Effects of ultraviolet-B radiation on leaf elongation, production and phenylpropanoid concentrations of Deschampsia antarctica and Colobanthus quitensis in Antarctica

Stratospheric ozone depletion by anthropogenic chlorofluorocarbons has lead to increases in ultraviolet-B radiation (UV-B; 280-320 nm) along the Antar ctic Peninsula during the austral spring. We manipulated UV-B levels around plants of Antarctic hair grass (Deschampsia antarctica; Poaceae) and Antar ctic pearlwort (Colobanthus quitensis; Caryophyllaceae) for one field seaso n near Palmer Station along the west coast of the Antarctic Peninsula. Trea tments involved placing frames over naturally growing plants that either (1 ) held filters that absorbed most biologically effective radiation (UV-B-BE ; 'reduced UV-B', 22% of ambient UV-B-BE levels), (2) held filters that tra nsmitted most UV-B,, ('near-ambient UV-B', 87% of ambient UV-B-BE levels), or (3) lacked filters ('ambient UV-B'). Leaves on D. antarctica exposed to near-ambient and ambient UV-B were 16-17% shorter than those exposed to red uced UV-B, and this was associated with shorter epidermal cells at the leaf base and tip. Leaves on C. quitensis exposed to near-ambient and ambient U V-B tended to be shorter (P = 0.18) and epidermal cells at the leaf base te nded to be smaller than those under reduced UV-B (P < 0.10). In order to fu rther explain reductions in leaf length, we examined leaf concentrations of insoluble (cell-wall bound) phenylpropanoids, since it has been proposed t hat wall-bound phenylpropanoids such as ferulic acid may constrain cell exp ansion and leaf elongation. In both species, HPLC analysis revealed that fe rulic and p-coumaric acid were major components of both insoluble and solub le phenylpropanoids. Although there were no significant differences in conc entrations between UVB treatments, concentrations of insoluble ferulic acid in D. antarctica tended to be higher under ambient and near-ambient UV-B t han under reduced UV-B (P = 0.17). We also examined bulk-leaf concentration s of soluble (methanol extractable) UV-B-absorbing compounds and found that concentrations were higher in plants exposed to near-ambient and ambient U V-B than in plants exposed to reduced UV-B. We also assessed the UV-B-scree ning effectiveness of leaves that had developed on plants at the field site with a fiber-optic microprobe. Leaf epidermal transmittance of 300-nm UV-B was 4.0 and 0.6% for D. antarctica and C. quitensis, respectively, which i s low compared to grasses and herbaceous dicotyledonous plants found in mor e temperate climates. While the leaves of Antarctic vascular plants are rel atively effective at screening UV-B, levels of UV-B in Antarctica are suffi cient to reduce leaf epidermal cell size and leaf elongation in these speci es, although the mechanisms for these reductions remain unclear.