EFFECTS ON MITOSIS, GROWTH, DIFFERENTIATION 257 



normal. There is undoubtedly some relationship, but it may be doubted if 

 it is simple. For example, in the study of Christiansen (1950) on the growth 

 of pea stems, in which the proper controls were run (see accompanying 

 tabulation), the inhibition of basal growth was 62% and the inhibition of 



3-Indoleacetate lodoacetate (0.6 mM) Elongation 



- _ 20 



+ 7.5 



+ - 50.9 



+ + 25.6 



the auxin effect was 41%. lodoacetate can interfere in the complexing of 

 3-indoleacetate with pea root proteins (completely at 0.5 mM) and in the 

 reaction of the auxin-protein complex with coenzyme-A (50% at 5 mM 

 (Siegel and Galston, 1953). The binding of auxin to protein in vivo seems 

 to be an energy-requiring process, and is inhibited by a variety of meta- 

 bolic depressants. Polar auxin transport in sunflower stems is moderately 

 reduced by lodoacetate (Niedergang-Kamien and Leopold, 1957) and this 

 could certainly play some part in growth inhibition under natural conditions. 

 Little attempt has been made to determine if the inhibitions observed 

 are indeed related to the block of the EM pathway. However, Commoner 

 and Thimann (1941 ) found that various cycle substrates can to a large extent 

 overcome the growth suppression produced by 0.05 mM lodoacetate. In 

 decreasing order of effectiveness, the following are active: fumarate, malate, 

 pyruvate, and succinate. Albaum and Eichel (1943) also found succinate 

 and pyruvate to be effective antagonists. It seems that lodoacetate acts 

 mainly on the EM pathway, but the order of potency of these substrates 

 is surprising and is not related to their penetrabilities (i.e., their p^,/s). 

 The odd observation of Thimann and Bonner (1948) that malonate and 

 maleate at 1 mM can overcome lodoacetate inhibition of coleoptile growth 

 is probably to be explained as due to the K+ ions introduced with these 

 anions (Cooil, 1952). It has been commonly observed that lodoacetate in 

 concentrations markedly depressing growth does not inhibit respiration 

 much or at all (Commoner and Thimann, 1941; Albaum and Eichel, 1943; 

 Christiansen et al., 1949). Commoner and Thimann believed there to be a 

 small fraction of the respiration associated with auxin-stimulated growth 

 which is particularly sensitive to lodoacetate, but it is doubtful if such an 

 interpretation can now be put on the results in light of the possible reasons 

 for the insensitivity of respiration to lodoacetate (see page 110). The dif- 

 ferential effect can be seen well in corn root tips, where the lodoacetate con- 

 centrations for 50% inhibition of elongation and respiration are 0.24 mM 



