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Papaya (Carica papaya L. cv. Eksotika) fruit softens differentially in relation to position of the tissue. The inner mesocarp tissue is softer, and its firmness decreases more rapidly during ripening than that of the outer mesocarp tissue. As the fruit ripens, pectin solubility and depolymerisation increase. Hemicellulose, too, appears to be depolymerised but, unlike pectins, this apparent degradation of hemicellulose is associated with an increase rather than a decrease in its level. Pectin and hemicellulose depolymerisation began in the inner mesocarp tissue at about the same time as β‐galactosidase (EC 3.2,1.23) activity started to increase and tissue firmness began to decrease more rapidly. In contrast, pectin solubilisation in both outer and inner mesocarp tissues occurred steadily throughout ripening at a comparable rate and paralleled closely the increase of polygalacturonase (PG; EC 3.2.1.67) and pectinesterase (EC 3.1.1.11). In general, irrespective of enzyme distribution, tissue softening during ripening was more closely related to changes in β‐galactosidase activity than to PG or pectinesterase activity. Papaya, β‐galactosidase appears to be an important wall degrading enzyme and may contribute significantly to differential softening, perhaps by complementing the action of polygalacturonase. Polygalacturonase activity increased with increasing depth of the mesocarp tissue, as did softening of the fruit.

1989. Polygalaeturonase aetivity and variation in ripening of papaya fruit with tissue depth and heat treatment -Physiol. Plant. 77; 93-98.Effects of tissue position (viz. outer vs inner mesocarp) and heat treatment (48°C, 20 min) on variations in polygalacturonase (EC 3.2.1.1.5 and EC 3.2.1.67) activity and ripening of fruits of Catica papaya L. ev. Backctoss Solo were investigated. Polygalaeturonase aetivity increased during ripening concotnitantly with an ineteasc in tissue softness and soluble polyuronide level. Throughout ripening, inner mesocarp tissue was softer and eontained higher polygalaeturonase activity than outer mesocarp tissue. Tittatable acidity as well as /i-galactosidase (EC 3.2.1.23) activity also increased during ripening; however, unlike polygalacturonase, their level or activity was lower in inner than in outer mesoeatp. Ascorbic acid could paitially account for the inerease in titiatable acidity during ripening but contributed very little to the differences in titratable aeid levels between outer and inner mesoearp. Heat treatment had no effeet on either fruit softness or tittatable aeidity, but it markedly redueed the increase in ascorbic acid and polygalacturonase activity during ripening. Ripening, as reflected by ehanges in tissue sottness and polygalacturonase activity, progressed outwatdly frotn the interior towatds the exterior of the fruit. The effect of heat treatment in supptessing polygalacturonase activity was relatively greater in inner than in outer mesocarp, suggesting that sensitivity of the enzyme to heat treatment may vary with stage of ripeness of the tissue.

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β‐Galactosidases (EC 3.2.1.23) from ripe papaya (Carica papaya L. cv. Eksotika) fruits having galactanase activities were fractionated by a combination of cation exchange and gel‐filtration chromatography into three isoforms, viz., β‐galactosidase I, II and III. The native proteins of the respective isoforms have apparent molecular masses of 67, 67 and 55 kDa, each showing one predominant polypeptide upon SDS‐PAGE of about 31 and 33 kDa for β‐galactosidases I and III, respectively, and of 67 kDa for β‐galactosidase II. The β‐galactosidase I protein, which was undetectable in immature fruits, appeared to be specifically accumulated during ripening. The β‐galactosidase II protein was present in developing fruits, but its level seemed to decrease with ripening. β‐Galactosidase I seemed to be an important softening enzyme; its activity increased dramatically (4‐ to 8‐fold) to a peak early during ripening and correlated closely with differential softening as related to position in the fruit tissue. The inner mesocarp tissue was softer, and its wall pectins were modified earlier and firmness decreased more rapidly during ripening compared to the outer mesocarp tissue. β‐Galactosidase II also may contribute significantly to softening because of its ability to catalyse increased solubility and depolymerization of pectins as well as through its ability to modify the alkali‐soluble hemicellulose fraction of the cell wall. The physiological significance of both β‐galactosidase isoforms may partly be attributed to their functional capacity as β‐(1,4)‐galactanases.