Cell wall metabolism and chilling injury during postharvest cold storage in zucchini fruit

• Chilling injury in zucchini fruit is associated with changes in cell wall metabolism.
• Cold storage induces the solubilization and depolymerization of pectin.
• Cold storage induces the activity of cell wall-modifying proteins.
• The lignification process is related with the onset of CI in zucchini fruit.

Postharvest cold storage in zucchini fruit extends the commercial life but causes the appearance of chilling injury (CI), characterized by the development of pits and damaged areas at the surface of the fruit. This physiological disorder has been related to alterations of the cell wall metabolism in several fruit. We have analyzed the relationship between the development of CI and the changes that take place at the cell wall due to cold storage in zucchini fruit, as well as the effect of a preconditioning treatment on these changes. Microscopical observations have shown that the surface depressions detected in chilling injured fruit were caused by cell death and cell collapse. Low temperature induced the solubilization of the more soluble pectins, as evidenced by the highest levels of neutral sugars and uronic acids found in control fruit after the cold storage in water-soluble fraction (WSF) and CDTA-soluble fraction (CSF). The results obtained in Na2CO3-soluble fraction (NSF) for both parameters was opposite; higher levels were detected in preconditioned fruit and lower in more damaged fruit. The same behaviour was found in the base soluble hemicelluloses, 1 M KOH- and 4 M KOH-soluble fractions (1KSF and 4KSF), and in the insoluble cellulose content. Zucchini fruit stored at low temperature showed an increase of the enzymatic activities pectin methylesterase (PME), polygalacturonase (PG), and cellulase (CEL), and an accumulation of mRNA corresponding to a expansin (EXP) gene. Preconditioned fruit showed the lowest levels of these enzymatic activities. Microscopic analysis of CI fruit correlated with the biochemical changes observed in cell wall. Lignin content was higher in control than preconditioned fruit, suggesting a possible role of the lignification process in CI development in zucchini.