06 Physiological disorders - Copy - PLB 172 1 of 8...

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PLB 172 1 of 8 PHYSIOLOGICAL DISORDERS OF FRESH HORTICULTURAL CROPS M.S. Reid, A.A Kader, M.E. Saltveit University of California, Davis 95616 Before and after harvest, the quality of per- ishable commodities can be reduced by a variety of external or internal blemishes resulting from improper environmental or cultural conditions. Blemishes without obvious causal fungal, bacte- rial, viral or insect agent are termed physiological disorders. Physiological disorders can be broadly classified into two groups - those caused by ad- verse temperature conditions, and those appar- ently resulting from physiological malfunction within the normal temperature range for the prod- uct. In this lecture we will also briefly discuss a further type of disorder, caused by environmental toxicants of various types. EFFECTS OF TEMPERATURE Freezing injury Reduction of the temperature below the freezing point of the tissue can result in freezing injury, a disorder whose symptoms include water- soaked areas in the tissue, and collapse and even disruption of the epidermis. Low temperatures may occur in the field, or in situations where tem- peratures in the storage environment fall, whether through faulty refrigeration thermostats, or ex- treme low temperatures during transportation. The presence of dissolved solids in the cell sap reduces the freezing point of plant tissues be- low that of pure water. This freezing point de- pression, which is a function of the osmolality of the cell solution is at most a few C and often less than 1C (Table 1). In general, the higher the soluble solids con- tent of the tissue, the lower the freezing point. For example, while high-sugar wine grapes will not freeze until the temperature falls below 5.0 C, leafy vegetables such as lettuce can freeze just below 0C. A knowledge of the way in which freezing occurs in plant tissues is important in understanding the symptoms which may develop. Table 1. Freezing points for some common fruits and vegetables. Commodity Freezing point range C Apple -2.2 ............... -1.7 Asparagus -1.4 ............... -1.1 Cherry -4.3 ............... -3.8 Cucumber -0.9 ............... -0.8 Grape -5.3 ............... -2.9 Lettuce -0.6 ............... -0.3 Onion -1.3 ............... -0.9 Orange -2.3 ............... -2.0 Potato -1.8 ............... -1.7 Tomato -1.0 ............... -0.7 When the temperature of tissues falls below the freezing point, ice crystals are not immediately formed because there is a substantial capacity for supercooling in plant tissues; for some time, the cell solution remains liquid even though it is be- low its freezing point. In many situations short periods of exposure to freezing conditions will not damage tissue because it supercools and rewarms without ice ever being formed. In certain situa- tions, however, super-cooling may be devastating because of the rapidity with which freezing occurs when the supercooled solution finally freezes, ei- ther by prolonged exposure to low temperature or when nucleated by vibration. Burton (1982) quotes a dramatic example of this effect. "When potatoes were widely stored in the open in field clamps, it was not uncommon for a grower to grade apparently sound but, unknown to him, su- percooled tubers for sale. They would be shov-
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