View of one branch filter (May 2004).
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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 two key enzymes of the phenolic pathway, CHS and PAL. 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 to be tested
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.
This research is being done with funding from the Maj and Tor Nessling Foundation (to Pedro Aphalo 2005-2006), Metsämiesten Säätiö (to Tuulia Venäläinen 2006), and Academy of Finland (to Pedro Aphalo 2007-2010).
Results. The information to be obtained from our experiments will be of great importance in assessing the results from numerous earlier experiments with plants. They will also be central to reassessing the effects of ozone depletion on plants. The new knowledge should also affect the design of future experiments to assess the effects of ozone depletion, worldwide.
We started the first field experiment with filters in the spring of 2004, as part of our research on the effects of UV-B on leaf litter quality and decomposition. Branch filters were installed on white birch and grey alder trees growing in an abandoned agricultural field near Jyväskylä. We have three treatments: 1) near ambient UV, 2) UV-B attenuation, and 3) attenuation of the whole solar UV band. The filters have equal and very high transmittance of visible light. The lower side is open for ventilation.
Click on the thumbnails for bigger images.
View of one branch filter (May 2004).
Another view of one branch filter (May 2004).
A filter on a branch, now with leaves (June 2004).
Six filters (three treatments) on the south facing side of an alder tree (June 2004).
Author: Pedro J. Aphalo.