Drought modulates ozone stress through BVOCs, antioxidant defenses, and metabolic responses in a tropical tree

Water limitation attenuated ozone-induced oxidative stress in Eugenia uniflora, maintaining physiological stability and promoting targeted biogenic volatile organic compounds (BVOCs) and metabolic adjustments rather than large-scale disruption. Tropical forests are increasingly exposed to multiple environmental stressors, yet the combined effects of drought and ozone on native tropical tree species remain poorly understood. We investigated the individual and interactive effects of water limitation and ozone exposure on Eugenia uniflora saplings using an integrated physiological, biochemical, metabolomic, and biogenic volatile organic compound (BVOC) emission approach. Saplings were subjected to four treatments: control, drought, ozone fumigation, and combined drought-ozone. Drought was imposed by withholding irrigation for ten days, while ozone exposure (80-120 ppb) was applied for six days, reaching a cumulative AOT40 of 930.7 ppb h. Despite reductions in soil moisture, no significant changes were observed in leaf relative water content, gas exchange, or photosynthetic pigment, indicating high physiological resilience. Ozone exposure alone induced depletion of antioxidant pools, reducing ascorbate and glutathione concentrations, while redox ratios remained stable. BVOC emissions comprised 15 compounds across five chemical classes, dominated by oxygenated sesquiterpenes and sesquiterpenes. Although total emission rates did not differ among treatments, multivariate analyses revealed shifts in BVOC composition, including methyl salicylate and β-ocimene under water limitation and farnesane under combined stress. Metabolomic profiling revealed coordinated adjustments in sugars, amino acids, and oleanolic acid associated with ozone exposure. Notably, antioxidant depletion observed under ozone alone was not detected under combined stress, suggesting a non-additive interaction between water limitation and ozone. Overall, E. uniflora maintained physiological stability while adjusting volatile emissions and metabolic pathways under concurrent stress, providing new insights into the mechanisms governing drought-ozone interactions in tropical trees.