Christopher B. Watkins

Physiological disorders of fresh crops occur as a result of altered metabolism in response to imposition of stresses and are manifested as visible symptoms of cellular disorganization and cell death. Most physiological disorders of temperate fruit occur after harvest, when they are removed from supplies of water, nutrients, hormones, and energy from the tree. Therefore, fruit have an altered ability to respond to stresses in the environment that interrupt, restrict, or accelerate normal metabolic processes in an adverse or negative manner. Interestingly, however, many postharvest management regimens beneficially utilize stress conditions such as temperature and atmosphere modification to maximize storage potential of fruit. During the postharvest period, stress is an external factor that will result in undesirable changes only if the plant or plant part is exposed to it for a sufficient duration or sufficient intensity, and, therefore, the postharvest period can be seen as a time of stress management (Kays, 1997).

Physiological disorders are distinct from the many other undesirable postharvest changes in quality, such as water loss, softening, loss of chlorophyll, and other ripening-related events associated with normal senescence, which affect storage potential and thus marketability of fruit. The definition also excludes a number of direct postharvest injuries that can occur as a result of mechanical damage (e.g., bruising), freezing injuries, and exposure to gases or chemical solutions (e.g., ammonia leaks in cold storage or salts and antioxi-dants used in postharvest treatments). Pathological disorders are also distinct, but it is not uncommon for diseases to be associated with physiological disorders, especially as secondary infections.

A common feature of all physiological disorders is that susceptibility to injury is affected greatly by cultivar, through characteristics such as skin diffusivity to oxygen and carbon dioxide, cell wall and membrane properties, mineral composition, and antioxidant status. Preharvest factors, which include climate, maturity at harvest, nutrition, and orchard management methods, also affect these characteristics. Considerable variation in the resistance of a given fruit to imposed stress occurs, and, therefore, severity and timing of disorder expression can be observed among apparently similar lots of the same cultivar or strain.

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