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Non-additive enhancement of the photosynthesis excited by simultaneous illumination with far-red light and light of shorter wavelengths is called as "second Emerson effect". Its action spectra are well-known as a photosynthetic yield's dependence on light wavelength in red (630-690 nm) spectral region at a constant-wavelength far-red illumination near 700-715 nm. However, the opposite dependence of the photosynthetic yield's of shorter constant-wavelength light (red or blue) on light wavelength in far-red (690-760 nm) spectral region was never studied. In this study the action spectrum of second Emerson effect was studied using a fast-Fourier dual-wavelength Pulse Amplitude Modulation (PAM) fluorometry. Chlorophyll fluorescence in ailanthus (Ailanthus altissima Mill.) leaves was excited with blue modulated light. Far-red induced decrease of fluorescence (fluorescence shift-FRIFS) was studied in response to illumination of leaves with a background light from 690 to 760 nm (10 nm step), calculating FRIFS = (F0-Fs)/F0, where F0-fluorescence measured without and Fs-with far-red light. Maximum FRIFS was observed at 720 nm (11.8%), but it still remained considerable at 740, 750 nm and a low FRIFS values were revealed at 690 and even at 760 nm. Measurements carried out with blue saturating flashes during and after far-red illumination showed the increase of quantum yield of Photosystem II (PSII), calculated as Fv/Fm at 720 nm background light. FRIFS had lower values under excitation with red modulating light. It is concluded that FRIFS is a result of a photochemical quenching caused by an additional selective far-red excitation of PSI in conditions when PSII is preferably excited by blue light thus leading the PSI to limit non-cyclic electron flow. The contradiction between the known absorption spectra of PSI-light harvesting complex I and the observed action spectrum of second Emerson effect (FRIFS spectrum) is discussed.

Far-Red Spectrum of Second Emerson Effect: A Study Using Dual-Wavelength Pulse Amplitude Modulation Fluorometry