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Under these conditions, according to the above hypothesis, Ps, Psme and Pc would probably be involved in tree
resistance, which could be related to the tree's ability to rapidly synthesize these compounds. After an early increase, their
concentration remains constant until the fungus was stopped, which could indicate that they were metabolized by the fungus
because of their toxic effect, as suggested by Lyr (1962). Their increase after day 30 could thus be due to the absence of
fungus activity. This increase also suggests that they are involved in wound healing processes, as could be taxifolin. In Sitka
Spruce as well as in hardwood trees, accumulation of phenolic compounds takes place parallel to suberin biosynthesis, a
component of the wound periderm (Biggs 1985, Woodward and Pearce 1988).
CONCLUSIONS
Tree response could be more a generalized wound response than a pure defensive response. Both stilbenes and
flavonoids could be involved in tree resistance. Taxifolin glycoside is a potential marker of tree resistance, all the more
interesting as it belongs to constitutive phloem, but contrary to pinocembrin and stilbenes, its high concentration would
indicate a low level of resistance. Whole phenolic composition of unwounded phloem could also be a marker of resistance.
However, several conditions need to be verified before definitive conclusions are possible about their use as markers. Indeed,
between clone differences could depend on tree physiological status or age, environmental conditions, locality or season. In
addition, if genetic markers are looked for, the corresponding compounds must be genetically dependent and the corresponding
genes must always be expressed.
SUMMARY
Scots pine phloem was investigated for phenolic markers of tree resistance to bark beetles. Our approach was based
on the study of relations between the reaction efficiency and phenolic composition in six different clones, and on the study of
the variations in phenolic composition during tree reaction development. Considerable modifications took place after
aggression. Compounds lacking in unwounded phloem appeared in the reaction zone. During the development of the
reaction zone, modifications of the phenolic composition clearly proceeded in two successive phases. Stilbenes and flavonoids
seemed to be involved in the induced resistance. Taxifolin glycoside in unwounded phloem looked interesting as
marker, as did the whole phenol composition of this tissue.
ACKNOWLEDGMENTS
The authors thank J. Garcia and P. Romary (INRA, Orleans, France) for their technical help and Dr. D. Sauvard
(INRA, Orldans) for help with the statistics. They are grateful to Prof. H. Sandermann and Dr. W. Heller (G.S.F. Neuherberg,
Deutschland) and to Dr. A. Scalbert (INRA Grignon, France) for providing pure compounds. They also thank Drs B.
Monties, A. Scalbert and M.T. Tollier (INRA Grignon, France)/'or their help in characterization. These studies were granted
by the Conseil R6gional de la Rdgion Centre (France) and by the French Ministry for Research and Technology.
LITERATURE CITED
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