Many physiological disorders have been identified in temperate fruit crops, especially the apple. Photographs of most of these are available in sources such as Lidster, Blanpied, and Prange (1999) and Snowdon (1990). Generally, symptoms of physiological disorders are well defined, but understanding of the biochemical processes involved in their development is incomplete.
Physiological disorders can be considered in three categories: those which develop (1) only on the tree, e.g., watercore and sunscald on apples; (2) on the tree and during storage, e.g., bitter pit of apples; and (3) only during storage. The third category includes most physiological disorders (Table P2.1) and is complex because of the many postharvest management techniques used to maintain quality. These disorders can be divided into those associated with senescence, low temperatures, and use of inappropriate atmospheres during storage.
Senescent disorders are related to harvest of overmature fruit and/ or fruit with nutritional imbalances such as high nitrogen and low calcium contents. Storage at higher than optimal temperatures can also contribute to disorder development.
Chilling injuries are visual manifestations of cellular dysfunction in crops exposed to chilling temperatures. There is some controversy as to whether certain low-temperature disorders occurring in temperate crops are manifestations of chilling injury, but the physiological and biochemical mechanisms of injury probably are identical in all susceptible commodities, and only the rate at which these changes occur differs. Temperature-exposure time interactions exist in development of chilling injury, and dysfunctions induced by chilling tem-
TABLE P2.1. Postharvest physiological disorders of apples, pears, peaches, nectarines, plums, and cherries and major effecting factors
Peaches and Nectarines
Climate, fruit maturity, nutrition
Senescent break- Bitter pit, corky
Storage temperature, fruit maturity, climate
Storage atmosphere down, bitter pit, lenticel blotch, Jonathan spot, lenticel spot
Superficial scald, low-temperature breakdown, soft scald, brown core
Low-oxygen injury, epidermal cracking, ribbon scald, brown heart, external carbon dioxide injury_
spot, cork spot, Anjou pit, breakdown, senescent scald, vascular, internal, and core breakdowns
Brown core, pithy brown core, flesh browning
Woolliness, internal breakdown
Low-oxygen injury, high-carbon dioxide injury
Co peratures can be repaired upon transfer of the crop to nonchilling temperatures before permanent injury has occurred. The primary response to direct chilling stress is thought to be physical in nature, centered on the cell membranes (Figure P2.1). The secondary events include the multitude of metabolic processes that are adversely affected as a consequence of the primary event and lead to visible symptoms and cell death. The subdivision between primary and secondary events is not arbitrary, as it is proposed that it allows the time-dependent secondary events ("effects") to be conceptually separated from the more instantaneous primary event ("cause").
Disorders associated with low oxygen and high carbon dioxide occur when fruit are subjected to atmospheres outside safe limits at any temperature-time combination. The safe limits can vary by fruit type, cultivar, and strain. Damage may be manifested as irregular ripening, initiation and/or aggravation of certain physiological disorders, development of off flavors, and increased susceptibility to decay. Tolerances of fruit to storage atmospheres are affected by metabolic and physical factors and can vary greatly among species, cultivars and strains, maturity and ripening stages, and growing conditions.
Lowered oxygen and elevated carbon dioxide concentrations affect respiration and associated metabolic pathways of glycolysis, fermentation, the tricarboxylic acid cycle, and the mitochondrial respiratory chain, as well as pathways involved in secondary metabolism such as production of ethylene, pigments, phenolics, and volatiles.
PRIMARY EVENT SECONDARY EVENTS
PHYSICAL PHASE I \ METABOLIC I \ MANIFESTATION CHANGE OF 1-/ DYSFUNCTION 1-/ OF INJURY
Ethylene production Respiration Energy production Amino acid incorporation Protoplasmic streaming Cellular structure
Discoloration Surface pitting Internal breakdown Loss of ripening capacity Wilting Decay
> Irreversible changes
Time at chilling-injury-inducing temperature
FIGURE P2.1. A schematic representation of responses of plant tissues to chilling stress (Source: Modified from Wang, 1990.)
Increased carbon flux through the fermentation pathway is a common feature of fresh crops exposed to anaerobic conditions, but direct evidence for injury by acetaldehyde and ethanol accumulations has not been demonstrated. Fruit exposed to high carbon dioxide, but usually not low oxygen, show high accumulations of succinate, which may be toxic to plant cells and is thought to be responsible for carbon dioxide injury. However, recent evidence with tissues of different susceptibilities to carbon dioxide injuries has not supported this view (Fernandez-Trujllo, Nock, and Watkins, 2001). It is likely that damaging levels of carbon dioxide result from progressive failure to maintain energy balance and metabolic cell function, rather than accumulation of any single injurious compound.
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