JXB Advance Access originally published online on June 4, 2004
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Journal of Experimental Botany, Vol. 55, No. 402, pp. 1569-1575, July 2004
Journal of Experimental Botany, Vol. 55, No. 402, © Society for Experimental Biology 2004; all rights reserved
RESEARCH PAPER |
Changes in pit membrane porosity due to deflection and stretching: the role of vestured pits
1Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, MA 02138, USA
2Laboratory of Plant Systematics, K.U. Leuven, Institute of Botany and Microbiology, Kasteelpark Arenberg 31, B-3001 Leuven, Belgium
* To whom correspondence should be addressed. Fax: +1 617 496 5854. E-mail: bchoat{at}fas.harvard.edu
The effect of increasing pressure difference (
P) on intervessel pit membrane porosity was studied in two angiosperm tree species with differing pit architecture. Fraxinus americana L. possesses typical angiosperm bordered pit structure while Sophora japonica L. exhibits well-developed vestures in intervessel pit chambers. It was hypothesized (a) that large P across intervessel pits would cause the deflection of pit membranes in the stems of F. americana resulting in significant increases in porosity and thus lower cavitation thresholds, and (b) that the presence of vestures would prevent the deflection of pit membranes in S. japonica. To determine if the porosity of pit membranes increased under mechanical stress, suspensions of colloidal gold, 5 nm and 20 nm in diameter, were perfused across intervessel pit membranes at P ranging from 0.25 MPa to 6.0 MPa. The effect of increasing P on membrane porosity was also tested by comparing air seeding thresholds (Pa) in stems perfused with water or a solution with lower surface tension. Air seeding and colloidal gold experiments indicated that pit membrane porosity increased significantly with P in F. americana. In S. japonica, increases in permeability to colloidal gold with P were small and maximum pore diameters predicted from Pa were independent of P, suggesting that vestures limited the degree to which the membrane can be deflected from the centre of the pit cavity. This provides the first experimental evidence that vestures reduce the probability of air seeding through pit membranes.
Key words: Air seeding, cavitation, deflection, pit membranes, porosity, vestures, xylem
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