40 Fundamentals of Auxin Action 



been recognized for many years (Fiedler, 1936). Most roots respond to 

 applied auxins by inhibitions of linear growth only; however, the use 

 of pea roots under the conditions specified for this test yields good 

 quantitative promotions of giowth with very low auxin concentrations 

 (Leopold and Guernsey, 1953). 



It should be pointed out that the promotion of growth by auxin 

 does not necessarily involve the same growth characteristics as does 

 the inhibition of growth. Therefore, measurements of auxin activity 

 using root inhibition tests such as those described in the next sec- 

 tion are all subject to some criticism in that they do not necessarily 

 measure the same characteristic of auxins as do tests involving growth 

 promotion. 



A small amount of calcium is essential for normal root growth 

 (Mevius, 1927), and this ion plus sugar and a buffer are included in 

 the test solution. The capacity of roots to respond positively to auxin 

 disappears 12 hours after cutting and consequently it is important 

 to begin the test promptly after cutting. 



PROCEDURE FOR THE PEA ROOT TEST 



Planting 



Place seeds of Alaska pea or other pea strain between wet towels in the dark- 

 room (25° C). 



Cutting 



Three to 4 days after planting cut uniform lengths of root tip (5 to 10 mm. 

 long). Roots should be carefully selected for straight, thin, uniform appearance. 

 Place directly into solutions. Add 10 to 20 sections per dish. 



Solutions 



The use of 10 ml. of solution per Petri dish keeps all sections at the surface. 

 All solutions contain 0.0025 M CaSOi and 1% sucrose, and, if desired, Mcllvaine's 

 buffer diluted 1:10 at pH 5.0. 



Measure 



Measure after 24 hours. 



As in the previous tests, results are expressed as the mean per 

 treatment plus or minus the Standard Error. A Standard Error of less 

 than 3 per cent of maximal growth should be obtained where 10 sec- 

 tions per dish are used. This is a considerably smaller Standard Error 

 than is usually found in the other straight-growth tests. 



The growth obtained is approximately proportional to the log- 

 arithm of the concentration of auxin. The minimum concentration 

 of indoleacetic acid detectable is 10^" M or about 0.000,001 mg./l. 

 This is a sensitivity 10,000 times greater than the other straight-growth 



