Interactive effects of biochar and vermicompost on drought tolerance in Camellia sinensis: Antioxidant defense, gene expression, and quality improvement

Drought stress (DS) significantly impairs seedling growth, development, and quality worldwide, underscoring the need for effective mitigation strategies. Biochar (BC) and vermicompost (VC) are promising amendments that can improve plant growth and help plants cope with stress. However, their combined effects on tea ( Camellia sinensis (L.) Kuntze) seedlings, particularly in relation to morphology, photosynthesis, and antioxidant responses under DS, remain poorly understood. In this study, we tested how BC and VC, both alone and together, affect tea seedlings under DS. Co-application of BC and VC improved growth, photosynthesis traits, and antioxidant activity. Specifically, this treatment increased chlorophyll a (117 %), chlorophyll b (219 %), carotenoids (200 %), PSII efficiency (104 %), photosynthesis rate (78 %), intracellular CO 2 levels (75 %), and transpiration (98 %) compared with drought alone. Important photosynthesis-related genes such as CsPsaG , CsPsbS , CsLHCA , CsLHCB , and CsCLH were also upregulated. Tea quality also improved. Levels of caffeine (10.4 %), polyphenols (33.9 %), theanine (140.5 %), amino acids (9.4 %) and catechin (21.95 %) all increased under the combined BC and VC treatment. Antioxidant enzymes such as catalase (40.8 %), superoxidase dismutase (69.6 %), peroxidase (85.3 %) and reduced glutathione (103.8 %) also became more active. Key antioxidant genes CsPOD , CsCAT , CsSOD , and CsAPX were expressed at higher levels. Scanning and transmission electron microscopy showed that combined BC and VC helped protect plant structures. They reduced drought-induced damage to stomata, guard cells, and mesophyll cells. Confocal microscopy imaging showed that cellular integrity was better preserved. Overall, the co-application of BC and VC is a promising and effective strategy to improve growth, physiology, and drought tolerance in tea seedlings. • Soil-applied biochar (BC) and vermicompost (VC) improved chlorophyll content, stomatal function, PSII efficiency and maintained cellular integrity. • Combined BC and VC enhanced tea antioxidant enzyme activities and mitigated oxidative damage. • Upregulation of antioxidant genes ( CsPOD , CsCAT , CsSOD , CsAPX ) contributed to improved drought tolerance. • BC and VC amendments offer a sustainable strategy to enhance tea quality under drought stress.