THE 2 SPECTROSCOPICALLY DIFFERENT SHORT-WAVELENGTH PROTOCHLOROPHYLLIDE FORMS IN PEA EPICOTYLS ARE BOTH MONOMERIC

The spectral properties of the protochlorophyllide forms in the epicot yls of dark-grown pea seedlings have been studied in a temperature ran ge, from 10 to 293 K with conventional fluorescence emission and excit ation spectroscopy as well as by fluorescence line narrowing (FLN) at cryogenic temperatures. The conventional fluorescence techniques at lo wer temperatures revealed separate bands at 628, 634-636, 644 and 655 nm. At room temperature (293 K) the 628 and 634-636 nm emission bands strongly overlapped and the band shape was almost independent of the e xcitation wavelength. Under FLN conditions, vibronically resolved fluo rescence spectra could be measured for the 628 and 634-636 nm bands. T he high resolution of this technique excluded the excitonic nature of respective excited states and made it possible to determine the pure e lectronic (0,0) range of the spectra of the two components. Thus it wa s concluded that the 628 and 634-636 nm (0,0) emission bands originate from two monomeric forms of protochlorophyllide and the spectral diff erence is interpreted as a consequence of environmental effects of the surrounding matrix. On the basis of earlier results and the data pres ented here, a model is discussed in which the 636 nm form is considere d as an enzyme-bound protochlorophyllide and the 628 nm form as a prot ochlorophyllide pool from which the substrate is replaced when the epi cotyl is illuminated with continuous light. (C) 1998 Elsevier Science B.V. All rights reserved.