Some Characteristics of Movement of Indoleacetic Acid in Coleoptiles of Avena. I. Uptake, Destruction, Immobilization, & Distribution of IAA During Basipetal Translocation

Indoleacetic acid (IAA) appears to be the major naturally occurring auxin controlling normal vegetative growth of a wide variety of higher plants. In many plants the auxin of the shoot is produced at or near the apex (8, 12, 32) and must be translocated from the site of production to the many sites where it influences growth and differentiation in the shoot. The translocation of auxin, like that of other organic molecules, can not be attributed to diffusion or purely physical processes; it requires the presence of living cells (28, 29, 23). The movement of auxin differs from the translocation of other organic substances in that it occurs predominantly from the apex toward the base of the shoot. This is not just-a reflection of auxin production at the apex. In Avena coleoptiles, Went showed that even when auxin is supplied at the base of a section it can not be collected at the apex (31). Since the initial work of Went (31) and van der Weij (28, 29), information has been obtained on the ability of plant tissues to translocate natural auxins (13) and on the inhibition of translocation by various substances (e.g. 5,16,17, 18). These studies, however, have been limited by the sensitivity and adaptability of some bioassay for detecting the auxin. Conclusions had to be drawn from the amount of auxin activity appearing at the opposite end of a section from a source of IAA. This experimental system is a complex one, and one would like to know how the uptake from the source, the destruction, and the retention of auxin within the tissue itself influence the amount of auxin recovered at the base of a section. Only with the advent of radioactively labeled IAA has this type of quantitative study become feasible. The information presented here on the normal basipetal transport of IAA by excised sections provides a basis for further studies. For example, the effect of altered oxygen tensions and the characteristics of an acropetal, apolar movement have already been examined and will be described in subsequent papers.