The light-response curves of P700 oxidation and time-resolved kinetics of P700(+) dark re-reduction were studied in barley leaves using absorbance changes at 820 nm. Leaves were exposed to 45 degrees C and treated with either diuron or diuron plus methyl viologen (MV) to prevent linear electron flow from PS II to PS I and ferredoxin-dependent cyclic electron flow around PS I. Under those conditions, P700(+) could accept electrons solely from soluble stromal reductants. P700 was oxidized under weak far-red light in leaves treated with diuron plus MV, while identical illumination was nearly ineffective in diuron-treated leaves in the absence of MV. When heat-exposed leaves were briefly illuminated with strong far-red light, which completely oxidized P700, the kinetics of P700(+) dark reduction was fitted by a single exponential term with half-time of about 40 ms. However, two first-order kinetic components of electron flow to P700(+) (fast and slow) were found after prolonged leaf irradiation. The light-induced modulation of the kinetics of P700(+) dark reduction was reversed following dark adaptation. The fast component (half time of 80-90 ms) was 1.5 larger than the slow one (half time of about 1 s). No kinetic competition occurred between two pathways of electron donation to P700(+) from stromal reductants. This suggests the presence of two different populations of PS I.