1602 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



Gorham and Clendenning (1952) found that the pH optimum lies at 

 5.0-6.5 without, and at 7.0-7.5 with added chloride. 



For chromate, Holt and French (1948) found pH 7.3 as optimum acidity; 

 the initial rate dropped by }/3 of the maximum at pH 5.7, and by J^ of the 

 maximum at pH 8.0. 



Effect of Temperature. Increase in temperature probably increases the 

 maximum rate, JJ"^^^', but since it also accelerates the deactivation of 



100 



200 

 TIME, seconds 



300 



Fig. 35.23. Effect of temperature on the rate of acidification of chloroplast sus- 

 pension in Hill's mixture in light; the chloroplasts had been in Hill's solution 3 

 minutes before illumination started (pH 6.8, 0.005 N KOH, 0.38 mg. chlorophyll) 

 (after French and Holt 1946). 



chloroplasts, the increase may be overcompensated by the fast decay even 

 in the initial rate measurement. French and Holt (1946) found that the 

 stimulating effect of increasing the temperature from 3° to 20° or 25° C. 

 was over after only 5 minutes of illumination (fig. 35.23). The tempera- 

 ture coefficient of the initial rate (with Hill's mixture as oxidant) was Qio = 

 3.5 (3-15° C). 



Arnon and WTiatley (1949) found the highest rate of oxygen production 

 at 15-20° C, dropping to equally low values at 10° C. and at 25° C. with 

 all three oxidants used — quinone, ferricyanide and phenol indophenol. 

 On the other hand, the total obtainable yield of oxygen was highest at 



