DUAL DAY LENGTH REQUIREMENTS 

 FOR FLORAL INITIATION 



ROY M. SACHS 

 Department of Floriculture, University of California, Los Angeles 



In the years 1930 to 1935 there was considerable interest in the inter- 

 action between vernalization and photoperiodism in the winter varieties 

 of wheat, rye, and barley; in four separate investigations attention was 

 called to the fact that some period of short-day (SD) preceding long- 

 day (LD) induction had a stimulatory effect upon reproductive de- 

 velopment (McKinney and Sando, 1930; Maximov, 1930; Forster 

 et ai, 1932; Purvis, 1934; McKinney and Sando, 1935). Prior to 

 this, winter cereals were considered to be simple long-day plants 

 (LDP) with a preceding low-temperature requirement. As a result of 

 their studies McKinney and Sando (1935) obtained sufficient evidence 

 for the Harvest Queen variety of winter wheat to suggest that it be 

 called a short-long day plant (SLDP). Their suggestion was based on 

 the fact that, following emergence of the vernalized or unvernalized 

 seedling, a 6-week period of SD induction decreased the number of 

 days to heading by as much as 33%. This was the first description of 

 a dual day length requirement for floral initiation. Little attention has 

 been paid to McKinney and Sando's paper even though their data 

 present serious difficulties for recent theories on the photoperiodic con- 

 trol of floral initiation (Borthwick et al., 1948a; Bakhuyzen, 1951; 

 Liverman and Bonner, 1953). Gott et al. (1955), continuing some 

 earlier work of Purvis (1934), have shown recently that floral initia- 

 tion in unvernalized winter rye is promoted by a sequence of SD fol- 

 lowed by LD induction, and Wellensiek (1953) has presented evi- 

 dence indicating that Campanula medium should also be placed in the 

 SLDP category. Winter wheat and rye are winter annuals and C. 

 medium is a biennial, so in all three cases floral initiation is promoted 

 by a cold treatment at some stage in the development of the plant. 

 Since SD induction in these plants can replace, partially or wholly, the 



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