Axial and radial wood maturation in three mistletoe 'giants' (Loranthaceae)

The formation of juvenile wood featuring greater cell variation relative to mature wood is a well-known phenomenon. Once wood attains maturity, it has both more consistent cell sizes and organization. This is especially well understood for large trees used for paper and timber production, but less so for shrubs and bushes. Despite its importance, there is very little information on the wood anatomy and wood maturation available for some lineages such as the Loranthaceae (Santalales), which are mostly composed of shrubby mistletoes. Here, we studied three of the largest known mistletoe species to analyse the variation of xylem structure within both the radial and the axial axis of the stem. Using classical anatomical techniques, we determined at which point, both in thickness and in distance from the apex, these parasitic plants start producing mature wood. We measured vessel element length, fibre length, vessel diameter, and vessel density on multiple points of either very thick or very long branches of three different mistletoe species: Struthanthus rhynchophyllus, Tripodanthus acutifolius and Psittacanthus robustus. Our findings suggest that Loranthaceae mistletoes reach wood maturity rather early, with very minor differences between juvenile and mature woods. These results open new avenues for further research on the wood anatomy of mistletoe's stems, enabling the use of smaller samples, such as those commonly present in herbarium vouchers.