Ozone affects birch (Betula pendula Roth) phenylpropanoid, polyamine and active oxygen detoxifying pathways at biochemical and gene expression level

We have studied ozone-induced reactions at biochemical and mRNA level in two birch clones that differ in their ozone-sensitivity. When exposed to a single 8 hour ozone pulse (150 ppb), first visible injuries appeared in 24 hours in the sensitive clone and lead eventually to partial tissue chlorosis and necrosis, while the insensitive clone was unaffected. Cell plasma membrane damage was measured by vital stain Evan's Blue permeability. After ozone-exposure, the relative number of vital stain permeable cells increased equally in both clones reaching maximum at 24 hours and decreasing thereafter The damaged cells were randomly distributed, but in some leaves of the sensitive clone cell death spread forming necrotic lesions. The total cellular activities of superoxide dismutase, peroxidase and glutathione reductase increased following the change in the relative Evan's Blue permeability. The enzyme activity increase was considerably higher in the sensitive clone suggesting that it is somehow related to the cell damage. PAL, that controls the phenylpropanoid biosynthesis, is a good indicator of the coordinated plant defense responses. Gene encoding PAL was induced rapidly but transiently in both clones during the ozone exposure. This indicates that defense responses were induced in both clones, when only the sensitive clone showed widespread cell death. This suggests that the ozone-induced defense reactions and cell death in these birch clones are two separately controlled processes. Increased putrescine levels have often been suggested to be involved in plant ozone tolerance. On the contrary to the induction of putrescine accumulation usually detected in ozone tolerant plants, the accumulation of free putrescine occurred in the ozone sensitive birch done.