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 vesselbearing 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 colimiting 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. Key-words : hydraulic architecture; vessel length; water transport; xylem structure and function; xylem flow