoe, traits which could provide more information about the function and survival of mistletoe under conditions of higher water potential gradient.