Abstract
The process of cold-induced ripening of pear (Pyrus communis L.) fruits consists of four consecutive steps ; (1) perception of the cold stimulus and transduction of the signal, (2) induction of ethylene synthesis, (3) action of the produced ethylene, and (4) softening by the action of hydrolytic enzymes and related changes in fruit tissues. The two pear strains, P12-9 and P12-111, derived from the cross between 'La France' and 'Le Lectier' do not respond to cold-induced ripening. In this study, we characterized the possible points of metabolic block(s) in these pear strains to the cold-induced ripening in relation to the synthesis and action of ethylene. In P12-9 fruits, ACC synthase was induced and ACC content was increased by the cold treatment, resulting in ethylene production ; whereas exogenous ethylene did not induce softening of P12-9 fruits, suggesting that ethylene was not perceived. P12-111 fruits produced ethylene under some conditions, but did not when exposed to cold treatment. These fruits did not soften even after an abundance of ethylene was produced during a prolonged cold storage. These findings indicated that P12-111 fruits lack the ability to perceive the cold stimuli and to respond to ethylene. Whether the subsequent softening step in both of the strains is normal or not remains to be elucidated.
