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First published online February 25, 2005; 10.1104/pp.104.058156 Plant Physiology 137:1139-1146 (2005) © 2005 American Society of Plant Biologists
Water Stress Deforms Tracheids Peripheral to the Leaf Vein of a Tropical Conifer1Department of Plant Science, University of Tasmania, Hobart, Tasmania 7001, Australia (T.J.B.); and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02183 (T.J.B., N.M.H.)
Just as a soggy paper straw is prone to yielding under the applied suction of a thirsty drinker, the xylem tracheids in leaves seem prone to collapse as water potential declines, impeding their function. Here we describe the collapse, under tension, of lignified cells peripheral to the leaf vein of a broad-leaved rainforest conifer, Podocarpus grayi de Laub. Leaves of Podocarpus are characterized by an array of cylindrical tracheids aligned perpendicular to the leaf vein, apparently involved in the distribution of water radially through the mesophyll. During leaf desiccation the majority of these tracheids collapsed from circular to flat over the water potential range –1.5 to –2.8 MPa. An increase in the percentage of tracheids collapsed during imposed water stress was mirrored by declining leaf hydraulic conductivity (Kleaf), implying a direct effect on water transport efficiency. Stomata responded to water stress by closing at –2.0 MPa when 45% of cells were collapsed and Kleaf had declined by 25%. This was still substantially before the initial indications of cavitation-induced loss of hydraulic conductance in the leaf vein, at –3 MPa. Plants droughted until 49% of tracheids had collapsed were found to fully recover tracheid shape and leaf function 1 week after rewatering. A simple mechanical model of tracheid collapse, derived from the theoretical buckling pressure for pipes, accurately predicted the collapse dynamics observed in P. grayi, substantiating estimates of cell wall elasticity and measured leaf water potential. The possible adaptive advantages of collapsible vascular tissue are discussed.
1 This work was supported by the National Science Foundation (grant no. IBN 0212792) and by a grant from the National Geographic Society (grant no. 7475–03). Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.058156. * Corresponding author; e-mail brodribb{at}fas.harvard.edu; fax 61–362–262698. Received December 12, 2004; returned for revision January 10, 2005; accepted January 10, 2005. This article has been cited by other articles:
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