~tuorescence and photochemical properties of phytochromes A and B (phyA and phyB) have been investigated in etiolated pea (Pisum sazi~,um L.) seedlings by comparing wild-type (wt) seedlings with mutants lacking phyB (lv-5) or phyA (fun1-1), and with the double mutant. The red-light (R)-absorbing form of phytochrome (Pr) is characterized by four major parameters: position of the emission maximum at 85 K (A .... ), total phytochrome content ( [Ptot] ); extent of the Pr photoransformation into the first product, lumi-R, at 85 K ( 3' ~ ), and into the far-red light (FR)-absorbing form (Pfr) at 273 K (T2). Phytochrome cannot be detected in the lv-5funl-1 double mutant, allowing specific characterization of phyA and phyB in the single mutants. Thus, for the first time we are able to compare in planta the properties of native phyA and phyB. Our results show the phyA population to be heterogenous both in distribution and properties. [PhyAtot] is highest in the tipper stem (5.23 rel. units), with Amax of 687 nm, "yj = 0.43 and ~/2 = 0.72, and 3.5-fold lower in the lower stem and roots, with Amax blueshifted by more than 1 nm, 3'J =0.3 and 3,=0.7. In contrast, phyB is present at a low level throughout the plant (0.3-0.4 rel. units), with A ...... of 683 nm, y~ of 0.05 and "Y2 of 0.3-0.35. It is also characterized by an earlier red drop in the dependence of "Y2 on the wavelength of the act il~ic light compared to phyA. Experiments with light-induced phytochrome destruction confirm previous findings that the majority of phyA, unlike phyB, is light-labile. The variation in properties of phyA is interpreted in terms of two species, phyA' and phyA". PhyA' is relatively abvndant, is distributed mainly in the upper stem, is light-labile and has a high % value ( = 0.5), whereas phyA" is less abundant, is distributed more or less evenly throughout the plant, is relatively light-stable and has a low "y~ value ( < 0.05). It is suggested that the lower photochemical actJ x ity of phyB at ambient temperatures (low ~/2) and its earlier red drop (which is likely to be due to the blue-shift in the absorption spectra of l't~yB Pr and Pfr) might explain, at least partially, the lower effectiveness of phyB under R and its lack of effect under FR.