Dominance of a 675 nm chlorophyll(ide) form upon selective 632.8 or 654 nm laser illumination after partial protochlorophyllide phototransformation

Abstract

The phototransformation pathways of protochlorophyllide forms were studied in 8–14-day-old leaves of dark-germinated wheat (Triticum aestivum L.) using white, 632.8 nm He–Ne laser and 654 nm laser diode light. The photon flux density (PFD) values (0.75–360 μmol photons m⁻² s⁻¹), the illumination periods (20 ms–10 s) and the temperature of the leaves (between −60 °C and room temperature) were varied. The 77 K fluorescence spectra of partially phototransformed leaves showed gradual accumulation or even the dominance of the 675 nm emitting chlorophyllide or chlorophyll form at room temperature with 632.8 nm of PFD less than 200 μmol photons m⁻² s⁻¹ or with 654 nm of low PFD (7.5 μmol photons m⁻² s⁻¹) up to 1 s. Longer wavelength (685 or 690 nm) emitting chlorophyllide forms appeared at illuminations under −25 °C with both laser lights or at room temperature when the PFD values were higher or the illumination period was longer than above. We concluded that the formation of the 675 nm emitting chlorophyllide form does not indicate the direct photoactivity of the 633 nm emitting protochlorophyllide form; it can derive from 644 and 657 nm forms via instantaneous disaggregation of the newly-produced chlorophyllide complexes. The disaggregation is strongly influenced by the molecular environment and the localization of the complex.