Photosynthetic electron sinks in transgenic tobacco with reduced amounts of Rubisco: little evidence for significant Mehler reaction

Transgenic tobacco (Nicotiana tabacum L. cv. W38) plants with an antisense gene directed against the mRNA of the small subunit of Rubisco were used to investigate the role of O 2 as an electron acceptor during photosynthesis. The reduction in Rubisco has reduced the capacity for CO 2 -fixation in these plants without a similar reduction in electron transport capacity. Concurrent measurements of chlorophyll fluorescence and CO 2 assimilation at different CO 2 and O 2 partial pressures showed close linear relationships between chloroplast electron transport rates calculated from chlorophyll fluorescence and those calculated from CO 2 -fixation. These relationships were similar for wild-type and transgenic plants, indicating that the reduced capacity for CO 2 fixation in the transgenic plants did not result in extra electron transport not associated with the photosynthetic carbon reduction (PCR) or photorespiratory carbon oxidation (PCO) cycle. This was further investigated with mass spectrometric measurements of 16 O 2 and 18 O 2 exchange made concurrently with measurements of chlorophyll fluorescence. In all tobacco lines the rates of 18 O 2 uptake in the dark were similar to the 18 O 2 uptake rates at very high CO 2 partial pressures in the light. Rates of oxygenase activity calculated from 18 O 2 uptake at the compensation point were linearly related to the Rubisco content of leaves. The ratios of oxygenase to carboxylase rates were calculated from measurements of 16 O 2 evolution and 18 O 2 uptake at the compensation point. These ratios were lower in the transgenic plants, consistent with their higher CO 2 compensation points. It is concluded that although there may be some electron transport to O 2 to balance conflicting demands of NADPH to ATP requirements, this flux must decrease in proportion with the reduced demand for ATP and NADPH consumption in the transgenic lines. The altered balance between electron transport and Rubisco capacity, however, does not result in rampant electron transport to O 2 or other electron transport acceptors in the absence of PCR and PCO cycle activity.