LEAF POWDERS AND ISOLATED CHLOROPLASTS 



65 



with l,l'-dipyridyl). The amount of hemoglobin was measured spectro- 

 photometrically, by oxidizing it to oxyhemoglobin. Thus, in practice, 

 the evolution of oxygen was determined by measuring the rate of for- 

 mation of oxyhemoglobin in anaerobic mixtures of chloroplasts with 

 ferric oxalate and hemoglobin, while the reduction oj ferric oxalate was 

 determined by measuring the same rate in aerobic mixtures of chloroplasts 

 with ferric oxalate and methemoglobin. 



The rates obtained by the second method were lower, but better re- 

 producible than those calculated from anaerobic experiments. The cause 

 of this difference was the slowness of the methemoglobin-ferrous oxalate 

 reaction, which "limited" the over-all process, making it less sensitive to 

 poisons (but more sensitive to temperature). Thus, values obtained in 

 anaerobic experiments, though less consistent, are more significant, being 

 free from such artificial limitation. 



10000 20000 30000 40000 



Light intensity, foot -candle 



Fig. 6. — Effect of varying light intensity on the rate of evolution of oxygen by 

 chloroplasts (after Hill and Scarisbrick 1940^). 



Fe concentration, 4 X 10"'' mole per 1. Chloroplast suspension, 0.4 ml. Circles, 

 extreme values; dots, mean values. 



The effect of light intensity on the rate of oxyhemoglobin production 

 by chloroplasts from Stellaria media is illustrated by figure 6. It shows 

 the phenomenon of "light saturation," typical of true photosynthesis 

 (Vol. II, Chapter 28) and occurring in the same region (~40,000 lux) 

 of intensities. The occurrence of light saturation shows that Hill's reac- 

 tion includes a nonphotochemical process of limited velocity, in addition 

 to the photochemical process proper. We know now that different enzy- 

 matic reactions may become "limiting" in photosynthesis under appro- 

 priate conditions, as, for instance, if slowed down by a specific poison. 



