518 M. J. Bukovac and S. H. Wittiver 



the methyl, 77-propyl, «-hexyl, and ?i-heptyl esters showed a significant 

 increase above the controls. The pronounced activity of some of the 

 gibberellates in lettuce seed germination may have resulted from their 

 hydrolysis in the germinating medium and/or on the seed surface. 

 In this regard the butyl cellosolve ester of A3 was equally as active as 

 gibberellin A3 in promoting the elongation of bean epicotyls, growth 

 of tomato ovaries, or lettuce seed germination, whereas ??-butyl gib- 

 berellate was inactive except in promoting the germination of lettuce 

 seed. Solubility and penetrating properties may have been favorably 

 altered by the side chain of the butyl cellosolve ester of A3. 



Other derivatives of gibberellins Aj, A3, and A4 Avere biologically 

 active. Compared with the parent compounds, hoAvever, the activity 

 was less and seldom exceeded 60 per cent. Activity disappeared when 

 the lactone group of ring A was removed. Likewise, activity was lost 

 following rupture of the D ring. Intact A and D rings and a reactive 

 carboxyl group appear to be essential for biological activity. The re- 

 port of Brian et al. (2) that aZ/ogibberic acid is not biologically active 

 has been confirmed. The necessity of ring A for activity has also been 

 confirmed in that gibberellenic acid is inactive (8). 



SUMMARY 



Gibberellins Aj, Ao, A3, and A4, the n-alkyl esters of A3, and some 

 derivatives of A^, A3, and A4 were bioassayed utilizing several test 

 systems with intact plants. In promotion of vegetative extension of 

 bean epicotyl and in celery petiole elongation, A3 was most active, 

 followed by Aj, A4, and A2. Reduced epicotyl elongation following leaf 

 blade applications of Ao and A4, as compared with A^ or A3, suggested 

 limitations in the transport of Ag and A4 in the bean plant. A marked 

 deviation from the usual order of growth extension activity among 

 the four gibberellins occurred in stem elongation of the cucumber. 

 Gibberellin A4 was strikingly the most active followed by Aj, A3, and 

 A2. Gibberellins A3 and Ai were more effective than Ao or A4 for ac- 

 celerating flowering of both 'Great Lakes' head and 'Grand Rapids' 

 leaf lettuce. Whereas gibberellin A3 was more active than Aj, there 

 were no differences between Ao and A4. Increasing the dosages of Ao or 

 A4 twenty-fold above Ai or A3 did not compensate for the markedly 

 greater acceleration of flowering resulting from the latter. Gibberel- 

 lins Al, A3, and A4 were more active than A2 in promoting the growth 

 of tomato ovaries at low dosages (3 X 10 "* or 3 X 10'^ ^^)' while 

 growth was comparable with all four gibberellins at 3 X ^^'^ ^^- ^^^ 

 gibberellins stimulated germination of lettuce seed in the dark. Photo- 

 induced dormancy of Weigcla was partially overcome by treatment 

 with gil)berellins A, and A3, whereas Ao and A4 were inactive. 



