Vernalization and Photoperiodism • 61 



starvation for carbohydrates since defoliation of the plants does 

 not have the same effect, nor does sucrose feeding during treatment 

 reduce devernalization (Schwabe, 1955b, 1957). Whether the de- 

 vernalization that occurs in the natural yearly cycle is actually due 

 to high temperatures and low light intensities (at the underground 

 growing points) is still uncertain. 



RELATIONS BETWEEN VERNALIZATION AND 



PHOTOPERIODISM 



Many of the plants studied, and also work with the gibberellins 

 (Chapter Six), may be used to support the idea of a close relation- 

 ship between vernalization and long-day requirements, but the 

 situation is probably more complex than this, varying greatly from 

 plant to plant. 



Petkus winter rye and biennial Hyoscyamus niger are "typical" 

 vernalizable plants in which the cold treatment brings about 

 quantitative or qualitative LDP responses. In other plants, vernal- 

 ization can even substitute partially or completely for a long-day 

 requirement. Vernalization of spinach seeds, for example, reduces 

 the critical daylength for flowering from 14 to about 8 hours 

 (Vlitos and Meudt, 1955), whereas cold treatments given to seed- 

 lings of certain strains of subterranean clover, Trifolium subter- 

 raneum, can completely remove any marked dependence on day- 

 length (Evans, 1959). 



Floral induction and development in several grasses depend 

 upon both photoperiod and vernalization. Plants of orchard grass, 

 Dactylis glomerata, studied by Gardner and Loomis (1953) require 

 low temperatures and short days (less than 13 hours light) for 

 floral induction, followed by higher temperatures and long days 

 for optimum flower development. The short-day and vernalization 

 requirements for induction can be satisfied separately but only in 

 that order, not in the reverse. In a sense, then, Dactylis glomerata 

 is one of the short-long-day plants (SLDP) mentioned in Chapter 

 Two, except that a period of low temperature must occur between 

 the two photoperiodic treatments or together with the first. 



In some plants, short-day treatments can substitute partially or 

 completely for vernalization, making them SLDP. Petkus winter 

 rye itself shows a response of this kind, although the situation is 



