A rapid gravitational effect on the translocation of fluorescein in mung bean hypocotyl sections.

Geotropic stimulation of excised stems of dark-grown mung bean seedlings (Vigna radiata L., Wilczek) results in a rapid increase in the movement of fluorescein in phloem cells of the hypocotyl. A significant effect is observed after subjecting stems to geotropic stimulation for 2.5 min. Maximum increase occurs after about 15 or more min of geotropic stimulation. The increase is confined to the lower side of the hypocotyl. Pretreatment with a 5x10(-4) M concentration of 2,4-dinitrophenol prevents the gravity-induced increase in fluorescein movement. It is suggested that the increased movement of fluorescein is due to the generation of a positive electrostatic charge in the plasma membrane of receptor cells by some unknown action of gravity on membrane molecules. The charge is presumed to be the causative factor that increases the movement of auxin and other negatively-charged substances into receptor cells.