140 



Ellen C. Weaver and Norman I. Bishop 



Another "CO2" mutant^ No. l8^ has a trace of R signal when ex- 

 posed to bright light, hut this may reflect the somewhat higher 

 overall rate of photosynthesis of which it is capable. However, 

 several other "CO2" mutants have an R signal of approximately wild 

 type proportions. Although a system capable of carrying out phot- 

 oreduction can produce an R signal, the converse is evidently not 

 true; and R signal is not a reliable indicator of photoreduction. 



We were fortunate, therefore ^ to be able to compare the behavi- 

 or of the mutants by optical means with the results already on 

 hand. The difference spectrophotometer in Prof. Melvin Calvin's 

 laboratory is somewhat similar to the one described by Dr. Bessel 

 Kok(6) in that it makes use of repetitive flashes of actinic light. 

 A spinning disc with a sector removed provides alternating light 

 and dark periods. The absorption is measured at a given wave- 

 length by a photomultiplier immediately after the flash, and again 

 just before the flash; the difference in the absorption (light 

 minus dark) is plotted against wavelength as the monochromator is 

 slowly driven. This particular machine is described in detail in 

 a forthcoming paperVn. The spectra displayed here utilize a 

 flash of 3 msec followed by a dark period of 30 msec; thus, only 

 reversible changes with time constants which fall within these 

 limits are detectable. 



WILD TYPE SCENEDESMUS 

 3 msec FLASH 

 30 msec DARK 



-'^Jj«Jv^*■)!^*W/«^ — ''■■'^'■./^fMJ^- 



_1 I L. 



500 540 



Fig. 2 Changes in absorption induced by 3 msec flashes in a sus- 



pension of Scenedesmus . The absorption was measured k 

 msec after the onset of the flash, and again at the end of the 30 

 msec dark period. A tungsten lamp was used with a Corning 2030 

 filter passing only the wavelengths between GhQ m^ and 75O mfj,. 

 The curve seen here is the first minus the second measurement, 

 (light minus dark). The base line is the absorption just before 

 the flash. 



