758 



PLANT GROWTH 



10 



TABLE 2 



SUMMARY OF METHODS FOR DETERMINATION OF AUXIN IN EXTRACTS 



1 Upper limit about 10 times this level; lower limit about one quarter for nos. i and 7, 



about one tenth for nos. 2-6. 



- Would not in general be used with aqueous solutions. 



^ Mixed with an equal volume of 3% agar to make enough blocks for 12 plants. 



a clear inhibition (about 20%) at lo^^M indoleacetic acid or about 0.2 \ig per 

 liter of solution. However, because the roots require several ml of solution, the 

 absolute amount of auxin detected, about i-io"^ [xg, is not much lower than with 

 the Avena agar test, which detects about 2-io"^ [xg, or the straight growth of ^y^na 

 mesocotyl sections detecting about 5-10 -^ [ig (see section Vllb, p. 801 for detailed 

 discussion). Because biological extracts often contain non-specific inhibitors in 

 addition to auxins, root inhibition is not satisfactory for bioassay of unknown 

 extracts. 



The sensitivities of several bioassays are compared with that of a chemical color 

 reaction in Table 2. 



Most of these tests have been used also for other growth factors and for growth 

 inhibitors, including chemical enzyme poisons and naturally occurring inhibiting 

 substances extracted from plants. The procedure in general is to add optimum 

 concentrations of auxin and/or nutrients and study the inhibition of growth as 

 function of concentration of inhibitor. Similar methods have been used to study 

 the influence of external conditions such as temperature, light, salt solutions, etc. 



(c) Assays for other growth factors 



For kinins difTerent methods are required, since kinetin and related kinins have 

 little or no action on cell elongation as such. Though standardized methods have 

 not yet been published, the most specific test would seem to rest on the ability of 

 kinins to bring about the development of buds in undifferentiated tissue. Isolated 



