100 • Chemical Control of Flowering 



THE GIBBERELLINS 



The single most striking property of the gibberellins, besides 

 the effects on flowering to be discussed, is their ability to cause 

 greatly accelerated growth in intact plants. This is evident mainly 

 in the stem, but occurs also in other parts and is especially obvious 

 in certain "dwarf" varieties. No other group of compounds, includ- 

 ing the auxins, is known to have such effects on a wide variety of 

 intact plants. Gibberellins also act on many of the same phenomena 

 affected by red and far-red light. Such action is not consistently 

 in one direction— in some cases, such as seed germination, gibberel- 

 lins appear to mimic the effect of red, but in others (for example, 

 stem elongation) they act in the same direction as far-red. It has 

 thus been suggested that gibberellins may be involved in the action 

 of the red, far-red system, but none of the specific hypotheses pro- 

 posed is as yet sufficiently grounded to be considered here. 



Several gibberellins have been isolated from higher plants, but 

 the group was originally discovered as products of a fungus 

 (Gibberella fujikuroi) causing a rice disease characterized by 

 excessive stem elongation. They are complex compounds that can 

 be regarded as derivatives of the hydrocarbon fluorene with lactone, 

 hydroxyl, and other substituents. The detailed structures of some 

 of them, notably gibberellin A 3 (gibberellic acid), are fairly well 

 established. Much of the work to be discussed has been done with 

 gibberellic acid, but other gibberellins have been studied as well, 

 and the general term "gibberellin" will often be used. Research on 

 the gibberellins has been pursued for several decades in Japan, 

 but became known outside that country only relatively recently. 

 The first generally available review, by Stowe and Yamaki in 1957, 

 has since been followed by others, and all should be consulted for 

 a thorough knowledge of this rapidly developing topic (Brian, 

 1959; Phinney and West, 1960; Stowe and Yamaki, 1960; Wittwer 

 and Bukovac, 1958). For an excellent discussion of gibberellin and 

 flowering, see Lang and Reinhard (1961). 



The first thorough publication on gibberellin and flowering 

 was that of Lang (1957), showing that a few drops of a dilute solu- 

 tion (chiefly gibberellic acid) given repeatedly to the growing point 

 or leaves brought about flowering of unvernalized biennial 



