Effect of Breaks in Temperature Control on Postclimacteric ‘Cripps

Without exception, exposing postclimacteric ‘Cripps Pink’ apples to 20°C resulted in a significant change in the ethylene production of the fruit on return to cool storage and initially on return to warm periods (Figure 4.4b, 4.5b and 4.6b). In general, on return to cool storage, ethylene production returned close to that prior to the initial temperature exposure, and remained at this level for approximately 5 days and then increased to approximately 1.5-2 times the initial ethylene production rate over the following 10 days and maintained this increased level of ethylene production for the remainder of the storage period. This ethylene production response of ‘Cripps Pink’ apples on return to coolstorage (at 0°C) subsequent to exposure to 20°C will be referred to in this document

Coupled with this observed induced increase in ethylene production was a lack of response in ‘Cripps Pink’ respiration rate on return to storage at 0°C after a short time period at 20°C. The respiration rate consistently returned to the values similar to that prior to the break and showed no subsequent change during storage at 0°C. This result demonstrates independence of the two physiological indictors (respiration rate and ethylene production), and hence suggests that both indicators should be measured in order to determine fruit physiological status.

An induced increase in ethylene production has been observed in other scenarios where fruit are considered to be stressed, such as bruising (Lougheed and Franklin, 1974). However, in this investigation, the increased ethylene production as a response to a break in temperature control of postclimacteric ‘Cripps Pink’ apples is both delayed (by approximately 5 days at 0°C on return to coolstorage) and sustained (for a period of up to 80 days after the initial break in temperature control). Both of these traits suggest that the response is not a result of stress but rather a shift in metabolic control and altered homeostasis of ethylene production.

No clear influences of harvest maturity (data not shown), time in storage prior to exposure to 20°C (assuming that fruit are postclimacteric) or previous exposures to 20°C (Figure 4.7) on the magnitude or rate of the response of the induced increase in ethylene production were evident. However, the duration of exposure to 20°C influenced the initial ethylene production of ‘Cripps Pink’ apples on return to cool storage (Figure 4.5b), with those fruit exposed for 1 day (treatment Q0A) producing half as much ethylene as to that prior to exposure, whereas those fruit exposed for 6 days (treatment L0A) returned to ethylene production levels similar to that prior to temperature exposure. Additionally, there is some evidence that a larger exposure time may induce a greater increase in ethylene production (Figure 4.5b).

Induced increases in ethylene production on return to cool storage have been observed in other investigations that contained variable storage temperature regimes for other apple cultivars and fruit. Johnston (2001) reported that returning ‘Royal Gala’ or ‘Cox's Orange Pippin’ apples to the optimal cool room temperature after exposure to 12°C for 7 days (after 50 days at the optimal storage temperature) resulted in fruit possessing a higher internal ethylene concentration that remained greater than the control fruit for up to 25 days in cool storage. Similarly, Alwan and Watkins (1999) studied the effect of intermittent warming on ‘Cortland’, ‘Delicious’ and ‘Law Rome’ apple cultivars and reported an increased ethylene production in intermittently warmed fruit, although these

observations were inconsistent. In other fruit, Zhou et al. (2001) induced an immediate increase in peach ethylene production on return to storage at 0°C after 1 day at 20°C and Cabrera and Salviet (1990) quadrupled cucumber ethylene production rates at 2.5°C by exposing fruit to 12.5°C for 18 hours every 3 days. The combination of these studies suggests that an increase in ethylene production at coolstorage temperatures may be a widespread response of climacteric fruit exposed to fluctuating temperature regimes.

The major pathway of ethylene biosynthesis in higher plants is methionine → S- adenosylmethionine (SAM) → ACC → ethylene (Yang and Hoffman, 1984). The steps in the pathway are catalysed by SAM synthase, ACC synthase (ACS) and ACC oxidase (ACO), respectively. Both ACS and ACO play a role in regulating ethylene biosynthesis (Alexander and Grierson, 2002). These enzymes are encoded by multigene families, with the expression of each gene in the family differently regulated by various developmental, environmental and hormonal signals (Jiang and Fu, 2000). Turnover of both ACS and ACO is rapid, allowing for rapid control of both enzymes and subsequent ethylene production (Fluhr and Mattoo, 1996).

It is possible that the processes of both exposing ‘Cripps Pink’ apples to a break in temperature control and/or returning the apples to cool storage, may turn on or off some genes in the ACO and ACS gene families, changing the balance of ACO and ACS and hence ethylene production. This hypothesis is supported by the responses observed for intermittently warmed peaches. When Zhou et al. (2001) exposed peaches to 20°C for 1 day and induced an increase in ethylene production (immediately) on return to storage at 0°C, with the increase in ethylene production being linked to increased activities of both ACS and ACO.

Alternatively, the change in ethylene production homeostasis may be related to compositional changes in the plasma membrane influencing the activity of ACO. Previously, acclimation to low temperatures (low temperature conditioning, section 2.1.1.1.6) by exposing fruit to low non-chilling temperatures prior to refrigeration treatments have been shown to induce compositional changes in the membrane lipids (Goto et al., 1984) that result in reduced sensitivity of the produce to refrigerated temperatures. The influence of temperature on membrane lipid composition has been confirmed in ‘Cox’s Orange Pippin’ apples (Bartley, 1986). It is a common belief that changes in membranes lipids, result in altered membrane fluidity and affect functionality of the associated proteins (Marangoni et al., 1996). Ramassamy et al. (1998) demonstrated that the ACO protein is located on the external face of the plasma membrane of ‘Golden Delicious’ apples. It is possible that fluctuations in temperature

may also induce membrane lipid compositional changes, and hence functionality. If this is the case, then changes in ethylene production rate are not unlikely due to the location of ACO on the plasma membrane. If this mechanism is proven to have a role in altering the homeostasis of ethylene production, then it is likely that the rate of change of temperature (from cool store to ambient and reverse) may also play a role in the observed effects on ethylene production.

More investigation is required to determine whether the shift in homeostasis of ethylene production of ‘Cripps Pink’ apples subjected to a break in temperature control is caused by a change in balance of ethylene producing enzymes (ACO and ACS), changes in membrane functionality, or a combination of both effects.