Coordination between leaf and stem hydraulic characteristics in Machilus species

We measured several physiological traits of seven Machilus species to determine the relationship between leaf water supply and demand as well as the relationship between xylem pit characteristics and hydraulic efficiency. Results showed that the seven Machilus species had much higher vein density (VD) than primitive angiosperms, within a moderately high range (9.8-14.1 mm/mm²), and a significant positive relationship between stomatal density (SD) and VD, indicating coevolution of leaf water supply and demand. Furthermore, SD was negatively correlated with guard cell length (GCL) and leaves with larger stomata had higher turgor loss points (TLP). Stem sapwood specific hydraulic conductivity (K_s) was relatively low (0.13-1.87 kg·m^-1·s^-1·MPa^-1), with rather large interspecific variation. Both VD and SD were positively correlated with K_s, reflecting the water relationship between plant leaves and stems. No significant relationship was found between K_s and pit membrane area (PMA), pit aperture area (OPPA), or pit aperture length to width (APF) ratio. Thus, although the Machilus species evolved relatively higher VD and there was coordination between xylem water supply and leaf structure, their xylem anatomical structure was still primitive, with low hydraulic efficiency, and the vessels had scalariform perforations, indicating they can only adapt to relatively humid habitats.