Electron flow to photosystem I from stromal reductants in vivo: the size of the pool of stromal reductants controls the rate of electron donation to both rapidly and slowly reducing photosystem I units.

Electron donation from stromal reductants to photosystem I (PSI) was studied using the kinetics of P700(+) (the oxidized primary donor of PSI) reduction in the dark after irradiation of barley ( Hordeum vulgare L.) leaves. The leaves were treated with diuron and methyl viologen to abolish both the electron flow from PSII and PSI-driven cyclic electron transport. The redox state of P700 was monitored using the absorbance changes at 830 nm (Delta A(830)). Two exponentially decaying components with half-times of about 3 s (the slow component) and about 0.6 s (the fast one) were distinguished in the kinetic curves of Delta A(830) relaxation after a 1-s pulse of far-red light. The complex kinetics of P700(+) reduction thus manifested two types of PSI unit differing in the rate of electron input from stromal reductants. The rates of both kinetic components assayed after 1-s pulses were increased about 20-fold by a short (2-5 min) heat-pretreatment of leaves, indicating the accelerated input of electrons to both types of PSI unit. The increased rates of electron flow to P700(+) were even observed 1.5 h after the action of heat had been completed. Both kinetic components were dramatically slowed down upon irradiation of heat-treated leaves for 20-30 s. Their rates were restored after a short (20-30 s) period of darkness. A 5-min leaf exposure at 38 degrees C was sufficient to stimulate by severalfold the reduction of P700(+) pre-oxidized by a brief light pulse. In contrast, the acceleration of P700(+) reduction after a 1-min irradiation was observed only if leaves were subjected to temperatures above 40 degrees C. Neither heat treatment of leaves nor light-dark modulations in the rates of the fast and the slow components of P700(+) dark reduction influenced the relative magnitudes of the two kinetic components, providing strong additional evidence in favor of two distinct types of PSI existing per se in barley leaves. The key role in the control of the activity of electron donation to P700(+) in both rapidly and slowly reducing PSI units was attributed to the amount of stromal reductants available for P700(+) reduction. The latter was expected to be reduced under illumination in the presence of methyl viologen, while increased again in the dark. The regeneration of the pool of stromal reductants in the dark was likely provided by starch breakdown within the chloroplast stroma, but not by import of reducing equivalents from the cytosol. This was evidenced by much lower rates, compared with 1-h dark-adapted leaves, of dark reduction of both components of P700(+) in leaves stored for 24 h in the dark and thus depleted of starch but containing large amounts of glucose, the respiratory substrate.