372 



C.P. Whittingham and P.M. Bishop 



by a fluorescent chlorophyll a. Excitation of the non-fluores- 

 cent chlorophyll a alone does not lead to photosynthesis. How- 

 ever, if both chlorophylls are excited simultaneously, the two 

 reactions can cooperate, and lead to an enhanced photosynthesis. 

 An increased photo synthetic activity (enhancement effect) due to 

 simultaneous illumination by two different wavelengths has now 

 been shown in a large number of organisms. The so-called acces- 

 sory pigments, e.g. chlorophyll b in Chlorella , and the phycobi- 

 lins in red and blue-green algae, effect photosynthesis by trans- 

 ferring their excitation energy by resonance transfer to fluor- 

 escent chlorophyll a but not to the non-fluorescent form. 



Myers and French (7) have shown that the enhancement effect 

 observed with Chlorella illuminated with two wavelengths does not 

 require that the two wavelengths be given simultaneously, but 

 that these may be given alternately in periods of several seconds 

 duration. This suggested the formation of an intermediate common 

 to the two reactions with a life of several seconds. Such a 

 mechanism could also be used to explain the observations of l-Jhit- 

 tingham and Brown (8), which extended those made earlier by Allen 

 and Franck (9). They observed, with Ankistrodesmus , that the 

 photosynthetic yield of oxygen from a light flash of 35 msec, dur- 

 ation was increased if it was preceded by a flash of 100 psec . 

 The yield from the pair of flashes varied with the interval sep- 

 arating them, rising at 20° C to a maximum when the interval was 

 0.7 sec. and decreasing as the time separation of the flashes was 

 increased, until after 10 or 15 sec. little enhancement was ob- 

 served. Further work by Whittingham and Bishop (10) has shown 

 that when the temperature was reduced to 4° C , the maximal yield 

 for a long flash preceded by a short flash was of the same order 

 of magnitude as the maximal yield of the same two flashes at 20° C , 

 except that the time between the flashes for maximum yield was 

 now 16 sec . 



At 4°C it was possible to investigate these flash effects with 

 chloroplasts isolated from spinach leaves, using different oxi- 

 dant systems. These consisted of:- 



a) basic ferricyanide - containing, in a total volume of 2.3 ml. 

 potassium ferricyanide 2 mp moles; Tris-HCl buffer at pH 7.4 40 

 pmoles; sodium chloride 70 pmoles; magnesium chloride 10 pmoles. 



b) ferricyanide with phosphorylating reagents - containing, in 

 addition to the basic medium, 2 pmoles ADP and 40 pmoles inorgan- 

 ic phosphate. 



c) ferricyanide with an uncoupling reagent - in which ammonium 

 chloride was added to the basic medium to give a final concen- 

 tration of O.OIM. 



