A Fluorescent Dye Method for Foliar Penetration Studies

Abstract

lish the location of the penetrant on the cellular level. Histochemical techniques have been employed with great success by Knight et al. (10), Rohrbaugh (14), Rice and Rohrbaugh (13), and others to demonstrate the sites of entry and routes of movement of oils. In the case of aqueous solutions, on the other hand, most evidence is indirect in nature, and histochemical evidence is limited to very few reports such as those of Palmiter et al. (12) and Wylie (24) for penetration and movement of iron. Many workers (3, 5, 15, 21, 22, 23) have compared stomatal with cuticular penetration by such indirect methods as measurement of plant response to growth regulators or nutrients, autoradiography or counting of tissues containing radioactive tracers, or measurement of nonabsorbed chemical. The importance of stomatal entry has not been clearly established by these methods. More direct evidence is obtained by using other techniques. For example, Aslander (2) and Crafts (6) studied the penetration of dilute sulfuric acid solutions by microscopic detection of injury resulting from the toxic action of the penetrant; however, conflicting observations were reported. Autoradiography of leaf sections has been used by Gustafson and Schlessinger (9) to demonstrate penetration of cobalt60 into bean leaves in the dark, but to date this promising method has not been widely applied in attempts to determine the route of penetration. Because most of the conventional methods yield only indirect evidence, a more direct method involving a fluorescent dye has been testedl. This type of tracer was used to study foliar absorption of dew water by Steubing (18), who followed entry and movement of berberine sulfate with the fluorescence microscope. By a similar method, Butterfass (4) observed foliar penetration via the club-shaped hairs of Vicia faba L. and Phaseolus coccineus L. Orgell3 used both fluorescent and nonfluorescent dyes to study penetration of isolated plant cuticles. In the work reported here, the fluorescent tracer method has been considerably expanded and improved for application to the problem of foliar absorption of aqueous solutions.