Susceptibility of Yellow Clingstone Peach Cultivars to Inking Disorder

Inking disorder is a peach quality defect in which fruits manifest a dark epidermis discoloration; this disorder reduces the commercial value of peach fruits. In red pigmented peach fruit, it has been proposed that inking is associated with mechanical damage and reactions involving metal ions (Fe, Cu, and Zn) and anthocyanins, but in yellow peach cultivars inking mechanism is unknown. The objective of the present investigation was to evaluate the effect of mechanical damage and contamination with Fe metal ions in the development of inking disorder in three yellow peach cultivars. The present study revealed that Fe₂(SO₄)₃ application increased total phenolic content and chlorogenic acid levels in all cultivars but did not induce inking symptoms in the absence of mechanical damage. In contrast, brushing treatments triggered inking development in 'Colegio' and 'MG8', allowing differentiation of cultivar susceptibility, whereas '229' showed complete resistance. Mechanical damage also significantly increased phenolic compounds, as well as phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) activities, with higher levels consistently observed in susceptible cultivars. Notably, the severity of inking increased when mechanical damage was combined with Fe(III) application. These results indicate that inking development requires both phenolic accumulation and their enzymatic oxidation. Fe(III) ions act as an enhancing factor by stimulating phenolic biosynthesis, thereby intensifying discoloration. The resistance observed in cv. '229' is associated with lower phenolic synthesis and oxidative activity. This study provides new insights into the physiological mechanisms of inking in yellow peaches and offers practical implications for postharvest management and cultivar selection.