The effects of UV exclusion on the soluble phenolics of young Scots pine seedlings in the subarctic

The characteristics of UV-absorbing compounds, particularly soluble phenoli cs, were studied in needles of 63-day-old seed-grown Scots pine (Pinus sylv estris L.) seedlings of two provenances in a UV exclusion field experiment at Pallas-Ounastunturi National Park in Finnish Lapland (68 degrees N, 270 m a.s.l.). The experiment used the following plastic filters in exclosure t reatments to manipulate the spectral balance of natural irradiance: (1) 'co ntrol' (a polyethene plastic filter); (2) 'UV-B exclusion' (a clear polyest er filter); and (3) 'UV-B/UV-A exclusion' (a clear acryl plate). Polyethene transmitted 89% of the ambient levels of total UV (280-400 nm), polyester transmitted 75% of the total UV, but only 0.6% of the UV-B (280-315 nm) com ponent, while acryl plate transmitted 0.2% of UV (280-360 nm). The research also included (4) 'Ambient' plants that were not subjected to any treatmen t exclosures. After the 58 day UV exclusion, Significant (p < 0.0001) diffe rences due to treatments were determined for a kaempferol derivative, kaemp ferol 3-glucoside, and a quercetin derivative, the quantities of which rang ed from 0.23 to 0.45, 0.42 to 1.34 and 0.39 to 0.75 mu mol g FW-1, respecti vely, depending on treatment and provenance. Overall, Scots pine seedlings grown at ambient UV radiation PAS300, Caldwell's generalized Plant Action S pectrum (PAS) normalized at 300 nm, 72 mW m(-2) or under a control had sign ificantly (p < 0.05) higher quantities of soluble phenolics than seedlings grown under UV-B or UV-B/UV-A exclusion treatments. There were no significa nt differences in the quantity of soluble phenolics between the two exclosu re treatments or between the two Scots pine provenances. The sums of diacyl ated flavonol glucosides ranging from 3.75 to 4.55 mu mol g FW-1 depending on treatment and provenance, were already present at very low UV-levels und er the UV-B/UV-A exclusion treatment. The present study indicated that solu ble phenolics, particularly the diacylated flavonol glucosides, may provide an effective preformed protection for young Scots pine seedlings against U V-B and UV-A radiation. (C) 1999 Elsevier Science Ltd. All rights reserved.