Conifers have two basic defensive systems called preformed (or constitutive) and induced (or dynamic) defenses:

A. Constitutive defenses are laid down before the tree is attacked and can consist of

1. tough epidermal layers on needles and stems;
2. thorns, spines and hairs (trichomes);
3. cells containing toxic or inhibitory chemicals (Slide); and
4. multicellular structures (ducts, blisters, and such) that contain toxic or inhibitory chemicals (Slide).

B. Induced defenses occur in response to attack and consist of two main types:

1. Hypersensitive reaction (HR), in which the tissues around the site of attack die and become saturated with toxic and/or inhibitory compounds such as terpenes and phenolics (Slide). Although HR may occur in response to mechanical wounding, is much more intense if the wound is infected by disease-causing microorganisms. In fact the main function of HR seems to be to arrest and contain the growth of microbes (Slide).
2. Acquired resistance (AR) where the tree builds up its defenses following attack and so becomes more resistant to subsequent attack. Sometimes acquired resistance can occur in tissues far from the point of initial attack, in which case it is called systemic acquired resistance (SAR). SAR has been observed in cucumbers, tomatoes and some other angiosperms attacked by insects. Recently, acquired resistance has also been demonstrated in conifers.

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© 1998 Alan A. Berryman