Document: HOW-3-32-21

Hydraulic properties of Galax urceolata, an evergreen, understory herb of the southern Appalachians.

SOBIERAJ II, J.H.* ¹, J.K.MCCARRON III ² and H.S.NEUFELD ¹

Appalachian State University, Boone, NC 28608 USA ¹
Kansas State University, Manhattan, KS 66502 USA ²

Abstract:
The ability of a plant species to persist and survive in a forest understory depends in large part on its shade tolerance. However, for shallowly rooted herbs, drought tolerance may also be a factor, as upper soil layers tend to dry out rapidly between rain events. The objective of this research was to learn more about the water relations of Galax urceolata (formerly G. aphylla), a shallowly rooted, evergreen, understory herb, and whether hydraulic properties changed with leaf ageing. Leaves on this plant usually persist for two complete growing seasons, but only briefly into the their third year, so that most of the time, plants have just two age classes of leaves. Using a Sperry apparatus, we measured maximum hydraulic conductivities (mg mm s^-1 KPa^-1) for detached first and second year leaves. No significant differences were found (0.379 + 0.044 vs 0.406 + 0.027 for first and second year leaves, respectively, N=18). Next, we measured conductivities of each leaf age class, but this time included the rhizome in the water path, in order to see if the rhizome was a significant resistance to water flow. We found no differences in conductivity for the two age classes, but inclusion of the rhizome in the pathway greatly reduced conductivities (0.064 + 0.005 vs 0.077 + 0.010, for first and second year leaves, respectively, N=11). We then investigated whether ageing might change the susceptibility of the petioles to cavitation. Using two different methods for introducing embolisms into the xylem, centrifugation and air injection (both yielded similar, but not identical results), we found no difference in susceptibility to cavitation between leaf age classes. Cavitation did not begin until leaf water potentials reached approximately -2.5 MPa, and conductances were still 57% + 3% of maximum at -4.0 MPa. Iin the field, water potentials in Galax rarely decrease below -1.6 MPa, thus this species would almost never suffer embolisms under typical growing season field conditions. Even under the most severe drought conditions, water flow could still be maintained to the leaves. The ability to resist cavitation in this species rivals that for plants more typical of desert habitats.

Keywords: Galax urceolata, hydraulic conductivity, water relations, evergreen herbs

This abstract is being presented at: 3:30 PM in session:
WATER RELATIONS