Rates and roles of cyclic and alternative electron flow in potato leaves

Abstract

Measurements of 810 nm transmittance changes in leaves, simultaneously with Chl fluorescence, CO2 uptake and O2 evolution, were carried out on potato (Solanum tuberosum L.) leaves with altered expression of plastidic NADP-dependent malate dehydrogenase. Electron transport rates were calculated: JC from the CO2 uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, JO from the O2 evolution rate, JF from Chl fluorescence parameters and JI from the post-illumination re-reduction speed of PSI donors. In the absence of external O2, JO equaled (1.005 ± 0.003) JC, independent of the transgenic treatment, light intensity and CO2 concentration. This showed that nitrite and oxaloacetate reduction rates were very slow. The Mehler-type O2 reduction was evaluated from the rate of electron accumulation at PSI after the O2 concentration was decreased from 210 to 20 mmol mol-1, and resulted in <1% of the linear flow. JF and JI did not differ from JC while photosynthesis was light-limited, but considerably exceeded JC at saturating light. Then, typically, J F = 1.2 JC and JI = 1.3 JC, and JF -JC and JI -JC depended little on CO2 and O2 concentrations. The results showed that the alternative and cyclic electron flow necessary to compensate variations in the ATP/NADPH ratio were only a few percent of the linear flow. The data do not support the requirement of 14H+/3ATP by the chloroplast ATP synthase. We suggest that the fast PSI cyclic electron flow JI - J C, as well as the fast JF - JC are energy-dissipating cycles around PSI and PSII at light saturation. © The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved.