Formation of short-wavelength chlorophyll(ide) after brief irradiation is correlated with the occurrence of protochlorophyll(ide)636–642 in dark-grown epi- and hypocotyls of bean (Phaseolus vulgaris)

Chlorophyll formation capacity along the seedling of bean ( Phaseolus vulgaris L. cv. Brede zonder draad) was investigated. After 7 days of irradiation a gradient was formed, where the primary leaf contained ca 300 times more chlorophyll per gram fresh weight than the lower hypocotyl section and ca 20 times more than the epicotyl. Similar chlorophyll gradients but at lower levels were seen when the seedlings were first placed in darkness for 7 days and then irradiated for 1, 2 or 7 days. Ultrastructural investigation of seedlings grown for 7 days in darkness and then irradiated for 24 h revealed a more developed inner membrane system with grana stacks in plastids of cells in the uppermost hypocotyl section compared to plastids of cells in lower hypocoty] sections. The higher up on the seedling the more the ratio increased of protochlorophyll(ide) emitting at 657 nm to short-wavelength protochlorophyll(ide). After flash irradiation of the different sections, fluorescence emission spectra with maxima at 680 and 690 nm, respectively, were observed, indicating the formation of short- and long wavelength chlorophyll(ide) forms. The lower the ratio of protochlorophyll(ide) emitting at 657 nm to the short-wavelength protochlorophyll(ide), the less long-wavelength chlorophyll(ide) was formed after irradiation. However, after continuous irradiation long-wavelength chlorophyll(ide) was formed. In dark grown roots, where only short-wavelength protochlorophyll forms were present, it was not possible to transform protochlorophyll to chlorophyll by flash irradiation. Possible explanations for this phenomenon are discussed.