PHOTOPERIODIC EFFECTS IN WOODY PLANTS 241 



SUMMARY AND CONCLUSIONS 



In woody plants such as Comus florida, Rhus typhina, long days 

 produce apparently unlimited vegetative growth, whereas short days 

 cause growth to stop completely after about 2 weeks of treatment. The 

 effect of the photoperiodic treatments is mediated through the young 

 leaves, those which have reached one-half to three-fourths of their 

 mature size. The manner in which the photoperiodic regulation of 

 growth is brought about seems to be complex. Experiments in which 

 the removal of leaves produced an increased growth suggested that 

 some inhibitory mechanism is involved in the process. On the other 

 hand, after effects, visible 1 year after the application of photoperiodic 

 treatments and after the destruction of all inhibitory substances 

 through a cold treatment, suggested that some growth-promoting 

 principle (the actual substance or a precursor of it) may be stored in 

 the tissues of long-day-treated plants. The study of growth-regulating 

 substances which can be extracted from stem tips revealed the presence 

 of both growth-promoting and growth-inhibiting substances. Treat- 

 ments inducing dormancy cause the level of auxins to decrease and 

 that of inhibitors to increase. The application of gibberellic acid, which 

 stimulates growth under both long and short days, also caused a rise 

 in the level of endogenous auxin. Thus, both indirect evidence and 

 actual extraction studies concurred in pointing out that the regulation 

 of vegetative growth of woody plants by photoperiodic treatments in- 

 volves changes in the levels of both the growth-promoting and the 

 growth-inhibiting substances. 



REFERENCES 



Barton, L. V. 1956. Growth response of physiological dwarfs of Mains 

 Arnoldiana. Sar. to gibberellic acid. Contribs. Boyce Thompson Inst., 18, 

 311-17. 



Downs, R. J., and H. A. Borthwick. 1956. Effect of photoperiod upon vegeta- 

 tive growth of Weigela fiorida var. variegata. Proc. Am. Soc. Hort. ScL, 68, 

 518-21. 



Garner, W. W., and H. A. Allard. 1923. Further studies in photoperiodism, 

 the response of the plant to relative length of day and night. J. Agr. 

 Research, 23, 871-920. 



