Developmental-dependent Responses of Beit-Alpha Cucumber Fruit to Ethylene Are Associated with Re-Dox Homeostasis

Continuous ethylene exposure induces severe watersoaking of beit-alpha cucumber fruit (Cucumis sativus L., cv. Manar). Our previous studies have shown that ethylene-induced watersoaking is a programmed cell death (PCD) associated with increases in reactive oxygen species (ROS). Responses of beit-alpha cucumber fruit to ethylene are developmentally dependent, occurring more rapidly in immature compared with mature fruit. The present study examined ethylene responses of the re-dox system in immature and mature beit-alpha cucumber fruit by evaluating ROS production, antioxidant capacity, and antioxidant enzymes. Immature (69.4 ± 0.8 g) and mature cucumber fruit (400.1 ± 5.5 g) were exposed to continuous air or 10 µL·L^-1 of ethylene for up to 8 d at 15 °C. As anticipated, watersoaking in ethylene-treated immature fruit was observed at 6 d (35.5 ± 2.1% of cross-sectional area), concomitant with decreased firmness and increased electrolyte leakage. 

Ethylene-treated mature fruit, however, exhibited significantly delayed and attenuated watersoaking (18.8 ± 1.1% at 8 d) including suppressed changes in firmness and electrolyte leakage. ROS production (H₂O₂-generating capacity) in ethylene-treated immature fruit  increased significantly from 0.29 ± 0.02 to 2.23 ± 0.22 μmol·mg^-1 H₂O₂ protein/min by 4 d, prior to watersoaking development, then decreased to 0.59 ± 0.09 μmol·mg^-1 H₂O₂ protein/min by 8 d, as watersoaking became more severe. ROS production in ethylene-treated mature fruit increased gradually through 8 d and reached 44% of the maximum value in ethylene-treated immature fruit. Ascorbate content and total antioxidant capacity (ORAC) were higher in mature fruit compared with immature fruit. ORAC in ethylene-treated immature fruit significantly increased concomitantly with enhanced ROS production through 4 d and maintained afterward. Ethylene-treated mature fruit exhibited a gradual increase in ORAC, comparable to the trend of ROS-generating capacity. Activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) decreased 60.9 and 54.9%, respectively, and activity of guaiacol peroxidase (GPX) increased about 15-fold in ethylene-treated immature fruit by 8 d. However, APX activity in ethylene-treated mature fruit remained high and unchanged through storage. Activities of SOD and GPX in ethylene-treated mature fruit exhibited trends similar to those of ethylene-treated immature fruit but were present at higher levels. These results suggest that enhanced antioxidant enzyme activities and antioxidant levels are associated with suppression of ROS production and watersoaking development in ethylene-treated mature cucumber fruit. We conclude that developmental increases in homeostatic re-dox responses explain the decline in expression of watersoaking responses in ethylene-treated cucumber fruit.