First published online April 2, 2004; 10.1104/pp.103.031203
Plant Physiology 134:1824-1833 (2004)
© 2004 American Society of Plant Biologists
WHOLE PLANT AND ECOPHYSIOLOGY
Hydraulic Analysis of Water Flow through Leaves of Sugar Maple and Red Oak1
Lawren Sack*,
Christopher M. Streeter and
N. Michele Holbrook
Biological Laboratories, Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138 (L.S., N.M.H.); Harvard Forest, Petersham, Massachusetts 01366 (L.S., C.M.S.); and The Arnold Arboretum of Harvard University, Jamaica Plain, Massachusetts 02130 (L.S.)
Leaves constitute a substantial fraction of the total resistance to water flow through plants. A key question is how hydraulic resistance within the leaf is distributed among petiole, major veins, minor veins, and the pathways downstream of the veins. We partitioned the leaf hydraulic resistance (Rleaf) for sugar maple (Acer saccharum) and red oak (Quercus rubra) by measuring the resistance to water flow through leaves before and after cutting specific vein orders. Simulations using an electronic circuit analog with resistors arranged in a hierarchical reticulate network justified the partitioning of total Rleaf into component additive resistances. On average 64% and 74% of the Rleaf was situated within the leaf xylem for sugar maple and red oak, respectively. Substantial resistance—32% and 49%— was in the minor venation, 18% and 21% in the major venation, and 14% and 4% in the petiole. The large number of parallel paths (i.e. a large transfer surface) for water leaving the minor veins through the bundle sheath and out of the leaf resulted in the pathways outside the venation comprising only 36% and 26% of Rleaf. Changing leaf temperature during measurement of Rleaf for intact leaves resulted in a temperature response beyond that expected from changes in viscosity. The extra response was not found for leaves with veins cut, indicating that water crosses cell membranes after it leaves the xylem. The large proportion of resistance in the venation can explain why stomata respond to leaf xylem damage and cavitation. The hydraulic importance of the leaf vein system suggests that the diversity of vein system architectures observed in angiosperms may reflect variation in whole-leaf hydraulic capacity.
1 This work was supported by the Arnold Arboretum of Harvard University (Putnam Fellowship to L.S.), the Andrew W. Mellon Foundation, and the National Science Foundation (grant no. 0139495).
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.031203.
* Corresponding author; e-mail lsack{at}hawaii.edu, fax 808-956-3923.
Received July 31, 2003;
returned for revision December 21, 2003;
accepted January 5, 2004.
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