- CHILLING INJURY IN TOMATO FRUIT 249
chilled fruit as there was no PG accumulation in chilled tomato. While
increasing PG activity in transgenicrin(ripening inhibitor) fruit caused
increased pectin solubilization (Giovannoni et al. 1989), and transgenic
tomato suppressed in PG activity showed reduced water-soluble pectin
(Carrington et al. 1993), other studies with PG-antisense fruit have
shown no correlation between pectin solubilization and PG activity
(Brummell and Labavitch 1997). Huber (1983) found that some fruit can
soften in the absence of a dramatic increase in PG activity and Lurie
et al. (1992) suggested that the levels of PG mRNA did not correlate
well with the measured firmness of tomato. Jackman et al. (1992) and
Marangoni et al. (1995) reported that enhanced softening in tomatoes
was not correlated with PG activity but was associated with higher PME
activity, especially when fruit were removed from cool storage.
PMEs play a minor role in fruit softening during ripening, how-
ever, their effect on cell wall tissue integrity and rigidity is important
(Brummell and Harpster 2001). Low PME expression in colorless non-
ripening (Cnr) tomato mutants is thought to be responsible for main-
taining a strong cell wall, indicating its role in maintaining fruit cell
wall integrity (Eriksson et al. 2004). PME activity in chill-injured fruit
is also a matter of debate. Both reduced PME activity (Buescher and Fur-
manski 1978) and increased PME activity (Brummell et al. 2004) were
reported for chilled stone fruit whereas in tomato, Rugkong et al. (2010)
observed that PME activity was not affected by cool storage. Art ́es et al.
(1996) attributed chill-induced woolliness in peaches and nectarines to
low PG and continuous PME activity.
Cell wall metabolism of tomato fruit is affected by exposure to CI-
inducing temperatures, in some ways that are similar to those associ-
ated with mealiness development in stone fruit (Rugkong et al. 2010).
Mealiness has been reported as a CI symptom in tomato (Jackman et al.
1992), and has been described as a dry, grainy, coarse look on the cut
surface (Ahrens and Huber 1988). Ultrastructural evidence shows an
extensive dissolution of the pectin-rich middle lamella in chill-injured
tomato (Marangoni et al. 1989) and fruit soften at an accelerated rate rel-
ative to that of non-chilled fruit (Marangoni et al. 1995; Van linden et al.
2008). Chill-injured tomatoes then developed mealiness where water
was translocated to the modified wall, resulting in a dry mouth feel of
chilling-injured fruit (Jackman et al. 1992).
Inhibition of fruit softening has also been reported in chilled tomato
(Watkins et al. 1990; Rugkong et al. 2010) where reduced expression
of genes encoding PG,PE1 (pectin esterase),and LeExp1 contribute to
reduced softening (Rugkong et al. 2011). Reduced expression of gene
encoding expansion or PG was also observed in cold-stored bananas