Temporal dynamics of leaf physiological and metabolic traits in Camellia reticulata during sequential freeze-thaw stress and subsequent recovery

Camellia reticulata is an economically and ornamentally valuable woody oil tree native to southwestern China, where it experiences extreme minimum temperatures near −4 °C. This species may face increasing freeze-thaw stress as it is introduced to higher altitudes and latitudes. This study investigated dynamic changes in photochemical activity, antioxidant status, carbohydrate metabolism, and glycerolipid profiles in C. reticulata during a freeze-thaw regime with two freezing exposures (C1 and C2) and subsequent recovery (R3). C1 did not reduce the maximum photochemical efficiency of photosystem II (Fv/Fm) but significantly inhibited photosynthetic electron transport. In contrast, C2 and R3 significantly reduced Fv/Fm and elevated the quantum yield of non-regulated energy dissipation, indicating persistent photodamage. Total antioxidant capacity (TAC) remained unchanged at C1 but decreased significantly at C2 and R3, while malondialdehyde levels remained stable, suggesting no severe membrane lipid peroxidation. Soluble sugars increased by 47.5% and cellulose decreased by 43.2% only at R3, whereas hemicellulose increased across all treatment groups, with starch and pectin unchanged. Phosphatidylcholine (PC) depletion and a reduced PC/phosphatidylethanolamine ratio indicated cold-induced membrane destabilization. However, adaptive lipid remodeling may mitigate membrane impairment. Phospholipid unsaturation rose significantly across all treatments, while glycolipid unsaturation increased at C1 and R3. Recovery specifically promoted the accumulation of phosphatidic acid, phosphatidylinositol, and phosphatidylinositol phosphate and increased total glycolipids, whereas triacylglycerol accumulated at both C1 and R3. Collectively, C. reticulata undergoes a coordinated metabolic network involving carbohydrate and lipid pathways to support plant regrowth despite decreased TAC. These findings elucidate the stage-specific metabolic mechanisms underlying freeze-thaw tolerance in C. reticulata and provide metabolic targets for improving cold resistance, offering a theoretical foundation for its introduction and domestication in colder regions.