Towards a better understanding of xylem: Helical thickenings in vessels of woody angiosperms are associated with thinner cell walls and lower vessel diameter

The vessels of various woody plants show helical sculpturing of the internal side of the secondary wall. The occurrence of these structures, termed helical thickenings (HT), is correlated with environmental parameters. Their adaptive benefit is, however, still not well understood. Suggestions for functional effects include mechanical stabilization, support of embolism refill or enhancement of water conductance. This study considers possible associations of HT with vessel wall thickness (VWT) and vessel diameter (VD) which are two xylem traits related to water transport and mechanical stabilization. Additionally, the relationship between HT and scalariform perforation plates (SPP) was studied, because a negative correlation between SPP and VWT was reported in the literature. The analysis also addressed the phylogenetic signal of HT. Available trait data for 337 species from 60 families and different biomes were used for statistical analyses. The results show (1) a significant negative correlation between HT and VWT across different biomes that likely indicates correlated evolution, (2) a tendency of HT to occur in narrower vessels (up to a diameter of about 100 µm), (3) an uneven distribution of HT and SPP among taxonomic groups of woody angiosperms, and (4) a moderate phylogenetic signal for HT which is evolutionary more labile than SPP. Based on these outcomes, the assumption of HT as a functional asset is supported which merits further study.