319 



Govindjee 



of this effect have given the first evidence for the now widely 

 postulated hypothesis of two photochemical systems participa- 

 ting in photosynthesis. The enhancement effect has been studied 



2a8 11 ""17 18-20 



by manometry ' '+"o^ polarography^-^ •^ , mass spectrometry 



and by the radiocarbon tracer technique. ^■'- Its finding both in 

 oxygen evolution and in CO2 uptake shows that it is characteris- 

 tic of photosynthesis as a whole. Experiments on the Hill 

 reaction^^"^^ suggested the involvement of the two pigment 

 systems in this process as well. Further evidence for the 

 existence of two such systems has come from difference spectro- 

 scopy ' X electron spin resonance studies 29-31^ studies 

 of fluorescence , and luminescence (afterglow) , absorption 

 spectroscopy^ ' ^" , as well as studies on mutants^^, from flash- „ 

 ing light experiments'^^ '^•"•j and from biochemical investigations. 



This paper summarizes some of our recent work on the "red dro|}' 

 and the enhancement effect in whole cells and in chloroplast 

 preparations. 



I. INVESTIGATIONS ON THE "RED DROP" IN PHOTOSYNTHESIS 



A. Algae Pioneer studies of Emerson and coworkers showed the 

 occurrence of "red drop" in several algae (Chlorella ' , Anacy - 

 stis ^^, Porphyridium^ '^^ and Navicula^*^'^^) . At 20°C, the decline 

 in the quantum yield of photosynthesis in Chlorella begins at 

 680 m|a, and the yield is halfway down to zero at 696 mp.. This 

 decline is affected by several factors: (a) Presence of appro- 

 priate background light ("enhancement effect" ). In this case, 

 the decline may disappear altogether, (see figure 1). 



I CHLO/teiL* to'c 



\ Willi imt^f^mtmlQfj 

 \ 



640 650 660 6T0 680 690 TOO 

 WAVE LENGTH. rr\ii 



Figure 2. Quantum yield of oxygen evolution <n 

 spinach chloroplaits, a* a function of wave - 

 length (ferricyanide * catalytic ataount of 

 2.6 DCPIP). (Data fron Bakri and Govindjee). 



Fi-iure I. Qjanium yifld of oxygen evolution aa 

 s function of waveleriKth in thick suapensiona 

 «f Ciilorella (corrected and uncorreced for «b- 

 >.iTption>; (data from Emerson). 



