Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of F V/F M on excitation wavelength

The F (0) and F (M) level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F (V)/F (M) (=(F (M) - F (0))/F (M)) have been derived: within each of the three spectra of F (V)/F (M), statistically meaningful variations were detected. Also, at distinct wavelength regions, the F (V)/F (M) differed significantly between leaf types. From spectra of F (V)/F (M), excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F (V)/F (M) between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F (V)/F (M) at these wavelengths. All three leaves exhibited low values of F (V)/F (M) around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550-650 nm region, the F (V)/F (M) in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C-4 species, than in barley, a C-3 species. Finally, low values of F (V)/F (M) at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F (V)/F (M) ratio to derive information on spectral absorption properties of PS I and PS II is discussed.