104 • Chemical Control of Flowering 



Most of the other situations in which gibberellin substitutes 

 for long days involve stem elongation. It causes flowering in the 

 LSDP Bryophyllum crenatum grown under short days, thus satis- 

 fying the long-day requirement; this again is a matter of bringing 

 about bolting (Biinsow et al., 1958). Another example is its action 

 on strawberry plants, in which it causes runner initiation, petiole 

 elongation, and flowering inhibition. These effects are all similar 

 to those of long days, and the postulated flower-inhibiting, growth- 

 promoting substance produced on long days may be related to gib- 

 berellin. (Thompson and Guttridge, 1959; see also Chapter Five 

 in this volume.) 



The action of gibberellin on stem development may well be 

 primary, with the promotion of flowering in rosette plants— both 

 LDP and biennials— an indirect result. Lang (1957), for example, 

 noted that although flower initiation in the rosette plants studied 

 occurred with the start of bolting under normal conditions— long 

 days, or vernalization followed by long days— bolting in gibberellin- 

 treated plants generally preceded flower initiation. In some rosette 

 plants, gibberellin causes bolting only, without flowering (Lona, 

 1956; see Wittwer and Bukovac, 1958). In many rosette plants, 

 normal flowering occurs only if the environmental requirements 

 are partially satisfied (see Brian, 1959; Chouard, 1960). Anatomical 

 investigations by Sachs, Lang, and collaborators (Sachs et al., 1959, 

 1960) show that the early effect of gibberellin treatment on several 

 rosette plants is the activation of the "subapical meristem," some- 

 what below the growing apex. The increased cell divisions in this 

 area are largely transverse; this, plus the subsequent cell elongation, 

 results in rapid stem growth. Gibberellin can also completely 

 reverse the effects of the complex growth-regulating compound 

 Amo-1618, which causes a dwarfed or rosette habit in normally 

 caulescent plants such as Chrysanthemum by inhibiting the 

 activity of trie subapical meristem. While such work bears no direct 

 relationship to flowering, it strengthens the view that gibberellin 

 may indirectly remove some inhibition on flowering through its 

 direct effect on stem growth. 



Gibberellin may either promote or inhibit later flower develop- 

 ment in SDP, but is entirely unable to bring about initiation under 

 noninductive conditions. In addition to the work already men- 

 tioned, a striking example of its ineffectiveness occurs with the 



