Climacteric fruits

The synthesis of endo-PG occurs in the ripening stage after an increase of ethylene production [21] and its appearance has been correlated with an increase in soluble pectin and softening [22]. Exo-PG is suggested to participate in the initiation of climacteric ethylene production [23]. Strawberry fruit has been accepted to be a non-climacteric fruit and ethylene... 

Application of ethylene-,4C to plants resulted in only a 2.4% conversion into soluble carbohydrates, 11% into ether-soluble materials, 6.9% into phytol, 31.7% into cellulose and lignin, and 9.6% into soluble protein and non-protein material, mainly phosphates. 9 Treatment of detached fruit (such as apples, bananas, peaches, figs, and pears) with synthetic auxins, especially (2,4,5-trichlorophenoxy) acetic acid, speeded up ripening, as indicated by color, taste, softness, and starch breakdown. 7 Other fruits have been similarly ripened, 8 and the treatments are effective both on climacteric and non-climacteric fruit. 

The tolerance limitation of fruit for irradiation establishes the maximum acceptable dose. If this dose controls decay organisms, the use of irradiation for a particular fruit may appear promising. Response to irradiation may be influenced by fruit maturity, variety, pre- and postharvest temperatures, handling, and extent of fungus growth. Climacteric fruits irradiated prior to the normal rapid increase in respiration usually show an immediate increase in respiration and the production of ethylene. These fruits are frequently retarded in ripening. 

Ethylene plays an important role in a number of plant developmental processes, including senescence and abscission of leaves and flowers, responses to wounding, and the ripening of climacteric fruits (Abeles, 1973). In each case ethylene is produced from methionine (Fig. 1). The two enzymes specific to the pathway, ACC synthase and ethylene forming enzyme, increase in activity in response to wounding and during ripening,... 

Physiological Responses to Ethylene. Classically, two types of fruit have been recognized with respect to their response to ethylene (1) (a) climacteric fruit, such as apples... 

Figure 1. Effect of ethylene on respiration of climacteric and nonclimacteric fruit. Ethylene causes greatest response in climacteric fruit when applied to mature fruit prior to the climacteric rise. In nonclimacteric fruit high concentrations of ethylene stimulate respiration for short time periods. This stimulation is observed at any time upon application of ethylene (3).

Figure 3 shows the hypothetical kinetics of growth, respiration and relative hormone levels in a climacteric fruit at different stages of its life cycle. Hypothetical hormone levels during development and ripening have been speculated on before (13). The rationale for this outline is based on the known influences of the various hormones on cell division,... 

During ripening of non-climacteric fruits like citrus, the process of colour change is named degreening and the natural loss of chlorophylls, accumulated into the chromoplasts of the epidermis (flavedo) and vesicles, and the concomitant manifestation and new biosynthesis of carotenoids, generally occurs very slowly (Eaks 1977). 

Some fruit and vegetables can be exposed to ethylene, the plant hormone produced by other commodities as climacteric fruits, during the postharvest storage and transport. This hormone can induce changes in phenolic metabolism and affect the phenolic metabolite composition and the antioxidant potential. 

Chlorophyll loss, a desirable trait for many climacteric fruits, results in quality loss for many vegetables. Chlorophyll degradation during the senescence of green vegetables can be reduced by low O2 and elevated CO2 (Ku and Wills, 1999). 

It is well established that 18 2 and 18 3 are the precursors of and C, aldehydes and alcohols by exogenous application of 18 2 or 18 3 and radiolabelled precursors, e.g. [93]. As for the hexyl and hexenyl esters, it is not clear if hexyl or hexenyl moieties of esters solely originate from C, aldehydes or alcohols. Incubation of apple fruits with hexanal or hexanol resulted in an increase in the formation of hexyl esters, e.g. [94], but increased levels of these esters at the ripe stage of apple fruits were much larger than the endogenous levels of aldehydes and alcohols [97]. Interestingly, methyl jasmonate application stimulated ester formation of the pre-climacteric apples, but had no effect on the ester formation of the post-climacteric fruits [17] however, it inhibited the hexyl ester formation in apples stored in a controlled atmosphere [96]. Fan et al. [17] speculated that the inhibitory effect found in the last report was owing to the toxic level of methyl jasmonate used. 

Ethylene gas is widely used commercially for ripening a variety of climacteric fruits and decoloring non-climacteric citrus fruits. However, the use of ethylene-releasing compounds to effect this response is confined to relatively few crops (Table 2), and normally when they are used as a preharvest application. This is probably due to the ease with which ethylene gas can be applied to harvested fruit, and the dangers of applying aqueous solutions to these disease-susceptible organs. 

Application of AA vapour has been shown to lead to major changes in fruitripening processes in non-climacteric and climacteric fruit (Fidler 1968). Yamashita et al. (1975,1976) showed that intact strawberries were able to synthesize carboxylic esters on addition of alcohols and acids or aldehydes and acids. In... 

Post-harvest application of AA to non-climacteric fruits has been reported to cause induction of CO2 production (a climacteric-like respiration) in orange (Fidler 1968 Pesis and Avissar 1989), fig (Hirai et al. 1968), strawberry and blueberry (Janes et al. 1978) and grape (Pesis and Marinansky 1992). In fig and orange, AA application leads to reduced acidity (Hirai et al. 1968 Pesis and Avissar 1989). 

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