Do woody plants operate near the point of catastrophic xylem dysfunction caused by dynamic water stress? : answers from a model.

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.

[1]  M. Zimmermann,et al.  The hydraulic architecture of eastern hemlock (Tsuga canadensis) , 1984 .

[2]  J. Grace,et al.  Concurrent measurements of stem density, leaf and stem water potential, stomatal conductance and cavitation on a spaling of Thuja occidentalis L. , 1984 .

[3]  M. Hipkins,et al.  Gas penetration of pit membranes in the xylem of Rhododendron as the cause of acoustically detectable sap cavitation , 1985 .

[4]  Melvin T. Tyree,et al.  A method for measuring hydraulic conductivity and embolism in xylem , 1988 .

[5]  M. Zimmermann.  Hydraulic architecture of some diffuse-porous trees , 1978 .

[6]  H. Jones,et al.  Relationships between Water Stress and Ultrasound Emission in Apple (Malus×domestica Borkh.) , 1986 .

[7]  J. Sperry.  Xylem Embolism in the Palm Rhapis Excelsa , 1985 .

[8]  M. Tyree,et al.  Ultrasonic acoustic emissions from the sapwood of cedar and hemlock : an examination of three hypotheses regarding cavitations. , 1984, Plant physiology.

[9]  Xylem Cavitation in Nodes and Internodes of Whole Chorisia insignis H. B. et K. Plants Subjected to Water Stress: Relations Between Xylem Conduit Size and Cavitation , 1986 .

[10]  Martin H. Zimmermann,et al.  The hydraulic architecture of balsam fir (Abies balsamea) , 1984 .

[11]  J. Sperry,et al.  SEASONAL OCCURRENCE OF XYLEM EMBOLISM IN SUGAR MAPLE (ACER SACCHARUM) , 1988 .

[12]  M. Tyree,et al.  The hydraulic architecture of Thuja occidentalis , 1983 .

[13]  M. Zimmermann,et al.  Spring filling of xylem vessels in wild grapevine. , 1987, Plant physiology.

[14]  M. Tyree,et al.  Cavitation Events in Thuja occidentalis L.? : Utrasonic Acoustic Emissions from the Sapwood Can Be Measured. , 1983, Plant physiology.

[15]  M. Tyree,et al.  Water stress induced cavitation and embolism in some woody plants , 1986 .

[16]  G. Byrne,et al.  Cavitation and resistance to water flow in plant roots , 1977 .

[17]  M. Tyree,et al.  Detection of Xylem Cavitation in Corn under Field Conditions. , 1986, Plant physiology.

[18]  Thomas J. Givnish,et al.  On the economy of plant form and function. , 1988 .

[19]  J. Grace,et al.  Water relations and ultrasound emissions of Pinus sylvestris L. before, during and after a period of water stress , 1986 .

[20]  M. Zimmermann.  Xylem Structure and the Ascent of Sap , 1983, Springer Series in Wood Science.

[21]  M. Tyree,et al.  Ultrasonic Acoustic Emissions from the Sapwood of Thuja occidentalis Measured inside a Pressure Bomb. , 1984, Plant physiology.