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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
78 Postharvest Diseases of Fruits and Vegetables
Several phytoalexinic compounds, such as umbelliferone, scopoletin
and esculetin, are produced in sweet potato roots infected by the fungus
Ceratocystis fimbriata, and it was noted that these compounds
accumulate more rapidly in roots resistant to this fungus than in
sensitive roots (Minamikawa et al., 1963).
The resistance of celery petioles to pathogens has been attributed over
the years to psoralens, linear furanocoumarins which are considered to
be phytoalexins (Afek et al., 1995a, b; Beier and Oertli, 1983). These
compounds, which have deleterious effects on the skin of humans, were
previously thought to be mycotoxins produced by Sclerotinia sclerotiorum
in celery (Ashwood-Smith et al., 1985). However, mechanical damage
during harvesting and storage, and other elicitors, such as low
temperatures and UV, have also been shown to induce furanocoumarin
production (Beier and Oertli, 1983; Chaudhary et al., 1985). The psoralen
content of celery increases during storage (Chaudhary et al., 1985), and
the levels of various psoralens found in older celery stalks are higher
than those in younger stalks (Aharoni et al., 1996). Furthermore,
infection of celery with S, sclerotiorum or Botrytis cinerea, the main
fungal pathogens of stored celery, was found to stimulate psoralen
production during storage (Austad and Kalvi, 1983). Recent studies with
celery (Afek et al., 1995b) indicate that (+) marmesin, the precursor of
linear furanocoumarins in this crop, and not psoralens, is the major
compound involved in celery resistance to pathogens; it has at least 100
times greater antifungal activity than psoralens. Also, increased
susceptibility of stored celery to pathogens is accompanied by a decrease
in (+) marmesin concentration and a corresponding increase in total
psoralen concentration. However, treatment of celery prior to storage
with gibberellic acid (GA3), a naturally occurring phytohormone in
juvenile plant tissue, resulted in decay suppression during 1 month of
storage at 2°C (see the chapter on Means for Maintaining Host
Resistance - Growth Regulators), although GA3 does not have any effect
on fungal growth in vitro (Barkai-Golan and Aharoni, 1980). It was
suggested that the phytohormone retards celery decay during storage by
slowing down the conversion of (+) marmesin to psoralens, thereby
maintaining the high level of (+) marmesin and low levels of psoralens
and, thus increasing celery resistance to storage pathogens (Afek et al.,
1995b).
Another phytoalexin found in celery tissue is columbianetin, which
probably also plays a more important role than psoralens in celery
http://arab2000.forumpro.fr