Protochlorophyllide and chlorophyll forms in dark-grown stems and stem-related organs

Protochlorophyllide contents and the spectral properties together with photoactivities of native protochlorophyllide forms have been studied in dark-forced stems of 26 and epicotyls or hypocotyls of 9 plant species. The 77 K fluorescence emission spectra show that a form emitting at 629-631 nm is general in these organs. Besides this short-wavelength form, other protochlorophyllide forms emitting at 636, 645 and around 650-655 nm are found with various relative amplitudes. The pigment contents show good correlation with the ratio of short- to long-wavelength forms, i.e., the higher this ratio is, the less protochlorophyllide is detected. In addition to protochlorophyllide, several dark-grown plants also contain chlorophylls. In some cases only one chlorophyll form appears with emission maximum at 678-680 nm; other plants have forms characteristic of the fully developed photosynthetic apparatus (with maxima at 685, 695 and 730-740 nm). Flash illumination can transform only the 645 and 650-655 nm protochlorophyllide forms, the shorter-wavelength-emitting forms being inactive. Plant species with dominating 629-636 nm protochlorophyllide forms cannot accumulate chlorophyll on continuous illumination of natural intensity, and they became photodamaged. The structural or molecular background of the appearance of the different protochlorophyllide and chlorophyll forms and the reasons for their photosensitivity are discussed.