therefore not to be dependent on concurrent deprivation of the senesc-
ing tissue.
If senescence in monocarpic plants represents a genetically pro-
grammed developmental phase, does senescence in harvested vegetables
show any similarities? Today there is growing agreement that senescence
of harvested plant organs proceeds in a similar, genetically controlled
manner as in intact plants. This means that although environmental fac-
tors may trigger the onset and progress of postharvest senescence, the en-
vironment does not regulate it. One difference between natural and
induced senescence is that induced senescence may be reversible if stress
conditions are removed before senescence has reached a certain point
(Smart, 1994).
Senescence in harvested plant organs as vegetables must therefore be
considered as a premature, induced process—if left to continue growth
on the plant, senescence will not occur after a similar period of time.
Both natural and induced senescence proceed in a sequential manner
probably with increasing dominance of catabolic processes leading to
cellular breakdown. However, postharvest senescence is not a passive
decay but rather an actively regulated process. Cells remain viable and
show tight metabolic regulation. Mitochondria retain their function since
respiration must continue until the end of senescence. The expression of
many genes is downregulated in senescing tissue, probably necessary to
prevent regeneration of cell components broken down during senescence
(Noodén et al., 1997). Concomitantly, there is a coordinated expression
of specific senescence genes, and the transcription of such genes seems
to be similar in both natural and induced senescence (Buchanan-Wol-
laston, 1997). The senescence syndrome is composed of several paral-
lel processes such as loss of chlorophyll and dismantling of chloroplasts,
degradation of soluble protein to amino acids and generation of oxygen
radicals resulting in lipid peroxidation and a general loss of cellular com-
partmentation leading to tissue breakdown.
The most evident symptom of senescence in harvested vegetables is
the loss of green color due to degradation of chlorophyll. Normally, light
delays loss of chlorophyll in detached leaves (Okada et al., 1992) but
most vegetables are stored in darkness. Cold storage of cabbage over
150 days resulted in a gradual loss of chlorophyll and degradation in
products with a color change from green to white (Heaton et al., 1996),
which is in agreement with Prange and Lidster (1991), who obtained
better chlorophyll retention in cabbage stored at low light intensities
compared with darkness. However, few, if any, investigations have de-
Physiology of Storage 107