Ultraviolet-B acclimation is supported by functionally heterogeneous phenolic peroxidases

Tobacco plants were grown in plant chambers for four weeks, then exposed to one of the following treatments for 4 days: (1) daily supplementary UV-B radiation corresponding to 6.9 kJ m^-2 d^-1 biologically effective dose (UV-B), (2) daily irrigation with 0.1 mM hydrogen peroxide, or (3) a parallel application of the two treatments (UV-B + H₂O₂). Neither the H₂O₂ nor the UV-B treatments were found to be damaging to leaf photosynthesis. Both single factor treatments increased leaf H₂O₂ contents but had distinct effects on various H₂O₂ neutralising mechanisms. Non-enzymatic H₂O₂ antioxidant capacities were increased by direct H₂O₂ treatment only, but not by UV-B. In contrast, enzymatic H₂O₂ neutralisation was mostly increased by UV-B, the responses showing an interesting diversity. When class-III peroxidase (POD) activity was assayed using an artificial substrate (ABTS, 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid)), both treatments appeared to have a positive effect. However, only UV-B-treated leaves showed higher POD activities when phenolic compounds naturally occurring in tobacco leaves (chlorogenic acid or quercetin) were used as substrates. These results demonstrate a substrate-dependent, functional heterogeneity in POD and further suggest that the selective activation of specific isoforms in UV-B acclimated leaves is not triggered by excess H₂O₂ in these leaves.