PARENT SESSION
Oral Session #74: Plant Water Relations: Hydraulics and use.
Presiding: B. Engelbrecht
Thursday, August 8. 9:00 AM to 11:30 AM. Greenlee Meeting Room, TCC.
Pit membrane porosity and vulnerability to embolism in four seasonally dry rainforest tree species .
Choat, Brendan*,1,2, Ball, Marilyn2, Luly, Jon1, Holtum, Joe1, 1 James Cook University, Townsville, QLD, Australia2 Research School of Biological Sciences, Canberra, ACT, Australia
ABSTRACT- Hydraulic architecture and vulnerability to embolism were examined in stems of two deciduous species, Brachychiton australis and Cochlospermum gillivrai, and two evergreen species, Alphitonia excelsa and Austromyrtus bidwillii, growing in a seasonally dry rainforest. The deciduous species were more vulnerable to water-stress induced xylem embolism. Brachychiton australis and C. gillivraei reached a 50% loss of hydraulic conductivity (kh) at - 3.17 MPa and - 1.44 MPa, respectively; a 50% loss of kh occurred at - 5.56 MPa in A. excelsa and - 5.12 MPa in A. bidwillii. To determine whether pit membrane porosity was responsible for greater vulnerability to embolism ('air seeding' hypothesis), the structure of xylem pit membranes was examined using SEM. The expected diameter of pit membrane pores was calculated from vulnerability curves using the capillary equation. SEM failed to reveal pores in the size range predicted for any of the species; pores could not be detected to a resolution of 20 nm. To test this result, solutions of different size particles were perfused through stem sections longer than the longest vessel. Particle sizes down to 20 nm gold colloids were blocked by the pit membranes in all four species. However, 5 nm particles of colloidal gold penetrated membranes of all species. The range of average pore sizes observed in the four species does not explain the variation in vulnerability to embolism via air seeding through pit membranes.
KEY WORDS: Cavitation, Pit Membrane Porosity, Air seeding