Sun-related physiological disorders such as sun damage on apples (Malus domestica Borkh) are caused by cumulative photooxidative and heat stress during their growing season triggering morphological, physiological, and biochemical changes in fruit tissues not only while it is on the tree but also after it has been harvested. The objective of the work was to establish the interaction of auxin (indole-3-acetic acid; IAA), abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), and ethylene (ET) and its precursor ACC (free and conjugated, MACC) during development of sun-injury-related disorders pre- and post-harvest on apples. Peel tissue was extracted from fruit growing under different sun exposures (Non-exposed, NE; Exposed, EX) and with sun injury symptoms (Moderate, Mod). Sampling was carried out every 15 days from 75 days after full bloom (DAFB) until 120 days post-harvest in cold storage (1^∘C, > 90%RH). Concentrations of IAA, ABA, JA, SA, were determined using UHPLC mass spectrometry, and ET and ACC (free and conjugated MACC) using gas chromatography. IAA was found not to be related directly to sun injury development, but it decreased 60% in sun exposed tissue, and during fruit development. ABA, JA, SA, and ethylene concentrations were significantly higher (P ≤ 0.05) in Mod tissue, but their concentration, except for ethylene, were not affected by sun exposure. ACC and MACC concentrations increased until 105 DAFB in all sun exposure categories. During post-harvest, ethylene climacteric peak was delayed on EX compared to Mod. ABA and SA concentrations remained stable throughout storage in both tissue. JA dramatically increased post-harvest in both EX and Mod tissue, and orchards, confirming its role in low temperature tolerance. The results suggest that ABA, JA, and SA together with ethylene are modulating some of the abiotic stress defense responses on sun-exposed fruit during photooxidative and heat stress on the tree.

Photooxidative stress, when combined with elevated temperatures, triggers various defense mechanisms leading to physiological, biochemical, and morphological changes in fruit tissue. Furthermore, during sun damage, apple fruit undergo textural changes characterized by high flesh firmness compared to unexposed fruit. Fuji and Royal Gala apples were suddenly exposed to sunlight on the tree and then sampled for up to 29 days. Cell wall components and lignin biosynthetic pathway analyses were carried out on the fruit tissue. At harvest, Fuji apples with different sun exposure levels, such as exposed to direct sunlight (Exp), shaded (Non-Exp), and with severe sun damage (Sev), were also characterized. In fruit suddenly exposed to sunlight, the expression levels of phenylpropanoid-related genes, phenylalanine ammonia lyase (MdPAL), chalcone synthase (MdCHS), and flavanone-3-hydroxylase (MdF3H), were upregulated in the skin and flesh of Exp and Sev. Exposure had little effect on the lignin-related genes caffeic acid O-methyltransferase 1 (MdCOMT1) and cinnamyl alcohol dehydrogenase (MdCAD) in the skin; however, the expression of these genes was highly induced in the flesh of Exp and Sev in both cultivars. Lignin deposition increased significantly in skin with sun injury (Sev); in flesh, this increase occurred late during the stress treatment. Additionally, the ethylene biosynthesis genes 1-aminocyclopropane-1-carboxylate synthase (MdACS) and 1-aminocyclopropane-1-carboxylate oxidase (MdACO) were highly expressed in the skin and flesh tissues but were more upregulated in Sev than in Exp during the time-course experiment, which paralleled the induction of the phenylpropanoid pathway and lignin accumulation. At harvest, flesh from Sev fruit exhibited higher firmness than that from Non-Exp and Exp fruit, although no differences were observed in the alcohol-insoluble residues (AIR) among groups. The fractionation of cell wall polymers revealed an increase in the uronic acid contents of the water-soluble pectin fraction (WSF) in Exp and Sev tissues compared to Non-Exp tissues, while the other pectin-rich fractions, that is, CDTA-soluble (CSF) and Na₂CO₃-soluble (NSF), were increased only in Sev. The amount of hemicellulose and cellulose did not differ among fruit conditions. These findings suggest that increases in the flesh firmness of apples can be promoted by photooxidative stress, which is associated with the induction of lignin accumulation in the skin and flesh of stressed fruit, with the involvement of stress phytohormones such as ethylene.