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Activities of noncyclic and alternative pathways of photosynthetic electron transport were studied in intact leaves of broad been ( Vicia faba L.) seedlings grown under white light at irradiances of 176, 36, and 18 µ mol quanta/(m 2 s). Electron flows were followed from light-induced absorbance changes at 830 nm related to redox transformations of P700, the photoactive PSI pigment. The largest absorbance changes at 830 nm, induced by either white or far-red light, were observed in leaves of seedlings grown at irradiance of 176 µ mol quanta/(m 2 s), which provides evidence for the highest concentration of PSI reaction centers per unit leaf area in these seedlings. When actinic white light of 1800 µ mol quanta/(m 2 s) was turned on, the P700 oxidation proceeded most rapidly in leaves of seedlings grown at irradiance of 176 µ mol quanta/(m 2 s). The rates of electron transfer from PSII to PSI were measured from the kinetics of dark P700 + reduction after turning off white light. These rates were similar in leaves of all light treatments studied, and their characteristic reaction times were found to range from 9.2 to 9.5 ms. Four exponentially decaying components were resolved in the kinetics of dark P700 + reduction after leaf exposure to far-red light. A minor but the fastest component of P700 + reduction with a halftime of 30-60 ms was determined by electron transfer from PSII, while the three other slow components were related to the operation of alternative electron transport pathways. Their halftimes and relative magnitudes were almost independent on irradiance during plant cultivation. It is concluded that irradiance during plant growth affects the absolute content of PSI reaction centers in leaves but did not influence the rates of noncyclic and alternative electron transport.Abbreviations : ∆ A 830 -absorbance changes at 830 nm; PSI and PSII-photosystems I and II.

The kinetic curves of dark reduction of P700 + (oxidized primary donor of PSI) after far-red light irradiation were studied on broad bean ( Vicia faba L.) leaves treated with antimycin A, methyl viologen, or diuron. Four components of P700 + reduction were found in untreated leaves, namely, an ultrafast component with a half-time of 25 ms, and fast (210 ms), middle (790 ms), and slow (6100 ms) components. The fast component disappeared in leaves treated with antimycin A or methyl viologen. At the same time, these substances did not affect other components of P700 + reduction. Treatment of leaves with diuron abolished both the ultrafast and fast components of P700 + reduction. As the length of far-red light exposure was increased, a lag phase appeared in the development of middle component in leaves treated with diuron, antimycin A, or methyl viologen. In thus treated leaves, an exponential pattern of the middle component was displayed with a certain delay after darkening. A conclusion was drawn that the minor ultrafast component of P700 + dark reduction in broad bean leaves was caused by electron donation to PSI from PSII, whereas the fast component of this process was determined by the operation of ferredoxin-dependent electron transport around PSI. The middle and slow components were supposed to be related to electron input to PSI from reductants localized in the chloroplast stroma. Key words: Vicia faba -alternative routes of photosynthetic electron transport -antimycin A -diuron -methyl viologenAbbreviations : PSI and PSII-photosystems I and II; P700-primary electron donor of PSI; ∆ A 830 -absorbance changes at 830 nm.

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