186 Fundamentals of Auxin Action 



gested, it would be reasonable to expect that they would be toxic. The 

 toxic effects of maleic hydrazide and N-phthalamic acid derivatives 

 may be due to their quinonoid structures also. This theory has been 

 criticized on the basis that the benzoic acid auxins never have open 

 positions para to one another (Thimann, 1952), and they are effective 

 herbicides. 



Van Overbeek el al (1951) have proposed that growth regulator 

 toxicity may be a result of an alteration of metabolism such that un- 

 saturated lactones are accumulated in the plant tissues. Such lactones 

 as coumarin, scopoletin, and umbelliferone are toxic to plants when 

 applied in high enough quantities, and it has been observed that such 

 compounds may accumulate in plant tissues following application of 

 herbicidal concentrations of 2,4-D (Fults and Johnson, 1950). Such 

 materials have been found to be more toxic to some dicotyledonous 

 plants than to monocotyledonous ones (Hamner and Sell, 1950). This 

 suggests the possibility that stimulation of such toxic metabolism may 

 account at least in part for herbicidal selectivity of auxins. 



In conclusion it may be said that the mechanism of auxin action 

 remains unsolved, but a variety of promising lines of evidence seems 

 to be emerging. Auxin has been found actually to react with some pro- 

 teinaceous material in a manner which is suggestive of a constructive 

 step in growth. Evidence has appeared that auxin may form high- 

 energy ester bonds with coenzyme A, which is suggestive of an energy- 

 controlling action by the growth hormone. A picture of how auxin 

 may react with its receptor compound has come from kinetic studies, 

 as well as a picture of the means by which other compounds may alter 

 auxin action. The means by which auxin may finally bring about cell 

 enlargement is being hotly debated between those who feel that the 

 action is principally based upon the cytoplasmic uptake of water and 

 those who feel that it is principally based upon the growth of the 

 cell wall. 



The clarification of this important problem will not only be 

 extremely interesting, but no doubt will greatly accelerate the advance 

 of practical and theoretical knowledge of auxins and growth. 



