Cell wall modifications, degrading enzymes and softening of carambola fruit during ripening

The fruit extracts of the slow-ripening cv. B 10 carambola (Averrhoa carambola L.) contained a number of cell wall hydrolases. The predominant ones appeared to be beta-(1,4)-glucanase (as carboxymethylcellulase), pectinesterase, beta-galactosidase, and polygalacturonase (PG). Other significant hydrolases the activity of which also increased with ripening were the glycosidases, alpha-arabinosidase, alpha-galactosidase, and alpha-mannosidase, and also the glycanases, beta-(1,4)-galactanase and xylanase. Throughout ripening, as pectins and hemicelluloses were being differentially modified, the levels of buffered-phenol cell wall materials, total polyuronides as well as arabinose, galactose, xylose, and glucose decreased. At early ripening phase (days 0-12) there was no apparent pectin solubilization, and the loosely-bound water- and chelator-soluble pectins were the first pectic polysaccharides to be affected. That of the former exhibited an upshift, whilst the latter, showed a distinct downshift in their molecular size profiles. At late ripening phase (days 12-24) when tissue firmness had declined substantially, dramatic changes involving pectins and hemicelluloses were evident. Pectins were solubilized, and this increased solubility was accompained by depolymerization of all pectin classes and a decrease in the level of the Na(2)CO(3)-soluble polyuronides. Coincident with this marked modifications of the tightly-bound, predominant polyuronide fractions and hemicellulose was the increase in activities of polygalacturonase and beta-(1,4)-glucanase, suggesting that these enzymes may contribute to wall modifications late during ripening. Some of the other wall hydrolases, namely, alpha-arabinosidase, alpha-galactosidase and certain isoforms of beta-galactosidase/galactanase, might be relevant to the early ripening changes when pectin solubilization was limited.