Abstract

The large internodal cells of the characean alga Chara corallina serve as an established model for investigating the intricate relationship between cytoplasmic dynamics and chloroplast function. This study aimed to elucidate how the rapid cytoplasmic flow, or streaming, influences the kinetics of chlorophyll (Chl) fluorescence induction, a non-invasive probe of photosynthetic electron transport and the metabolic state of the chloroplast. Specifically, the research focused on the S-M-T transitions of the Chl fluorescence induction curve, which are sensitive to the exchange of metabolites between the chloroplast and the surrounding cytoplasm. Using microfluorometry on individual chloroplasts in the internodal cells, the study monitored the induction changes under various experimental conditions that modulate cytoplasmic streaming and the concentration of key metabolites. The key finding is that the characteristic induction changes are highly dependent on the efficiency of metabolite export from the chloroplasts and their subsequent rapid distribution by cytoplasmic streaming. The results suggest that the streaming prevents the localized accumulation or depletion of photosynthetic products and intermediates near the chloroplast envelope, thereby maintaining a stable and efficient rate of photosynthesis. This work highlights the critical role of cytoplasmic streaming in regulating intracellular communication and metabolic homeostasis in large plant cells, providing new insights into the biophysical constraints on photosynthetic efficiency.