AUXIN TRANSPORT IN THE LONG SHOOTS OF PINE

BROWN, C. L. (Forest Genetics Laboratory, College Station, Texas.), and RALPH H. WETMORE. Auxin transport in the long shoots of pine. Amer. Jour. Bot. 46(8): 586-590. 1959.-Data are presented which show that endogenous auxin in the stem segments of pine is actively transported under extremely low oxygen tension and in amounts equal to that in the normally aerated controls. In distinct contrast, stem segments, previously evacuated of endogenous auxin, which readily transport IAA applied in donor blocks at physiological concentrations in air, do not do so in an atmosphere of pure nitrogen. It is concluded that it is the initial phase, the uptake of IAA from the block into the tissue, which is limiting at low oxygen tension, while the second phase, the transport sensu stricto, is almost independent of oxygen. THE PHENOMENON of polar auxin transport, first clearly demonstrated by Went (1928) in the Avena coleoptile and intensively studied by Van der Weij (1932, 1934) has since been confirmed by other workers in many species of higher plants (Avery et al., 1937; Clark, 1937; Went and Thimann, 1937; Oserkowsky, 1942; Jacobs, 1950, 1951, 1952; and Leopold and Guernsey, 1953). Although several early hypotheses were proposed to explain the possible mechanism of auxin transport (Went and Thimann, 1937, pp. 101-104), no single one seems to have gained much favor over another and the actual mechanism still remains one of the intriguingly unsolved problems associated with growth correlations in higher plants. Since the early demonstration by Van der Weij (1934) that applied auxin in donor blocks could move basipetally through Avena coleoptiles quite independently of the external auxin gradients, several workers have shown a relationship between auxin transport and energy supplied through respiration (Bonner, 1936; Clark, 1937; DuBuy and Olson, 1940; Niedergang and Skoog, 1952; Gregory and Hancock, 1955; Niedergang-Kamien and Skoog, 1956; and Niedergang-Kamien and Leopold, 1957). The latter authors, Niedergang-Kamien and Leopold (1957), by the use of different classes of metabolic inhibitors, have shown the specific inhibition of auxin transport at concentrations which did not inhibit respiration to the same extent. Thus, these authors made the appropriate suggestion that auxin transport is dependent upon energy production and transfer rather than respiration per se. The primary purpose of the present paper is to show that the basipetal movement of native or endogenous auxin in the long shoots of pine, in ' Received for publication March 14, 1959. This work was performed during 1956-1957 while the senior author held a Research Assistantship awarded by the Maria Moors Cabot Foundation, Harvard University, Cambridge, Massachusetts. The authors wish to express their sincere appreciation to Professor K. V. Thimann, Harvard Biological Laboratories, Harvard University, for his keen interest and stimulating conversations throughout the course of these studies. contrast to that of applied auxin, can occur under extremely low oxygen tensions. This observation must be taken into account in interpreting previous auxin transport studies using externally applied auxin. EXPERIMENTAL PROCEDURES.-Intensive investigations on the production and distribution of diffusible auxin in the elongating shoots of loblolly pine (P. taeda L.), were made previously in connection with another study and will be reported in detail elsewhere. For the purpose of this paper it is only pertinent to point out that the amount of diffusible auxin obtained from different portions of the elongating shoots is lowest at the distal end, progressively increases through the region of internodal elongation, and reaches its highest peak in the basal maturing portion of the shoot, i.e., in the sterile bract region just below the young developing needle fascicles or brachyblasts. Isolated stem segments 1.0 cm. in length from the latter region, when diffused for a standard 3-hr. period into 1.5%o agar blocks (11 X 8 X 1.5 mm.) normally yield an average of 10.00 Avena curvature. In the tests reported here this is equivalent to a concentration of 0.025 mg./l. of IAA. Although a vigorously growing pine shoot initially exudes copious amounts of resin at freshly cut surfaces, it nevertheless has proved to be excellent material for transport studies because of the consistent high levels of endogenous auxin, and the absence of any appreciable enzymatic auxin inactivation at the cut surfaces. Two types of transport experiments were conducted: (1) the movement of endogenous auxin into agar blocks, and (2) the transport of exogenous IAA through stem segments (after evacuation of the native auxin) using the standard donor/receiver block technique. Both types of transport were performed in an atmosphere of purified nitrogen, accompanied by appropriate controls in air. All tests were performed at room temperature (260 ? 2?C.), and light was excluded by covering the diffusion chamber with black cloth. For the transport studies in nitrogen, a simple "air-tight" Plexiglass chamber approximately 10 X