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Antimicrobial defence in the sapwood of Eucalyptus nitens : studies of the reaction zone

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Barry, Karen May (2001) Antimicrobial defence in the sapwood of Eucalyptus nitens : studies of the reaction zone. PhD thesis, University of Tasmania.

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Abstract

Decay arising from pruning wounds in plantation-grown Eucalyptus spp. is a concern
for the Australian solid wood products industry, as the potential for decay spread within
the tree is unknown at present. Mechanisms of antimicrobial defence in the xylem of
Eucalyptus nitens were investigated. Key factors investigated within the host-fungal
interaction included phenolics, moisture content, mineral content and anatomical
alterations (tyloses and suberization).
A purple reaction zone was regularly observed at the sapwood interface of decay lesions
developing from naturally-infected pruning wounds in plantation-grown
E. nitens. Total phenols levels were increased substantially and vessel tyloses were
abundant in the reaction zone. A log incubation experiment showed that after 9 months
the reaction zone remained intact while sapwood became decayed. The reaction zone
was significantly drier than healthy sapwood, with lower levels of potassium, and a
lower pH than both sapwood and heartwood. These trends are more similar to conifer
reaction zones than deciduous angiosperms. The mode of reaction zone defence appears
to be a dynamic process (also similar to conifers) rather than a static process typical of
deciduous angiosperms.
The initial stages of antimicrobial defence formation were also studied. Xylem defence
responses occurring in young pot-grown E. nitens saplings were analysed within the
first few weeks following stem wounding and inoculation with the decay fungus
Ganoderma adspersum (which displayed relatively slow colonization of the wounded
xylem). Events occurring most quickly after wounding and inoculation were tylose
formation and phenol accumulation, which indicates that they are key defences. NMR
imaging revealed that changes in xylem moisture content were not rapid after
wounding.
Reaction zone extracts were inhibitory to test-fungi in thin-layer chromatography
bioassays. Analysis of extracts by liquid chromatography — electrospray ionization mass
spectrometry revealed that a diverse range of hydrolyzable tannins were present in E.
nitens wood, including over thirty gallotannins, ellagitannins and phenols. A number of
ellagitannins (particularly pedunculagin) were considerably more abundant in the reaction zone than the healthy sapwood and may contribute to the effectiveness of the
reaction zone as an antimicrobial barrier. The process of reaction zone formation may
be an accentuation of processes that occur when heartwood forms. Heartwood extracts
were qualitatively similar to reaction zone extracts but actual increases in ellagitannins
were not as great.
Conditions such as season of wounding and the type of fungi which become established
in wounds, appear to effect the degree of defence response. This was indicated by
phenol levels. More extensive decay lesions were generally associated with more
phenolic production. Sterile inoculations and weakly-aggressive fungi were associated
with no or little reaction zone formation, while aggressive fungi elicited more
discolouration and phenolic accumulation in advance of infection. This suggests that
non-aggressive fungi are less tolerant of the inherent xylem microenvironment and
active defence responses are not "required" or elicited.
Xylem defence responses appear to be relatively successful in E. nitens. This may be
facilitated (at least in part) by a fast and considerable accumulation of phenolic
compounds. Qualitative studies of phenolic compounds may be a useful indicator of
defensive capacity.

Item Type: Thesis (PhD)
Keywords: Eucalyptus
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Chapter 2 is the equivalent of the peer reviewed version of the following article: Barry, K.M., Pearce, R.B. and Mohammed, C.L. 2000. Properties of reaction zones associated with decay from pruning wounds in plantation-grown Eucalyptus nitens. Forest Pathology. 30: 233-245 which has been published in final form at http://dx.doi.org/10.1046/j.1439-0329.2000.00206.x This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Chapter 3 is the equivalent of the peer reviewed version of the following article: Barry, K.M., Pearce, R.B., Evans, S.D., Hall, L.D. and Mohammed, C.L. Initial defence responses in sapwood of Eucalyptus nitens (Maiden) following wounding and inoculation. Physiological and Molecular Plant Pathology 58(2) 63-72 which has been published in final form at http://dx.doi.org/10.1006/pmpp.2000.0314

Chapter 4 is the equivalent of the peer reviewed version of the following article: Barry, K.M., Davies, N.W. and Mohammed, C.L. Identification of hydrolysable tannins in the reaction zone of Eucalyptus nitens wood by high performance liquid chromatography–electrospray ionisation mass spectrometry. Phytochemical Analysis. 12(2): 120-127 which has been published in final form at http://dx.doi.org/10.1002/pca.548 This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Chapter 6 is the equivalent of the peer reviewed version of the following article: Barry, K.M., Davies, N.W. and Mohammed, C.L. Effect of season and different fungi on phenolics in response to xylem wounding and inoculation in Eucalyptus nitens. Forest Pathology 32(3) 163-178 which has been published in final form at http://dx.doi.org/10.1046/j.1439-0329.2002.00281.x This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Date Deposited: 25 Nov 2014 00:50
Last Modified: 11 Mar 2016 05:54
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