Characterization of photorespiration and photosynthesis in soybean cotyledons during seedling development, cotyledon senescence and rejuvenation

We measured light and dark carbon dioxide exchange rates (CER) in germinating, presenescent, and senescing soybean cotyledons and in cotyledons rejuvenated by epicotyl removal on different days after planting. Soybean cotyledons achieved rates of net photosynthesis equivalent to rates that have been reported for true leaves. Light CER was greater than dark CER throughout most of the developmental sequence studied, indicating that soybean cotyledon photsynthesis provides for more than replacement of carbon respired in the dark. Rejuvenated cotyledons exhibited light CER equivalent to the maximal rates measured in presenescent cotyledons.;In other aspects of cotyledon physiology, we observed significant differences between rejuvenated and presenescent soybean cotyledons. Rejuvenated cotyledons accumulated chlorophyll to levels greater than the maximal amount measured in presenescent cotyledons. A second difference was the observation that during rejuvenation, the Post-illumination outburst of CO;To investigate the relationship between the PIB and light CER, we labelled cotyledons with ;We measured the amount of ribulose-1,5-bisphosphate carboxylase oxygenase (RubisCO) in the cotyledons by rocket immunoelectrophoresis. Light CER was highly correlated with RubisCO concentration during most of the developmental sequence studied. Deviations from this relationship were seen during the earliest times of rejuvenation and in 4-day and 6-day old cotyledons. Light CER increased immediately after rejuvenation, without a change in RubisCO concentration. In 4-day and 6-day old cotyledons, light CER was much lower than expected based on the amount of RubisCO present.