146 Fundamentals of Auxin Action 



Molecular Structure and Translocation 



An interesting aspect of the translocation of auxins in the carbo- 

 hydrate transport system is the large effect which molecular structure 

 may have on this function. The studies in which observations of 

 translocation for several auxins have been made are very few, but some 

 general patterns may be perceived in the available data. First, in the 

 phenoxy acid series tested by Osborne and Wain (1950), it appears 

 that chlorination of the phenyl ring may increase translocation. For 

 example, 4-chlorophenoxyacetic acid was translocated less readily than 

 2,4-D. The extreme mobility of 2,4,5-T in plants is well known 

 (Young and Fisher, 1950). Second, a-propionic acid side-chains may 

 be associated with poor translocation. For example, a-phenoxypropi- 

 onic acid had a largely local effect, though with some movement to the 

 growing point (Osborne and Wain, 1950). A similar auxin with a 

 2-chloro substitution was somewhat more mobile, and the 2,4-dichloro 

 derivative was reported to induce responses like 2,4-D. This a-propi- 

 onic derivative of 2,4-D has been reported as being very poorly trans- 

 ported in the Avena test (Collins and Smith, 1952), and the (L) isomer 

 of a-indolepropionic acid is also poorly transported (Kogl and Ver- 

 kaik, 1944). 



The presence of nitrogen in indoleacetic acid appears to be 

 related to its translocation in the plant, for substituting a carbon or 

 an oxygen atom for the nitrogen essentially eliminates translocation, 

 even though auxin activity remains (Thimann, 1951). 



Translocation Rates 



Rates of entry and translocation of 2,4-D through bean seedlings 

 have been estimated through some ingenious experiments by Day 

 (1952). Movement of the material through the epidermis is estimated 

 at about 30 micra per hour. This might be considered to represent the 

 rapidity of absorption of the auxin through the surface of the leaves. 

 This slow rate does not continue, however, after the auxin has reached 

 the vascular system. Once in the phloem the rate of movement is 

 estimated to be between 10 to 100 cm. per hovu'. This rate is in the 

 same range as the rate of carbohydrate translocation, estimated to be 

 about 85 cm. per hour (Vernon and Aronoff, 1952). The rate of move- 

 ment of 2,4-D in kerosene was considerably slower, 4 cm. per hour. 

 It might be recalled that the polar transport of auxin in the Avena 

 coleoptile is approximately 1 cm. per hour. 



Several clear distinctions can be drawn between auxin transport 

 which occurs in conjunction with carbohydrate translocation and the 



