Phytohormone Crosstalk Under Elevated CO2: Linking Sugar Signaling, Stress Resilience, and Metabolic Quality in Tea

Elevated atmospheric CO2 (eCO2) is commonly treated as a photosynthetic substrate, yet in horticultural crops it also alters carbon status and signaling processes associated with stress resilience and quality-related metabolism. Tea (Camellia sinensis) is particularly valuable because young-shoot metabolites are closely related to carbon–nitrogen balance and defense. This review develops an evidence-based framework linking guard-cell CO2 sensing and photosynthetic carbon influx with soluble-sugar and starch pools and candidate signaling through hexokinase (HXK), trehalose-6-phosphate (T6P), SNF1-related protein kinase 1 (SnRK1), and target of rapamycin (TOR). These crosstalk pionts intersect with phytohormone networks: abscisic acid (ABA)–Ca2+–reactive oxygen species (ROS)–nitric oxide (NO) signaling contributes to stomatal and water-stress responses; salicylic acid (SA)–NO promotes eCO2-induced flavonoid biosynthesis; jasmonic acid (JA)–ethylene contributes to activate defense and volatile metabolism; and auxin–ethylene influences growth and defense. Nitrogen availability and sink strength may determine whether additional carbon supports assimilation and quality metabolism or promotes carbohydrate feedback and photosynthetic acclimation. We relate these pathways to catechins, theanine, caffeine, and aroma volatiles while distinguishing tea-supported relationships from mechanisms derived mainly from model plants. Finally, we propose cumulative CO2 enrichment above baseline (Δppm·h) as a treatment-reporting descriptor and hypothesis-generating variable integrating enrichment magnitude and duration. Its biological relevance should be tested across nitrogen supply, genotype, developmental stage, and stress context using time-based gas exchange, stomatal phenotyping, hormone profiling, carbon–nitrogen flux analysis, and multi-omics. This model presents sugar–phytohormone crosstalk as a candidate regulatory junction linking carbon enrichment with stress responses and quality-related metabolism in tea plants.