Difference spectra of bacterial cell sediments of R. etli, A. caulinodans, R. leguminosarum, and X. autotrophicus are shown. Solid lines, difference spectra obtained from subtracting absorbance spectra after far-red irradiation (780 nm) from absorption spectra after red irradiation (655 nm); dotted lines, spectra after 1 h of dark incubation subtracted from spectra after far-red irradiation (a positive signal around 700 nm indicates Pfr-to-Pr dark conversion); dashed lines, spectra after 1 h of dark incubation subtracted from spectra after red irradiation (a positive signal around 750 nm indicates Pr-to-Pfr dark conversion).