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

Abstract

Measurements of 810 nm transmittance changes in leaves, simultaneously with Ch1 fluorescence, CO2 uptake and O-2 evolution, were carried out on potato (Solanum tuberosum L.) leaves with altered expression of plastidic NADP-dependent malate dehydrogenase. Electron transport rates were calculated: J(C) from the CO2 uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, J(O) from the O-2 evolution rate, JF from Ch1 fluorescence parameters and JI from the post-illumination re-reduction speed of PSI donors. In the absence of external O-2, J(O) equaled (1.005 0.003) J(C), 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 O-2 concentration was decreased from 210 to 20 mmol mol(1), and resulted in 1 of the linear flow. J(F) and J(I) did not differ from J(C) while photosynthesis was light-limited, but considerably exceeded JC at saturating light. Then, typically, J(F)=1.2 J(C) and J(I)=1.3 J(C), and J(F)-J(C) and J(I)-J(C) depended little on CO2 and O-2 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 J(I)-J(C), as well as the fast J(F)-J(C) are energy-dissipating cycles around PSI and PSII at light saturation.