HIGH CO2 LEVELS REDUCE ETHYLENE PRODUCTION IN KIWIFRUIT

The effects of CO2 on ethylene production rates and on the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene have been i nvestigated in kiwifruit [Actinidia deliciosa (A. Chev.) Liang and Fer guson, cv. Hayward] pericarp tissues. Carbon dioxide concentrations up to 10% notably reduced ethylene production rates but did not affect A CC levels. Carbon dioxide levels from 10 to 28.5% slightly reduced wou nd-induced ACC accumulation but resulted in only a slight additional r eduction in ethylene production. However, in the presence of saturatin g amounts of ACC, ethylene formation was stimulated by high CO2 levels . The apparent V-max and K-m for ACC of the in vivo conversion of ACC to ethylene were both increased by high CO2 although to a different ex tent. When CO2 concentrations were increased from 0.2 to 28.5%, the V- max varied from 50 to 90 nl g(-1) h(-1) while the K-m for ACC increase d from 15 to 72 mu M. Thus, the V-max over K-m ratio, which represents the efficiency of ACC conversion to ethylene when ACC concentrations are very low, was reduced by 40% in the presence of 5% CO2 and by 60% in the presence of 28% CO2. The concentrations of ACC, at which ACC ox idase was inhibited by high CO2, are comparable to the concentrations found in the fruit. It is therefore suggested that the reduction of et hylene production rates in fruits by high CO2 might be exerted at leas t partially at the level of the conversion of ACC to ethylene.