PHOTOPERIODIC EFFECTS IN WOODY PLANTS; 

 EVIDENCE FOR THE INTERPLAY OF 

 GROWTH-REGULATING SUBSTANCES ' 



J. p. NITSCH and COLETTE NITSCH 



Laboratoire du Phytotron, Gif-sur-Yvette (Seine-et-Oise), France 



Although the effect on vegetative growth of the relative duration of day 

 and night was recognized early in the history of photoperiodism 

 (Garner and Allard, 1923), the majority of plant physiologists have 

 neglected this aspect somewhat and until recently focused their atten- 

 tion on the flowering phenomenon. Woody plants, that is trees and 

 shrubs which take years to reach the flowering stage, are excellent 

 materials for investigating photoperiodic effects on vegetative growth. 



PHOTOPERIODIC CONTROL OF GROWTH AND DORMANCY 



If, for example, one divides a lot of actively growing dogwoods 

 {Cornus florida L.) into two groups and places one of them under long 

 days of 1 5 hr or more and the other one under short days of 1 2 hr or 

 less, one will observe that vegetative growth will continue almost in- 

 definitely under long days, but it will cease rapidly under short days. 

 In the latter case, both the elongation of the stem and the development 

 of new leaves will be arrested; instead of leaves, cataphylls (scales) 

 will be produced, and will enclose the terminal meristems: in short, the 

 plant will become dormant. In the dogwood (Waxman, 1957), in 

 Platanus occidentalis L., or in Rhus typhina L. (Nitsch, 1957b), stem 

 elongation completely stops after 2 weeks of short days. The onset of 

 dormancy is a complex phenomenon. Part of it is a growth process 

 and, therefore, can be investigated with the techniques which have 

 been developed in the field of plant growth. Such an attempt is pre- 



1 Research supported in part by the National Institutes of Health, Bethesda, 

 Maryland (grant No. RG 4840) and in part by the National Science Founda- 

 tion (grant No. G 4046), 



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