Variation of phenolic compounds in Betula pendula leaves exposed to ozone

Abstract

1. Purpose
Tropospheric ozone is presently causing forest decline and the predictions indicate 1-2% increases for ozone concentrations per year. Ozone have an effect on metabolism and secondary chemistry of the trees (Waterman and Mole, 1989). Betula pendula is the most important commercial deciduous forest tree in Finland. Out purpose of this study is to examine the impact of ozone to growth and secondary chemistry in young birch leaves in response to ozone.
2. Methods
We micropropagated eight random genotypes from naturally regenerated birch forest and placed one-year-old clones in randomized block design into open-field ozone exposure and ambient-ozone control blocks in spring of 1999. The target level for elevated ozone was 1.5 times the ambient. On August 29-30, 2000, we measured all the seedlings for the growth and injuries of leaves. Leaves were extracted with methanol and 27 phenolic compounds were analyzed by HPLC-DAD and HPLC-MS. Condensed tannins were analyzed by the acid-butanol test.
3. Results
Ozone treatment is earlier found to alter the shoot/root (s/r) ratio in birch (Oksanen and Rousi, 2001). Our results indicate that there is large intraspecific variation in this response: s/r ratio increased in some genotypes (clone 4, 17 and 26) (=s/r+), in some it decreased (clone 7, 19 and 20) (=s/r-) and in some it was unaffected (clone 6 and 30). In the s/r+ clones, 10 phenolic compounds (quercetin-rhamnoside, chlorogenic acid etc.) increased in ozone-exposed leaves significantly compared to the control leaves (P<0.05). S/r- clones and unaffected clones did not show significant increase or decrease of phenolics in ozone-exposed leaves.
Significant positive correlations were found between leaf injuries (the number of necrosis spots) and 18 out of 28 different phenolics. S/r ratio-decreased and -unaffected clones showed positive correlations between leaf injuries and chlorogenic acid derivatives (P<0.001), while s/r ratio-increased clones showed positive correlations between leaf injuries and some myricetin-glycosides (P<0.001).
4. Discussion
Ozone can induce membrane-damaging lipid peroxidation and initiate chain reactions generating free radicals and injuries in plant leaves. In our study, s/r ratio-increased birch clones tended to protect leaves from ozone damage by producing higher amounts of antioxidants, such as chlorogenic acid derivatives and flavonoid-glycosides. S/r ratio-decreased and -unaffected clones, on the other hands, seemed to preserve their growth and might not produce extra phenolic compounds. Thus, our results indicate that there are different chemical defensive systems within birch populations in response to ozone enhancement.