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

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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 µmol quanta/(m2 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 µmol quanta/(m2 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 µmol quanta/(m2 s) was turned on, the P700 oxidation proceeded most rapidly in leaves of seedlings grown at irradiance of 176 µmol quanta/(m2 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.