Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Means for Maintaining Host Resistance 139
effect on fungal development in vitro, or in vivo on inoculated fruits, and
its effect on decay development was attributed to the enhancement of
fruit resistance. A pre-harvest combination of GA3 and iprodione
treatments may further reduce the percentage of fruits rendered
unmarketable by the black spot disease (Perez et al., 1995).
Retardation of senescence by growth regulators has also been reported
for leafy vegetables. Fresh herbs are prone to accelerated senescence
because of their high rate of metabolism, which is further increased
following harvesting and handling procedures. Their accelerated
senescence is expressed in decreased levels of RNA, proteins, fats,
carbohydrates, organic acids and vitamins, and increased enzymatic
activity of RNase, protease, lypase and other hydrolytic enzymes, as well
as those associated with fat oxidation. These changes are accompanied by
chloroplast destruction and chlorophyll decomposition, resulting in a
rapid yellowing of the leaves (Aharoni, 1994). During harvesting, grading
and packaging, when the leaves lose water because of evaporation, and
suffer from mechanical injury, the senescence process is further
enhanced and is accompanied by increased ethylene and respiration
levels. The endogenous ethylene continues to accelerate chlorophyll
destruction and leaf senescence (Meir et al., 1992). Along with their
aging, the leaves lose their natural resistance to pathogens and tend to
rot. In many cases, the senescence process is controlled by plant
hormones; it is accompanied by decreases in the levels of
growth-stimulating hormones - gibberelin, cytokinin and auxin, and
increases in those of growth-inhibiting hormones - ethylene and abscissic
acid (ABA). It is not surprising that application of synthetic hormones -
gibberelin, cytokinin and a very low concentration of auxin - was very
efficient in delaying leaf senescence, while application of ABA or ethylene
accelerated it (Aharoni and Richmond, 1978). Delay of senescence by
gibberelin and cytokinin is attributed to their antagonistic effects on
ethylene and to their ability to neutralize the stimulation of chlorophyll
destruction caused by ethylene (Aharoni, 1989).
The delay of aging conferred by GA3 has also been found in several
fresh herbs which tend to yellow, such as chives, dill, chervil, coriander
and parsley (Aharoni, 1994). In a laboratory experiment with chives, a
dip in GA3 solution (10 ppm) was found to prevent the accelerated
respiration caused by the cutting stress, and then to prevent the
climacteric respiration associated with leaf senescence. A marked effect
on respiration was achieved by spraying chives in the greenhouse, close
to harvesting. Such a treatment delays chlorophyll destruction, retards
http://arab2000.forumpro.fr