Ozone effects on root-disease susceptibility and defence responses in mycorrhizal and non-mycorrhizal seedlings of Scots pine (Pinus sylvestris L.).

Interactions between ozone and biotic stress caused by a pathogen, Heterobasidion annosum (Fr.) Bref., in mycorrhizal [Hegeloma crustuliniforme (Bull, ex St. Am.) Quel.] and non-mycorrhizal Scots pine seedlings were investigated using a semi-axenic model system. Ozone exposure (200 nl 1 , 8 h d for 28 d) increased disease incidence significantly, but mycorrhizal infection completely prevented this negative effect. The presence of the pathogen on the root systems was necessary for the induction of changes in the soluble and wall-bound secondary compounds of roots and needles; ozone alone did not induce such changes. Mycorrhizal infection appeared to have a dampening effect on the induction of these compounds. H. annosum induced a significant accumulation of the two pine stilbenes both locally and systemically in the more susceptible seedlings. In these seedlings ozone had a significant positive effect on the accumulation of both stilbenes in the roots, but it reduced pinosylvin and had no effect on pinosylvin 3-methyl ether in the needles. The catechin content of the roots decreased in the same infected seedlings, but to a larger degree upon ozone treatment. One compound of as yet unknown structure accumulated gradually in the infected roots over the experimental period, and could thus be associated with resistance. Its accumulation was little affected by ozone treatment. Among the root cell wall-bound phenolics analyzed, only lignin-like material showed significant changes. The presence of the pathogen was again necessary for induction, but ozone had an inhibitory effect on this response. Pure pinosylvin applied through the hypocotyls of excised seedlings was shown to be phytotoxic, with the needles displaying discoloration and wilting as observed after pathogenic inoculation, and being characterized by a lower chlorophyll content and increased transpiration. Accumulation of pinosylvin in the needles was detected at amounts comparable to those found in the main experiment.