Identification of Ferredoxin-Dependent Cyclic Electron Transport around Photosystem I Using the Kinetics of Dark P700+ Reduction

Abstract

Kinetic curves of absorbance changes induced by far-red light (FR, 830 nm) ( ∆ A 830 ), which reflect redox transformations of PSI primary electron donor, P700, were examined in intact barley ( Hordeum vulgare L.) leaves. In intact leaves, FR induced the biphasic increase in absorbance related to P700 photooxidation. Leaf treatment with methyl viologen or antimycin A suppressed the slow phase of P700 photooxidation, which was attained in such leaves within the first second of light exposure. With FR turned off, the previously increased absorbance at 830 nm dropped down to its initial level, thus reflecting ê700 + reduction. In the control leaves, the kinetics of ê700 + reduction consisted of three exponentially decaying components, with the corresponding half-times of 8.8 s (the slow component, with its magnitude comprising 24% of the total ∆ A 830 signal), 0.73 s (the middle component, 49% of ∆ A 830 ), and 0.092 s (the fast component, 26% of ∆ A 830 ). The rate of the fast component of P700 + reduction, following FR irradiation of leaves, was about ten times lower than that of the noncyclic electron transfer from PSII to PSI computed from ∆ A 830 relaxation after the abrupt offset of white light. The treatment of leaves with methyl viologen or antimycin A completely abolished the fast component of ∆ A 830 relaxation after FR exposure. It was concluded that the fast component is determined by the operation of ferredoxin-dependent cyclic electron transport around PSI. This study represents the first report on the identification of this pathway of electron transport in vivo and the estimation of its rate.

Induction kinetics of photosystem I-activated P700 oxidation in plant leaves and their dependence on pre-energization

“…In consistency with previous reports [15,16], the kinetics of dark reduction of P700 + , accumulated in the FR light, fit well to a sum of three exponentials. The fast component with a time constant τ 1 = 150-300 ms is thought to be associated with CET, while slower components (τ 2 ~ 1.5 s and τ 3~1 0 s) corre spond to electron flows toward PSI from different stromal sources.…”

“…By contrast, CET induced by FRL in dark adapted leaves was characterized by low initial rate. Indeed, the fast component in the dark reduction of P700 + , attrib uted to CET [16], increased within 1-1.5 s of FR illu mination, prior to the subsequent gradual decline (Fig. 4b).…”

“…The relative amplitude of the fast component, i.e., A 1 /(A 1 + A 2 + A 3 ) was used as a measure of CET activity. The association of the fast component with CET has been revealed [16].…”

“…The decomposition of the kinetic curve of P700 + dark reduction into sev eral exponential terms provides the means to compare the rates of electron fluxes to P700 + along the cyclic route and from alternative sources [13][14][15]. Another approach to CET quantification makes use of the leaf exposure to white light (WL) under control conditions and in the presence of methyl viologen (MV), an agent that effectively consumes all electrons available on the acceptor side of PSI, thus excluding CET [15,16].…”

Activities of Noncyclic and Alternative Pathways of Photosynthetic Electron Transport in Leaves of Broad Bean Plants Grown at Various Light Irradiances

“…It is remarkable that bean leaves, unlike those of barley, demonstrated four, not three, exponentially decaying components in the kinetics of P700 + dark reduction. In addition to slow, middle, and fast components found earlier in barley leaves [34], a superfast component with a half-time similar to that for noncyclic electron transport was observed in leaves of broad been seedlings.…”