Biological Evaluation of Gibherellins 517 



/xl. of 3 X 10 '"^ ^l) oi' all gibherellins resulted in comparable activities 

 with epicotyls significantly longer than nontreated controls. The first 

 measurable response on bean epicotyl elongation was observed with 

 ten /xl. of gibberellin A3 at 3 X ^^'^' M, for Aj and A4 at 3 X 10-"' M, 

 and 3 X 10"* M for Ao. Gibherellins A^ and A3, however, applied to 

 the leaf blade induced significantly greater growth than Ag or A.,. If the 

 length of the epicotyl following application of each of the gibherellins 

 to the terminal bud was divided by the length of the epicotyl follow- 

 ing application to the leaf blade, the ratios for Aj and A3 approached 

 unity (Table 2). Gibherellins Ao and A4, in contrast, were much more 

 active when applied to the terminal bud than to the leaf blade, and 

 growth from the former was 2 to 3 times greater. These data suggest 

 possible limitations in the absorption and transport of A2 and A4 from 

 the primary leaves, and that the rate of penetration and transport of 

 exogenously applied gibherellins from the treatment site to the site 

 of action may be an important consideration in assessing activity by 

 a selected assay. 



The strikingly greater effect of gibberellin A4, in contrast to A^, 

 Ao, or A3, on stem elongation of the cucumber is an intriguing de- 

 parture from the usual response pattern. Such specificity among the 

 gibherellins for any vegetative elongation response has not heretofore 

 been recorded, and has not been duplicated in other bioassays em- 

 ployed. The response of the cucumber to gibberellin A4 may be uti- 

 lized as a bioassay for differentiation between naturally occurring 

 A4 and Ai, Ao, and A3 in higher plants; and further serves as a guide 

 for the study of the controlling mechanisms in flower sex expression 

 of cucurbits (19). 



Quantitative as well as qualitative differences were recorded among 

 the four gibherellins. Ao did not stimulate the growth of tomato 

 ovaries at 3 X 10"^ M, but was highly effective at 3 X 10"^ ^^- In con- 

 trast, relatively large quantities (400 ^g. per plant) of Ao and A4 failed 

 to promote seedstalk elongation in 'Great Lakes' head lettuce, whereas 

 20 ixg. of either A^ or A3 per plant induced bolting without heading. 

 The presence of gibberellin A^ in some higher plants (11, 12) may 

 alter responses to exogenous applications of the fungal extracted gib- 

 herellins. Marked specificity among the latter has already been noted 

 for vegetative extension, the most commonly observed gibberellin ef- 

 fect. As more species and responses are examined these relationships 

 will undoubtedly be multiplied. 



Esterification of the carboxyl group of gibberellin A3 resulted in 

 a complete loss of biological activity when elongation of bean epi- 

 cotyls or growth of tomato ovaries constituted the assay. For promo- 

 tion of germination of lettuce seed in the dark, however, the ethyl, 

 77-butyl, and ??-amyl esters equalled the activity of gibberellin A3, while 



