Luminescence indicators of bean leaves at different times of treatment with sodium fluoride
The regulation of the light stage of photosynthesis of higher plants (light absorption, migration of excitation energy over the pigment matrix, charge separation in reaction centers, and electron transport between the photosystems) plays an important role in improving the efficiency of the entire photosynthetic process. One of the regulatory mechanisms that coordinate the work of photosystem I (PS I) and photosystem II (PS II) is the redistribution of excitation energy between them depending on the intensity and spectral composition of the actinic light. Sodium fluoride can affect the efficiency of the interaction of PS I and PS II. The fluorescence kinetics of chlorophyll a of the leaf and the light induced changes of fluorescence parameters were measured using a PAM-2500 pulse fluorometer (Walz, Germany). The plant was placed in a dark chamber after 3, 6 or 9 hours with sodium fluoride treatment; the leaf was fixed in a holder on the fluorometer and maintained for 5 min in total darkness to standardize the experimental conditions. The fluorescence was excited by a pulsed measurement light (λ = 630 nm, Δλ = 5 nm, I = 10 μmol photons/(m2 s)); immediately after the measurement light was turned on, the fluorescence baseline F0 was determined. The maximum level of fluorescence, Fm, was determined at the leaf illumination by a saturating flash of light (λ = 630 nm, τ = 0.5 ms, I =3400 μmol photons/(m2 s)). The leaf was then illuminated by actinic light (λ = 455 nm, I = 150 μmol photons/(m2 s)); saturating flashes of light occurred every 20 s. The coefficients of photochemical and non-photochemical quenching (qP and qN, respectively) were recorded. NaCl solution of equal concentration was used as a control. The effect of sodium fluoride was manifested in a decrease in the stationary values of the photochemical quenching coefficient with an increase in the time of NaF treatment. At the same time, the value of qP in the control samples did not change. The values of the non-photochemical quenching coefficient under these conditions in the samples treated with sodium fluoride, on the contrary, increased. Results obtained are interpreted proceeding, on the one hand, from the influence of NaF on redistribution of excitation energy between photosystem II and photosystem I and its inhibitory effect on the ATPase complex and Kalvin–Benson cycle, on the other.
Department of Physics, Lomonosov Moscow State University
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