Abstract

Increased amounts of epicuticular waxes and UV-absorbing compounds, such as flavonoids, and smaller leaf/needle surface area are plant defence mechanisms against UV-B radiation. The response of the needle epicuticular waxes of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies Karst.) seedlings to increased UV-B were investigated in short-term and long-term greenhouse experiments. In a more realistic long-term field experiment with mature Scots pines, the methanol-extractable UV-absorbing compounds were also analysed.

Some significant changes were observed in the wax tube distribution (WTD, %) and the amount of waxes in Norway spruce seedlings in the short-term Belgian greenhouse experiment (UV-B_BE 0, 11.3 and 22.6 kJ m^-2 d^-1), but no changes were detected in Scots pine seedlings. No changes in waxes were observed in the long-term Finnish greenhouse experiment (UV-B_BE 0, 2.2–6.6 and 5.6–16.8 kJ m^-2 d^-1), where both the Norway spruce and the Scots pine seedlings seemed to respond by having smaller needle surface areas. A field experiment (UV-BBE 0.5–2.4 kJ m^-2 d^-1 and 0.7–5.1 kJ m^-2 d^-1) with mature Scots pines revealed no significant changes in WTD during the three growing seasons or the amount of waxes during the third growing season.

In the long-term field experiment the amount of UV-absorbing compounds varied significantly between seasons and/or needle age classes. Elevated amounts of these compounds were already observed in the three-day-old needles and also in the oldest (c + 2) needles when the waxes were still undeveloped or already somewhateroded. No significant differences in the amount of UV-absorbing compounds were observed between the treatments during the first and second growing seasons. During the third growing season, needles of all ages contained significantly or slightly less UV-absorbing compounds in supplemental UV-B than in the ambient treatment, possibly due to cumulative effects of UV-B in already inhibited pigment synthesis. This suggests that these defence mechanisms are not efficient enough to prevent the UV-B-induced damage in the long term.