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AOBPreview originally published online on February 1, 2007
Annals of Botany 2007 99(4):609-623; doi:10.1093/aob/mcm003
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© The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

An Anatomical Assessment of Branch Abscission and Branch-base Hydraulic Architecture in the Endangered Wollemia nobilis

G. E. Burrows1,*, P. F. Meagher2 and R. D. Heady3

1 Institute for Land, Water and Society, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia
2 Mount Annan Botanic Garden, Mount Annan Drive, Mount Annan, NSW 2567, Australia
3 School of Resources, Environment and Society, The Australian National University, Canberra, ACT 0200, Australia

* For correspondence. E-mail gburrows{at}csu.edu.au

Received: 2 October 2006    Returned for revision: 5 December 2006    Accepted: 8 December 2006    Published electronically: 1 February 2007

Background and Aims: The branch-base xylem structure of the endangered Wollemia nobilis was anatomically investigated. Wollemia nobilis is probably the only extant tree species that produces only first-order branches and where all branches are cleanly abscised. An investigation was carried out to see if these unusual features might influence branch-base xylem structure and water supply to the foliage.

Methods: The xylem was sectioned at various distances along the branch bases of 6-year-old saplings. Huber values and relative theoretical hydraulic conductivities were calculated for various regions of the branch base.

Key Results: The most proximal branch base featured a pronounced xylem constriction. The constriction had only 14–31 % (average 21 %) of the cross-sectional area and 20–42 % (average 28 %) of the theoretical hydraulic conductivity of the more distal branch xylem. Wollemia nobilis had extremely low Huber values for a conifer.

Conclusions: The branch-base xylem constriction would appear to facilitate branch abscission, while the associated Huber values show that W. nobilis supplies a relatively large leaf area through a relatively small diameter ‘pipe’. It is tempting to suggest that the pronounced decline of W. nobilis in the Tertiary is related to its unusual branch-base structure but physiological studies of whole plant conductance are still needed.

Key words: Hydraulic architecture, xylem, tracheids, compression wood, branch abscission, cladoptosis, Huber value, Wollemi pine, Wollemia nobilis, Agathis, Araucaria, Araucariaceae


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