Ecophysiological responses of subarctic scots pine to ultraviolet (UV) radiation

The focus of our field studies in the subartic of Finnish Lapland from 1997 onwards has been to investigate the effect of solar UV radiation on the ecophysiology of Scots pine. Our studies have indicated that solar UV-B_BE exclusion during 1997-2002 caused transient effects on the growth of both Scots pine seedlings and saplings, which may not be related to changes in Photosystem II efficiency, however. Studies of the phylloplane fungi of Scots pine needles showed that Aureobasidium pullulans (de Bary) Arnaud and Cladosporium sp. populations were found to decrease, and Phoma sp. to increase, with UV exposure, but there was no UV effect found for total populations. It was also shown that both UV-B radiation and increasing temperature enhance the nitrate reductase (NR) activity of Scots pine needles. On the other band, our earlier studies have shown that the defence mechanisms (increase in the concentration of soluble phenolics, thickening of the epi- and hypodermal cell layers) of the Scots pine were functioning against ambient solar UV radiation. The daily UV dose rates during 1997-2002 were not very high and in our experiments, the exposure periods of three growing seasons were probably not long enough to cause greater effects that may be cumulative in nature and occur only after longer experimental periods.