FLUORESCENCE-TIME CURVES 



1383 



preparing the way for inhibition and activating the inhibitor, respectively. 

 A second nonphotochemical process removes the inhibition and thus ter- 

 minates the wave. According to this concept, the dark pause has a twofold 

 effect; it completes the removal of the active inhibitor (if this was not 

 completed in light), and replenishes the amount of the "precursor," if the 

 latter has been exhausted in light. (We recall that the "precursor" may 

 be a negative quantity — the absence of a catalytic component needed to 

 prevent the formation of an inhibitor in light.) The first effect will be 

 most conspicuous if illumination has been interrupted during the initial 

 fluorescence burst, i. e., after an illumination time of the order of a second, 



lOO 



2 4 6 8 10 



DARK TIME, sec. 



Fig. 33.24. Survival of fluorescence-promoting substances in the dark meas- 

 ured by the starting height of curves (after Franck, French and Puck 1941): 3 

 sec. illumination, followed by dark periods of various durations. Curve corre- 

 sponds to equation y = e-o-«6( Half-life = 1.7 sec. 



while the second effect will be more important if the illumination had lasted 

 long enough to approach the steady state {i. e., for at least several minutes). 



Kautsky, Hirsch and Davidshofer (1932) noticed that, if light is turned 

 off when fluorescence has just reached its peak (point 5 in figure 33.19a), 

 the curve AB can be repeated after a few seconds, but that, if point C has 

 been reached, several minutes of dark rest are necessary for the repetition 

 of the whole curve ABC. In the first case, the rate of disappearance of the 

 light-produced inhibitor is measured; in the second ca.se, the rate of re- 

 generation of the precursor (or the rate of deactivation of the protective 

 catalyst). 



The removal of the inhibitor in the dark can be studied quantitatively, 

 by determining the initial intensity of fluorescence (height of point A in 

 fig. 33.19a), after dark intervals of different duration. The results of such 

 a study are shown in figure 33.24. It indicates that, in Hydrangea, the 

 active inhibitor survives for about 10 seconds at room temperature; its 

 disappearance is a first-order process, with a half-time of 1.7 seconds. The 

 survival of this fluorescence-promoting and photosynthesis-inhibiting ma- 



