INVOLVEMENT OF LIPOXYGENASE-MEDIATED LIPID CATABOLISM IN THE START OF THE AUTOCATALYTIC ETHYLENE PRODUCTION BY APPLES (cv GOLDEN DELICIOUS) : A RIPENING HYPOTHESIS

A ripening sequence is proposed for apples cv Golden Delicious which starts with the maturity-controlled degradation of fatty acids by lipoxygenase into hexanal, followed by the oxidation of the aldehyde into hexanoic acid (this transformation was found to be performed in the presence of lipoxygenase), and then into smaller carboxylic acids. Because of the fact that pre-climacteric apples use small carboxylic acids as a substrate for the production of minor amounts of ethylene,

increased availability of the acids will lead to a slowly increasing formation of ethylene, and finally (possibly in cooperation with the carboxylic acids) to the other "ripening responses" (auto-catalytic ACC-mediated ethylene production, increased respiration, aroma formation, etc.).

The earlier ripening hypotheses (changes in sensitivity towards ethylene, resistance against ripening, the "tree factor") are rationalized by invoking a "tree effect". As long as the fruits are attached to the tree, ample amounts of nutrients are constantly supplied by the mother plant, fulfilling energy requirements and allowing normal growth. But when this food supply is cut off, the fruit will have to start living on its own reserve-materials. Degradation processes, amongst them lipoxygenase activity, will thus gain in importance, and will drive the fruit irrevocably towards ripening.