Protochlorophyllide forms and energy transfer in dark-grown wheat leaves. Studies by conventional and laser excited fluorescence spectroscopy between10 K-100K

The fluorescence properties and role in energy transfer of protochlorophyll ide (Pchlide) forms were studied in dark-grown wheat leaves by conventional and laser excited high resolution methods in the 10 K-100 K temperature ra nge. The three major spectral bands, with emission maxima at 633, 657 (of h ighest intensity) and 670 nm as Bands I, II, and III were analyzed and inte rpreted as the contributions of six different structural forms. Band I is t he envelope of three (0,0) emission bands with maxima at 628, 632 and 642 n m. Laser excitation studies in the range of Band II at 10 K reveal the pres ence of a spectrally close donor band besides the acceptor, Band II. The in tensity 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 67 4 nm. Excitation spectra show that besides the Pchlides with absorption aro und 650 nm within Band II, another significant population of Band I with ab sorption around 640 nm is also coupled by energy transfer to the acceptor o f Band II. The spectral difference between the two donor forms indicate dif ferent dipolar environments. Upon increasing the temperature, the intensity of Band II and its satellite, Band III decrease, while Band I remains unaf fected. Band II shows also a broadening towards the blue side at higher tem peratures. Both the quenching of fluorescence and the spectral change was e xplained by a thermally activated formation of a non-fluorescent intermedia te state in the excited state of Pchlide acceptors.