The color of light emitted by the primary LED distinguishes the BLUE from the RED version of the MINI-PAM-II fluorometer (Fig. 1). The BLUE version (MINI-PAM-II/B) possesses a blue LED emitting maximally around 475 nm which is replaced by a red LED emitting maximally around 655 nm in the RED version (MINI-PAM-II/R). Both versions have a second LED providing far red light for specific excitation of photosystem I.
The second difference between the two versions is the spectral window for fluorescence detection. The BLUE version detects fluorescence at wavelengths > 630 nm but the RED version detects fluorescence at wavelengths > 700 nm (Fig. 2).
The detection window for fluorescence of the BLUE version extends to 640 nm but the RED version detects only fluorescence at wavelengths longer than 700 nm (Fig. 2). In principle, its extended range for fluorescence detection makes the BLUE version more sensitive than the RED version because photosystem II fluoresces at wavelength between 650 and 700 nm. In fully green leaves, however, a large part of this short wavelength fluorescence (650 - 700 nm) is reabsorbed by chlorophyll so that the gain in sensitivity of the BLUE version is moderate. When reabsorption (and also the fluorescence signal) is low, like in extremely bleached leaves, the increased sensitivity of the BLUE version can be advantageous.
The MINI-PAM-II can be used to investigate lichens or photosynthetic microbial mats. Cyanobacteria present in these mats often absorb poorly in the blue. Therefore, the RED version is normally preferred in studies of cyanobacteria.
Blue actinic light of the MINI-PAM-II/B excites the broad short wavelength band of the major light-harvesting complex of photosystem II in higher plants (LHC II). Red light of the MINI-PAM-II/R excites the comparably minor long-wavelength band of the LHC II. Hence, if LHC II excitation is important, the BLUE version is recommended.
Blue is absorbed by blue light photoreceptors which can stimulate plant responses like chloroplast relocation and stomatal movements. Therefore, the BLUE version can be advantageous when blue light responses are of interest. Blue light-driven chloroplast relocation, however, can affect the fluorescence signal by changing the efficiency of light absorption which is difficult to distinguish from the effects of high-energy fluorescence quenching on fluorescence.