Phytochrome is a key photoregulation pigment in plants which determines the strategy of their development throughout their life cycle. The major achievement in the recent investigations of the pigment is the discovery of its structural and functional heterogeneity: existence of a family of phytochromes (phyA-phyE) differing by the apoprotein was demonstrated. We approach this problem by investigating the chromophore component of the pigment with the use of the developed method of in vivo low-temperature fluorescence spectroscopy of phytochrome. In etiolated plants, phytochrome fluorescence was detected and attributed to its red-light absorbing form (Pr) and the first photoproduct (lumi-R), and a scheme of the photoreaction in phytochrome, a distinction of which is the activation barrier in the excited state, was put forward. It was found that the spectroscopic and photochemical characteristics of Pr depend on the plant species and phytochrome mutants and overexpressors used, on localization of the pigment in organs and tissues, plant age, effect of preillumination and other physiological factors. This variability of the parameters was interpreted as the existence of at least two phenomenological Pr populations, which differ by their spectroscopic characteristics and activation parameters of the Pr ->> lumi-R photoreaction (in particular, by the extent of the Pr ->> lumi-R photoconversion at low temperatures, gamma1): the longer-wavelength major and variable by its content in plant tissues Pr' with gamma1 = 0.5 and the shorter-wavelength minor relatively constant Pr" with gamma1 < or = 0.05. The analysis of the phytochrome mutants and overexpressors allows a conclusion that phytochrome A (phyA), which dominates in etiolated seedlings, is presented by two isoforms attributed to Pr' and Pr" (phyA' and phyA", respectively). Phytochrome B (phyB) accounts for less than 10% of the total phytochrome fluorescence and belongs to the Pr" type. It is also characterized by the relatively low extent of the Pr photoconversion into the far-red-light absorbing physiologically active phytochrome form, Pfr. Fluorescence of the minor phytochromes (phyC-phyE) is negligible. The recently discovered phytochrome of the cyanobacterium Synechocystis also belongs to the phenomenological Pr" type. PhyA' is a light-labile and soluble fraction, while phyA" is a relatively light-stable and, possibly, membrane (protein)-associated. Experiments with transgenic tobacco plants overexpressing full-length and C- and N-terminally truncated oat phytochrome A suggest that phyA' and phyA" might differ by the post-translational modification of the small N-terminal segment (amino acid residues 7-69) of the pigment. PhyA' is likely to be active in the de-etiolation processes while phyA" together with phyB, in green plants as revealed by the experiments on transgenic potato plants and phytochrome mutants of Arabidopsis and pea with altered levels of phytochromes A and B and modified phenotypes. And finally, within phyA', there are three subpopulations which are, possibly, different conformers of the chromophore. Thus, there is a hierarchical system of phytochromes which include: (i) different phytochromes; (ii) their post-translationally modified states and (iii) conformers within one molecular type. Its existence might be the rationale for the multiplicity of the photoregulation reactions in plants mediated by phytochrome.