Effect of cytoplasmic streaming on photosynthetic activity of chloroplasts in internodes of Chara corallina

Abstract

Cytoplasmic streaming plays an important role in cell processes since it promotes solute exchange between the cytoplasm and organelles and enables lateral transport for extensive distances. The role of cyclo sis in chloroplast functioning should be most conspicuous under conditions mimicking natural mosaic illu mination and consequent alternation of cell regions with dominant dark and photosynthetic metabolism. Based on this assumption, we examined the light response curves and the induction kinetics of fluorescence based parameters of chloroplast photosynthetic activity on small regions (d ~ 100 µm) of Chara corallina Klein ex Willd. internodal cells exposed to local and overall illumination under conditions of normal cyto plasmic streaming and after suppression of cyclosis by cytochalasin B, an inhibitor of actin microfilaments. Under control conditions, the whole cell illumination caused non photochemical quenching (NPQ) of chlo rophyll fluorescence, which approached the saturation at a photon flux density of about 40 µmol/(m 2 s). By contrast, illumination of a small (2 mm wide) cell part did not cause significant NPQ at light intensities up to 100 µmol/(m 2 s), indicating that the chloroplast photosynthetic activity was substantially higher under con ditions of localized illumination. After the inhibition of cyclosis by cytochalasin B, the light response curves were represented by nearly identical sigmoid curves, irrespective of the illumination pattern. When the cyclo sis was restored in the cells washed from the inhibitor, the light response curves measured under overall and localized illumination returned to their original divergent shapes. These and other data indicate that different photosynthetic activities of chloroplasts in cells exposed to entire and partial illumination are directly related to the flow of compositionally nonuniform cytoplasm between the cell parts with prevalent photosynthetic and respiratory metabolism.