Blue-light-induced cortical fiber reticulation concomitant with chloroplast aggregation in the alga Vaucheria sessilis.

Point illumination with low-intensity blue light induces the chloroplasts and other organelles, which normally stream in the cytoplasm of Vaucheria sessilis (Vauch.) D.C. (Xanthophyceae), to aggregate in the illuminated region of the cell. Aggregation is passive and results from the "trapping" of the organelles as they stream into the blue light. Prior to illumination, longitudinal fibers along which the organelles appear to move, can readily be seen through a light microscope fitted with differential interference contrast optics. Upon actinic irradiation, these fibers appear to become destabilized, branching and forming a cortical fiber reticulum in the light. The reticulation process always precedes chloroplast aggregation. Aggregation itself occurs after a lag period which is inversely related to fluence rate. The lag period at high fluence rates (>400 mW m(-2)) may be as short as 20 s. Studies of wavelength dependence show that wavelengths near 480 nm are maximally effective while those longer than 530 nm are inactive.