93 



¥. L. Butler and N. I. Bishop 



modified, instriinient was similar in principle to that used by 

 Duvsens ^^^ - The fluorescence was excited by a weak (^0 ergs/ 

 cmVsec), 650- nm monochromatic beam which was chopped at 36O 

 cycles/sec. The sample could be simultaneously illuminated with 

 a high intensity (1000 to 2000 ergs/cmVsec), monochromatic acti- 

 nic beam. A cut-off filter, placed between the sample and photo- 

 tube, blocked the low-intensity, measuring beam and the high- 

 intensity, actinic beam but transmitted the fluorescence of wave- 

 lengths longer than 710 nm. It was previously shown with a green 

 leaf (3) that the light-induced fluorescence changes, even when 

 limited to wavelengths longer than 730 nm, were due to changes in 

 the yield of chlorophyll a fluorescence. The photometer incorpo- 

 rated a tuned amplifier which was tuned to the chopping frequency 

 of the measTiring beam so that the alternating fluorescence excit- 

 ed by the chopped, low-intensity beam was measured but the con- 

 stant fluorescence excited by the steady, high-intensity, actinic 

 beam was not. The actinic beam changed the fluorescence yield 

 which resulted in a change in the intensity of fluorescence 

 excited by the chopped beam. The intensity of fluorescence 

 excited by the actinic beam was monitored with a D.C. voltmeter 

 which measured the drop across the anode resistor. The intensity 

 of the actinic light was adjusted at each wavelength such that 

 the intensity of fluorescence excited by this beam was about 20- 

 fold greater than that excited by the measuring beam. 



Absorption spectra were measured at -196°C with a single-beam 

 recording spectrophotometer similar to one described previously 

 do). The spectrophotometer could also record derivative spectra 

 by differentiating the signal from the photometer electrically. 



Measurements were made on green bean leaves, on spinach chlo- 

 roplasts and on suspensions of Scenedesmus. The Scenedesmus were 

 wild-type cells and two classes of mutants which have been des- 

 cribed previously by Bishop (^^^2). One class of mutants ("GO2" 

 mutants) will not fix CO2 in the light but will evolve O2 in a 

 quinone Hill reaction. The other class of mutants ("O2" mutants) 

 will not evolve O2 but will photoreduce CO2 in a hydrogen 

 atmosphere. 



RESULTS MP DISCUSSION 



Action spectra for the effects of light on the fluorescence 

 yield of a green leaf in air and in nitrogen are shown in Fig. 1. 

 The fluorescence yield of the leaf is somewhat greater in nitro- 

 gen presumably because Q is more reduced. The horizontal lines 

 show the relative intensity of fluorescence in the absence of 



