Seasonality but Not Tree Age Determines Secondary Plant Metabolite Composition in Scots Pine Needles

Abstract

Abstract Background and Aims: Secondary plant metabolites, including organic acids and phenolic compounds, play key roles in plant physiology and response to environmental stressors. How the production of these compounds varies across tree age and seasonality are of considerable interest to understanding plant stress response. Methods Monthly, we collected fallen needles of Scots pine (Pinus sylvestris L.) across stands of five different age classes (20, 40, 60, 80, 100 years) for one year and measured organic acids and phenolic compounds. Results Seven low molecular weight organic acids and thirteen phenolic compounds were detected. No differences were observed across stand age. Significant seasonal differences were detected. Most compounds peaked during the growing season. Succinic acid was the most prevalent organic acid, comprising 78% of total organic acids (351.27 ± 34.27 ug g− 1), and was 1.5 to 11.0 times greater in the summer than all other seasons. Sinapic acid was the most prevalent phenolic compound (42.15 µg g− 1) representing 11% of total phenolic compounds and was 39.8 times greater in spring and summer compared to autumn and winter. Growing season peaks in needle concentrations were observed for all thirteen phenolic compounds and two organic acids (lactic, succinic). Citric acid exhibited a definitive peak in late winter into early spring. Conclusion Our results highlight the change in secondary metabolite production in response to seasonality, likely in response to UV-B radiation and the onset of the growing season. These compounds protect against environmental stressors through metabolic reactions within needles as trees acclimate to seasonal conditions.