Low-Temperature Fluorescence Spectrum of Plants

The measurement of the low-temperature fluorescence spectrum (77 K) was performed according to the methodology of Wiciarz et al. [56 (link)]. The frozen plant material was homogenized to leaf powder and then mixed with Hepes 0.05 M + 0.33 M sorbitol buffer (pH 7.5) in the proportion of 0.1 g/1 mL. The measurement was performed at 77 K using an LS 55 spectrofluorimeter (Perkin Elmer, Waltham, MA, USA). The sample was placed in a quartz glass capillary and excited with radiation at a wavelength of 437 nm. The fluorescence spectrum was recorded in the range of 650–800 nm using the FL WinLab 4.0 software (Perkin Elmer, Waltham, MA, USA). The curves were normalized to the maximum PSII fluorescence value. Based on the spectra, the values of the F730/F690 ratio were calculated, which allows to estimate the relative share of PSI and PSII in the thylakoid membranes.