320 



Govindjee 



(b) Temperature . Lowering the temperature shifts the beginning 

 of the decline to longer wavelengths. At 10°C, the quantum 

 yield starts declining only at 690 m\x and is halfway down at 

 730 m[i. (c) Growing the algae in " earth extract " (an undefined 

 organic medium). This too, shifts' the beginning of the decline 

 to longer wavelengths and causes the enhancement phenomena to 

 disappear. (d) Growing the algae in glucose medium^^ does not 

 shift the beginning of the decline appreciably, although there 

 is a somewhat higher activity at longer wavelengths, as compared 

 to that of cells grown completely in inorganic medium. The 

 persistence of the drop suggests that no significant amount of 

 energy can be supplied to photosynthesis from exogenous respira- 

 tion, (e) When Chlorella was grown in heavy water , the decline'^5 

 in the yield began at 670 m\i (instead of 680 m|a) ; no enhancement 

 was observed in these cells. An interpretation of this effect 

 must await studies of the absorption spectra of cells grown in 

 D2O. 



B. Chloroplasts from Higher Plants. 



23 24 

 Experiments made on the Hill reaction, using quinone , NADP 



or ferricyanide (in the presence of a catalytic quantity of the 



dye 2, 6 DCPIP^^^. (figure 2), revealed a long-wave decline 



("red drop") similar to that found in photosynthesis of whole 



cells. An enhancement effect also has been observed, at least 



with quinone^-^ and NADP^^. (see section V). This lends support 



to the previously suggested concept that the Hill reaction and 



photosynthesis have the same photochemical mechanism. 



C. Bacteria. 



Uptake of COo + H2 was followed in Rhodospirillum rub rum 

 suspensions at certain selected wavelengths, beyond the major 

 peaks of bacteriochlorophyll , i.e. in a region where one would 

 have expected a decline analogous to the "red drop" in photo- 

 synthesis. The quantum yield found at 940, 960 and 980 m\i 

 were all in the range of 0.10, similar to those found at the 

 shorter wavelengths and suggesting absence of a "red drop". 

 Blinks and van Neil^^ noted the absence of an enhancement phen- 

 omenon in bacterial photosynthesis. 



D. Discussion. 



The question "Does the existence of a "red drop" £er se prove 

 the presence of two photochemical systems?" must be answered in 

 the negative. If, for a certain reaction, say - the Hill 



