337 



C. S. French 



reviewed by Clayton^ ^■'-^ On comes off 



and of French and Fork^ '-''' 



from a reaction involving both photoproducts . Following an expo- 

 sure to a wavelength previously activating one of the pigment sys- 

 tems, the lingering O2 evolution decreases slowly. However if the 

 other photoproduct is continuously produced by exposure to a back- 

 ground light which activates the other pigment system the decrease 

 in rate is much more rapid, as seen in figure 2. Thus by speeding 



Wilh 650rr.ii 



Fig, 2. (a) The rate of O2 exchange from a 20-second 

 exposure at 650 m(a in Scenedesmus . A background light 

 of 690 mn increases the rate of decay of the lingering 

 O2 evolution so much that the opposing light-stimulated 

 O2 uptake becomes evident, 



(B) The decline in rate of O2 evolution in 

 Scenedesmus after a 2-minute exposure to 700 mp. With 



650-mp background the decline is much steeper. French 



(9) 



up the utilization of the 02-evolving products this reaction can 

 be completed quickly. When this is done with a background beam 

 the light-stimulated uptake of O2 shows more clearly in the post- 

 illumination part of the time-course curve, as is also evident in 

 figures 1 and 2. 



Because the increased rate of 0^ uptake at very low light in- 

 tensity starts before photosynthesis does, its magnitude for dif- 

 ferent wavelengths can be measured by the small negative deflec- 

 tion at the start of a light exposure. The action spectrum for 

 respiratory stimulation was measured by plotting the initial nega- 

 tive spike and also by the magnitude of the increased O2 uptake 

 following an exposure. The spectra of figure 3 show that the in- 

 creased O2 uptake is induced by light absorbed by chlorophyll 

 while, as has long been known^^> ■'-^ »■'--''' , O2 evolution is far great- 

 er in light absorbed by the accessory pigment system. An inter- 

 esting characteristic of these spectra is that they show very lit- 

 tle if any evidence for the participation of phycoerythrin in the 



