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ARS Home » Plains Area » Fort Collins, Colorado » Center for Agricultural Resources Research » Water Management and Systems Research » Research » Publications at this Location » Publication #374416

Research Project: Improving the Sustainability of Irrigated Farming Systems in Semi-Arid Regions

Location: Water Management and Systems Research

Title: Tip-to-base xylem conduit widening as an adaptation: Causes, consequences, and empirical priorities

item OLSON, MARK - Universidad Nacianal Autonoma De Mexico
item ANFODILLO, TOMMASO - Universita Di Padova
item Gleason, Sean
item MCCULLOH, KATHERINE - University Of Wisconsin

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2020
Publication Date: 9/28/2020
Citation: Olson, M.E., Anfodillo, T., Gleason, S.M., McCulloh, K.A. 2020. Tip-to-base xylem conduit widening as an adaptation: Causes, consequences, and empirical priorities. New Phytologist. 229(4):1877-1893.

Interpretive Summary: Water transport systems in the leaves of all vascular plants (including all crop species) are designed in such a way that they provide the greatest conductance (liquid water) per unit carbon that has been invested in the network (Murray's law). However, the vascular systems in stems and branches appear to deviate from this rule. This manuscript describes the design of vascular systems in plants ("upstream" from leaves), identifies the most likely explanations for their seemingly counterintuitive design, and also provides a list of empirical priorities that are needed to better understand vascular networks in plant species. A better understanding of the underlying selection forces that have shaped plant vascular networks are needed before we should expect meaningful progress towards improving these systems in crop species.

Technical Abstract: As a vascular plant grows in height or increases in length (e.g., lianas), the distance along which water must be moved from the soil to the leaves also grows, meaning that hydraulic resistance could reasonably be expected to increase and conductance per unit leaf area to drop, and thus photosynthetic productivity to suffer with height growth. How natural selection forges adaptive responses to the hydraulic challenge imposed by increasing conductive pathlength is central to understanding how plants have diversified in habit, in size, and across the ecological diversity of the terrestrial biosphere. Here, we focus on the apparently universal tendency for plant water conducting conduits to widen from their maximally narrow termini at the distal end of the conductive stream toward the base of a plant. As our epigraph shows, this pattern of tip-to-base increase in conduit diameter has been noticed for centuries. Despite being known for so long, tip-to-base conduit widening has yet to be recognized as one of the most functionally consequential patterns in all of plant biology, and therefore one whose causes are essential to understand. We discuss data that are consistent with the pattern being adaptive, and therefore shaped by natural selection, and identify crucial data that are still needed to test the hypothesis of its adaptiveness. We conclude by highlighting some of the far-reaching implications of the pattern being an adaptive one. Before examining these issues, we briefly explain what we mean by “tip-to-base conduit widening.”