Flesh translucency

Translucency is a physiological disorder where the pineapple fruit flesh shows water-

soaking symptoms. Translucency as opposed to opaqueness, lacks the presence of small air bubbles in the intercellular spaces of the fruit flesh tissue and has a higher SG (Sideris and Krauss, 1933a). The SG can be employed as a non-destructive method for the detection of fruit translucency. Translucent fruit are extremely fragile, making these fruit very prone to mechanical injury during harvest, postharvest handling (Py et al., 1987) and shipping as fresh fruit. In addition, translucent fruit are more susceptible to diseases (Gortner et al., 1963) and pre-harvest sunburn (Keetch, 1977). Highly translucent fruit have flat and overripe off-flavours and a significantly lower edible quality (Bowden, 1967).

The incidence of peduncle leakage is correlated with translucency severity, and this leakage keeps the broken peduncle wet and may lead to unsightly, dark bluish grey Pénicillium growth postharvest (Paull and Reyes, 1996). Translucency occurs before harvest (Bowden, 1969; Rohrbach and Paull, 1982; Paull and Reyes, 1996) and generally the basal flesh is the first to show symptoms; in severe cases the whole fruit is affected (Paull and Reyes, 1996). Translucent fruit are more common in the cooler season (Fig.

10.8). Waxing can reduce the rate of translucency development after harvest in low shell-colour fruit. Soler (1993, 1994a) regards translucency as being due to early flesh senescence; however, it is apparently also related to the weather 3 or more months before harvest (Paull and Reyes, 1996). Fruit with larger crowns have a lower incidence and severity of translucency.

Fruit with increasing translucency have increased pH, TSS/acid ratio and fruit weight and decreased total esters, and acids decline. The decline in organic acids during ripening is more pronounced in translucent (Sideris and Krauss, 1933a) than in opaque fruit, while sugars show little change. Translucent fruit usually have a higher TSS-to-TA ratio than opaque fruit, due to lower acidity (Sideris and Krauss, 1934; Bowden, 1969).

The cause or causes of translucency are unknown. Translucency in Hawaii has been associated with clones, high nitrogen, large vigorous plants, spring-ripened fruit, treatment with fruit-enlarging agents, irrigation rate, planting density, larger crowns and environmental factors (Paull and Reyes, 1996). Paull and Reyes (1996) found that both crown weight and fruit translucency at har

Fig. 10.8. Variation in crown weight and fruit-translucency severity at different times of the year (from Paull and Reyes, 1996).

vest are correlated to the monthly average air temperature 2-3 months before harvest, and the correlation between crown weight and translucency severity was significantly negative. However, removing the crown either at an early or at a late stage of pineapple fruit development did not have any significant effect on fruit weight or translucency (Chen, 1999). The significantly negative correlation between the air temperature 2-3 months before harvest and translucency (Paull and Reyes, 1996) is possibly related to an increase in heat tolerance of fruit flesh, due to their higher fruit temperature (Chen, 1999).

Pineapple fruit translucency has been suggested to be related to an increase in cellwall hydrolases (Soler, 1993) and membrane permeability (Soler, 1994a). High calcium concentration may decrease the secretion or activities of cell-wall hydrolases (Huber, 1983) and membrane permeability; however, calcium concentration and the divalent cation-binding capacity of cell walls in pineapple fruit flesh decline with development (Chen, 1999). Pineapple fruit translu-cency increases with increasing fruit weight (Bowden, 1969), possibly due to a decrease in calcium concentration, since larger fruit may need more calcium to stabilize cell membranes. When fruit cannot acquire sufficient calcium, cell membranes may lose integrity and lead to leakage and translucency.

Basal fruitlets show translucency first. These tissues have higher sugar content than the interfruitlet tissue and the flesh at the top of the fruit, respectively, suggesting that translucency is related to maturity. Chen (1999) reported that plant defoliation conducted 3 or 4 weeks before harvest decreased the pineapple fruit flesh TSS and translu-cency. There was a linear cause-effect relationship between percentage of defoliation and TSS and translucency. Defoliation did not significantly affect the membrane permeability of pineapple fruit-flesh cells, suggesting that translucency caused by sugar unloading is different from translucency caused by heat stress, and the former was much more significant at the later stages of fruit development.

The intercellular spaces are filled with liq uid in translucent pineapple fruit-flesh tissues, suggesting that a higher osmotic pressure exists in the apoplast than in the symplast, which steepens the pressure gradient between the phloem end and the flesh cells, where unloading is occurring. The higher apoplastic osmotic pressure and water uptake due to increased sugar storage in the apoplast could favour the occurrence of pineapple fruit translucency. CWI activity in pineapple fruit flesh increases rapidly 4 weeks before harvest and is followed almost immediately by the first symptom of translu-cency. A positive correlation also exists between the CWI activity and the severity of translucency (Chen, 1999). The CWI activities in the tissues that first show translucency - the basal fruitlet and basal flesh - are significantly higher than in the interfruitlet tissue and the fruitlets and associated flesh of the top of the fruit, respectively. This suggests that CWI in the pineapple fruit flesh could be a positive factor in the occurrence of translucency.

Translucency can occur when the fruit shell is still green (Py et al., 1987). The green-shell translucency is found more frequently during high-temperature periods (Green, 1963) and may be due to lesions in the cell membrane caused by either high temperatures or too abundant supplies of sap brought about by a sudden improvement in the water-supply during a period of intense photosynthetic activity, or both (Py et al., 1987). Soler (1993, 1994b) showed that green-shell translucency, at a biochemical level, is characterized by an increase in catalase, a-and P-galactosidase activities and ascorbic acid synthesis, which might be related to translucency by the modification of membrane galactolipids and permeability changes. Pineapple fruit flesh became susceptible to high fruit temperature-induced leakage and translucency at the later stages of fruit development (Chen, 1999). Sideris et al. (1935) suggested that flesh translucency and shell degreening were independent phenomena associated with fruit senescence, with the flesh of green ripe fruit senescing before shell degreening started. These green-shell translucent fruit degreen rapidly when treated with ethephon.

Pineapple fruit translucency, which normally occurs at the later stages of fruit development, is therefore due to a combination of effects, related to both high fruit temperature and fruit maturity. Associated factors affecting the severity of translucency include a decrease in calcium concentration of fruit flesh and of divalent cation fruit-flesh cell-wall-binding ability, which leads to a loss of cell-membrane integrity and cell-wall rigidity. These changes are followed by an increase in membrane permeability and an enhanced susceptibility of fruit flesh to high temperature. Increased sucrose accumulation and the activity of CWI favouring apoplastic phloem unloading cause an increase in the solute concentration and liquid volume in the apoplast, which in turn leads to translucency.

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