hours after harvest (Abeles et al., 1992). Harvest always introduces
wounding stress and it is therefore conceivable that postharvest degra-
dation of chlorophyll is induced by an enhanced stress-mediated ethyl-
ene production. Trace amounts of ethylene (less than 0.1 ppm) in the
storage atmosphere are physiologically active and therefore promote en-
dogenous ethylene production and senescence. Senescing plant organs
are able to produce sufficient amounts of ethylene to accelerate senes-
cence of the entire content in a closed box of vegetables as well as dur-
ing exposure in a retail cabinet provided the temperature is approaching
room temperature. Careful handling in order to reduce mechanical dam-
age (Abeles et al., 1992), misting to maintain turgor (Barth et al., 1992),
modified atmosphere packaging of perishable products to inhibit ethyl-
ene synthesis and protection from exogenic ethylene sources (Yamauchi
and Watada, 1993), and removal of waste or infested products during
the whole postharvest chain will therefore reduce degreening and delay
senescence.
The involvement of free radicals in the process of leaf senescence is
well established (Leshem, 1988). The chloroplast is a main cellular com-
ponent where reactive oxygen species are generated and inability of the
chloroplast to eliminate free radicals may result in an accumulation of
lipid peroxidation products. The main cellular components susceptible
to damage by free radicals are polyunsaturated fatty acid in membranes,
proteins, carbohydrates, nucleic acids and pigments such as chlorophyll
and carotenoids. Living systems are protected from active oxygen
species by enzymes such as superoxide dismutase, peroxidase and cata-
lase. Besides these enzymatic antioxidants, glutathione, tocopherols,
ascorbic acid, carotenoids and other reducing compounds also contribute
to the elimination of reactive oxygen species. It has been proposed that
initiation of senescence results in generation of oxygen radicals (Leshem,
1988; Dhindsa et al., 1981), with the progress of lipid peroxidation in-
hibited by the ability of the antioxidant system to quench various oxy-
gen radicals. Recently, Toivonen and Sweeney (1998) have reported that
the difference in retention of green color between two broccoli cultivars
could be related to the rate of superoxide dismutase and peroxidase ac-
tivities, which were approximately 30% higher in the cultivar retaining
a stable chlorophyll content over four days at 13°C. The chlorophyll
breakdown in three edible herbs was related to the oxidative defense
system and not to proteolysis, indicating a close relationship between
chlorophyll breakdown and lipid oxidation (Philosoph-Hadas et al.,
1994).
Physiology of Storage 109