Protochlorophyllide transformations and chlorophyll accumulation in epicotyls of pea (Pisum sativum)

Abstract

Low‐temperature fluorescence emission spectra of epicotyls of 6.5‐day‐old dark‐grown seedlings of pea (Pisum sativum L.) showed the dominance of short‐wavelength protoch lorophyllide forms with emission maxima at 629 and 636 nm, respectively. The presence of long‐wavelength protochlorophyllide with emission maxima around 650 nm was just detectable. Accordingly, irradiation with millisecond flashes gave a minute formation of chlorophyllide. The chlorophyll(ide) formation varied along the epicotyl. Irradiation with continuous light for 1.5 h resulted in an evident accumulation of chlorophyll(ide) in the upper part of the epicotyl. Only small amounts accumulated in the middle section. The conversion of protochlorophyllide to chlorophyllide was temperature dependent and almost arrested at 0°C. The chlorophyll(ide) formed had one dominating fluorescence peak at 681 nm. Irradiation for 24 h gave almost 100 times more chlorophyll in the upper part of the epicotyl than in the lower part. Electron micrographs from the upper part of the epicotyl irradiated for 6 h showed plastids with several developing thylakoids, while the plastids in the lower part of the epicotyl had only a few thylakoids. The dominance of short‐wavelength protochlorophyllide forms indicated the presence of protochlorophyllide not bound to the active site of NADPH‐protochlorophyllide oxidoreductase (EC 1.3.1.33). The inability of the short‐wavelength form to transform into chlorophyllide with flash light denotes a dislocation from the active site. The time and temperature dependence of the chlorophyll(ide) formation in continuous light indicates that a relocation is required of the short‐wavelength protochlorophyllide before chlorophyllide formation can occur.