68 KENNETH V. THIMANN 



of malate, which evidently needs further study to elucidate the con- 

 ditions under which it occurs. 



The inhibition of growth by iodoacetate and its promotion by four- 

 carbon acids has been confirmed also for the growth of intact seedlings 

 by Albaum and coworkers (1941, 1943)- They found certain differ- 

 ences in the time relations, however, which may be due to the use of 

 whole seedlings with accompanying complications of transport, inactiva- 

 tion in the plant, and varying uptake by the roots, or may be due as they 

 suggest to changes in the enzyme systems with age of the seedlings. 



Recently Bonner and Wildman have reported (1946) that fluoride 

 inhibits growth, and like iodoacetate the inhibition can be exerted with- 

 out any marked reduction in total respiration. They were led to this by 

 their finding (mentioned above) that the auxin-protein in the cytoplasm 

 of leaves contains a phosphatase and no other enzyme — a very sugges- 

 tive fact. Fluoride and iodoacetate both act on the phosphorylating break- 

 down of glucose, in fact on directly subsequent reactions, though it is 

 true that in the extracted systems so far studied they act only in higher 

 concentrations than are effective on plant growth. We know from the 

 work of James and his coworkers at Oxford that sugar breakdown in 

 leaves follows closely (though not identically) the chain of reactions 

 worked out in yeast and elsewhere. It is tempting therefore to postulate 

 that both interfere with growth through the sugar-phosphorylating 

 chain, and this would then be the system that delivers energy for growth. 

 The four-carbon acids might participate by allowing a phosphorylation 

 to occur at the expense of oxidizing malate, etc., as described for suc- 

 cinic, pyruvic, and ketoglutaric acids by Colowick et al. (1940, 1941) 

 and by Ochoa (1943, 1944). 



In attempting to get some evidence on this we have carefully com- 

 pared the action of these two poisons on growth. Some time ago I 

 noticed that iodoacetate, although a growth inhibitor, increases the 

 auxin curvatures in the pea test, although it inhibits straight growth of 

 the same pea seedling internodes at the same cencentration. The curva- 

 ture may be increased by 50% while growth is decreased by some 50%. 

 This characteristic is shown only very slightly, if at all, by fluoride. The 

 highest nontoxic concentrations of both give opposite effects. Also, W. 

 Bonner and I have found that the inhibition by fluoride is not released 

 by the organic acids, though it can be released by certain hexose phos- 

 phates. Our tentative conclusion is that iodoacetate and phosphate 

 inhibit growth in different ways or through different systems. The one 



