Vernalization and Photoperiodism — 48 — A Symposium 



To shorten the photoperiod, plants are commonly covered with frames 

 of black paper or curtains of black cloth during part of the day, while it 

 may be conveniently lengthened by means of electric illumination of com- 

 paratively low intensity. These procedures are quite similar to those used 

 in experimental work. 



Tests have shown that plants respond the same whether artificial light 

 is given at the beginning or end of the dark period (Post, 1942). This is 

 in agreement with the early observations by Garner and Allard (1920). 

 A minimum light intensity of 3-5 f .c. seems to be sufficient for most plants, 

 though more uniform results apparently are secured with 10 f.c. or higher 

 illumination. Recent studies have shown that intermittent supplementary 

 flash lighting for J^-1 minute, followed by an equal or a somewhat longer 

 dark period, depending on the kind of plant treated, is equally as satisfac- 

 tory as continuous illumination and, of course, more economical (Hume, 

 1940; Snyder, 1940). Quite identical results can be obtained also, so it is 

 said, by interrupting near its middle the dark period with usual lighting for 

 a few minutes only (Parker and Borthwick, 1942). 



A desired reaction of plants, whether in delaying or hastening bloom- 

 ing, can be best secured from a reduction or an increase in the length of 

 day when there is previous information on the normal seasonal develop- 

 ment and of the reaction of particular plants to specific photoperiods and 

 its relation to temperature and other factors. Moreover, the condition of 

 the plants at the start, and the length of the treatment, determine to a large 

 extent the resultant growth and development. 



An interesting application of photoperiodism, conjointly with tempera- 

 ture, on the induction of flowering in embryo-cultured peach seedlings has 

 been reported by Lammerts (1943). If derived from parental varieties 

 with a long chilling requirement, such seedlings form rosettes and go dor- 

 mant early in the fall. This may be overcome by exposing them to long 

 photoperiods or to continuous light at a relatively high temperature ( Mini- 

 mum 70-75° F.), which lengthens the growth period. With the addition of 

 a brief chilling treatment, this hastens development, resulting in flower pro- 

 duction two years after pollination, thus helping to speed up breeding work. 



Relation to Temperature: — The modifying effects of temperature on 

 photoperiodism have been known ever since the observations by Garner 

 and Allard, who state (1923, p. 912) that "temperature undoubtedly is the 

 most important environmental factor in relation to the action of the light 

 period on plant growth." When certain varieties of soybeans were grown 

 in greenhouses during winter months (short days) with mean temperatures 

 of 55° F and 72° F, respectively, flowering was markedly hastened by the 

 higher temperature. Subsequent studies with several varieties of soybeans, 

 conducted both outdoors and in the greenhouse, led them to conclude that 

 "under field conditions in Washington, D. C. variations from year to year 

 in date of flowering of both early and late varieties of soybeans, when 

 planted on any particular date, are due chiefly to differences in temperature, 

 while length of day is the primary external factor responsible for the fact 

 that one variety is always relatively early and another late in attaining the 

 reproductive stage" (Garner and Allard, 1930). 



