Fluorescence lifetimes of protochlorophyllide in plants with different proportions of short-wavelength and long-wavelength protochlorophyllide spectral forms[para]

Photochemistry and Photobiology, Aug 2003 by Mysliwa-Kurdziel, Beata, Amirjani, Mohammad R, Strzalka, Kazimierz, Sundqvist, Christer

The slow component with a lifetime between 5.1 and 7.0 ns, which was observed for the long-wavelength Pchlide^sub 650-656^ form, probably reflects the fluorescence from the aggregated Pchlide forms, where the excited electron can move among a number of molecules and find a suitable relaxation path.

In this study, the fraction corresponding to the slow-fluorescence lifetime component was always higher than that of the fast component. This parameter provides some information about the relative intensity of fluorescence that is emitted by fluorescing molecules showing a given lifetime component; however, it is not directly related to the proportion of these fluorophores. If we assume that two fluorescence lifetime components represent two groups of Pchlide molecules with different surroundings and thus undergoing different de-excitation pathways competing with fluorescence like energy transfer, pigment-pigment interaction or photochemistry, the number of molecules within these groups (ground state equilibrium) can be characterized using preexponential factors (28). The preexponential factor and, thereby, fractions of Pchlide molecules related to the fast component were higher than that of the slow component, whatever the emission wavelength for wheat and wild-type pea leaves, and more or less equal for maize but lower for the lip1 mutant (Fig. 9). An increase of the preexponential factor corresponding to the fast-lifetime component was observed for wild-type pea within the emission range between 640 and 670 nm. However, it should be pointed out that reduction of the Pchlide fluorescence lifetime and, therefore, the origin of the fast-fluorescence lifetime component observed for different emission wavelengths might be caused by different processes (photochemistry, aggregation and energy transfer).

The highest preexponential factors corresponding to the fast component measured for the excitation at 440 and the emission at 656 nm was found for wild-type pea, followed by wheat and maize. This can be interpreted as the Pchlide giving rise to the fast-lifetime component in wild-type pea, which exists in aggregates as large as those of maize and wheat, which are dominated by the Pchlidef,^sub 650-656^ form. A part of the 633 nm fluorescence might then have a different localization. The lip1 mutant is known to miss regular PLB and aggregated Pchlide (24), which fits well with the lowest value of the preexponential factor for the fast component.

The preexponential factor connected to the slow-fluorescence lifetime component measured at 633 nm was the highest for the lip1 mutant. This fluorescence lifetime can be related to Pchlide^sub 628-633^, which is not involved in energy transfer because this can be considered low in lip1 due to the lack of acceptor. In all plant species the value of this component was considerably lower than the fluorescence lifetime of Pchlide in organic solvents at 77 K (around 9-10 ns). This indicates a Pchlide-POR interaction leading to a de-excitation pathway different from fluorescence. It is consistent with the hypothetical attachment of Pchlide^sub 628-633^ to POR, causing a partial loss of photochemical activity (1). The short-wavelength Pchlide form can then represent a monomeric Pchlide form, which still can be bound to the POR protein but without a possibility for phototransformation or it can be attached to a different protein.