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

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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 β-(1,4)-glucanase (as carboxymethylcellulase), pectinesterase, β-galactosidase, and polygalacturonase (PG). Other significant hydrolases the activity of which also increased with ripening were the glycosidases, α-arabinosidase, α-galactosidase, and α-mannosidase, and also the glycanases, β-(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 accompanied by depolymerization of all pectin classes and a decrease in the level of the Na2CO3-soluble polyuronides. Coincident with this marked modifications of the tightly-bound, predominant polyuronide fractions and hemicellulose was the increase in activities of polygalacturonase and β-(1,4)-glucanase, suggesting that these enzymes may contribute to wall modifications late during ripening. Some of the other wall hydrolases, namely, α-arabinosidase, α-galactosidase and certain isoforms of β-galactosidase/galactanase, might be relevant to the early ripening changes when pectin solubilization was limited.