359 



David C, Fork 



ever, when the 1,3-second white exposure was given again the O2 

 spike had increased 2,77 times, A 3-minute dark interval, substi- 

 tuted for the 700 mp exposure, increased the O2 spike 1,49 times, 



Butler(17) reported an opposing effect of 650 and 705 my light 

 on the yield of chlorophyll fluorescence in a bean leaf. The con- 

 stant fluorescence excited by a very weak 650 my light was greatly 

 increased by a 5-second exposure to a much brighter 650 mp light. 

 After this treatment the weak light excited a continued high fluo- 

 rescence which could be lowered to its former level by an exposure 

 to 705 mii light. If fluorescence can be equated to light energy 

 which is wasted by the photochemical apparatus, then a parallel 

 can be drawn between Butler's observations and the observations 

 made on chloroplasts. Light at 650 mp excites largely chlorophyll 

 b and chlorophyll a 670. At low intensities this light can, be- 

 cause of overlap with the long-wavelength chlorophyll (the chloro- 

 phyll driving light reaction 1 of Muller, et al.^1), sustain a low 

 level of photosynthesis with little loss (low fluorescence), 

 Chloroplasts without a Hill oxidant show a small amount of O2 evo- 

 lution in weak 650 my light, A 5-second exposure to bright 650 my 

 light causes an accumulation of the product of the accessory pig- 

 ment reaction (resulting in high fluorescence) since little energy 

 can be absorbed by long-wavelength chlorophyll to drive the oxida- 

 tion of the reduced accessory-pigment product (Mijller, Weikard and 

 Witt'sl8,19' reduced plastoquinone-complex causing increased ab- 

 sorption at 515 mn). Bright 650 mu- light gives a transient O2- 

 production spike which cannot be sustained for the same reason 

 that the reduced product accumulates. A 700 mp exposure which ex- 

 cites long -wavelength chlorophyll regenerates the oxidized plasto- 

 quinone-complex needed to produce another O2 spike and allows weak 

 650 mn light to be used more effectively (decreased fluorescence), 

 A dark interval, shorter in air than N2, regenerates material 

 needed to produce another O2 spike, Butler likewise found the in- 

 creased fluorescence excited with bright 650 mn light persisted 

 longer in N2 than in air. 



Butler(20) has observed a shift to shorter wavelengths in the 

 peak position of the action spectrum for the inhibition by far red 

 of the fluorescence produced upon exposing a leaf to bright 650 mi-i 

 light. The peak of this action spectrum was shifted to shorter 

 wavelengths when lower intensities of far-red light were used, A 

 variation in the peak position from about 750 to 705 mp occurred, 



A similar effect of intensity was found in the action spectrum 

 for the regeneration of the O2 spike by far-red light. The action 

 spectrum for the light-induced recovery of the O2 spike was re- 

 ported(15) to have a peak at 730 mp. This action spectrum was de- 

 termined with an intensity of 437 ergs cm"2 sec"^ at 700 mp. When 

 this action spectrum was determined with low intensities 



