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We have studied scald development by comparing changes in gene expression, C2H4 evolution, and α-farnesene and conjugated trienol contents in scald-resistant cultivars, i.e., `Gala' and `Braeburn', and scald-sensitive cultivars, i.e., `Red Delicious' and `Granny Smith'. We also carried out similar comparisons between controls and treatments that diminished scald symptoms in sensitive cultivars. The data show that scald development is critically dependent on the initiation of the climacteric rise in C2H4 evolution, since treatments that suppress the latter inhibit scald development at low temperatures coincident with a suppression of α-farnesene and conjugated trienols. However, in scald-resistant cultivars, there is an increase in α-farnesene and conjugated trienols, although to a lower degree than in the sensitive cultivars. This indicates that factors other than the auto-oxidation of α-farnesene are also involved in scald development. Analytical data show that malonyl dialdehyde (MDA) increases only in scalded areas, which, in turn, suggests that oxidative reactions are involved in scald development. Storage of `Granny Smith' at temperatures above 7 °C prevents the development of scald without affecting the accumulation of α-farnesene and conjugated trienols. This in turn suggests that chilling temperatures induce as yet unknown enzymes that contribute to scald development. In short, the data show that in addition to cultivars, low temperature stress and the induction of the C2H4 climacteric play a crucial role in scald development. Preliminary data show that treatment of `Granny Smith' apples with olive oil emulsions suppress scald development symptoms.
We have studied the effects of MCP and low O2, applied singly and in combination, on apple fruit ripening at 1, 7, and 18 °C. The single application of 2 ppm MCP is more effective in delaying the onset of the C2H4 climacteric than is 1% O2. However, the combined application has a much larger effect than the single applications of either MCP or 1% O2. For instance, at 7 °C, the onset of the C2H4 climacteric occurs at 15, 50, and 90–95 days for the controls, 1% O2 and 2 ppm MCP, respectively, whereas the combined application of 2 ppm MCP and 1% O2 suppressed the initiation of the C2H4 climacteric for 200 days, the duration of the experiment. The retardation of the climacteric onset by the treatments is associated with the suppression of ACC-synthase (ACS1) and the putative receptor ERS1. The accumulation of their transcripts is critically dependent on the rate of C2H4 evolution. As expected, the combined application of MCP and 1% O2 completely suppressed the expression of both genes. Yet when the fruits were transferred to 18 °C in air, they ripened normally. A similar pattern of inhibition in response to the above treatments was also observed with a C2H4-dependent MAPK. The expression of ETR1, ETR2 and ACC-oxidase was not affected by the treatments. The nature of this strong effect of the combined application of MCP and low O2 is not clear. It should be pointed out that MCP does not inhibit the induction of hypoxic proteins such as ADH.
`Granny Smith' apples were stored at 1 °C, 5 °C and 20 °C, then treated at the preclimacteric stage with 2 ppm MCP, various O2 concentrations, and MCP + low O2. All the treatments greatly retarded the onset of the C2 H4 climacteric, and hence ripening. MCP + low O2 was much more effective than were the applications of MCP and low O2 singly. Even at 20 °C, 4.04 kPa O2 inhibited the rise in C2 H4 evolution for 145 d. Neither low O2 nor MCP inhibited the System 1 C2 H4 evolution. The suppression of the climacteric rise in C2 H4 evolution was accompanied by a strong inhibition of the accumulation of ERS1 C2 H4 receptor and ACS transcripts. On the other hand, ETR1 receptor was constitutively expressed. When climacteric fruits were treated with MCP, and with low O2 + MCP the rate of C2 H4 evolution decreased sharply. This occurred simultaneously with a decrease in ERS1 mRNA. Moreover, the decrease in ERS1 mRNA paralleled the decrease in C2 H4 evolution. The data thus indicate that the initiation and sustainment of the C2 H4 climacteric requires the presence of functional C2 H4 receptors. The expression of ETR2 and ERS2 is also under investigation.