Xylem vessel relays contribute to radial connectivity in grapevine stems (Vitis vinifera and V. arizonica; Vitaceae)

Premise of the study: Xylem network connections play an important role in water and nutrient transport in plants, but also facilitate the spread of air embolisms and xylem-dwelling pathogens. This study describes the structure and function of vessel relays found in grapevine xylem that form radial and tangential connections between spatially discrete vessels. Methods: We used high-resolution computed tomography, light microscopy, scanning electron microscopy, and single-vessel dye injections to characterize vessel relays in stems and compare their distributions and structure in two Vitis species. Key results: Vessel relays were composed of 1-8 narrow diameter (~25 µm) vessel elements and were oriented radially, connecting vessels via scalariform pitting within a xylem sector delineated by rays. The functional connectedness of vessels linked by vessel relays was confirmed with single-vessel dye injections. In 4.5-cm sections of stem tissue, there were 26% more vessel relays in V. vinifera compared with V. arizonica. Conclusions: Because of their spatial distribution within Vitis xylem, vessel relays increase the connectivity between vessels that would otherwise remain isolated. Differences in vessel relays between Vitis species suggest these anatomical features could contribute to disease and embolism resistance in some species.