14 PLANT GROWTH SUBSTANCES 



Tang and Bonner (55) showed that the optimum range of activity fell 

 between pH 6.2 and 6.7. It is reported that the enzyme does not attack 

 indoleacetamide, indolebutyric acid, indolepropionic acid, indolecar- 

 boxylic acid, or tryptophan, and therefore seems to possess a considerable 

 degree of substrate specificity. 



It would be desirable to have chemical methods to determine quanti- 

 tatively the different auxins. Up till now such methods have been de- 

 veloped only for indoleacetic acid. Salkowski (51), analyzing the products 

 of putrefaction of proteins, found that one of the products formed gave 

 a red color with nitrite or ferric chloride and a mineral acid. Both 

 reactions have been carefully studied (39) and the optimum conditions 

 for the reactions determined. Methods were developed whereby indole- 

 acetic acid can be determined over a range of 5 to 200 gammas per 

 milliliter. Especially recommended is the nitrite method, where the 

 coefficient of variability is less than three per cent. The ferric chloride 

 reaction has often been used for the identification of indoleacetic acid 

 in plant material, as, for example, in the media of fungi. Wildman and 

 Bonner (70) found that a considerable portion of the Avefia tip growth 

 activity could be explained by the presence of indoleacetic acid. 



After discussing the question as to which auxins occur in plant tissues, 

 another problem connected with their production, storage, transport, 

 and action presents itself. It had been observed that a part of the auxins 

 is readily extracted from the tissue, but that considerable additional 

 amounts of auxins could be obtained by continued ether extraction 

 or by treatment with hydrolytic agents. Thimann and Skoog (61) have 

 shown that many ether extractions spaced over several months are 

 required to extract all the ether-soluble auxins present in Lemna. They 

 concluded from their experiments that the continued production of 

 auxin in ether was due to an enzymatic liberation from an inactive form, 

 probably a protein. These observations have since been confirmed by a 

 number of investigators. Determination of free auxins prevents this 

 enzymatic production by boiling previously frozen and ground material, 

 or by carrying out the ether extraction at zero degrees. 



Wildman and Bonner (69) obtained from spinach leaves a fraction 

 which is homogeneous in ultracentrifugation and electrophoresis experi- 

 ments and has phosphatase action. This fraction gave, on treatment with 

 alkali or proteolytic enzyme, a growth hormone which is probably 

 identical with indoleacetic acid. The bound auxin seems a part of the 



