494 RADIATION BIOLOGY 



for control of flowering can be noted for irradiation times greater than 

 2 hr in contrast to those less than 1 hr. The rates of the biological 

 changes and the rate of the dark reaction appear to be of the same order 

 of magnitude, so that the resultant deviations from reciprocity are a 

 function of the two. 



Energy-response curves are determined by a combination of (1) the 

 functional dependence of pigment change on irradiation; (2) the dis- 

 tribution of "sensitivities" in the irradiated population to physiological 

 response at a given pigment level j and (3) the range in amounts of the 

 pigment required for a detectable response (a threshold concentration), 

 on the one hand, and for saturation of response, on the other. 



The probit (Finney, 1947) of the proportion of seed germinating varies 

 linearly with the logarithm of the irradiance. This type of response is 

 often encountered in biological assay but has not been explained. In 

 fact, Eddy (1953) writes: "It seems rather that it must be treated simply 

 as an approximate empirical relationship, expressing the fact that the 

 range of variation of survival times is large in comparison with that often 

 encountered in measurement of simpler properties." The precision of 

 test is low, owing to relatively small populations, when the response is 

 small or approaches saturation. 



The number of pigment molecules in each cell or each object, such as a 

 seed, is surely large (many powers of 10). A photoreaction progressively 

 shifts this number, and at any one irradiance all seed are alike in the 

 proportion of the pigment changed if the change is unimolecular wdth 

 regard to energy and if the pigment is not variably screened by scattering 

 or absorption. The objects might vary in their total pigment of both 

 forms and in their response to a given concentration of effective pigment. 

 All these variables appear as "sensitivities," and for seed germination 

 "sensitivities" vary from germination response to very small irradiances 

 to germination response requiring very large irradiances. Of greatest 

 importance is the fact that an irradiance much greater than that required 

 for essentially complete germination of seed does not markedly change 

 the inhibitory irradiance required for 50 per cent response. In other 

 words, saturation for germination approximately corresponds to complete 

 conversion of the pigment from one form to the other. Some response, 

 moreover, is noted for very little change. 



In a unimolecular reaction, change of 90 per cent of the reactant takes 

 about twenty times the energy required for a 10 per cent conversion. 

 This is also approximately the case for seed-germination response upon 

 irradiation. If, then, the 10 and 90 per cent conversions are used to 

 establish a scale factor, the seed germination for intermediate irradiances 

 follows an approximately unimolecular change. 



The relative biological response, such as internode elongation, seed 

 germination, or flowering, is limited by the photoisomerization of the 



