Abstract:

We discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate ;runaway embolism,' whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-physiology, and evolution of woody plants.

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Do Woody Plants Operate Near the Point of Catastrophic Xylem Dysfunction Caused by Dynamic Water Stress? <sup><a href="#fn1" rid="fn1" class=" fn">1</a></sup>

M Tyree

J Sperry

Abstract

We discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate `runaway embolism,' whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-physiology, and evolution of woody plants.

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Sperry JS. Relationship of Xylem Embolism to Xylem Pressure Potential, Stomatal Closure, and Shoot Morphology in the Palm Rhapis excelsa. Plant Physiol. 1986 Jan;80(1):110–116.[PMC free article] [PubMed] [Google Scholar]
Sperry JS, Holbrook NM, Zimmermann MH, Tyree MT. Spring filling of xylem vessels in wild grapevine. Plant Physiol. 1987 Feb;83(2):414–417.[PMC free article] [PubMed] [Google Scholar]
Tyree MT, Dixon MA. Cavitation Events in Thuja occidentalis L.? : Utrasonic Acoustic Emissions from the Sapwood Can Be Measured. Plant Physiol. 1983 Aug;72(4):1094–1099.[PMC free article] [PubMed] [Google Scholar]
Tyree MT, Dixon MA, Thompson RG. Ultrasonic Acoustic Emissions from the Sapwood of Thuja occidentalis Measured inside a Pressure Bomb. Plant Physiol. 1984 Apr;74(4):1046–1049.[PMC free article] [PubMed] [Google Scholar]
Tyree MT, Dixon MA, Tyree EL, Johnson R. Ultrasonic acoustic emissions from the sapwood of cedar and hemlock : an examination of three hypotheses regarding cavitations. Plant Physiol. 1984 Aug;75(4):988–992.[PMC free article] [PubMed] [Google Scholar]
Tyree MT, Fiscus EL, Wullschleger SD, Dixon MA. Detection of Xylem Cavitation in Corn under Field Conditions. Plant Physiol. 1986 Oct;82(2):597–599.[PMC free article] [PubMed] [Google Scholar]

Department of Botany, University of Vermont, Burlington, Vermont 05405

^{Various parts of this research were funded by: Natural Sciences and Engineering Research Council of Canada, grant No. A6919, U.S. Department of Agriculture grant No. 86-FSTY-9-0226 and U.S. Department of Agriculture grant No. 85-CRSR-2-2564. Foreign travel costs were covered by Visiting Fellowships to both authors from the Smithsonian Tropical Research Institute and by a grant from the Lintilhac Foundation.}

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Abstract

We discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate `runaway embolism,' whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-physiology, and evolution of woody plants.

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