Interactions between the Amount and Molecular Forms of Polygalacturonase, Calcium, and Firmness in Tomato Fruit

Abstract

A range of tomato (Lycopersicon esculentum Mill.) cultivars was examined for changes during ripening in firmness, endopolygalacturonase (PG) activity and the molecular forms of polygalacturonase, Ca concentration, and the extractability of the Ca. Firm cultivars were firmer than the soft cultivars throughout ripening, and generally they contained less PG activity at each stage examined. In all cultivars, PG was predominately or entirely in the high molecular weight form (PG1) early in ripening, with the PG2 forms being increasingly prominent as ripening progressed. Differences in firmness were established while PG1 was the predominant PG. Uronic acid polymers in isolated cell walls were degraded rapidly by endogenous PG when citrate was present to complex Ca. In the presence of sufficient citrate, cell wall uronic acids of a firm and soft cultivar were equally susceptible to hydrolysis, suggesting that differences in the digestion of the walls by PG were dependent upon differences in Ca content or distribution. However, neither total, water, nor saline-extractable Ca showed consistent correlations with fruit firmness, and they also showed no progressive change during ripening.

Factors Affecting Drip Loss from Thawing Thornless Blackberries

“…However, there was no indication of any relationship between endogenous soluble or insoluble Ca and drip loss in this study (Table 4). Brady et al (2) were unable to show a consistent relationship between total, water-extractable, or saline-extractable endogenous Ca and firmness in tomato fruit. However, the application of preharvest sprays containing Ca salts to blackberries and raspberries has been shown to improve fruit firmness following postharvest holding (7,14).…”

“…fruit must be allowed to mature before harvest and to ripen thereafter for attainment of desirable culinary quality. Additionally, their general response to chilling exposures has been characterized (1,2,3,5). This report deals with changes in ACC concentration in the skin and cortex of 'Honey Dew' melons in response to: a) duration of chilling exposure; b) subsequent warming; c) degree of ripeness of the melons; and d) an ethylene treatment applied before the melons were stored at a chilling temperature.…”

“…This report deals with changes in ACC concentration in the skin and cortex of 'Honey Dew' melons in response to: a) duration of chilling exposure; b) subsequent warming; c) degree of ripeness of the melons; and d) an ethylene treatment applied before the melons were stored at a chilling temperature. The latter approach is based on the observation that such a treatment reduces the chilling sensitivity of 'Honey Dew' melons (2,5). We also investigated the relation between the occurrence of solar yellowing (SY), a yellow pigmentation of the surface of 'Honey Dew' melons induced by exposure to sunlight (2), and ACC accumulation during chilling.…”

“…of fresh and frozen fruit are well-established (2,17). Calcium addition to strawberries prior to freezing has been shown to firm the thawed product (15) and reduce drip losses (4).…”

Ca2+ in Cell Walls of Ripening Tomato and Peach

“…A major difference in softening and texture between tomatoes and peach could be a result of the difference in CW-ML Ca2+ content and, hence, pectin retention. Whether Ca2+ is directly involved in softening of fruit is still a matter of debate (4); however, the presence of wall Ca2+ deters their degradation by polygalacturonase (4,6) and undoubtedly contributes to differences in peach and tomato in terms of flesh texture.…”