LIGHT AND THE PIGMENT 121 



Red light inhibits flowering maximally when it is appHed about 

 8 hr after the beginning of the dark period. Far-red not only fails 

 to reverse this inhibition but is itself inhibitory at this time. As a 

 matter of fact, a sufficient intensity of far-red will inhibit as much as 

 will red. Far-red also inhibits, although not as much, when it is 

 applied at the beginning of the dark period, and in some experiments 

 red actually promotes at this time. (In experiments with cocklebur 

 at Beltsville, far-red promoted at the beginning of the dark period.) 

 The inhibitory effects of far-red at the beginning of the dark period 

 can be reversed by red, as can the promotive effects of red by far-red. 

 After about 16 hr (using abnormally long nights) no light interruption 

 has any eff"ect, although in cocklebur highly significant inhibitions 

 can sometimes be observed following such treatment. 



At present we cannot choose between various alternative explana- 

 tions for these results but obviously plant response to light varies 

 considerably from species to species. 



7. Overlapping of the Action Spectra in Pigweed (19) 



There are a number of experimental results which might be very 

 difficult to interpret if we did not understand that the absorption 

 spectra of R-phytochrome and F-phytochrome overlap considerably 

 (Fig. 7-3). Both absorb some light at all wavelengths from 300 to 

 800 m/x. An excellent and most instructive example is an experiment 

 done at Beltsville using seedlings of Chenopodium rubrum, a pigweed. 

 Six days after seeds are placed on moist filter paper in a petri dish, 

 the seedlings will respond to a long dark period, and a few days later 

 the flowering response can be evaluated using a stage system similar 

 to the one used with cocklebur. In the Beltsville experiment, a 16-hr 

 dark period was interrupted in the middle with enough red light to 

 completely inhibit flowering, and then the plants were placed in the 

 spectrum and irradiated for various time intervals. Intensity across 

 the spectrum varied slightly, but not enough to worry about. The 

 results for 4-min and 64-min illumination times in the spectrum are 

 shown in Fig. 7-10. The 4-min curve is a typical action spectrum, 

 closely resembling the far-red spectra of Fig. 7-3. The 64-min curve, 

 however, has been shifted far to the right, and only at 795 m/x is 

 flowering repromoted more than for the 4-min illumination. Thus 

 at long exposure times the repromotion is lost. 



