AUXIN RELATIONSHIPS IN THE ALASKA PEA (PISUM SATIVUM)

SCOTT, Tom K. , AND WINSLOW R. BRIGGS. (Stanford U., Stanford, Calif.) Auxin relationships in the Alaska pea (Pisum sativum). Amer. Jour. Bot. 47(6): 492-499. Illus. 1960. -The distribution of "free" auxin in the 9-day-old 'Alaska' pea epicotyl was determined by short-term ether extraction and by the standard agar diffusion technique. The apical bud appeared to be the only source of "free" auxin. In the upper (growing) internode "free" auxin as determined by diffusion was found to decrease significantly from apex to base, while "free" auxin as determined by extraction remained constant. Below this region, both diffusible and extractable auxin remain constant through one internode and then both decrease simultaneously to the base of the plant. In the growing region, a fraction of diffusible auxin must move from the transport system but remain readily extractable. Upon removal of the apical auxin source all "free" auxin will ultimately be found in the transport system from which it gradually disappears basally. A PRIMARY requirement for understanding the role of auxin in the elongation of plant tissue is an exact knowledge of the distribution of auxin in its various forms. The 2 usual methods for obtaining native auxin from plant tissue are solvent extraction for estimating total auxin content and auxin diffusion for determining the amount of auxin actually moving in a transport system. Controversies have arisen in the past between authors using different auxin-assay techniques. One such controversy regarding auxin distribution following phototropic induction has recently been highlighted in a review by Reinert (1959). Other difficulties in interpreting native auxin studies are considered in the discussion. Therefore, it seems very important to determine in 1 plant at 1 time the distribution both of diffusible and of extractable auxin, and to determine the precise relationships between these 2 and growth. There are numerous studies of the correlation between the distribution of auxin and growth (Thimann, 1934; Avery, 1935; Van Overbeek, 1938, 1941; Went, 1942; Briggs et al., 1955; Steeves and Briggs, 1959). However, in view of the rapid advance of knowledge of plant growth hormones during recent years, it seemed that a re-evaluation might be useful. An important feature of such a study must be that it consider only that auxin which is assumed readily available for growth. In the first 5 papers cited above, to be considered in detail in the discussion, one can not be certain what portion of the extracted auxin is normally growth-active. In the remaining 2 papers, on the other hand, recovery of diffusible auxin was hampered by the presence of a cut-surface auxin inactivation system. Thus, it is difficult to compare the results of each of the several techniques, either as used on a single plant or on different plant species. A consideration of the role of auxin in the intact plant must necessarily include a study 1 Received for publication October 18, 1959. 2 This work was supported by the National Science Foundation under grant NSF G-2832 to the junior author. both of moving and static components. No single study has provided complete information on both. The purpose of the present investigation was to seek answers to several questions. First, and perhaps foremost, what relationships exist between diffusible and so-called "free"-extractable auxin in the pea seedling? If the 2 forms of auxin are not equivalent, in what ways may they be distinguished? Is it possible to make a complete separation of "free"-extractable auxin and diffusible auxin, or does one include all or part of the other? If the distribution of the 2 forms is altered in the seedling, how are the changes reflected in space and time? Finally, if such changes exist, with which is growth correlated? MATERIALS AND METHODS. Auxin determinations were made from 'Alaska' pea seedlings (Pisum sativum L.) grown under constant environmental conditions. Seeds were germinated on wet filter paper at 25? C. after a soaking period of 6-8 hr. in dechlorinated tap water. Upon germination (2 days), the seedlings were placed on a sheet of perforated "plexiglas" so that the roots had access to a container of dechlorinated tap water below. The plants were then transferred to a constant-light and constant-temperature cabinet. The temperature of the cabinet was maintained at 25? -270C. and a light intensity of 75-100 ft. -c. was provided by a 20 w. "standard cool white" fluorescent lamp operated on a 20-hr. light-4-hr. dark cycle. On the ninth day following soaking, individual plants 12-16 cm. in length (fig. 1) were selected for auxin-distribution studies. Internodal length ranged from approximately 1 cm. in the first and second internodes (1 and 2, fig. 1) to 3-5 cm. in the top internodes (3, 4, and 5, fig. 1). At this age, 5 leaves are present in addition to the cotyledons, with a sixth enclosed in the apical bud. The 2 basal leaves are reduced and scale-like, while the third appears normal in character though somewhat smaller in size than those above. Leaves 4 and 5 have the normal pair of ovate leaflets in addition to a terminal tendril. There are large foliar