534 



PHOTOCHEMISTRY OF PIGMENTS IN VIVO 



CHAP. 19 



Figure 60 shows "time curves" corresponding to different light 

 intensities. At 1000 f.-c, the rate is constant, that is, no inhibition 

 occurs after 30 minutes of illumination. At 4000 f.-c, the initial rate is 

 higher than at 1000 f.-c, but inhibition sets in after about 20 minutes, 

 causing a gradual reduction of photosynthesis to a final rate which is 

 lower than that at 1000 f.-c. At 6100 f.-c, the initial rate is about the 



10 20 30 40 50 60 



Time of illumination, minutas 



Fig. 60.— Inhibition of oxygen liberation in Chlorella by intense light (after Myers 

 and Burr 1940). Light intensities are in foot-candles. Numbers above the curves are 

 rates of pressure change (in mm. per 10 min.). 



same as at 4000 f.-c. (showing that light-saturation has been reached) 

 but the inhibition is more rapid and more severe. At 12,900 f.-c, after 

 about 20 minutes, evolution of oxygen is replaced by its consumption; 

 at 18,400 f.-c, the oxygen consumption in light exceeds that in the dark, 

 that is, not only is photosynthesis completely suppressed, but photautoxi- 

 dation is added to normal respiration. At 27,700 f.-c, the total oxygen 

 absorption is 2.5 times larger than that caused by respiration alone, and 

 the decline in pressure sets in almost immediately upon the start of 



