- CHILLING INJURY IN TOMATO FRUIT 247
Low temperature storage alters tomato’s textural properties resulting
in unusual changes in firmness. Different research groups observed vari-
ation in chilling effects on firmness changes including increased soft-
ening (Efiuvwevwere and Thorne 1988), inhibited softening (Rugkong
et al. 2011), or both excessive softening and persistent firmness follow-
ing exposure to low temperatures (Hobson 1987). Inconsistent reports of
changes in firmness of tomatoes stored at chilling temperatures could
result from differences in the definition of firmness, in methods used
to measure firmness, and in the degree to which fruit had been chilled
(Jackman et al. 1990). Chilling-related softening could be attributed to
formation of pectate salts (Buescher 1974), associated changes in ion
leakage (Autio and Bramlage 1986), loss of tissue integrity (Moline
1976), and/or altered cell wall metabolism (Rugkong et al. 2010) that
could be influenced by exposure time and the extent of chilling
temperature.
Mechanisms of chilling-induced softening can be confounded with
normal ripening-associated changes in softening. Excessive softening is
reported as a CI symptom in tomatoes over a wide range of tempera-
tures (2.5–10◦C). Since ripening can initiate softening, and not all stud-
ies carefully separate chilling-induced softening from ripening-related
softening, it is difficult to determine if the reported chilling-induced
softening (especially at warmer temperature such as 10◦C) is not con-
founded by ripening-related softening. An attempt was made by Biswas
et al. (2014a) to differentiate chilling-induced softening from ripening-
related softening using color as an indicator of ripening. By plotting
firmness data (as a dependent variable) against color (an independent
variable), significantly different trajectories in firmness were observed
between chilled (2.5◦Cor6◦C) and non-chilled (20◦C) fruit. It was sug-
gested that loss of stiffness of fruit stored at 20◦C was coordinated with
ripening (color change), whereas fruit stored at 2.5◦Cor6◦Cfailedto
coordinate softening with color change, thus softening was chilling
induced (Biswas et al. 2014a).
During normal ripening, softening in tomato is characterized by
a reduction in cellular turgor pressure (Shackel et al. 1991) and
catabolism of different cell wall components (Brummell and Harp-
ster 2001). Cell wall disassembly is a complex process involving
both enzymatic and non-enzymatic mechanisms. Studies show that
many classes of cell wall-modifying enzymes coordinate this cell
wall modification in a synergistic manner, including polygalacturonase
(PG), pectin methylesterase (PME),훽-galactosidase (훽-gal), endo-1,4-훽-
glucanase (EGase), xyloglucan endotransglucosylase/hydrolase (XTH),
and the cell wall-localized protein, expansin (EXP) (Brummell and