323 



Govindjee 



Franck et al which suggests several other possible interpreta- 

 tions of S-shaped light curves obtained in white light under 

 anaerobic conditions). 



S-shaped light curves are well-known in bacterial photosyn- 

 thesis-'*-'; recently they have been found also for the production 

 of ATP by chloroplasts.-''^ We observed^^^ an S-shaped light 

 curve for oxygen production by spinach chloroplasts, (The 

 reaction mixture contained excess of ferricyanide and traces of 

 2,6 DCPIP). 



A method to observe enhancement in non-linear light curves 

 is through the experiment shown in figure 4. (1) The rate of 

 reaction as a function of light intensity of the far-red light 

 (A) is first measured (lowest curve, figure 4) (2) The same is 

 done in the presence of another beam (B) of far-red light, of 

 the same wavelength (middle curve, figure 4) (3) The same is 

 done in the presence of supplementary beam (C); the intensity 

 of which is adjusted to give the same rate of reaction as that 

 produced by beam (B) . Enhancement is then the difference 

 between the upper curve and the middle curve or 



E = ^,2 - "^1 (1) 



(■^2,2' - V 



where E = Emerson enhancement, R. . = rate in both 

 supplementary and far red beams, R, = iate in supplementary 

 beam alone, R2 = rate in far red beam alone, and R2 2' ^^ ^^^ 

 rate in combined far-red beams. 



C. Light Curves with Continuously Decreasing Slopes (early 

 saturation effects) . 



53 

 Light curves for various Hill reactions, in general show a 



decreasing slope with increasing light intensity, (i^.£. only a 

 very short linear range is available) . This raises the 

 question of the validity of the results °»-^ in which no 

 Emerson enhancement was reported for the Hill reaction. The 

 650 m|j. light curve for NADP reduction shows the above mentioned 

 early bending of the light curve. ^'^°* ■'^ Due to early "satura- 

 tion" effects, two beams of 650 mp. light, given together, give 

 a rate smaller than the sum of the two separate rates. 54_ True 

 enhancement can be calculated by taking this fact into account 

 by means of the equation: 



