The inter-relations among xylem anatomy, hydraulic conductivity and leaf water status in five sub-tropical tree species.

Abstract

The wood anatomy, hydraulic properties and leaf water status of five sub-tropical plant species were studied. The specimens studied were growing in a private, irrigated garden. Consequently, any differences in xylem anatomy would be a result of phylogenetic and not environmental factors. Podocarpus latifolius, being a gymnosperm, had only narrow, short tracheids as the conducting conduits. The size of the vessels of the four angiosperms increased in the following order: Tecomaria capensis, followed by Cinnamomum camphora, Trichilia dregeana and finally Barringtonia racemosa had the widest vessels. T. capensis and T. dregeana had the shortest vessel lengths. Those of C. camphora were slightly longer and the vessels of B. racemosa were the longest. Measurements of hydraulic conductance (Kh), twig specific conductivity (TSC) and xylem specific conductivity (XSC) followed similar trends to those of conduit diameters. The measurements of leaf specific conductivity (LSC) , however, did not follow these trends. T. dregeana, which had a far higher Kh than P. latifolius, did not have a significantly different LSC. This is because the twigs of T. dregeana supported a far greater leaf area than did the twigs of P. latifolius. There was also no significant difference in LSC among T. capensis, C. camphora and B. racemosa, although their LSC's were all significantly higher than those of P. latifolius and T. dregeana. The consequence of efficient xylem anatomy thus seems to be, not only a greater supply of water to the leaf but also, and perhaps more importantly, it allows a greater leaf area to be produced. The length of the vessels was also shown to have a large effect on the hydraulic conductivity of the twigs. The Kh values measured on excised twigs were found to range between 40% and 87% of the Kh calculated using the Hagen- Poiseuille equation. Conduit size distributions were also found to be important in calculating the Kh. The more efficent xylem anatomy of B. racemosa resulted in little decrease in plant water potential even with large increases in transpiration. P. latifolius on the other hand showed a considerable decrease in leaf water potential with just a slight increase in transpiration. The other three species showed decreases in leaf water potential inter-mediate to these two extremes. The inferred root-to-leaf condutivity, shown by the inverse of the slopes of the water potential versus transpiration curves, were lower than the LSC measurements taken on excised twigs in the laboratory. The difference between the inferred and the measured LSC's could give an indication of resistances such as those within the root and at the soil-root interface.