Re-assessing the ecological consequences of ozone depletion: a field comparison of the action spectra used in UV-B research with plants and vegetation

UV filters in the field

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Summary

An action spectrum measures the size of a biological response as a function of the wavelength (colour) of the light applied. Action spectra are used for estimating the relationship between ozone depletion and the biologically effective change in ultraviolet (UV) radiation. Action spectra are also used for calculating the equivalence between treatments with lamps in labs or in the field and ozone depletion. Finally action spectra can provide very valuable information about the photophysiology of the response studied. Because the suitability of the published action spectra most frequently used has been questioned, we propose to test the contending spectra using broad band filters that absorb in different portions of the ultraviolet spectrum. We do not attempt to describe the fine structure of the spectra, something that is currently impossible to do in the field. Instead we will compare the differences between UV-A and UV-B bands. We will use phenolic metabolites and growth as model systems. We will measure concentrations of flavonoids, phenolic acids and tannins, and their precursors and we will measure gene expression for severak key enzymes of the phenolic pathway, CHS and PAL among others. Additionally we will measure growth of the seedlings and morphology of seedlings and leaves. Experiments will be done outdoors, as responses to UV-B measured indoors have usually little relation to what happens in natural and managed ecosystems.

Hypotheses tested

  1. Accumulation in response to UV radiation of individual secondary metabolites from different steps of a single metabolic pathway may follow different action spectra.
  2. Concentrations of precursors are also affected by UV radiation.
  3. Effects of UV-A and UV-B on the expression of genes for key regulatory enzymes of the pathway only partly explain the action spectra for accumulation of secondary compounds.
  4. Responses to UV of phenolic concentrations and epidermal UV-protection follow different action spectra than responses of growth and morphology.

Possible problems to elucidate/confirm Both experiments with lamps, even outdoors, and exclusion experiments with filters are unrealistic as a simulation of ozone depletion because different metabolic responses to UV-B follow action spectra of different shapes. This implies that using a light source with an unnatural spectrum will alter, for example, the relationship between damage and repair/protection reactions.

Experiments. Three types of experiments will be done: 1a) UV exclusion and attenuation with filters over branches of trees (grey alder, white birch) growing in the field. 1b) Similar filters over seedlings growing outdoors. 2) Experiments with seedlings with sunlight and UV from lamps filtered separately, to achieve a factorial design with UV-A and UV-B at different doses. 3) Experiment with seedlings growing under sunlight filtered with filters with different cut-out wavelengths, to construct broad-band dose response curves in outdoors conditions.

Funding. This research is being done with funding from the Maj and Tor Nessling Foundation (to Pedro Aphalo 2005-2006, to Titta Kotilainen 2007), Metsämiesten Säätiö (to Tuulia Venäläinen 2006), and Academy of Finland (to Pedro Aphalo 2007-2010).

Results. The information obtained from our experiments has been important in reassessing the results from numerous earlier experiments with plants. They will also help when reassessing the effects of ozone depletion on plants observed in different experiments or using different experimental protocols. The knowledge obtained should also affect the design of future experiments to assess the effects of ozone depletion, worldwide.

Current status and impact

Several of the participants in this project participated in the COST action “UV4growth” FA0906, 2009-2014. P. J. Aphalo was a work group coordinator. As part of these activities P. J. A. was the lead editor of a handbook of methods for UV research with plants with contributions from 17 authors from across Europe and America, which we hope will help make future experiments on the effects of ultraviolet radiation more reliable and make it easier for researchers new to the field to understand the methodological difficulties, compromises and “traps” involved in the design and execution of such experiments.

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