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

temperatures. Some show serious internal discoloration after prolonged storage at
several degrees above their actual freezing point, while others may not show injury
at –1.7 °C, the average freezing point of most potato varieties.
Freezing injury of celery can be readily recognized at harvest time by the flabby
water-soaked condition of the leaves and leaf stalks. Frozen leaves, if not attacked
by bacteria, dry out and become papery. A second type of freezing symptom is
the appearance of isolated sunken lesions on the leafstalks. These two types of injury
are most often apparent at harvest but are of little importance on the market.
3.3. Mineral nutrient disorders
In this section the key nutrients causing physiological disorders and their role in
vegetables will be discussed. A mineral nutrient can function as a constituent of
an organic structure, as an activator of enzyme reactions or as a charge carrier
and osmo-regulator.
3.3.1. Calcium
Effect of Preharvest Factors 13
Calcium is a relatively large, divalent cation. The Calcium content of plants varies
between 0.1 and > 0.5% of dry weight depending on the growing conditions, plant
species, and plant organ. Genotypically differences in Ca 2+ requirements are closely
related to the Ca 2+ binding sites in the cell walls. Ca deficiency in plant tissues causes
many physiological disorders which lead to significant losses in plant production.
Calcium shortage in plants is related to poor Ca uptake, its limited movement
to above – ground plant parts and strong competition for Ca between leaves and
generative plant parts (fruits, seeds) (Wojcik, 1998).
Calcium readily enters the apoplasm and is bound in an exchangeable form to
cell walls and at the exterior surface of the plasma membrane. Its rate of uptake
into the cytoplasm is severely restricted and seems to be only loosely coupled to
metabolic processes. The mobility of Ca from cell to cell and in the phloem is
very low. It is the only mineral nutrient other than Bo which functions mainly outside
the cytoplasm in the apoplasm. Most of its activity is related to its capacity for
co-ordination by which it provides stable but reversible intermolecular linkages;
predominantly in the cell walls and the plasma membrane. These Ca 2+ mediated
linkages respond to local changes in environmental conditions and are part of the
control mechanism of growth and developmental processes. Calcium is a nontoxic
mineral nutrient, even in high concentrations, and is very effective in
detoxifying high concentrations of other mineral elements in plants.
There are two distinct areas in the cell wall with high Ca 2+ concentrations, the
middle lamella and the extension surface of the plasma membrane. In both areas
Ca 2+ has essential structural functions, namely, the regulation of the membrane
permeability and related processes and the strengthening of the cell walls.
The fundamental role of Ca 2+ in membrane stability and cell integrity is reflected
in various ways. It can be demonstrated most readily by the increased leakage of
low molecular weight solutes (e.g. in tomato fruits; Goor, 1968) and in severely
deficient plants, by a general disintegration of membrane structures (Hecht-Buchholz,