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Climate change-related drought stress is expected to shift carbon partitioning toward volatile organic compound (VOC) biosynthesis. The effect of drought stress on VOC synthesis remains unknown in several tree species. Therefore, we exposed eastern white pine (Pinus strobus) plants to severe drought for 32 days and performed physiological analysis (chlorophyll content, leaf water content, and root/shoot index), biochemical analysis (non-structural carbohydrates, proline, lipid peroxidation, and antioxidant assay), and total root VOC analysis. Drought stress decreased the relative water and soil moisture contents. Root proline accumulation and antioxidant activity increased significantly, whereas leaf chlorophyll synthesis and fresh weight decreased significantly in drought-treated plants. A non-significant increase in sugar accumulation (leaves and roots), proline accumulation (leaves), antioxidant activity (leaves), and lipid peroxidation (leaves and roots) was observed in drought-treated plants. Drought stress caused a non-significant decline in root/shoot ratio and starch accumulation (leaves and roots) and caused a significant increase in root abscisic acid content. Drought-treated plants showed an increase in overall monoterpene synthesis (16%) and decline in total sesquiterpene synthesis (3%). Our findings provide an overall assessment of the different responses of VOC synthesis to severe water deficit that may help unravel the molecular mechanisms underlying drought tolerance in P. strobus.

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Elevated atmospheric CO2 (eCO2) concentration initially enhances photosynthesis, growth and ecosystem productivity, but the excessive use of nitrogen due to the increased productivity causes uncertainty in long-term ecosystem responses. We exposed Korean red pine, Chinese ash, and Korean mountain ash to current atmospheric CO2 concentration (aCO2), 1.4 times higher CO2 concentration (eCO21.4), and 1.8 times higher CO2 concentration (eCO21.8) in an Open-Top Chamber (OTC) experiment for eight years (2010-2017) to investigate the effect on the morphological and physiological properties of trees. We also assessed whether nitrogen limitation occurred with time by comparing leaf and soil nitrogen concentration. CO2 fertilization effect was observed on tree growth for the first two years (p < 0.05), but there was no difference thereafter. For photosynthetic properties, CO2 effects were species-specific; no effects on Korean red pine and Chinese ash vs. significant effect on Korean mountain ash. However, maximum photosynthetic and carboxylation rates significantly decreased by 24.3% and 31.3% from 2013 to 2017, respectively. Leaf nitrogen significantly decreased by 21.0 % at eCO21.4 and 18.5 % at eCO21.8 compared with aCO2 treatment. This study showed the decline of leaf nitrogen and species-specific responses to long-term high CO2 concentration, which will effect on species competition and ecosystem succession.

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