AN ASSAY METHOD FOR GROWTH-PROMOTING 



SUBSTANCES UTILIZING STRAIGHT GROWTH 



OF THE AVENA COLEOPTILE 



By ROBERT L. WEINTRAUB 



Division of Radiation and Organisms, Smithsonian Institntion 



(With One Plate) 



The most delicate available methods for the determination of plant 

 growth-promoting substances involve the direct measurement of the 

 effects of these substances on the growth of suitable plant test objects. 

 The most commonly employed indicator is the decapitated Avcna 

 coleoptile. Two general techniques are available ; one makes use of the 

 elongation (straight growth) resulting from the application of the 

 substance in question symmetrically with respect to the long axis 

 of the coleoptile, the other utilizes the curvature produced by uni- 

 lateral application of the growth-promoting substance. The latter 

 method, which has been more widely used, has been described repeat- 

 edly (see e.g., Boysen-Jensen, 1936; Went and Thimann, 1937; 

 Avery, Burkholder, and Creighton, 1937) and need not be detailed 

 here. As Went and Thimann (1937, p. 51) point out, "The con- 

 venience of curvature methods rests vipon two facts : ( i ) the residual 

 growth, after decapitation, is the same on both sides of the plant and 

 thus is automatically eliminated from the measurement — no controls 

 are necessary ; and ( 2 ) only one measurement need be made ; there is 

 no zero reading." It should be noted, however, that the first condi- 

 tion, namely, the uniformity of the residual growth, is true only dur- 

 ing the first 2 hours following decapitation (cf. fig. 20. Went and 

 Thimann, 1937), and this limits the length of the test period. 



Thus the curvature test measures not the maximum amount of 

 curvature (growth) which can be induced by the applied substance, 

 but rather the mean rate of curvature during a given period. During 

 this period the rate is not constant and may even change in sign (cf. 

 Schneider and Went, 1938). The factors which cause a reduction in 

 the curvature rate and therefore in the amount of curvature at the 

 end of the test period are : ( i ) gravity, which causes a geotropic curva- 

 ture in the opposite direction; (2) the effect of "physiological regen- 

 eration" of the tip. and (3) the lateral transport of the applied growth- 



Smithsonian Miscellaneous Collections, Vol. 97 No. 11 



