Vessel diameter is related to amount and spatial arrangement of axial parenchyma in woody angiosperms.

Parenchyma represents a critically important living tissue in the sapwood of the secondary xylem of woody angiosperms. Considering various interactions between parenchyma and water transporting vessels, we hypothesize a structure-function relationship between both cell types. Through a generalized additive mixed model approach based on 2,332 woody angiosperm species derived from the literature, we explored the relationship between the proportion and spatial distribution of ray and axial parenchyma and vessel size, while controlling for maximum plant height and a range of climatic factors. When factoring in maximum plant height, we found that with increasing mean annual temperatures, mean vessel diameter showed a positive correlation with axial parenchyma proportion and arrangement, but not for ray parenchyma. Species with a high axial parenchyma tissue fraction tend to have wide vessels, with most of the parenchyma packed around vessels, whereas species with small diameter vessels show a reduced amount of axial parenchyma that is not directly connected to vessels. This finding provides evidence for independent functions of axial parenchyma and ray parenchyma in large vesselled species and further supports a strong role for axial parenchyma in long-distance xylem water transport. In this paper, we explored the amount and spatial position of axial parenchyma in xylem tissue in relation to vessel size, factoring in a range of parameters using a modelling approach, including maximum tree height. Our most important finding was that as vessels increase in diameter, so too does the amount of axial parenchyma. At the same time, we show that the arrangement of axial parenchyma becomes more abundantly associated with vesselsallying more tightly as a mass of living cells around the vessels. This finding suggests that large vessels require more support from the living cell network, which has been suggested previously but without any direct evidence. As our analyses are based on a large, global dataset, the results obtained further our understanding of xylem adaptation across various biomes, and the relationship between parenchyma and the water transport system, provoking the traditional mindset that water transport in plants is merely a physical process.