Auxin-transport in the hypocotyl of Phaseolus vulgaris L.

OF THE MANY intriguing aspects of organic development, few are as ubiquitous, as striking, and as baffling as the phenomenon of polarity. In any attempted analysis of biological polarity, it is useful to try to distinguish between various manifestations of polarity and the propertv or properties which are the determinative factors in polaritv. For instance, the basal formation of roots on stem cuttings seems, in many cases, to be a regenerative manifestation of basipetally moving auxin. A reconnaissance of polarity phenomena in plants (suclh as is provided by Bloch, 1943; Gautheret, 1944) impresses a researcher trained in auxin work with the large number of polarity manifestations which seem explicable in terms of current knowledge of the transport properties of auxin and of auxin's manifold effects. Of course, as previous writers have pointed out, an explanation of many polarity phenomena as manifestations of a polarized movement of auxin still does not explain what controls the latter. But such an explanation can be useful by indicating cause-and-effect relationships. In the experiments which followed the first collection of auxin from coleoptile tips (Went, 1926), it was soon noted that auxin moved in polar fashion through stem tissues (Went, 1928). Van der Weij (1932, 1934) carried out extensive experiments on auxin movement in Avena coleoptiles. He found basipetal polarity with very slight acropetal movement. Auxin-transport could even occur against an auxiin gradient. This polarity of auxin-transport has been confirmed for Avena coleoptiles and has been reported for various other shoot structures, including hypocotyls (cf. Went and Thimann, 1937, p. 97; Skoog, 1938; Went, 1939, 1941; Went and WVhite, 1939). As long as physiological concentrations of auxin are used, and aside from cases where the auxin is apparently carried in the transpiration stream (Hitchcock and Zimmermann, 1936), all present evidence agrees in showing a strong basipetal polarity of auxin movement in stem tissues. The experiments reported in this paper were designed primarily to shed light on the following questioiis and hypotheses (and the results will be discussed under these headings): I. In a well developed hypocotyl does the polarity of auxin transport have the same strength all along the hypocotyl, or is there a gradient of decreasing transport along the hypocotyl (contrary to its morphological and anatomical characteristics) ? II. Does the initial differentiation of the meristem of the primary root occur after the polar 'Received for publication July 14, 1949. Much of this work was done while a member of the Society of Fellows of Harvard University. My appreciation of the excellent technical assistance of Miss B. Edson is oLadlv mentioned. transport of auxin is apparent? This question is asked for the following reasons. At present, the factors determining the initial axial polarity of the proembryo (i.e., the differentiation of a root at one end of the proembryo and of a shoot at the other end) are quite unknown. But research on the effects of auxin on root-initiation in stem-cuttings and root-cuttings (Gautheret, 1946; Warmke and Warmke, 1946) has shown that a relatively high concentration of auxin stimulates root-differentiation, while a lower concentration promotes the differentiation of shoots. If auxin present in the proembryo were transported in polar fashion, it would presumably "pile-up" at one end of the proembryo; when this localized concentration of auxin became high enough, it would, according to this hypothesis, stimulate at that region the differentiation of the primary root. Hence, as a first check on the validity of this hypothesis it is important to know if auxin moves in polar fashion in the embryo even before the root-meristem is differentiated. MATERIALS AND METHODS.-Seeds of "Red kidney bean" were soaked in water, planted in sand in flats on 6-7 successive days, grown in the laboratory at 24 ? 1 C. and dug up on the morning of the test. The seedlings were washed in water and surface-dried on paper-towels. Between 9:00 and 10:30 A.M., cylindrical sections 5 mmi. long were cut from the top 7 mm. of the hypocotyl by making two transverse cuts with a sharp razor blade. Five mm. was chosen as the best length for the transport-sections. Five mm. was long enouglh to minimize auxin-movement in an enveloping water film (particularly after the seedlings bad been surface-dried)-an artifact which van der Weij (1932) could not eradicate using 2 mm. sections. It was not so long that much inactivation could occur (van Raalte, 1937) or that the transport-time would be inconveniently long. Van der Weij (1932), for instance, found a transportvelocity of 8-15 mm./hr. in Avena coleoptiles; hence, auxin applied at one end of a 5 mm. section might be expected to appear at the collecting block at the other end about 1/2 hr. later. Also, this lelngth was near enough to those used by previous workers (e.g., Dijkman, 1934; van Raalte, 1937; Skoog, 1938; Went, 1939; Went and White, 1939) to mnake our results more comparable. Indole-acetic-acid (IAA) at 2 p. p. m. in 1.5 per cent agar was placed on one end of each hypocotyl section, plain 1.5 per cent agar at the collecting end (2 p. p. m. was decided on because it is not unphysiologically hiigh). Good contact between tissue and agar was insured by placing a small drop of distilled water on each cut-surface just before adding the agar blocks. The hylpocotyl sections, with agar blocklets added, were placed in damp chambers at room temperature (24 + 1? C.) and with ordinary, diffuse liglht-