Abstract—
Low‐temperature (80 K) fluorescence belonging to the red light‐absorbing phytochrome form (Pr) with the emission maximum at 684 nm in stems and at 682 nm in roots was detected and described in etiolated cress seedlings. By monitoring fluorescence intensity, Pr concentration changes in stems and roots were followed during etiolated growth of seedlings. In both tissues, the changes have the same characteristics, reaching maximum concentration in stems, 37 ± 4 μ g/g fresh weight (f.w.), and in roots, 6.2 ± 1.2 μ g/g f.w., 48 h after sowing. Red (660 nm) preillumination of the sample leads to a decrease of the phytochrome content in 3 h of incubation in darkness to 29 ± 6% in stems and 59 ± 5% in roots compared with that in the red‐far‐red illuminated control samples. Assuming identical fluorescence properties, the proportion of the labile and stable pools was calculated to be 90% and 10% in stems and ˜50% in roots, respectively. It was furthermore deduced that the absolute concentration of the stable pool is about the same in both tissues, 3.1–3.7 μg/g f.w., while that of the labile pool is approximately 10 times higher in stems than in roots. Comparison of the results with data on the dependence of the photophysical and photochemical properties of phytochrome on its localization in etiolated seedlings (Sineshchekov and Sineshchekov, 1990, J. Photochem. Photobiol. 5, 197–217) suggests that the labile and stable pools correspond to the Pr states with low and high activation barriers of the photoreaction, Ea, 3–4 kJ/mol, and ca 35 kJ/mol, and with high and low photochemical activity at 80 K. The two pools also differ in the position of their fluorescence emission and excitation spectra, those of the stable pool being shifted 2 nm to the blue region.
