different ripening processes, such as autocatalytic ethylene production, skin yellowing,
r
C02 and softening. It is also possible that each ripening pathway may have a commonethylene receptor, and that there may be different signal transduction pathways with different response capacities for each ripening pathway.
Autocatalytic ethylene production requires upregulation of the key regulatory ethylene biosynthetic enzymes, l -aminocyclopropane- l -carboxylic acid (ACC) synthase and ACC oxidase, and perhaps activation of receptors and signal transduction pathways that facilitate upregulation of these enzymes (Yang and Hoffman, 1 984; Payton et al., 1 996; Lelievre et al., 1 997; Klee et al., 1 999). The mechanism by which cold treatments induced earlier autocatalytic ethylene production in GS than in non-treated fruit at 20DC has been largely elucidated. Cold-treated GS fruit have been shown to accumulate ACC and ACC oxidase at 0-4DC (Jobling et aI. , 1 99 1 ; Larrigaudiere and Vendrell, 1 993; Lelievre et aI., 1 995; Larrigaudiere et al. , 1 997), and have increased ACC oxidase activity upon transfer from cold to warm temperatures (Larrigaudiere and Vendrell, 1 993; Larrigaudiere et aI., 1 997).
In contrast to GS, the ethylene biosynthesis of PR at shelf-life temperatures was not stimulated by prior exposure to low temperatures. IEC's of PR fruit transferred to 20DC after 1 0-30 days at O.SDC were similar to those for PR held continuously at 20DC with or without ethylene treatment. Furthermore, the maximum IEC's attained in PR at 20DC were similar for fruit from all treatments. Therefore, because PR does not have ethylene biosynthesis induced by cold treatment, and it has a maximum IEC at 20DC that is 70-
80% less than that in GS (after cold treatment) and other cultivars such as 'Cox's Orange Pippin' and 'Royal Gala' at 20DC (Chapter 4), it is suggested that PR is a mutant genotype of apple with reduced capacity for ethylene biosynthesis and action.
The nature of the mutation in PR that imposes reduced ethylene production and softening relative to other apple cultivars is currently not known. This mutation was most dominant at shelf-life temperatures (20-3SDC), as rapid phase softening was not initiated at these temperatures irrespective of prior cold or ethylene treatment. Other apple cultivars, such as 'Honeycrisp', 'NJ55' and 'PA14-238', have also been identified
as slow softening apple cultivars (Gussman et al., 1 993; Tong et aI., 1 999). However, it is not known if these cultivars are like PR in not initiating rapid phase softening at shelf life temperatures. Slow softening and low ethylene production mutants have also been identified in several other fruits, including nectarines, tomatoes and kiwifruit
(Tigchelaar et al., 1978; Brecht et al., 1 984; Hewett et al., 1 999). Research is required to determine the molecular basis that causes PR to soften slowly, as done for several tomato ripening mutants (Lelievre et aI., 1 997).
Rapid softening is an undesirable trait in apples, as firmness is rapidly reduced to an unmarketable level during postharvest handling. Temperature strongly influenced both the initiation and rate of rapid phase softening, although the extent of this effect was cultivar dependent, and was dependent on the ethylene physiology of the cultivar. Further studies are required to determine if the softening behaviour of other
commercially important apple cultivars at different temperatures also conform to the three classes identified in this study, and to determine the role of ethylene sensitivity in determining the softening response of different cultivars to temperature. Research is also required to determine the molecular basis for the slow softening characteristic of PR, especially at shelf-life temperatures.
5.6 References
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