486 



RADIATION BIOLOGY 



ACTION SPECTRA 



EQUIVALENCE OF PHENOMENA 



The incident energy or quanta in different wave-length regions required 

 to reduce the effectiveness of a dark period otherwise adequate for a given 

 physiological expression to some given level can be measured. These 

 measurements in the case of photoperiodism have not been expressed in 

 terms of energy absorbed by the pigment in the effective reaction. They 

 are of value, nevertheless, as a first guide to the equivalence of the initial 



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5400 5800 6200 6600 7000 

 WAVE LENGTH. A 



Fig. 10-1. Response curve for suppression of floral initiation in Biloxi soybean in the 

 red region of the spectrum. Resulting energy at each wave-length station is plotted, 

 and superimposed on energy lines are total numbers of flower primordia formed by 

 lots of four plants which received a specific wave-length band at different energy levels. 

 The curves were drawn freehand through energy lines, crossing between points at 

 which sharp breaks occurred in the number of flower primordia produced. The solid 

 curve was taken as the one representing the response, and the broken-line curve illus- 

 trates gradation effect with decreasing energy. 



reaction in the several responses of plants. Quantitative information is 

 restricted to plants, and even qualitative results are very limited for 

 animals. 



Such direct results, however, require correction for absorption by 

 inactive pigments and scattering by tissue. This action spectrum gives 

 the reciprocal of the relative absorption spectrum of the effective pig- 

 ment, provided that (1) the total absorption is small, (2) absorbed quanta 

 are equally effective independent of wave length, and (3) the light path is 

 nearly the same at all wave lengths. The first and third requirements 

 have been met in some cases for plants and could be met with some small 

 animals such as mosquito larvae and aphids. 



In plants the action spectrum can be found by irradiation of the leaf, 

 followed by measurement of responses (Parker et al., 1946). It is very 

 convenient in this work to make use of dark-period interruption on a 



