Increased pine root exudation of phenolic acids and amino acids under drought

Belowground carbon (C) allocation by trees particularly through root exudation is a key pathway influencing long-term C storage in forest soils. Future climate change scenarios, such as drought and elevated CO2 (eCO2) strongly affect trees' physiology and influence belowground C allocation. However, how the rate and chemical composition of root exudation change especially under the combined effect of drought and eCO2 remains poorly understood. We conducted an experiment to examine the single and combined effects of drought and eCO2 on the belowground C allocation of Pinus brutia saplings. Root exudates were collected, quantified, and metabolically profiled. Our results revealed that under eCO2, C assimilation increased up to 2.2-fold, increasing plant biomass while root exudation rate and composition remained unchanged under eCO2 compared to ambient CO2 (aCO2). Additionally, when trees were exposed to drought stress, root exudation rate increased 4.3-fold under aCO2, and up to 10.4-fold when combined with eCO2; however, overall, eCO2 had no effect on root exudation. Root starch reserves decreased under drought, whereas soluble sugars increased, with the largest increase under combined drought and eCO2. Further, drought altered exudate composition, increasing several metabolites, mainly phenolic acids (e.g., gallic acid and caffeic acid), amino acids and key osmoprotectants such as trehalose, proline and betaine. Overall, these observations show that pines strongly increased their root exudation under drought and that exudate metabolites shifted towards more specialized metabolites linked to stress metabolism.