1594 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



indophenol dye as oxidant. Its shape is similar to that for photosynthesis 

 (fig. 35.20). 



Gilmour and co-workers (1953) measured hght curves for the reduction 

 of ferricyanide by beet chloroplasts and found them to be rectangular hy- 

 perbolas {i. e., 1/H is a linear function of 7). Their saturation level was 

 rather low (corresponding to about 800 mm.^ O2 per milligram chlorophyll 

 per hour, or a turnover time of 360 sec, cf. table 35.X). 



100 



80 - 



< 

 a: 



UJ 



> 



I- 

 < 



_i 



UJ 



60 



40 



20 



10 20 30 40 50 60 70 80 90 100 



RELATIVE INTENSITY 



Fig. 35.20. Light curves of Hill reaction in chloroplast suspension: solid line, before 

 irradiation; broken line, after irradiation with 253.7 raix (after Holt, Brooks, and 

 Arnold 1951). 



Maximum Rate in Saturating Light. According to chapter 28 (section 

 A5), the usual value of Pniax:/Chlo — the saturating rate of photosynthesis 

 in strong light with ample supply of carbon dioxide, related to unit amount 

 of chlorophyll — corresponds to about one molecule O2 liberated per ♦mole- 

 cule chlorophyll about every 20 sec. (much higher values have been found 

 only with aurea leaves). This corresponds to about 4000 mm.^ O2 per mg. 

 chlorophyll per hour. Table 35. X gives in comparison* some of the maxi- 

 mum (initial) rates of oxygen production by chloroplastic matter, as re- 

 ported by several observers with different preparations and different oxi- 

 dants. 



With a given chloroplast preparation, the oxygen production in saturat- 

 ing light depends on the nature of the oxidant. This can mean that the 

 rate-limiting process involves the supply (or the preliminary transforma- 

 tion) of the specific oxidant prior to its participation in the photochemical 

 reaction; or it may indicate a difference in the probability of back reac- 



