Chemical structure and biological activity 



METHODS 



In order to obtain a reasonably complete picture of the growth effects of the 

 different substances, three methods of testing have been used : 



(i) The Jiax root test {S-test, Aherg, 1950 ; 1953b; 1955) showing preferen- 

 tially the inhibition of root growth by auxins, and also the restorative effects 

 of anti-auxins upon 2:4-D or lAA inhibited roots. The activity of a certain 

 substance in this test will sometimes be given as the molar concentration 

 (C^q) needed for 50 per cent inhibition, or as the negative logarithm of this 

 value (pCso). 



{ii) A wheat root test closely corresponding to the flax root test and showing 

 much more pronounced root-growth stimulations at application of anti- 

 auxins than the latter one (cf. Burstrom, 1950; 1951a,b; 1955; Hansen, 

 1954; Aberg, 1952; 1955). The wheat seedlings, var. 'Diamant IT, are 

 used when the median root has reached a length of 10 mm, the test period 

 (18 hours), the temperature (25°C), and the basic solution (pH 5-9) being 

 the same as in the flax root test. During the germination period the seedlings 

 grow on moist filter paper in large Petri dishes, and during the test period 

 they are placed in perforated cork disks floating on the solution. Only the 

 median root is measured. The average dispersion of the G values (growth as 

 per cent of control) from independent experiments is 7-9J^0-5 per cent 

 (of G) as judged from 25 series, each comprising 7-9 experiments. No clear 

 correlation between the inter-experimental dispersion and the magnitude of 

 the G value is apparent for this material, which fact may be connected with 

 the occurrence of steep parts of the action curves within the whole of the G 

 range covered (63-197) (cf. Aberg, 1955). 



(Hi) An Avena cylinder test performed mainly as described by Aberg and 

 Khalil (1953), the seedlings being reared, however, on a net of stainless steel 

 placed in a large glass dish immediately over the water surface, and the test 

 period being 18 hours. The average growth of the control sections during 

 this period amounts to 2-833b0-04 mm {a = 0-41, N = 120) or 28 per cent 

 of the initial length. The average inter-experimental dispersion of the G 

 values (expressed in per cent of control growth) is 10-7j^O-7 per cent (of G) 

 as judged from 11 series, each comprising 7-10 experiments. 



WORKING HYPOTHESIS 



There is at present no generally accepted theory as to the mechanism of the 

 physiological action of auxins and related substances. As a guide for our 

 orientation among the multitude of experimental data we will, however, 

 use the following working hypothesis which seems to give a coherent picture 

 of a major part of them. 



The auxins are thought to interact with some 'receptor' of protein nature, 

 and to exert their effects only when bound to these 'active sites' or 'growth 

 centres' of the protoplasm. The binding is assumed to be effected by manv 

 weak bonds ('multipoint attachment'), thus allowing for a high degree of 

 specificity and taking advantage of the analogy with some better known 

 enzyme-substrate complexes (Veldstra, 1953; Aberg, 1953b). 



The question .of the function of the hypothetical auxin-receptor complex 



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