Chilling-associated softening of tomato fruit is related to increased pectinmethylesterase activity

Mature-green tomatoes chilled 15 days (5 degrees C; RH >85%) were softer than nonchilled during subsequent ripening (22 degrees C) by both whole fruit and pericarp tissue puncture (p<0.05), but not by flat-plate compression. No differences in total polygalacturonase (PG) or PG isozyme activity were evident although total activity was greater in nonchilled after 10 days ripening. Softening of nonchilled fruit correlated (P<0.05) with extracted PGI activity, while chilling-associated softening correlated (p<0.05) with higher initial extracted pectinmethylesterase (PME) activity. Extracted peroxidase remained constant throughout ripening but was greater (p<0.05) in pre-chilled fruit consistent with chill-induced membrane dysfunction. Transmission electron microscopy showed the middle lamella from pre-chilled tomatoes was swollen and less defined. Loss of turgor from translocation of water to the PME-modified cell wall was suggested to be responsible for softening as a consequence of chilling.