Water transport in vesselless angiosperms: conducting efficiency and cavitation safety

Two structure-function hypotheses were tested for vesselless angiosperm wood. First, vesselless angiosperm wood should have much higher flow resistance than conifer wood because angiosperm tracheids lack low-resistance torus-margo pits. Second, vesselless wood ought to be exceptionally safe from cavitation if the small cumulative area of pits between tracheids confers safety (the pit area hypothesis). Data were obtained from branch wood of 19 vesselless angiosperms: Amborella trichopoda, Trochodendron aralioides, Tetracentron sinense, and 16 Winteraceae from Tasmannia, Zygogynum, Bubbia, Pseudowintera, and Drimys. Contrary to the first hypothesis, vesselless and conifer species with narrow tracheids (below ca. 18 mu m) had similar area-specific resistivities. The reason was that vesselless angiosperms had an intertracheid pit resistance (mean 16 +/- 2 MPa s m(-1)) that was nearly as low as that of conifers (6 +/- 1 MPa s m(-1)) and much lower than that of eudicot intervessel pits ( 336 +/- 81 MPa s m(-1)). Low pit resistance was associated with greater pit membrane porosity inferred from scanning electron microscopy observations and silicone penetration and may represent incipient pit membrane loss. Pit resistance was often greater in wider angiosperm tracheids and obscured any drop in wood resistivity with tracheid width. In support of the second hypothesis, vesselless woods averaged a cavitation pressure of -3.4 +/- 0.3 MPa, which is low for their wet habitats. In agreement with the pit area hypothesis, resistance to cavitation increased with decreasing total pit area between conduits. However, vesselless angiosperms were more vulnerable for a given pit area than eudicots, consistent with their more permeable pit membranes. Small total pit area between conduits may allow angiosperm tracheids to have more porous membranes for conducting efficiency without creating a cavitation problem.