Transport of the Auxin, Picloram, Through and Coleus and Stem Sections of Petioles of Bean Pea

The transport of the synthetic auxin, picloram (4-amino-3,5,6-trichloropicolinic acid) was investigated in sections of petioles of Phaseolus vulgaris L. and Coleus blumei Benth. and stems of Pisum sativum L. Transport of 14C-picloram was basipolar in all tissues, although the degree of polarity was dependant on age. The velocity of picloram movement was calculated at between 0.75 and 1.11 mm/hr. The amount moved in a given time, the flux, was dependant on the ooncentration applied and the length of the sections used. Picloram did not appear to be metabolized by the tissues during the transport experiments. When compared to the movement of other growth flegulators, picloram transport bears marked similarities to that of 2,4-dichlorophenoxyacetic acid. Picloram (4-amino-3,5,6-trichloropicolinic acid). a synthetic growth regulator with a unique chemical structure (3) acts as a potent auxin in a variety of test systems when used at low concentrations (7). Picloram, which has been used extensively as a herbicide in recent years, appears to be absorbed and translocated readily after foliar application (3.4). In the present study the transport of picloram was investigated for 2 specific reasons. Extensive investigations of the phytotoxic effects of high concentrations of picloram have established that this compound is not metabolized rapidlv, if at all. bv plant tissue. One of the problems encountered in physiological studies of growth regulator transport is the occurrence of varying degrees of degradation of the transported molecule during uptake and within the transport system itself (2. 10, 11). Picloram, because of its apparent chemical stabilitv within the tissue, affords the possibility of studying auxin transport in a more simplified system. It is also important to determine whether the capacitv of a substance to be transported in plants in a polar manner is a fundamental propertv of any molecule which exhibits auxin activity. Kefford and Caso (7) have pointed out that picloram pos-esses a chemical structure which is markedly different from that of any other natural or synthetic auxin. It is desirable that synthetic growth regulators with a phytotoxic action which in part is dependant upon their chemical stability within plants, are also investigated in terms of their usefulness as tools in a further understanding of a basic physiological 1 This work was supported by grants from the National Research Council of Canada and the Ontario Department of University Affairs. phenomena. The technique of applying an isotopically labeled growth regulator in agar blocks to either end of a tissue section and measuring the subsequent transport into plain-agar receiver blocks placed at the opposite end allows an accurate determination of the course of transport over short time periods, together with an estimation of the amount of uptake from donor blocks (1,2,8,10,11,12). McCready and Jacobs (11) have pointed out that most of the information concerning the movement of the native auxin, IAA, comes from such experiments, whilst ideas on the movement of phytotoxic growth regulators are derived largely from experiments using more nearly intact plants. In the present study, several initial experiments. which established the polarity of picloram movement in plant ticsues, were carried out with sections excised from Coleus and pea plants. The basipolar movement of isotopically labeled picloram was then investigated more fully in sections cut from the petioles of primary leaves of beans. Materials and Methods Tissue sections were excised from the expanding petioles of 6 to 8 day old plants of Phaseoluts vltlgaris L. Contender. In experiments to investigate the transport of picloram in petioles of different ages. 4 to 26 day old plants were used. In some experiments, sections were prepared from petioles of Coleus blurinei Benth and from just below the apex of 8 day old plants of Pisum sativtzum L. Little Mfarvel. All the plants used were grown in a greenhou-e. Gels of 1.5 % agar containing the potassium salt of 14C-carboxyl-labeled picloram (specific activity 4.25 pic/mg) were sectioned into cylindrical blocks of 20 ,l volume. A donor block containing 14Cpicloram was applied to one end of a tissue section. 2045 www.plantphysiol.org on January 6, 2018 Published by Downloaded from Copyright © 1968 American Society of Plant Biologists. All rights reserved.