WAVE LENGTH DEPENDENCE 



AND 



THE NATURE OF PHOTOPERIODISM 



by 

 H. A. Borthwick/ M. W. Parker,^ and S. B. Hendricks' 



United States Department of Agriculture 



Knowledge has been obtained by physical methods about photoreactions 

 that regulate the concentrations of compounds in photoperiodically sensi- 

 tive plants of both the long-day and short-day types. The action spectrum 

 of a reaction that under certain conditions prevents initiation of flowers 

 by short-day plants has recently been determined with a spectograph spe- 

 cially designed in this laboratory to irradiate entire leaflets at high intensity 

 and with great spectral purity (17, 18). This is the only work of this type 

 from which quantitative results have been obtained. This reaction which 

 opposes floral initiation apparently proceeds in short-day plants any time 

 they receive radiation of suitable quality and quantity, but it actually pre- 

 vents floral initiation only when irradiation reduces the dark periods below 

 a certain minimal length and when the total energy applied is adequate. 

 For soybean, Soja max, var. Biloxi, and cocklebur the minimal length of 

 dark period is between eight and ten hours. In these experiments radia- 

 tion of known ranges of wave length was applied for short durations near 

 the middle of 14-hour dark periods for soybean and 12-hour dark periods 

 for cocklebur. This procedure divided each of these long dark periods, 

 which were conducive to flowering into two shorter ones neither of which, 

 under the experimental conditions, exceeded the eight to ten hours that 

 were necessary if floral initiation was to occur. By application of various 

 levels of energy to different lots at each region of the spectrum the minimum 

 energy that would prevent floral initiation for each wave band was deter- 

 mined. Action spectrum curves, figure 1, were constructed from data (18). 



Two regions of maximum effectiveness for the prevention of floral 

 initiation in these plants were found in the visible spectrum, a narrow one 

 in the violet near 4000 A. and a rather broad one extending from about 

 5600 A. to 7200 A. in the orange-red region. Minimum effectiveness was 

 found for radiation of about 4800 A. The long wave length limit of effective- 

 ness was very abrupt at about 7200A. and infra red was completely ineffec- 

 tive. Radiation at 6400 A. was about 60 times more effective than that 

 at 4800 A. for soybean and about 200 times more effective for cocklebur. 

 This difference in ratios for the two plants was mainly the result of lower 

 effectiveness of blue radiation for cocklebur than for soybean. 



Experiments of the same type are in progress with long-day plants. 



, _ ^ S.e"'°''_ Botanist, 2 Physiologist, Division of Fruit and Vegetable Crops and Diseases, and 

 3 Principal Chemist, Division of Soils, Fertilizers and Irrigation, Bureau of Plant Industry Soils 

 and Agricultural Engineering, Agricultural Research Administration, U. S. Department of Agri- 

 culture. 



