Estimating the contribution of Photosystem I to total leaf chlorophyll fluorescence

Chlorophyll a fluorescence characteristics were investigated in 12 species and 2 hybrids from the genus Flaveria exhibiting C3, C3–C4 intermediate, or C4 photosynthesis, and in the C4 species Zea mays. At room temperature, the variable fluorescence divided by the maximum fluorescence (FV/FM) of dark-adapted leaves decreased from C3 to C4 plants. This trend was qualitatively paralleled by an increase of the 735 nm peak relative to the 685 nm peak (F735/F685) of fluorescence emission spectra measured at low temperature (77 K). The variations were analysed using a quantitative model that takes into account higher PS I fluorescence in C4 plants than in C3 plants. The model predicts a linear correlation between 1/(FV/FM) and F735/F685, and was experimentally confirmed. From linear regression analysis, the FV/FM of PS II was calculated to be 0.88. By comparing the FV/FM of PS II with the FV/FM from leaves, the PS I contribution to total F0 fluorescence at wavelengths greater than 700 nm was determined to be about 30% and 50% in C3 and C4 plants, respectively. The corresponding values for the FM fluorescence were 6% and 12%. It is concluded that the effects of PS I fluorescence are significant and should be taken into account when analysing fluorescence data.