Differential changes in respiratory metabolism and energy status in the outer pericarp and core tissues affect the ripening of ‘Xuxiang’ kiwifruit

In 'Xuxiang' kiwifruit, the outer pericarp and core soften at different rates, leading to a hard-core and reducing consumer acceptance. Studies have reported that the differences in respiration intensity result in different ripeness levels in various zones of fruit; however, no one has delineated the association between the softening rates and respiratory metabolism in 'Xuxiang'. Therefore, the current work analyzed the respiration and energy status of the outer pericarp and core of 'Xuxiang' and assessed the association of these features with the firmness decrease during postharvest ripening. Compared to the core, the outer pericarp showed a quicker firmness decrease, starch degradation, and citric acid reduction during postharvest ripening. The outer pericarp showed lower nicotinamide adenine dinucleotide kinase (NADK) activity and nicotinamide adenine dinucleotide phosphate (NADP) and NADPH (reduced form of NADP) levels but higher nicotinamide adenine dinucleotide (NAD) and NADH (reduced form of NAD) levels than the core. It also showed higher adenosine triphosphate (ATP), diphosphate (ADP), and monophosphate (AMP) levels and energy charge (EC) than the core. Among the two tissue zones, the outer pericarp displayed an earlier expression of energy metabolism-related genes (AdAAC1, AdAtpB, and AdAOX2). Finally, the Pearson correlation analysis associated the decline in outer pericarp firmness during postharvest ripening with the respiratory substrates, respiration, and energy status in 'Xuxiang' kiwifruit. These observations suggest that the outer pericarp exhibited higher respiration and energy levels than the core, at least in part, which resulted in a rapid decline in the outer pericarp firmness in 'Xuxiang'.