Surface structure, wax and methanol-extractable compounds in Scots pine and Norway spruce needles enhanced UV-B

Heli Kinnunen

Department of Biology

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-BBE 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-BBE 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 somewhat eroded. 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.

Table of Contents
Acknowledgements
Abbreviations
Definitions
1. Introduction
2. Aim of the research
3. Materials and methods
3.1. Short-term greenhouse experiment
3.2. Long-term greenhouse experiment
3.3. Field experiment
3.4. Wax response
3.5. Methanol-extractable pigments
3.6. Statistics
4. Results
4.1. Seedlings
4.1.1. Norway spruce
4.1.2. Scots pine
4.2. Mature Scots pines
5. Discussion
5.1. Wax response
5.1.1. Seedlings in greenhouse experiments
5.1.2. Mature trees in field conditions
5.1.3. Comparison between the greenhouse and field experiments
5.1.4. Different analysing methods
5.2. UV-absorbing compounds
5.2.1. Responses to supplemental UV-B
5.2.2. Cumulative effects
5.2.3. Changes in the amount of UV-absorbing compounds between seasons and/or needle ages
5.2.4. Individual UV- absorbing compounds
5.2.5. Advantages and disadvantages of defence mechanisms
6. Conclusions
References
List of Tables
3-1. Summary of the three enhanced UV experiments.
3-2. The monthly means of UV-A and biologically effective UV-B doses in the field experiment during the three growing seasons. Solar radiation and temperatures are expressed as monthly means and maximum values. .
List of Figures
3-1. Provenances of the studied Scots pine and Norway spruce seedlings (I, II).
4-1. The wax tube distribution (WTD, %) of the needles of mature Scots pines under ambient and supplemental UV-B during the exposure years 1996–1998. The error bars represent SD (n=7).
4-2. Methanol-extractable UV-absorbing compounds (A300/cm2) in the needles of mature Scots pines under ambient and supplemental UV-B during the exposure years 1996–1998. The error bars represent SD (n=7).
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