Alleviation of Chilling Injury in Tropical and Subtropical Fruits

Chilling injury in tropical and subtropical fruits can be alleviated by low temperature preconditioning, intermittent warming, heat treatment, controlled atmosphere storage, treatments with calcium or other chemicals, waxing, film packaging, genetic modification, or applications with ethylene, abscisic acid, polyamines, methyl jasmonate, methyl salicylate, or other natural compounds. The effectiveness of each treatment varies with the commodity, the maturity of the fruit, and the dosage of the treatment. Low temperature conditioning and intermittent warming maintain high levels of phospholipids, increase the degree of unsaturation of fatty acids, increase the levels of spermidine and spermine, and stimulate the activities of free radical scavenging enzymes. Heat treatment induces heat shock proteins (HSP), suppresses oxidative activity, and maintains membrane stability. Methyl jasmonate and methyl salicylate stimulates the synthesis of some stress proteins, such as HSP, pathogenesis-related (PR)-proteins, and alternative oxidase (AOX) and can activate lipoxygenase gene expression and induce synthesis of abscisic acid and polyamines. Polyamines may act as free radical scavengers and membrane stabilizers. All of these processes can enhance chilling tolerance of tissues and alleviate chilling injury of tropical and subtropical fruits. INTRODUCTION Most fruits originated from the tropical or subtropical regions are chillingsensitive (Gross et al., 2002). These crops are injured after a period of exposure to chilling temperatures below 10 to 15°C but above their freezing points. Some horticultural crops of temperate origin are also susceptible to low temperature injury. Those temperate crops have lower critical threshold temperatures (generally below 5 to 10°C). At these chilling temperatures, the tissues weaken because they are unable to carry on normal metabolic processes. Various physiological and biochemical alterations and cellular dysfunctions occur in chilling-sensitive species in response to chilling stress (Wang, 1982). These alterations include stimulation of ethylene production, increase in respiratory rate, interference in energy production, increase in activation energy, slowing of protoplasmic streaming, increase in permeability, reduction in photosynthesis, enzyme inactivation, membrane dysfunction, and alteration of cellular structure. If chilling stress is prolonged, these alterations and dysfunctions will lead to the development of a variety of chilling injury symptoms such as surface lesions, internal discoloration, water-soaking of the tissue, off-flavor, decay and failure to ripen normally (Saltveit and Morris, 1990). CHILLING INJURY SYMPTOMS Common symptoms of chilling injury in tropical and subtropical crops are pitting, discoloration, water-soaked appearance, internal breakdown, failure to ripen, off-flavor, and decay (Paull, 1990; Saltveit and Morris, 1990). Symptom development depends not only on species and cultivars, but also on maturity, types of tissues, and environmental factors. Surface pitting is the most common form of chilling injury in many tropical and subtropical fruits and vegetables. Pitting can occur on citrus fruits, cucumbers, eggplant, melons, okra, papayas, pomegranates, sweet peppers, sweetpotatoes, and tamarillos. Proc. III IS on Trop. and Subtrop. Fruits Eds.: M. Souza and R. Drew Acta Hort. 864, ISHS 2010 268 Failure to ripen is another common symptom of chilling injury in tropical and subtropical crops. It can occur in chilled avocados, bananas, mangos, melons, papayas, sapodilla, and tomatoes. Some specific chilling injury symptoms occur in certain commodities, such as subepidermal brown streaking of vascular tissues in bananas, membranous staining in lemons, and mahogany browning in potatoes. Another common chilling injury symptom is internal discoloration. This symptom can occur in such tropical and subtropical crops as avocados, pineapples, sweet potatoes and taro. FACTORS AFFECTING CHILLING INJURY SUSCEPTIBILITY Factors affecting the susceptibility to chilling injury include the origin of the crop, genetic makeup of the commodity, stage of development or maturity, metabolic status of the tissue, and a number of environmental factors such as temperature, light, relative humidity, and atmospheric composition. The origin of a crop or the genetic makeup of a plant determines whether the species is sensitive or resistant to chilling (Patterson and Reid, 1990). In chilling-sensitive species, the critical threshold temperature may vary with the stage of development or maturity. For example, avocados, honeydew melons, mangos, papayas, and tomatoes are more sensitive to chilling when they are less mature (Paull, 1990). The metabolic status and the chemical composition of the tissue at the time of chilling can affect the resistance of tissue to chilling. Chilling-resistant tissues tend to have a higher degree of unsaturation of fatty acids in the membrane lipids than chillingsensitive tissues (Tabacchi et al., 1979). High levels of reducing sugars and proline were also found to correlate positively with the resistance to chilling injury (Purvis, 1981; Purvis and Grierson, 1982). Among the environmental factors, temperature is obviously the main element which can influence the severity of chilling injury. Light can also enhance chilling injury under certain circumstances (van Hasselt, 1990). Chilling-induced structural alteration of proteins associated with photosystem II might be the primary event for light-induced damage (Moll and Steinback, 1986). Relative humidity in the storage environment is another factor affecting the severity of chilling injury. Generally, chilling injury symptoms are more severe under low relative humidity than under high relative humidity. RESPONSE OF TROPICAL AND SUBTROPICAL FRUITS TO CHILLING TEMPERATURE Prolonged exposure of tropical and subtropical crops to chilling temperature results in various physiological and biochemical alterations which lead to a variety of chilling injury symptoms. There are two hypotheses to explain how chilling temperatures induce various responses and cause detrimental effects. The first hypothesis suggests that there is a single unifying primary response to chilling temperature for all chillingsensitive species. This primary response would lead to secondary events and then develop into a variety of chilling injury symptoms. Several events have been proposed as likely candidates for the primary response including a phase transition in membrane lipids, an alteration in the kinetics or substrate specificity of a regulatory enzyme, a change in the cytoskeletal structure, or an increase in cytosolic calcium (Raison and Orr, 1990). After prolonged exposure of sensitive species to chilling, the primary response would lead to secondary events which include loss of membrane integrity, leakage of solutes, loss of compartmentation, decrease in the rate of mitochondrial oxidative activity, increase in the activation energy of membrane-associated enzymes, cessation of protoplasmic streaming, reduction in energy supply and utilization, decrease in photosynthetic rate, disorganization of cellular and subsellular structure, dysfunction and imbalance of metabolism, accumulation of toxic substances, stimulation of ethylene production, increase in respiration rate, and manifestation of a variety of chilling injury symptoms. The second concept puts forth the idea that chilling injury originates from a multitude of responses to low temperature. Because of the diversity of tropical and subtropical fruits and vegetables in the structure and expression of chilling injury