O2-dependent electron flow, membrane energization and the mechanism of non-photochemical quenching of chlorophyll fluorescence

Abstract

Recent progress in chlorophyll fluorescence research is reviewed, with emphasis on separation of photochemical and non-photochemical quenching coefficients (qP and qN) by the 'saturation pulse method'. This is part of an introductory talk at the Wageningen Meeting on 'The use of chlorophyll fluorescence and other non-invasive techniques in plant stress physiology'. The sequence of events is investigated which leads to down-regulation of PS II quantum yield in vivo, expressed in formation of qN. The role of O2-dependent electron flow for ΔpH- and qN-formation is emphasized. Previous conclusions on the rate of 'pseudocyclic' transport are re-evaluated in view of high ascorbate peroxidase activity observed in intact chloroplasts. It is proposed that the combined Mehler-Peroxidase reaction is responsible for most of the qN developed when CO2-assimilation is limited. Dithiothreitol is shown to inhibit part of qN-formation as well as peroxidase-induced electron flow. As to the actual mechanism of non-photochemical quenching, it is demonstrated that quenching is favored by treatments which slow down reactions at the PS II donor side. The same treatments are shown to stimulate charge recombination, as measured via 50 μs luminescence. It is suggested that also in vivo internal thylakoid acidification leads to stimulation of charge recombination, although on a more rapid time scale. A unifying model is proposed, incorporating reaction center and antenna quenching, with primary control of ΔpH at the PS II reaction center, involving radical pair spin transition and charge recombination to the triplet state in a first quenching step. In a second step, triplet excitation is trapped by zeaxanthin (if present) which in its triplet excited state causes additional quenching of singlet excited chlorophyll.

Concerning a Dual Function of Coupled Cyclic Electron Transport in Leaves

“…It (20). It appears that, in isolated chloroplasts, in the absence of so-called Mehler reagents and of C02, the maximum electron transport from water to 02 is not much more than about half of the electron transport made possible in leaves by photorespiratory and photosynthetic CO2 turnover behind closed stomata.…”

“…Overreduction, with its damaging consequences, is prevented in water-stressed leaves exposed to high irradiances by the concerted action of the transthylakoid proton gradient, which controls electron flux from PSII, and by electron drainage at the reducing end of the electron transport chain (9). Phosphoglycerate (formed during carboxylation and oxygenation of ribulose bisphosphate), 02, and oxidized ascorbate all function as main electron acceptors of linear electron flow (2,20,25). In C4 plants, cyclic electron transport also contributes to the higher ATP requirement of C4 photosynthesis when CO2 is available (7).…”

“…If H+/e is only 2, then insufficient ATP is formed for carbon reduction, the stoichiometry of which requires a little more than 1.5 ATP/2e in C3 plants and 2.5 ATP/2e in C4 plants. An auxiliary reaction capable of supplying extra ATP, such as cyclic electron transport or linear electron transport to 02 in the Mehler reaction (2,20) or to nitrite in nitrogen metabolism, would be required for both types of plants. However, if the H+/e ratio were stoichiometrically fixed to a value of 3, more ATP would be formed than is required for carbon assimilation in C3 plants but still not enough for assimilation of C4 plants (cf.…”

Increased Sensitivity of Photosynthesis to Antimycin A Induced by Inactivation of the Chloroplast ndhB Gene. Evidence for a Participation of the NADH-Dehydrogenase Complex to Cyclic Electron Flow around Photosystem I

“…However, the obligatory character or the flexibility of the Q-cycle during CO 2 fixation remains a matter of debate (Davenport and McCarty, 1984;Ort, 1986;Heber and Walker, 1992;Cramer et al, 1996). Other mechanisms, like cooperation with mitochondrial respiration (Krö mer, 1995;Hoefnagel et al, 1998) and Mehler reactions (also known as waterwater cycle) (Schreiber and Neubauer, 1990) have also been suggested to re-equilibrate the chloroplastic ATP to NADPH ratio by generating extra-ATP, but their contribution during CO 2 fixation remains to be established.…”

Aspects of bulblet growth of lily in vitro

“…In the course of CO 2 starvation, all endogenous electron acceptors become reduced and oxygen is the main available electron acceptor. Oxygen can serve as electron acceptor in the Mehler reaction (Mehler 1951;Schreiber and Neubauer 1990).…”