Biological Evaluation of Gibberellins 



511 



between Ao and A^, gibberellin A3 was slightly more active than A^. 

 All plants of both varieties treated with any of the gibberellins flow- 

 ered in fewer days (Table 2) than the controls. 



'Great Lakes' lettuce plants treated with A^ or A3 did not head and 

 seedstalks commenced to elongate within ten days (Figure 5). Plants 

 treated with Ao and A4 headed, as did the controls, and the seed- 

 stalks, as with controls, later emerged through the heads. At the time 

 of first visible flower primordia, seedstalk heights for plants treated 

 with Ai, Ao, A3, and A4 were 41, 28, 84, and 29 cm., respectively, and 

 30 cm. for controls. 



The effects of different levels (two applications of 1, 10, 50, 100, 

 and 200 /xg. per plant) of each of the gibberellins on flowering of let- 

 tuce were next determined. Increasingly greater amounts of A^ or A3 

 resulted in progressively taller seedstalks, and hastened flowering in 



Fig. 5. Response of 'Great Lakes' head lettuce to treatment with gibberellins. 

 Left to right: control, A^, A,, A3, and A,. Note the absence of heading in Ai and 

 A3 treated plants. (Three applications of 10 fi\. of 3 X 10"' M solutions at 2-week 

 intervals.) 



