Lateral transport of photosynthetically active intermediate at rest and after excitation of Chara cells

Abstract

Cytoplasmic streaming is vital for plant cells; however, its relation to cell functions remains largely undisclosed. Microfluorometry of chloroplasts in vivo and measurements of cell surface pH under localized illumination of cell regions located upstream the cytoplasmic flow, at a distance of few millimeters from the analyzed area, is a new means to reveal the role of liquid flow for signal transmission in large cells, such as internodes of characean algae. Properties of photoinduced signals transmitted along the cell can be clarified by comparing the effects of pointed illumination under conditions of continuous and briefly arrested cytoplasmic flow. Chlorophyll fluorescence measurements with the use of saturation pulse method showed that excitation-induced cessation of cytoplasmic streaming, concomitant with the period of localized illumination, caused a significant delay and deceleration of the lateral transmission of the photoinduced signal and, in addition, diminished the peak of maximal fluorescence F m′ in the cell response to propagated signals. The relative extent of the peak suppression was small in cell regions producing light-dependent external alkaline zones and increased substantially for cell regions with slightly acidic external pH. These and other results indicate the possible role of cytoplasmic pH in controlling chlorophyll fluorescence and photosynthetic activity in vivo. When the period of streaming cessation coincided with localized illumination, the velocity of cytoplasmic flow recovered slower than after arrest of the flow without additional illumination. The results are promising for further analysis of regulatory and protective functions of cytoplasmic streaming in photosynthesizing plant cells.