372 



LIGHT AND LIFE 



Fig 



2 4 6 8 10 12 14 16 18 

 MAGNITUDE OF SLOW ONSET 

 8. Data replotted from Fig. 7. 



When these data are compared with the evidence regarding the 

 relative responses of the two ESR signals to light intensity and tem- 

 perature, the following relationships emerge. In the temperature 

 range which is optimum for photosynthesis (18-30°C) , ESR signal 

 II, which is present in the dark, increases in the light at a relatively 

 slow rate, to attain a total signal intensity of about 8 units. When 

 the light ceases, the signal decays with a single, relatively slow rate 

 constant. 



At lower temperatures matters are different. The total ESR signal 

 intensity in the light now exceeds 8 units and is due to tw'O super- 

 imposed effects: (a) signal II increases from a dark value of about 

 3 units to a maximum of 8 units, and (b) signal I rises from a dark 

 value of zero to a level which increases with decreasing temperature. 

 On illumination, both components increase in intensity at the same 

 rate, so that at all temperatures a single rate constant is observed for 

 onset. At low temperatures the decay process occurs with two time 

 constants. Signal II decays slowly from its maximum value of 8 units. 

 Signal I decays more rapidly. 



Direct evidence of these relationships can be seen in the upper 

 part of Fig. 9, which represents the change in meter deflection during 

 a brief decay of the light-induced signal at 12.5°C. This figure 

 shows a remarkable "undershoot" effect, in that the signal intensity 

 exhibited by a briefly darkened sample continues to decline for a 

 fraction of a second after being again illuminated. This result is 

 to be expected from the fact that the rate of onset for both ESR com- 

 ponents that contribute to the overall meter deflection is relatively 



