Appropriate calcium treatments help maintain the appearance, nutritional quality, and postharvest quality of apples, reducing losses during storage. This study investigated the effects of different calcium preparations on the fresh-cut quality and ultrastructure of ‘Starkrimson’ apples. The treatments included control (CK), calcium chloride (T1), sorbitol-chelated calcium (T2), and calcium nitrate (T3). The results demonstrated that sorbitol-chelated calcium significantly inhibited the decline in fresh-cut firmness and pectin content while reducing the increase in cellulose content and minimizing ultrastructural damage. Apples treated with sorbitol-chelated calcium maintained the best fresh-cut hardness and soluble pectin contents, which were 35.71% and 15.42% higher than that of CK on the 12th day, and the cellulose was 27.08% lower than that of CK. Under transmission electron microscopy, the pulp cell surface in the T2 group remained intact, with no bending or deformation, and the middle lamella was well preserved. Additionally, T2 treatment promoted the expression of aroma-related genes during fruit storage. Sorbitol-chelated calcium effectively preserved color and significantly reduced the browning and microbial spoilage of fresh-cut apples, particularly postharvest pathogen growth. The study demonstrates that sorbitol-chelated calcium preserves fresh-cut apple quality by reinforcing cell wall integrity through calcium-mediated crosslinking, suppressing pectin degradation and cellulose accumulation, and activating aroma-related genes (AAT1, AAT2, LOX) to enhance volatile synthesis, thereby reducing microbial spoilage and enzymatic browning during storage.

Excess copper (Cu) in the soil has become a key environmental stress factor constraining the high quality and yield of apple. Melatonin (MT) has significant potential in regulating plant stress resistance. This study used apple rootstock M9T337 in a hydroponic experiment with four treatments: control (CK), 100 μmol·L ⁻¹ MT (MT), 30 μmol·L ⁻¹ CuSO ₄ (Cu), and Cu+MT. Isotopic labeling, noninvasive micro-test technology, and quantitative real-time PCR (qRT-PCR) were used to investigate the mechanisms by which MT affects the growth, antioxidant system, and carbon–nitrogen metabolism of apple rootstock under Cu stress. Copper stress inhibited seedling growth, deteriorated root morphology, decreased root activity, and led to carbon-nitrogen metabolism imbalance. Exogenous MT application alleviated these inhibitory effects. Compared to Cu treatment, Cu+MT treatment decreased H ₂ O ₂ and malondialdehyde content and increased seedlings biomass, leaf superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase activities, net photosynthetic rate (P _n ) and rubisco activity, and nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate synthase activities and root tip NO ₃ ⁻ flux transitions from efflux to uptake. Moreover, Cu+MT treatment increased ¹³ C and ¹⁵ N accumulation compared to Cu treatment. qRT-PCR showed that MT upregulated the expression of MT synthesis genes (e.g., MdTDC ), copper detoxification genes (e.g., MdCCS ), carbon metabolism genes (e.g., MdSUSY1 ), and nitrogen metabolism genes (e.g., MdNRT1.1 ), while downregulating the expression of copper absorption genes (e.g., MdCOPT2 ) and copper transport genes (e.g., MdYSL3 ). MT effectively alleviates Cu stress inhibition in apple rootstock by enhancing the antioxidant capacity, regulating key enzyme activities and gene expression in carbon–nitrogen metabolism, and optimizing the allocation of photosynthetic products and nitrogen. These results provide a theoretical basis for managing Cu pollution in apple orchards.