Selective permeability of Chara nodal complex for cell-to-cell passage of photometabolites exerting opposite action on chlorophyll fluorescence

—Plasmodesmata (PD) are crucial for intercellular communication and long-distance transport of signaling substances and photoassimilates. These nanosized cytoplasmic strands piercing cell walls between adjoining cells allow the passage of many low-molecular-weight substances. However, it is not yet known if small molecules such as NAD(P)H and H₂O₂ released from chloroplasts under local light stress permeate equally well across the PD. In this work, the actual and maximal chlorophyll (Chl) fluorescence yields, F′ and F_m′ were measured with PAM microfluorometry on internodal Chara cells bathed with the media in physiologically relevant pH ranges (pH 7.0 and 9.5). In the cells exposed to continuous dim light and subjected to a local pulse of high light, the streaming cytoplasm carried the metabolites of two types. At pH 7.0, the released metabolites transiently elevated F′ by promoting plastoquinone reduction. Under CO₂-depleted environment at pH 9.5 adjusted with CHES buffer, the cytoplasm was additionally enriched with a metabolite, presumably H₂O₂ that quenched both F′ and F_m′. Strong rapid H₂O₂-mediated quenching of Chl fluorescence in vivo was verified by means of pointed pericellular application of this chemical. The intercellular permeation of the metabolites having opposite influence on Chl fluorescence was assessed by applying the local light pulse to one internodal cell in the paired internode sample and by recording Chl emission in the adjacent internode. The results demonstrate that the metabolite enhancing F′ fluorescence readily permeates through the nodal complex, whereas the transnodal permeation of the metabolite responsible for quenching of F_m′ and F′ is prevented.