Propagation of photoinduced signals with the cytoplasmic flow along Characean internodes: evidence from changes in chloroplast fluorescence and surface pH

Emerging evidence suggests that cytoplasmic streaming can regulate the plasma-membrane H⁺ transport and photosynthetic electron flow. Microfluorometric and surface pH measurements on Chara corallina internodes revealed the transmission of photoinduced signals by the cytoplasmic flow for a distance of few millimeters from the site of stimulus application. When a 30-s pulse of bright light was locally applied, the downstream cell regions responded with either release or enhancement of non-photochemical quenching of chlorophyll fluorescence, depending on the background irradiance of the analyzed cell area. Under dim background irradiance (<20 μmol m^?2 s^?1), the arrival of the distant signal from the brightly illuminated 400-μm-wide zone elevated the maximal fluorescence F _m ^′ in the analyzed downstream area, whereas at higher background irradiances it induced strong quenching of F _m ^′ . At intermediate irradiances the increase and decrease in F _m ^′ appeared as two successive waves. The transition between the F _m ^′ responses of opposite polarities occurred at a narrow threshold range of irradiances. This indicates that inevitable slight variations in irradiance at the bottom chloroplast layer combined with the cyclosis-transmitted signals may contribute to the formation of a photosynthetic activity pattern. The rapid cyclosis-mediated release of non-photochemical quenching, unlike the delayed response of opposite polarity, was associated with opening of H⁺ (OH^?)-conducting plasma membrane channels, as evidenced by the concurrent alkaline pH shift on the cell surface. It is proposed that the initial increase in F _m ^′ after application of a distant photostimulus is determined, among other factors, by the wave of alkaline cytoplasmic pH.