76 Fundamentals of Auxin Action 



zinc deficiency may limit tryptophan formation in turn by limiting 

 serine formation. Serine is considered to be condensed with indole to 

 form tryptophan, and either tryptophan or serine were found to re- 

 lieve the symptoms of zinc deficiency, while indole did not. 



Several developmental steps are believed to be associated with the 

 production of large amounts of auxin. Synapsis, the stage at which 

 reduction division occurs in the floral parts, is coincident with a surge 

 of growth in grown plants and an apparent increase in auxin content 

 (Wittwer, 1943). Pollination, too, is associated with a large production 

 of auxin (cf. figure 91, p. 216). It is recognized that the pollen brings 

 a certain amount of auxin to the ovary, but Muir (1951) has demon- 

 strated that shortly after pollination thirty times as much auxin is 

 produced in the tobacco ovary as could be accounted for from the 

 pollen itself. He suggests that pollination stimulates the release of an 

 auxin-forming enzyme system in the ovary. 



While auxin is produced in the ovary shortly after pollinati(jn, 

 this source dwindles after the commencement of embryo growth in the 

 apple (Luckwill, 1948). The embryo itself becomes the primary source 

 of auxin in the developing fruit. An elegant demonstration of the em- 

 bryo as an auxin-producing center has been carried out with the 

 strawberry (Nitsch, 1950). It was demonstrated that the removal of 

 the achenes containing developing embryos prevented normal fruit 

 enlargement, and the role of the achene could be replaced by auxin 

 pastes (see figure 50, p. 110). Fruit shape was controlled by removal 

 of the achenes from some sections of the fruit, resulting in decreased 

 growth in the parts so deprived of the auxin supply. 



Biochemistry of Formation 



From the early observations of Skoog and Thimann (1940) it is 

 evident that the production of auxin during long term extraction 

 procedures is primarily an enzymatic process. The addition of certain 

 enzymes to various plant materials has greatly stimulated auxin pro- 

 duction. Again it will be recalled that the effect of temperature dur- 

 ing extraction on the yields of auxin clearly indicate an enzymatic 

 auxin production (cf. figure 4, p. 17). It has been observed that the 

 appearance of large amounts of auxin associated with pollination of 

 tobacco ovaries is due to an enzymatic production (Muir, 1951). 



An enzyme actually capable of producing auxin has been sepa- 

 rated from spinach leaves (Wildman et al, 1947). This enzyme prepara- 

 tion is capable of converting tryptophan into indoleacetic acid. More 

 detailed studies (Wildman and Bonner, 1948) have shown that such 

 an enzyme is distributed through the Avena coleoptile in a manner 



