After 48 hours at 2°C, hypocotyls from chili-sensitive Cucumis sativus s ins showed a burst of 02 uptake. The alternative pathway became engaged to close to 45% fill capacity during this postchilling respiratory burst. However, it only accounted for up to 50% of this increased respiratory 02 uptake. By 24 hours after chiling, when the seedlngs were fully recovered from visible symptoms of chilling 4ury, the flux through the alteative pathway was back to the level (about 10%) found before chilng.Blocking chilling-induced ethylene production with aminoethoxyvinylgycine had no effect on this increased utilization of the alternative pathway.The direct effects of temperature on respiration rates and the effects of inhibitors suggested that there was a rapid increase in altermative pathway activity and decrease in the cytochrome pathway activity. The possibility that the alternative pathway represents a compensatory mechanism for the more labile cytochrome pathway is discussed.Chill-sensitive plants that have been exposed to low temperatures exhibit a burst of respiration, as measured by 02 uptake, after they are returned to nonchilling temperature (4, 9). This accelerated respiration might be associated with recovery from chilling damage. However, before any role can be assigned, it must be determined how the plant accomplishes this respiratory rise. Studies with corn seedlings indicated that respiration was uncoupled at the peak of this respiratory burst, since DNP2 had no effect on the rates of 02 uptake (3). This uncoupled respiration could be a consequence of alternative respiratory pathway activity, which branches at ubiquinone, thus bypassing two of three phosphorylation sites of the Cyt-linked respiratory chain (6,7,14,15). This can result in thermogenesis (5,11,14), and it may also permit a greater turnover of glycolysis and the Krebs cycle before the feedback inhibitory level of ATP is reached. In this study, the possible involvement of the alternative pathway in the postchilling burst of cucumber seedling respiration was investigated. The postchilling burst of 02 uptake in cucumber seedlings is sensitive to SHAM (12). This burst coincided with the appearance of the chief chilling-injury symptom of these seedlings, i.e. collapse with a flaccid appearance. The results described represent a more detailed analysis of this system showing the relative contributions of the Cyt and alternative pathway using the titration method devised by Bahr and Bonner (1). MATERIALS AND METHODSCucumber seedlings (Cucumis sativus L. cv. Marketmore 70) were used throughout this study. Seeds were imbibed in water for 2 h at 22°C and sown on vermiculite. They were then grown for 8 days at 27 ± 1°C in continuous light (160 mE/cm2. s) at a RH of 80 ± 5%. Seedlings were then either transferred to 2 ± 1GC, 95% RH in the dark or kept at 27°C but in the dark. This was to eliminate any photooxidation effects of chilling in the light. In some experiments, AVG was used to control ethylene production. VT' was then plotted against g(i...