Abstract
Supplemental narrow-band 311 nm UV-B radiation was applied in order to study the effect of this specific wavelength on tobacco as a model plant. UV-B at photon fluxes varying between 2.9 and 9.9 μmol m−2 s−1 was applied to supplement 150 μmol m−2 s−1 photosynthetically active radiation (PAR) for four hours in the middle of the light period for four days. Narrow-band UV-B increased leaf flavonoid and phenolic acid contents. In leaves exposed to 311 nm radiation, superoxide dismutase activity increased, but phenolic peroxidase activity decreased, and the changes were proportional to the UV flux. Ascorbate peroxidase activities were not significantly affected. Narrow-band UV-B caused a dose-dependent linear decrease in the quantum efficiency of photosystem II, up to approximately 10% loss. A parallel decrease in non-regulated non-photochemical quenching indicates potential electron transfer to oxygen in UV-treated leaves. In addition to a flux-dependent increase in the imbalance between enzymatic H2O2 production and neutralization, this resulted in an approximately 50% increase in leaf H2O2 content under 2.9–6 μmol m−2 s−1 UV-B. Leaf H2O2 decreased to control levels under higher UV-B fluxes due to the onset of increased non-enzymatic H2O2- and superoxide-neutralizing capacities, which were not observed under lower fluxes. These antioxidant responses to 311 nm UV-B were different from our previous findings in plants exposed to broad-band UV-B. The results suggest that signaling pathways activated by 311 nm radiation are distinct from those stimulated by other wavelengths and support the heterogeneous regulation of plant UV responses.
Highlights
A key function of plant stress tolerance is the maintenance of relatively low cellular concentrations of reactive oxygen species (ROS)
In leaves exposed to 311 nm radiation, superoxide dismutase activity increased, but phenolic peroxidase activity decreased, and the changes were proportional to the UV flux
Plants respond to several potential stressors by enhancing peroxidase activity [9,10], but this line of defense is of special importance in response to UV-B because short-wave (λ < 305 nm) radiation is capable of photoconverting H2O2 to hydroxyl radicals [11]
Summary
A key function of plant stress tolerance is the maintenance of relatively low cellular concentrations of reactive oxygen species (ROS). The response of these two enzymes to 311 nm UV-B (increasing SOD and unchanged APX, Figure 4) was opposite to previous observations in leaves acclimated to broad-band UV-B (decreasing or unchanged SOD and increasing APX [7,8,15,38]) This discrepancy may be explained by the potential photocleavage of H2O2 to hydroxyl radicals by shorter (285–305 nm), but not longer, UV-B wavelengths [11], leading to the stricter control of cellular H2O2 levels in the presence of shorter-wavelength UV-B than under 311 nm UV-B only. In addition to causing a 25% decrease in photochemical yields, exposure of Arabidopsis leaves to broad-band UV-B produced an increase in non-enzymatic antioxidant capacities [38]; the positive effect observed in this study (Figure 6) is a consequence of high UV photon flux rather than a unique characteristic of 311 nm UV-B, as opposed to the effect of 311 nm treatment on antioxidant enzymes (Figure 4), which appear to be driven by this wavelength. Based on the present study, fluxes in the 3–6 μmol m−2 s−1 range may be suitable for this purpose, but this is expected to depend on plant species, age, and growth conditions
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