Co-evolution of axial parenchyma and vessels in relation to water transport in woody angiosperms reveals coordinated functional adaptation to climate across biomes

Wood parenchyma plays crucial roles in maintenance of woody plants. Functions range from storage and transport of metabolites and solutes to defence and maintenance of the plant hydraulic system. Ray and axial parenchyma (RAP) are oriented 3-dimensionally in radial and axial files. RP forms a twoway transport network connecting inner-bark to wood while AP forms a vertically-oriented support link to the vessels, dead conduits that transport water under negative pressure. Although this anatomically and functionally integrated coupling between AP and vessels has been shown, we have yet to understand the evolution driving this coordination, which is related to how phylogenetically conserved vessels, ray and axial parenchyma are within clades. We tested hypotheses with a dataset
of 2,332 globally distributed species by comparing the performance of pure drift, Lambda effect and Brownian motion models of trait evolution with the selection-inertia, Ornstein-Uhlenbeck model. Phylogeny confirms that the proportion, spatial arrangement of AP and ultimately the level of support to vessels was found to co-evolve with vessels, which, in turn, is related to climate and maximum plant
height as a coordinated assemblage; RP was found to be independent of climate. Highest evolutionary rates were found in AP and maximum plant height, while vessels were more conserved among clades. We provide further evidence to support the role of axial parenchyma in long-distance water transport,
facilitating co-optimisation of hydraulic efficiency and safety in angiosperm woody plants.