Fruit and Vegetable Quality

scribed the influence of light on color retention during handling and re-
tailing of vegetables.
Degreening is the most clear indication of ageing and decreasing
freshness of vegetables besides loss of turgor and decay. Improved
knowledge about the processes that initiate and regulate chlorophyll
degradation is therefore of paramount importance in postharvest han-
dling of vegetables to design techniques to prevent or delay the onset of
senescence. This is closely connected to the question about duration of
freshness after harvest. Chlorophyll degradation is not visible to the eye
until senescence is advanced, making color judgements less suitable for
evaluation of freshness. Search for a suitable nondestructive “early
senescence marker” is therefore of high priority, especially for mini-
mally processed, refrigerated vegetables.
The degradation of chlorophyll is assumed to start with the removal
of the phytol chain by the enzyme chlorophyllase resulting in the for-
mation of chlorophyllide. The following steps include replacement of
the central magnesium by hydrogen to form pheophorbide, and finally
cleavage of the porphyrin ring system by a dioxygenase with formation
of colorless low molecular weight compounds stored in the vacuole
(Heaton and Marangoni, 1996). Ethylene seems to play an important
role both for initiating and coordinating chlorophyll breakdown in some
plants, preferably dicots. It is also a common experience that exogenous
ethylene enhances degreening of most vegetables (e.g., mixed loads con-
taining both ethylene-producing fruit and vegetables) but the pathway is
still unclear. Ethylene is reported to increase chlorophyll degradation
with enhanced chlorophyllase activity and accumulation of chlorophyl-
lide in the flavedo of mature green oranges (Amir-Shapira et al., 1987;
Trebitsh et al., 1993). Investigations made into breakdown of chloro-
phyll in harvested vegetables stored in air, air10 ppm ethylene or
modified atmosphere (Yamauchi and Watada, 1991, 1993, 1998) indi-
cate the presence of different pathways for chlorophyll degradation.
The role of ethylene as an activator of senescence-related genes is un-
clear. Ethylene is probably involved in plant responses to external sig-
nals such as wounding, pathogen and environmental stress but ethylene
seems not to activate senescence genes directly (Buchanan-Wollaston,
1997). Senescence of detached leaves was enhanced by wounding with
the first peak of ethylene as a response to detachment (Philosoph-Hadas
et al., 1989). Ethylene is usually produced at very low levels in vegeta-
tive tissue. Wounding induces enzymes in the ethylene biosynthetic path-
way, resulting in a burst of ethylene that may occur immediately or some

108 POSTHARVEST HANDLING AND STORAGE OF VEGETABLES