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The major veins of mesomorphic leaves revisited: tests for conductive overload in Acer saccharum (Aceraceae) and Quercus rubra (Fagaceae)¹

²Department of Organismic and Evolutionary Biology, Harvard University, Biological Laboratories, 16 Divinity Avenue, Cambridge, Massachusetts 02138 USA; ³Harvard Forest, Harvard University, P.O. Box 68, 324 N. Main Street, Petersham, Massachusetts 01366 USA

Many leaves survive the severing of their major veins in apparently excellent health. According to the classical explanation, the leaf minor veins provide "conductive overload," an excess of parallel conductive paths, rendering the major veins hydraulically dispensable. Whether such an excess of conductive paths exists has important implications for vascular design and for leaf response to vascular damage. We subjected leaves of Acer saccharum and Quercus rubra to cutting treatments that disrupted the major vein system and determined leaf survival, stomatal conductance (g), quantum yield of photosystem II (_PSII), and leaf hydraulic conductance (K_leaf). For A. saccharum, the cuts led to the death of distal lamina. For Q. rubra, however, the treated leaves typically remained apparently healthy. Despite their appearance, the treated Q. rubra leaves had a strongly reduced K_leaf, relative to control leaves, and g and _PSII were reduced distal to the cuts, respectively, by 75–97% and 48–76%. Gas exchange proximal to the cuts was unaffected, indicating the independence of lamina regions and their local stomata. Analogous results were obtained with excised Q. rubra leaves. These studies demonstrate an indispensable, vital role of the major veins in conducting water throughout the lamina.

Key Words: biological networks • drought response • herbivory • hydraulic architecture • leaf hydraulic conductance • stomatal conductance • temperate forest • vascular architecture

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