Comparative analysis of end wall resistivity in xylem conduits

The hydraulic resistivity (R, pressure gradient/flow rate) through end walls of xylem conduits was estimated in seven species of diverse anatomy and affinity including a vessel-bearing fern, a tracheid-bearing gymnosperm, and angiosperms with versus without vessels. Conduit lengths were measured with a silicone injection method which was easier and more accurate than the usual paint injection. The R declined linearly with the removal of end walls as stems were shortened from 10 to 0.3 cm. This relationship gave the minimum R with no end walls present, or the lumen resistivity (R-L). This was indistinguishable from the Hagen-Poiseuille value. The maximum R with all end walls present gave R-C, the resistivity of end wall and lumen in series. Average end-wall resistivity (R-W) was the difference R-C - R-L and the 'wall fraction' was R-W/R-C. Wall fraction was approximately constant, averaging 0.54 +/- 0.07. This suggests that end wall and lumen resistivities are nearly co-limiting in vascular plants. Average conduit length was proportional to the diameter squared across species (r(2) = 0.94). Together with a constant wall fraction, this was consistent with the end wall resistance (r(w), pressure difference/flow rate) being inversely proportional to conduit length. Lower r(w) in longer conduits is consistent with their having more end wall pits than shorter conduits.