RESPIRATORY CHARACTERISTICS AND CALCULATED ATP PRODUCTION OF APPLE FRUIT IN RELATION TO TOLERANCE OF LOW O-2 CONCENTRATIONS

The applicability of respiratory characteristics to determine optimal O-2 concentrations for the storage of apples was tested. A comparison was made between gas exchange rates of apples directly after harvest a nd after a period of storage. Apples of three harvest dates were used. Optimal O-2 concentrations were based on gas exchange data and gas ex change models fitted on the data, using the Anaerobic Compensation Poi nt (ACP) and the Respiratory Quotient Breakpoint (RQB). A third way wa s comparing total ATP production with estimated maintenance energy req uirements, revealing the Maintenance Oxygen Concentration (MOC). ATP p roduction was calculated using gas exchange models. MOC was defined as the oxygen concentration with the minimal ATP production rate necessa ry for maintaining cell viability. The optimal O-2 concentrations as e stablished by ACP, RQB and MOC differed considerably. Because ACP valu es differed from normally advised values, the ACP was unsuitable for a quick determination of the optimal O-2 concentration of the apples us ed. The RQB, however, might be suitable, but than the limit used to es tablish the RQB should be more than 0.5 units higher than the RQ measu red in normal air. The ACP and the RQB were decreased to lower O-2 con centrations after storage, suggesting that optimal concentrations decr eased during storage. In contrast the MOC was increased after storage, in agreement with results found in practice. Model calculations indic ated the lowest optimal O-2 concentration for the second (optimal) har vest using the ACP, the RQB and the MOC. It is suggested that research on the relationship between Maintenance Energy Requirements (MER) and cell injury will clarify an important part of the changes in optimal O-2 concentrations (or the tolerance to low O-2 concentrations) during ageing or maturation or harvested plant tissues.