The functional significance of tracheids co-occurring with vessels in xylem of Eudicots suggests a role in embolism tolerance

Abstract

One of Sherwin Carlquist’s greatest scientific legacies is how far he advanced our understanding of the structural and functional relationships within the xylem. His research was critical in illuminating the occurrence and importance of tracheids co-occurring with vessels in the xylem of some eudicots. Here, we highlight key concepts and findings from his work on these topics, and how testing has advanced with the advent of recent methods that produce spatially explicit data on embolism formation. There are three classifications of tracheids within vessel-bearing xylem: vasicentric, true, and vascular. Carlquist viewed tracheids as critical in safely transporting water. We review the literature on tracheid function, focusing on transport safety and efficiency, and discuss how the presence of tracheids might affect other xylem traits in the context of tradeoffs. Available data indicate that Carlquist was correct in his view that tracheids are resistant to embolism, and they contribute to the safe transport of sap. Tracheids provide connections (bridges) among vessels that remain sap-filled within highly embolized vessel networks and, in this role, they promote embolism tolerance. There is still much to learn, and integrating tracheids into models of sap flow, understanding their pit traits and connections, and how they affect xylem functional tradeoffs will transform our understanding of plant sap transport. Diverse xylem structural arrangements suggest many potential ways for xylem networks to function. In species containing both vessels and tracheids, the hydraulic network appears to be able to tolerate high levels of vessel embolism within seasonal environments.