Changes in respiration and ethylene production of apples in response to internal and external oxygen partial pressures

Variations in respiration rate and ethylene production of 'Cox's Orange Pippin' and 'Granny Smith' apples in response to reduced oxygen partial pressures were characterised by studying O-2, CO2 and C2H4 partial pressure differences between the internal and external atmospheres (Delta p(O2), Delta p(CO2) and Delta p(C2H4), Pa) of individual fruit maintained in different O-2 atmospheres at 20 +/- 1 degrees C. Delta p(O2) decreased at low O-2 levels, reflecting he decreased rate of O-2 uptake in low O-2 partial pressures. Oxygen uptake relative to that in air (relr(O2)) approximately followed Michaelis-Menten kinetics, with a half-maximal rate at 3.3 kPa O-2 for internal oxygen (p(O2)(i)) and 10.4 kPa O-2 for external oxygen p(O2)(e). An equation was developed to describe aerobic and anaerobic components of CO2 production relative to that in air (relr(CO2)) as a function of p(O2)(e) or p(O2)(i). The relationship between rate of C2H4 production relative to that in air (relr(C2H4)) and P-O2(i) was reasonably described by a Michaelis-Menten type hyperbolic curve, confirming the expectation that small changes in O-2 partial pressure would have a much greater effect on relr(C2H4) at low P-O2(i) than they do at high p(O2)(i). In contrast, presence of the skin as a diffusion barrier resulted in development of an apparent 'lag phase' in the relationship between relr(C2H4) and p(O2)(e) such that it became essentially sigmoidal. These differences are attributed to differences in O-2 partial pressure between internal and external atmospheres.