76 



PHOTOCONTROL OF GROWTH 



more than 48 hr from sowing. Also with short periods of irradiation 

 Evenari and Stein (1957) confirmed that the promotive effect of blue 

 light increases progressively during the first 16 hr of imbibition. With 

 one type of filter they obtained inhibition during the first hours of 

 imbibition, and this was followed by a promotive effect with longer 

 periods of imbibition. 



We have carried out experiments to determine how the responses of 

 the seeds to blue light vary both with the imbibition period and with 

 the duration of exposure. It was found that although a short period of 

 irradiation with blue inhibits during the first 2-3 hr of imbibition, a 

 longer exposure of 1-2 hr during this period is markedly stimulatory. 

 As the period of imbibition increases, short periods of irradiation be- 

 come promotive, whereas longer periods (1-2 hr) of irradiation be- 

 come less promotive and ultimately become slightly inhibitory, after 

 about 10 hr of imbibition (Fig. 1 ). Short periods of irradiation do not 

 become inhibitory, at least up to 20 hr of imbibition. 



IMBIBITION PERIOd(hRS.) 



> ' 



Fig. 1. Effect of irradiation with blue for various periods at different 

 times during the period of imbibition. Blue fluorescent source. Intensity 

 100 ^w/cm-. The germination percentages represent differences from 

 "dark" control (taken as 0% ). Exposure periods (minutes) : A, 10; B. 30; 

 C, 60; D, 120. 



Having determined the time at which the inhibitory effect becomes 

 predominant, we investigated the interaction between the promotive 

 effects of red light and the inhibitory effects of blue. Borthwick et al. 

 (1954) were able to obtain only slight reversal of the promotive effect 



