ABSTRACT
Watersoaking is an ethylene-induced disorder observed in some members of the Cucurbitaceae including cucumber (Cucumis sativus L.), watermelon (Citrullus lanatus Thunb. Matsum and Nakai), and tropical pumpkin (Cucurbita moschata Duch.). Previous studies have found that immature beit-alpha cucumber (cv. Manar) exhibit watersoaking after 6d of continuous exposure to 10 µLL(-1) ethylene in air (21 kPa O(2)). The present study was designed to investigate the early dynamics of ethylene responses in immature cucumber fruit in order to provide insight into the watersoaking triggering mechanism. Changes in respiration, epidermal color, firmness, reactive oxygen species (ROS) production and electrolyte leakage were evaluated as a function of time under different ethylene concentrations and exposure duration. Ethylene concentrations exceeding 10 µLL(-1) did not accelerate changes in any of the evaluated responses. The first detectable change was a significant rise in respiration on day 2, followed by a significant rise in ROS on day 4, and significant degreening, mesocap softening, and increased electrolyte leakage on day 6; the latter responses coincident with incipient watersoaking. Varying the duration of exposure to ethylene indicated that the critical exposure time is between 2 and 4d. Notably, all deleterious responses to ethylene were suppressed under a hypoxic atmosphere. A model is proposed in which ethylene induces a sharp increase in respiration with a concomitant sharp rise in ROS, which the immature fruit is incapable of quenching. The resulting production of excess ROS leads to discoloration and membrane deterioration, leading to the release of cytoplasmic content, rapid softening, and the visual symptom of watersoaking.
