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

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Means for Maintaining Host Resistance 143
significant role in calcium intake and affect the efficiency of the
treatment. However, the amount of calcium taken in during treatment
also depends on the growing conditions and environmental factors, and
can vary with the cultivar and with the state of maturity of a given
cultivar. Experiments with 'Golden Delicious' apples showed that calcium
absorption in fruits picked too early (2 weeks before the prime harvest
period) and treated with CaCb solution was low and did not inhibit
decay. However, fruit picked 2 weeks after prime harvest absorbed large
quantities of calcium that markedly reduced decay development, but
caused severe calcium injury. The optimum calcium treatment at prime
harvest reduced decay 40% with no injury (Conway and Sams, 1985).
Most of the calcium that penetrates into the host tissue seems to
accumulate in the middle lamella region of the cell wall. The
calcium-induced resistance of storage organs (fruits or tubers) to
postharvest pathogens has been attributed to an interaction between the
cell wall pectins and Ca ions. By binding to pectins in the cell wall, Ca
ions contribute to maintaining the structural integrity of the cell wall.
Pectins are chains of polygalacturonic acid residues into which rhamnose
is inserted. The rhamnose insertion causes kinks in the chain, which
allow the attachment of Ca ions. The cations form bonds between
adjacent pectic acids or between pectic acids and other polysaccharides,
forming cross bridges which make the cell walls less accessible to the
action of pectolytic enzymes of the pathogen (Preston, 1979). When
apples were infiltrated with Ca, or potato tubers were fertilized with high
Ca or infiltrated with Ca, the quantity of Ca bound to the cell walls
increased (Conway et al., 1987; McGuire and Kelman, 1984), and cell
walls became less vulnerable to the pectolytic enzymes of the pathogen.
The reduction in decay caused by P. expansum in apple fruits has thus
been correlated, at least in part, to the improved structural integrity of
the tissues imparted by an increase in the Ca content of the cell walls
(Conway et al., 1987).
Analysis of the pectic fractions of grape berries cv. Sauvignon, known
to be resistant to Botrytis cinerea, showed that the content of host
cell-wall pectins decreased following infection with this pathogen. At the
same time, the proportions of water and a chelator-soluble protein
increased. The changes in the pectin composition of the berries were
markedly less pronounced when the berries were infiltrated with calcium
prior to infection (Chardonnet et al., 1997). It was suggested that
dimethylated proteins produced by B. cinerea are probably protected by a
rapid calcium chelator.
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