Effects of light on respiration and oxygen isotope fractionation in soybean cotyledons

Light effects on electron flow through the cyanide-resistant respiratory pa thway, oxygen isotope fractionation and fetal respiration were studied in s oybean (Glycine max L.) cotyledons. During the first 12 h of illumination t here was an increase in both electron partitioning through the alternative pathway and oxygen isotope fractionation by the alternative oxidase. The la tter probably indicates a change in the properties of the alternative oxida se. There was no engagement of the alternative oxidase in darkness and its fractionation was 27%, In green cotyledons 60% of the respiration flux was through the alternative pathway and the alternative oxidase fractionation w as 32%. Exposing previously illuminated tissue to continuous darkness induc ed a decrease in the electron partitioning through the alternative pathway. However, this decrease was not directly linked with the low cellular sugar concentration resulting from the lack of light because 5 min of light ever y 12 h was sufficient to keep the alternative pathway engaged to the same e xtent as plants grown under control conditions.