Effects of ultraviolet-B radiation on phenylpropanoids and growth of vascular plants in Antarctica

Stratospheric ozone depletion by chlorofluorocarbons has led to enhanced levels of ultraviolet-B radiation (UV-B; 280–320 nm) along the Antarctic Peninsula during the austral spring. The influence of solar UV-B radiation during ozone depletion events on the growth and phenylpropanoid concentrations of Deschampsia antarctica and Colobanthus quitensis , the only two vascular plant species native to Antarctica, was examined in field experiments along the west coast of the Antarctic Peninsula. Experiments involved placing filters that either absorbed most biologically effective UV-B (UV-B_BE; “reduced UV-B,” 13% of ambient UV-B _BE) or transmitted most UV-B_BE (“near-ambient UV-B,” 83% of ambient UV-B_BE) over plants. Exposure to near-ambient UV-B consistently reduced the growth and biomass production of both species. These reductions were associated with slower leaf elongation rates, shorter leaves and less total leaf area, but not reductions in photosynthesis per unit leaf area. These plants also had smaller epidermal cells than those under reduced UV-B. Since it has been proposed that UV-B-induced accumulation of wall-bound hydroxycinnamic acids (HCAs) may constrain epidermal cell expansion and leaf elongation under UV-B exposure, concentrations of wall-bound HCAs were also examined. High-performance liquid chromatography (HPLC) analysis revealed that ferulic and p-coumaric acids were the major insoluble (wall-bound) and soluble (cytoplasmic/vacuole-contained) HCAs in leaves and concentrations of wall-bound HCAs tended to increase with exposure to UV-B. These results show that ambient levels of UV-B along the Antarctic Peninsula reduce growth primarily through slower leaf expansion, and this may be associated with increases in wall-bound HCAs. In a parallel growth-chamber experiment, the UV-B-dose response of D. antarctica and C. quitensis was examined by challenging plants for 16 h with 7.3 μW cm −2 of UV-B_BE, which is similar to UV-B_BE levels during moderate ozone-depletion events along the Peninsula. Epidermal transmittance of 300-nm UV-B increased and phenylpropanoid concentrations decreased with UV-B exposure time in D. antarctica. In contrast, epidermal transmittance of UV-B and phenylpropanoid concentrations both declined with UV-B exposure time in C. quitensis. These counter-intuitive results suggest that the relationship between whole-leaf phenylpropanoid concentrations and leaf UV-B screening effectiveness is complex, and that current models need to be reevaluated.