The role of oxygen in ethylene-induced watersoaking in immature beit-alpha cucumber fruit

Watersoaking is an ethylene-induced disorder that affects members of the Cucurbitaceae. Our understanding of the cellular mechanisms contributing to watersoaking is incomplete. This study was conducted to address the role of oxygen in watersoaking using immature beit-alpha cucumber fruit. Ethylene at 10 iL.L-1 induced watersoaking, and higher concentrations did not accelerate the disorder. At least 4 d of ethylene exposure (10 iL.L-1) induced watersoaking and accompanying symptoms including degreening, softening, and enhanced electrolyte leakage. Continuous ethylene exposure induced accumulation of reactive oxygen species (ROS) at 2-4 d and maximum levels of ethylene receptor transcripts (Cs-ERS, Cs-ETR1, and Cs-ETR2) at 1 d. Histochemical staining revealed that watersoaking appears closely associated with H2O2. Of three ethylene receptor genes, Cs-ETR1 in mesocarp and Cs-ERS in exocarp were the most markedly up-regulated in response to ethylene. Watersoaking in immature cucumber fruit was initiated in hypodermal tissue, followed by ingress to mesocarp. Altered gas-exchange properties of fresh-cut slices did not affect the spatial pattern of watersoaking. The intensity of watersoaking, however, was markedly diminished in slices compared with intact fruit. In intact fruit, hyperoxia (40 kPa O2) accelerated ethylene-induced watersoaking while hypoxia (2 kPa O2) suppressed these symptoms. In fresh-cut slices, ethylene-induced symptoms were negated by hypoxia but unaffected by hyperoxia. Ethylene-mediated increases in H2O2 occurred 2 d earlier than incipient watersoaking under normoxia and hyperoxia, but not hypoxia. O2 .- production decreased in ethylene-treated fruit as watersoaking developed. Antioxidant capacity of cucumber fruit increased in response to ethylene at 6 d and 4-6 d in exocarp and mesocarp, respectively. Cucumber fruit preconditioned (2 kPa O2 for 8 d) prior to ethylene exposure under normoxia exhibited softening, ion leakage and tissue disruption, but no watersoaking. Preconditioning reduced ethylene-induced ROS and H2O2 generation. The data collectively show that watersoaking is a tissue-specific ethylene response and total ROS and H2O2 generation capacity appears to contribute to ethylene-induced watersoaking of immature cucumber fruit as influenced by pO2. Transcriptional regulation of ethylene receptors was noted as an early cellular response prior to incipient watersoaking. Up-regulation of ETR1-like receptors could represent a means of offsetting the delirious effects of excess ethylene.