Production Practices and Quality Assessment of Food Crops. Vol. 1

14 R. M. Madakadze and J. Kwaramba
1979) and a loss of cell compartmentalization. Calcium stabilizes cell membranes
by bridging phosphate and carboxylate groups of phospholipids (Caldwell and Hang,
1981) and proteins, preferentially at membrane surfaces (Legge et al., 1982). There
can be an exchange between Ca 2+ at these binding sites and other cations (e.g. K + ,
Na + , or H + ) although the latter cannot replace Ca 2+ in its membrane stabilization role.
To fulfill its functions at the plasma membrane, Ca 2+ must always be present in
the external solution where it regulates the selectivity of ion uptake and prevents
solute leakage from the cytoplasm. The membrane protecting effect of Ca 2+ is most
prominent under various stress conditions such low temperature (Zsold and Karvalry,
1978) and anaerobiosis (Cristiansen et al., 1970).
The degradation of pectates in cell walls is mediated by polygalacturonase, which
is drastically inhibited by high Ca 2+ concentrations. In Calcium – deficient tissue
polygalacturonase activity is increased (Konno et al., 1984), and a typical symptom
of Calcium deficiency is the disintegration of cell walls and the collapse of the
affected tissues, such as the petioles and upper parts of the stems (Bussler, 1963).
The proportion of Ca in the cell walls is of importance for determining the susceptibility
of the tissue o fungal infections and for the ripening of fruits. In tomato
fruit the Ca 2+ content of the cell walls increases to the fully-grown immature stage
but this is followed by a drop in the content just before the onset of ripening (softening
of the tissue) (Rigney and Wills, 1981). A shift in the binding stage of Ca 2+
occurs in which water soluble Ca 2+ is favoured over wall bound Ca 2+ . This shift is
associated with a sharp increase in ethylene formation in the fruit tissue.
High growth rates of low transpiring organs, increases the risk that the tissue
content of Ca 2+ falls below the critical level required for the maintenance of
membrane integrity, leading to typical Calcium deficiency – related disorders such
as tip burn in lettuce, blackheart in celery, blossom-end rot in tomato or watermelon,
and bitter pit in apple (Shear, 1975; Bangerth, 1979). Low tissue levels of Ca 2+ in
fleshy fruits also increase the losses caused by enhanced senescence of the tissue
and by fungal infections. Vitrescence, a physiological disorder in melons has been
reported as being caused by Ca deficiency (Jean Baptiste et al., 1997). The symptoms
are an intense colour, with a vitreous aspect and a delinquescent texture).
3.3.2. Potassium
Potassium is a univalent cation. It is characterised by high mobility in plants at
all levels, within individual cells, within tissues, as well as in long-distance transport
via the xylem and phloem. Potassium is the most abundant cation in the
cytoplasm, and potassium salts make a major contribution to the osmotic potential
of cells and tissues of the glycolytic plant species. Cytoplasmic K + concentrations
are maintained in the relatively narrow range of 100 to 120 mM. The various
functions of K + in cell extension and other turgor regulated processes are related
to the K + concentration in vacuoles. Potassium ions act as a charge carriers of
high mobility that form weak complexes in which they are readily exchangeable
(Wyn Jones et al., 1979). The high concentrations of K + in the cytoplasm and the
chloroplasts are required to neutralize the soluble (e.g. organic acid anions and
inorganic anions) and insoluble macromolecular anions and to stabilize the pH