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 CO2-fixation in thes e plants without a similar reduction in electron transport capacity. Concur rent measurements of chlorophyll fluorescence and CO2 assimilation at diffe rent CO2 and O-2 partial pressures showed close linear relationships betwee n chloroplast electron transport rates calculated from chlorophyll fluoresc ence and those calculated from CO2-fixation, These relationships were simil ar for wild-type and transgenic plants, indicating that the reduced capacit y for CO2 fixation in the transgenic plants did not result in extra electro n transport not associated with the photosynthetic carbon reduction (PCR) o r photorespiratory carbon oxidation (PCO) cycle. This was further investiga ted with mass spectrometric measurements of O-16(2) and O-18(2) exchange ma de concurrently with measurements of chlorophyll fluorescence. In all tobac co lines the rates of O-18(2) uptake in the dark were similar to the O-18(2 ) uptake rates at very high CO, partial pressures in the light. Rates of ox ygenase activity calculated from O-18(2) uptake at the compensation point w ere linearly related to the Rubisco content of leaves. The ratios of oxygen ase to carboxylase rates were calculated from measurements of O-16(2) evolu tion and O-18(2) uptake at the compensation point. These ratios were lower in the transgenic plants, consistent with their higher CO2 compensation poi nts, 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 consu mption in the transgenic lines, The altered balance between electron transp ort and Rubisco capacity, however, does not result in rampant electron tran sport to O-2 or other electron transport accepters in the absence of PCR an d PCO cycle activity.