Protochlorophyllide forms and energy transfer in dark-grown wheat leaves.: Studies by conventional and laser excited fluorescence spectroscopy between 10 K-100K

The fluorescence properties and role in energy transfer of protochlorophyllide (Pchlide) forms were studied in dark-grown wheat leaves by conventional and laser excited high resolution methods in the 10 K-100 K temperature range. The three major spectral bands, with emission maxima at 633, 657 (of highest intensity) and 670 nm as Bands I, II, and III were analyzed and interpreted as the contributions of six different structural forms. Band I is the envelope of three (0,0) emission bands with maxima at 628, 632 and 642 nm. Laser excitation studies in the range of Band II at 10 K reveal the presence of a spectrally close donor band besides the acceptor, Band II. The intensity in Band III originates mostly from being the vibronic satellite of Band II, but contains also a small (0,0) band with absorption maximum at 674 nm. Excitation spectra show that besides the Pchlides with absorption around 650 nm within Band II, another significant population of Band I with absorption around 640 nm is also coupled by energy transfer to the acceptor of Band II. The spectral difference between the two donor forms indicate different dipolar environments. Upon increasing the temperature, the intensity of Band II and its satellite, Band III decrease, while Band I remains unaffected. Band II shows also a broadening towards the blue side at higher temperatures. Both the quenching of fluorescence and the spectral change was explained by a thermally activated formation of a non-fluorescent intermediate state in the excited state of Pchlide acceptors.