MODE OF ACTION OF OXYGEN AND CARBON DIOXIDE ON POSTHARVEST PHYSIOLOGY OF 'BARTLETT' PEARS

Studies were conducted to investigate the effects of reduced oxygen and elevated carbon dioxide concentrations on respiratory metabolism and ethylene biosynthesis in relation to skin color, flesh firmness, and composition of 'Bartlett' pears. The oxygen concentration at which aerobic respiration shifts to anaerobic respiration varied between 0.3 and 1.7% at temperatures between 0°C and 25°C, respectively, and it increased with ripening (possibly due to reduced oxygen diffusion coefficients within the fruit). Reduced oxygen and elevated carbon dioxide concentrations decreased respiration rate by partially inhibiting the activity of certain enzymes in the glycolytic pathway and Krebs cycle. Decreased oxygen is more effective than increased carbon dioxide levels in reducing ethylene production, while elevated carbon dioxide concentrations are more effective in inhibiting ethylene action. Carbon dioxide concentrations above 5% may induce flesh browning and the severity of this physiological disorder depends on maturity at harvest, concentrations of oxygen and carbon dioxide, and storage duration. No consistent and significant effects of 5, 10, or 20% carbon dioxide added to air were found on the activities of phenylalanine ammonia-lyase and polyphenol oxidase enzymes or on the levels of chlorogenic acid, catechin, epicatechin, and caffeic acid.