Combined effects of elevated ozone, temperature, and nitrogen on stem phenolic concentrations of Scots pine (Pinus sylvestris) seedlings

Abstract

Phenolic metabolites in forest trees play a key role in the defence against biotic and abiotic stressors, yet we lack information about the effects of combined abiotic factors on phenolic compounds in conifers. We studied the effects of combined abiotic factors (ozone × temperature, ozone × nitrogen, temperature × nitrogen, and ozone × temperature × nitrogen) on phenolic concentrations in stems of Scots pine (Pinus sylvestris L.) seedlings in a 3-year-long field experiment in central Finland. In current-year stems, elevated ozone increased the concentrations of total phenolics, soluble proanthocyanidins, and total proanthocyanidins, while warming reduced the concentrations of piceatannol glucoside, pinosylvin, isorhamnetin + kaempferol-3-rhamnoside, and monocoumaroyl isoquercitrin 1. Complex interaction effects on current-year stems showed that nitrogen addition increased the concentrations of some flavonoids in ambient ozone and temperature levels. In the stems from the previous years, ozone decreased the concentrations of total phenolics, total proanthocyanidins, and several flavonoids in ambient temperature, while warming increased their concentrations in combination with elevated ozone. Our results suggest that phenolic defence responses in Scots pine seedlings are affected by all three factors, but that the level of phenolics in the stems from previous years may increase under the combined exposure to elevated temperature and ozone — an expected climate trend in the Northern Hemisphere.