This study was conducted at the Taidong Base of the Shandong Institute of Pomology, Tai’an City, Shandong Province, China, from May to October 2020. The effects of spraying calcium fertilizer on the photosynthetic characteristics, mineral element content, sugar and acid metabolism, and quality of apples were assessed. Fuji Apple Tianhong.2/SH/Malus robusta (M. robusta) was treated with the calcium fertilizers Niucui (Ca ≥ 100 g/L, B: 1–10 g/L) and Naipu 9 (Ca: 90 g/L, Mg: 21 g/L, N: 110 g/L) at the young fruit stage (25 May), the expansion stage (5 July), and the coloring stage (25 September). Water was sprayed as the control application. The results revealed that spraying calcium fertilizer increased the chloroplast pigment content and photosynthetic capacity of the apple leaves. Compared with the control, the net photosynthetic rate of apple leaves sprayed with Niucui increased by 4.3–34.6%, and that of leaves sprayed with Naipu 9 increased by 15.0–57.4%. Spraying calcium fertilizer promoted the accumulation of Ca, Mg, and B mineral elements in leaves and fruits while inhibiting the accumulation of Cu. Spraying calcium fertilizer improved the quality of apple fruit. Compared to the control fruit, the single fruit weight, fruit hardness, soluble solid content, and Vitamin C content of fruit sprayed with Niucui and Naipu 9 increased by 6.5 and 12.1%, 3.6 and 16.1%, 6.3 and 12.0%, and 30.5 and 29.4%, respectively. Spraying calcium fertilizer increased the sugar content and decreased the acid content of the apples. Naipu 9 was more effective at increasing fruit sugar content, especially sucrose and sorbitol, while Niucui was more effective at reducing the acid content, especially malic acid. This was observed because Naipu 9 increased the sugar metabolism enzyme activity, while Niucui increased the acid decomposition-related enzyme activity. In conclusion, according to the nutrition quality and flavor of the fruit, Naipu 9 application was determined to be the best fertilizer for the Fuji apple.

Seaweed extracts, which are naturally degradable and environmentally friendly, have become important components of organic fertilizers for fruit. In this study, the effects of seaweed-extract-based organic fertilizers on the levels of mineral elements, sugar–acid components, and hormones in ‘Fuji’ apples were evaluated. Eleven-year-old ‘Fuji’ apple (‘Yanfu 3’/M9/Malus robusta) trees were sprayed with seaweed-extract-based fertilizers SE1 (contained ≥20 g/L of algal polysaccharides, ≥100 g/L of amino acids, and ≥30 g/L of EDTA-Ca) and SE2 (contained ≥100 g/L of algal polysaccharides, ≥160 g/L of Ca, and ≥10 g/L of Mg) from early May to early October at two-week intervals in 2020. Water spraying was used as a control treatment. The preliminary results showed that SE1 and SE2 significantly increased the net photosynthetic rate, chlorophyll content and Rubisco enzyme activity in ‘Fuji’ apple leaves. SE1 and SE2 improved the quality of ‘Fuji’ apples. Compared to the single-fruit weight, soluble solid content (SSC), soluble sugar content, sugar–acid ratio, vitamin C (VC) content, and free amino acid content of control apples, those of SE1-treated and SE2-treated apples increased by 10.74% and 8.94%, 12.16% and 9.76%, 21.96% and 11.90%, 45.12% and 28.55%, 56.12% and 47.96%, and 17.15% and 13.42%, respectively. SE1 and SE2 significantly improved the levels of fructose, glucose, sucrose, and sorbitol, but decreased the levels of malic acid, oxalic acid, and tartaric acid in ‘Fuji’ apples. Meanwhile, SE1 and SE2 increased the levels of potassium, calcium, magnesium, iron, zinc, and boron in ‘Fuji’ apples. SE1 and SE2 significantly improved the levels of indole-3-acetic acid, zeatin riboside, and gibberellic acid, but significantly decreased the level of abscisic acid in ‘Fuji’ apples. In conclusion, spraying seaweed-extract-based foliar fertilizer is beneficial to the nutrient accumulation and flavor development in ‘Fuji’ apples.

Many studies have demonstrated that anthocyanin synthesis in apple peel is induced by light, but the color of bagged apple peel continues to change under dark conditions after light induction has not been characterized. Here, transcriptional and metabolic changes associated with changes in apple peel coloration in the dark after different light induction treatments were studied. Apple pericarp can achieve a normal color under complete darkness followed by light induction. Metabolomics analysis indicated that the expression levels of cyanidin-3-O-galactoside and cyanidin-3-O-glucoside were high, which might be associated with the red color development of apple peel. Transcriptome analysis revealed high expression levels of MdUFGTs, MdMYBs, and MdNACs, which might play a key role in light-induced anthocyanin accumulation under dark conditions. 13 key genes related to dark coloring after light induction was screened. The results of this study provide new insights into the mechanism of anthocyanin synthesis under dark conditions.