Developmentally dependent responses of cucumber (Cucumis sativus L.) fruit to exogenous ethylene

Studies with members of the Cucurbitaceae have demonstrated the ability of ethylene to induce fruit watersoaking. The present study explored the effect of mini-cucumber fruit developmental stage on the formation of watersoaking when treated with ethylene. Fruit were harvested at four levels of development: immature (4–8 days after anthesis, DAA), mature (10–14 DAA), breaker (16–20 DAA) and yellow (35–40 DAA). Fruit were stored at 15°C in air or either 10 µl l−1 ethylene or 1300 µl l−1 propylene for 12 d. Immature fruit treated with ethylene for 9 d exhibited mesocarp watersoaking, epidermal sloughing and lower surface hue angle, mesocarp pH, and firmness. Mature fruit behaved similarly but exhibited reduced watersoaking. In contrast, breaker and yellow fruit after 9 d of ethylene exposure accumulated β-carotene and produced aromatic compounds, but did not exhibit watersoaking. Ethylene increased respiration in fruit of all stages of development; however, ethylene production was detectable only in decaying fruit. Decay incidence increased in ethylene-treated fruit and was inversely proportional to developmental stage at harvest; however, isolated decay organisms were found to be non-pathogenic in nature. Breaker and yellow fruit retained greater cellular viability in the presence of ethylene, compared to non-watersoaked tissue of immature and mature fruit; however, watersoaked tissue did not exhibit fluorochromasia, indicating that ethylene induced programmed cell death (PCD).;Immature mini-cucumber fruit treated with air or 10 µl l −1 ethylene were monitored for cellular and molecular hallmarks of PCD. Total nuclease activity in control fruit remained constant during storage in air; however, ethylene-treated fruit exhibited a 4-fold increase from 3 d to 7 d, concomitant with incipient watersoaking. Four nucleases of 37, 34.5, 32.5 and 31 kD were detected by SDS-PAGE followed by activity staining. The induction of 37 and 31 kD nucleases correlated with increased total nuclease activity and incipient DNA laddering. DNA ladder intensity increased through 7 d concomitant with increases in the activities of 37 and 31 kD nucleases.;The data strongly indicate that the response of mini-cucumber fruit to ethylene is developmentally dependent. Upon ethylene exposure, older fruit share ripening characteristics with the climacteric melon, whereas younger fruit exhibit symptoms of PCD prior to incipient watersoaking.