Investigations on spruce decline in the Bavarian forest

The primary damaging reactions in spruce needles may operate as follows: 1) Trees under "stress" produce the plant hormone ethylene. 2) Ethylene and ozone react extremely fast forming hydrogen peroxide and formaldehyde, compounds which may damage the wax layer. 3) Ozone as a very aggressive oxidant will inactivate membrane bound enzymes through oxidation of their thiol groups. Thus the translocation of sugars from the chloroplast into the phloem may be inhibited or blocked. The result will be an "over-reduction" of the electron transport chain resulting in the formation of reactive oxygen species in the light. These reactive oxygen species will induce lipid peroxidation and pigment co-oxidation. 4) The visible effects are bleached needles and an impairment of structural resistance against fungal infections. 5) In addition ozone will directly reduce the content of antifungal compounds such as p-HAP. 6) Furthermore p-HAP may be involved in the bleaching reaction after its release from picein. 7) Finally, fungi may penetrate the needles and eventually grow faster in bleached needles. Infected needles will become necrotic and abscise.