ISOLATED CHLOROPLASTS II9 



with Chlorella cells in the presence of quinone, using the 

 same type of illumination as was used in photosynthesis in 

 the earlier experiments of Emerson and Arnold (described 

 in Chapter 4). The results obtained were similar to those for 

 photosynthesis and it was concluded that the chloroplast 

 reaction must also involve a dark process whose reaction 

 velocity is of the order of -y^ second just as was found for 

 photosynthesis. 



{d) Effect of temperature 



The activity of chloroplast preparations is rapidly lost at 

 temperatures above 45° C. This indicates a greater degree 

 of thermo-lability than is characteristic of many enzyme 

 systems but corresponds with the sensitivity of photo- 

 synthesis to temperature found by Blackman. The detailed 

 comparison of the temperature characteristics of photo- 

 synthesis and of the chloroplast reaction is not straight- 

 forw^ard because it is likely that some of the dark reactions 

 are not common to both processes. With abundant hydrogen 

 acceptor and at high light intensities the temperature co- 

 efficient between 10° and 20° C. is of the same order as that 

 for photosynthesis with high concentrations of carbon 

 dioxide. This observation together with those on the affect 

 of intermittent illumination make it possible that the same 

 dark reaction is limiting under these conditions in photo- 

 synthesis and in the chloroplast reaction. 



(e) Inductio?i phenomena 



In most plants a lag period in photosynthesis or 'induc- 

 tion' effect is observed subsequent to an aerobic dark period, 

 the half time varying from plant to plant. If the dark period 

 is anaerobic the lag period is prolonged. With the chloro- 

 plast reaction no induction period has been observed. Even 

 when chloroplasts were illuminated in the presence of an 

 oxygen concentration as low as 1/100,000 mm. Hg pressure 

 the full rate of oxygen production was observed as soon as 

 it was possible to record the rate (within about ^^q second). 

 In these experiments by Franck and colleagues oxygen was 

 determined using the extremely sensitive method of fluor- 

 escence quenching. On the other hand with photosynthesizing 



