Single-vessel flow measurements indicate scalariform

During vessel evolution in angiosperms, scalariform perforation plates with many slit-like openings transformed into simple plates with a single circular opening. The transition is hypothesized to have resulted from selection for decreased hydraulic resistance. Previously, additional resistivity of scalariform plates was estimated to be small – generally 10% or less above lumen resistivity – based on numerical and physical models. Here, using the singlevessel technique, we directly measured the hydraulic resistance of individual xylem vessels. The resistivity of simple-plated lumens was not significantly different from the Hagen–Poiseuille (HP) prediction (+6 3.3% mean deviation). In the 13 scalariform-plated species measured, plate resistivity averaged 99 13.7% higher than HP lumen resistivity. Scalariform species also showed higher resistivity than simple species at the whole vessel (+340%) and sapwood (+580%) levels. The strongest predictor of scalariform plate resistance was vessel diameter (r2 = 0.84), followed by plate angle (r2 = 0.60). An equation based on laminar flow through periodic slits predicted single-vessel measurements reasonably well (r2 = 0.79) and indicated that Baileyan trends in scalariform plate evolution maintain an approximate balance between lumen and plate resistances. In summary, we found scalariform plates of diverse morphology essentially double lumen flow resistance, impeding xylem flow much more than previously estimated. Key-words: ecological wood anatomy; hydraulic conductivity; plant water transport; vessel evolution; xylem flow resistance; xylem transport. Abbreviations: A, cross-sectional lumen area; a, major axis of ellipse; b, minor axis of ellipse; D, vessel diameter; h, width of slit plus bar thickness; k, width of slit/h; L, length of scalariform slit opening; Le, spacing between perforation plates; Lv, length of the vessel (=stem segment measured); p, perimeter of vessel lumen; P, pressure head; rL, resistance of the lumen; RL, resistivity of the lumen; rp, resistance of the plate; Rp, resistivity of the plate; rtotal, resistance of vessel and capillary tube; rcapillary, resistance of capillary tube; rvessel, resistance of vessel; Rv, resistivity of the vessel; t, time; V, volume; m, viscosity of water.