Induction Changes of Chlorophyll Fluorescence in Chara Cells Related to Metabolite Exchange between Chloroplasts and Cytoplasmic Flow

Induction changes in chlorophyll fluorescence (Fl) are associated with photosynthetic electron transfer, generation of a proton gradient, and carbon metabolism in the CO2 fixation cycle. The reactions of photosynthesis are also accompanied by the outflow of photoproducts from illuminated chloroplasts and their long-distance transport. The exchange of metabolites across the chloroplast envelope membranes is carried out by transporters that are active in the light and cease to operate in darkness. Inactivation of light-dependent envelope transporters in Chara cells interrupts spatial signaling manifested as a transient Fl rise in response to illumination of a distant cell area. The dark adaptation was found to down-regulate the entry of metabolites from the streaming cytoplasm into shaded chloroplasts, without substantial retardation of the metabolite export from illuminated plastids. Fluorescence induction curves were found sensitive to illumination or darkening of the sample area residing outside the analyzed photometric region. The overall low-intensity illumination of the Chara internode caused significantly larger Fl changes than a similar narrow-field illumination of the Fl detection area. The results indicate that a slow increase in Fl during the induction period in characean cells results not only from photosynthetic activity of chloroplasts in the examined cell region but also from interactions between the analyzed and neighboring cell areas. When the cytoplasmic streaming was arrested by cytochalasin D, similar induction changes were induced by local and global illumination, indicating a disruption of long-range interactions. The results suggest that the liquid flow not only carries metabolites from illuminated to shaded cell parts but also facilitates the export of photometabolites from chloroplasts to the cytoplasm.