Hydraulic conductance and tracheid anatomy in six species of extant seed plants

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Abstract
Hydraulic conductance of tracheids was studied in either petioles or young stems of six species of seed plants having various types of intertracheid pitting. Measured conductances were compared with estimates based on Hagen–Poiseuille flow through ideal capillaries and with predictions from a biophysical model incorporating observed anatomical characteristics of tracheids and intertracheid pits. Conductance of the xylem, expressed as a percentage of the ideal capillary flow prediction, varied from an average of 88% for a species containing only very narrow tracheids to less than 35% for species with large-diameter tracheids. The biophysical model allowed fairly close predictions of conductance for all species except one, where an estimate of the pit membrane resistance could not be experimentally obtained. For individual tracheids, conductance was largely a function of lumen diameter, pit membrane resistivity, and the exposed area of the pit membranes, as determined by pit shape, size, and frequency. For wide tracheids, scalariform-pitted elements showed a linear increase in conductance with an increase in lumen diameter; however, for tracheids with large circular pits, the conductance increase afforded by a wider lumen declines as lumen diameter increases. These model simulations demonstrate the increasing significance of intertracheid pitting in obstructing flow as lumen diameter increases.