Histoautoradiographic Studies of Herbicidal Translocation

Abstract

AN understanding of the physiological action of various herbicides 1A is based in part upon a knowledge of (1) the exact path of movement of the substance in the plant, (2) the rapidity of transfer from one tissue to another, and (3) the relative proportion of the total conductive tissue that is effective in transfer under any given set of conditions. Although the intercellular distribution of radioactive isotopes in both plant and animal tissues has been studied by the use of histoautoradiography (6), only one brief paper (8) describes the use of this method with plant growth regulators. A histoautoradiograph is obtained by bringing a thin tissue section (1 to 25 [t) containing a radioisotope into close contact with a photographic emulsion which is exposed and later developed. Microscopic examination is then made to correlate the pattern of developed emulsion grains with the tissues of the section. Two of the major problems encountered in this procedure are the loss of labeled substance from the tissue and diffusion from the initial location to other locations within the tissue during fixation and dehydration. These artifacts can be minimized by (1) the use of precipitants that react with the herbicide to form insoluble and immovable deposits, together with fixatives that do not dissolve the compound being studied (15, 17); (2) by the application of a freeze-drying technique of the Altmann-Gerch type (9, 10, 12); or (3) by a combination of these two methods. The present paper deals with the evaluation of these techniques in the study of the intercellular movement of labeled 2,4-dichlorophenoxyacetic acid (2,4-D) and 3-amino-1,2,4-triazole (amitrole).