Hydraulic design criteria for compound leaves

Differences in the distribution of a given amount of vascular tissue volume can greatly affect the hydraulic conductance of the resulting network. In plants, two main factors that determine this distribution are the number of xylem conduits at subsequent levels, and conduit diameter relative to subsequent levels (conduit taper). Murray’s law (ML), which xylem seems to follow when the conduits are not providing structural support to the plant, predicts the volume distribution that maximizes the conductance/volume for a given change in the conduit number. The Inverse Murray’s law (IML) maximizes the conductance/volume at a given conduit taper. We tested whether xylem in compound leaves more closely followed ML or IML in six tropical species. Leaves from four species complied with ML, while two species were consistent with IML. Dalbergia retusa deviated from both laws and had a decrease in the vessel cross-sectional area in the petiolules versus petioles. A similar pattern has been observed in stem wood, suggesting that vessels in Dalbergia leaves may structurally support the leaf. Schefflera morototoni leaves deviated from ML, but complied with IML, suggesting that this species may more easily adjust the number of vessels in each rank than their diameters. For the three species that complied with ML, but not IML, the vessel diameters may be more easily altered in new tissue than the number. This small survey indicated that plants vary in their ability to adjust vessel taper and number, but that they generally achieve either the IML or ML optimum.