EFFECTS OF GIBBERELLIN ON THE 



PHOTOPERIODIC RESPONSES OF 



SOME HIGHER PLANTS ' 



S. H. WITTWER and M. J. BUKOVAC 

 Department of Horticulture, Michigan State University, East Lansing 



One of the unique effects of gibberellin on higher plants is its alteration 

 of photoperiodic responses. That stem elongation and flowering may be 

 accelerated in several facultative long- or long-short-day plants, and 

 induced in some having an obligatory long-day requirement has been 

 reported (Bukovac and Wittwer, 1958; Biinsow and Harder, 

 1956a,b, 1957; Lang, 1956b, 1957; Langridge, 1957; Lona, 1956a,b; 

 Wittwer and Bukovac, 1957a,b). Furthermore, flowering without a 

 cold treatment has been induced with gibberellin in some cold-requir- 

 ing biennials if they were exposed simultaneously, or subsequently, to 

 long photoperiods (Carr et al, 1957; Lang, 1956a,b, 1957); and in 

 others irrespective of day length (Bukovac and Wittwer, 1957). By 

 contrast, flowering has not been accelerated or induced with gibberellin 

 in certain classical short-day plants grown under long days (Lang, 

 1956b, 1957), and gibberelhn has retarded flower formation when 

 some short-day plants were grown under inductive photoperiods (Har- 

 der and Bunsow, 1956, 1957; Lona, 1956b). 



EXPERIMENTAL 



The influence of foliar sprays of gibbereflin on the time of flowering 

 of photoperiodically sensitive plants was observed by growing them 

 under both short (9 hr) and long (18 hr) day lengths. Photoperiods 

 were maintained on a 24-hr cycle, and extended beyond the normal 

 day length with incandescent lamps. Prior to treatment with gibberel- 

 lin, long-day plants were held under a short photoperiod, and short- 

 day plants under a long photoperiod, until they were of an age and 



1 Journal article No. 2165 of the Michigan Agricultural Experiment Station. 



373 



