Microfluidic communications in characean internodes at neutral and alkaline external pH

Intracellular communications mediated by cytoplasmic streaming compensate for the slowness of diffusion on large scale distances. In characean internodes, cyclosis serves to smooth concentration gradients related to local structural distinctions, irregular spotted illumination, and patterned profiles of external pH. In dimly lit Chara cells, the fluidic transmission of reducing equivalents from the spot of bright light incidence to a remote analyzed area transiently elevates the actual yield of chlorophyll fluorescence (F') under natural acidic zones with little effect on F' under alkaline bands. Here, the natural formation of alkaline zones was imitated by placing the internodal cell part into a solution with a pH of 9.5. Using PAM microfluorometry, we found that chloroplasts located under an alkaline solution retained the perception of reducing equivalents transported with the fluid flow but, in addition, became responsive to another transportable metabolite that promoted strong quenching of both F'_m and F' fluorescence. The superposition of oppositely directed F' responses to distinct cyclosis-transported metabolites resulted in the seeming suppression of microfluidic interactions between distant chloroplasts. The action potential generation did not affect F'_m fluorescence (an indicator of non-photochemical quenching, NPQ) when the cell was bathed at neutral pH but induced strong NPQ in the high pH solution. We propose that the restricted CO₂ supply at high external pH induces the rearrangement of electron transport to alternative pathways, which elevates the background level of NPQ-promoting metabolite (supposedly H₂O₂), thus enhancing the chloroplast sensitivity to H₂O₂ portions delivered with the fluid flow from the region subjected to intense local light.