The photosynthetic electron transport chain consists of 3 large protein complexes Photosystem II (PSII), Cytochrome (cyt b6/f), Photosystem I (PSI). P700 is the term used to describe the chlorophyll within the reaction center of PSI as this is the wavelength of light to which the photosystem is most reactive. Upon illumination using a strong light source, the photosynthetic electron transport chain is almost entirely reduced.
The electrons from this reduction, in turn, activate the enzyme ferredoxin-NADP+ reductase which leads eventually to NADP reduction and CO2 fixation. This initial reduction process is represented by the OJIP steps of the Kautsky induction curve during prompt fluorescence measurements.
The oxidation of P700 causes an increase in absorbance at wavelengths falling in the 800nm – 850nm band. M-PEA measures the transmission of P700 using a modulated LED with a peak wavelength of 820nm and a highly sensitive photodiode to monitor the absorbance of the PSI complex during prompt fluorescence measurements.
Since the 820nm LED is not actinic, M-PEA is able to use high light intensities without disturbing the PSII complex. Therefore, M-PEA presents a convenient, reliable method of measuring chlorophyll a fluorescence and transmission at 820nm simultaneously, thus allowing the study of the electron transport process during the Kautsky induction at both ends of the photosynthetic electron transport system.
M-PEA is also fitted with a far-red light source which can be used to preferentially excite the PSI complex. Re-reduction occurs via the intersystem electron transport chain by PSII activity, with electrons originating from hydrolysis.
The M-PEA uses an optically filtered, modulated 820nm LED for high-quality P700 absorbance measurements. P700 activity is recorded using an optimised low-noise, fast-response PIN photodiode and 16-bit A/D converter providing an excellent signal-to-noise ratio.