Wavelength-dependent photooxidation and photoreduction of protochlorophyllide and protochlorophyll in the innermost leaves of cabbage (Brassica oleracea var. capitata L.).

Abstract

The photoreduction and photooxidation processes of different protochlorophyll(ide) forms were studied in the innermost leaves of cabbage (Brassica oleracea var. capitata L.) under monochromatic irradiations. Room-temperature fluorescence emission spectra were measured from the same leaf spots before and after illumination to follow the wavelength dependence of the photochemical reactions. Short-wavelength light of 7 µmol photons m(-2) s(-1) (625-630 nm) provoked mainly bleaching, and longer wavelengths (630-640 nm) caused both bleaching and photoreduction, while above 640 nm resulted in basically photoreduction. When bleached leaves were kept in darkness at room temperature, all protochlorophyll(ide) forms regenerated during 72 h. Oxygen-reduced environment decreased the extent of bleaching suggesting the involvement of reactive oxygen species. These results confirm that the short-wavelength, 628 nm absorbing, and 633 nm emitting protochlorophyll(ide) form in etiolated cabbage leaves sensibilizes photooxidation. However, the 628 nm light at low intensities stimulates the photoreduction of the longer wavelength protochlorophyllide forms. Kinetic measurements showed that photoreduction saturates at a low PFD (photon flux density) compared to bleaching, suggesting that the quantum yield of photoreduction is higher than that of bleaching.