1346 INDUCTION PHENOMENA CHAP. 33 



light of comparatively low intensity — above the compensation point, but 

 well below the intensity required for the light saturation of photosynthesis. 

 Horizontal shading is used in the diagram for areas above the dotted curve, 

 indicating carbon dioxide production in excess of that due to respiration 

 and photosynthesis. Vertical shading is used for areas below the dotted 

 curve, indicating a deficit in the expected carbon dioxide production by 

 respiration. It is not clear to what extent the duration of the gush 

 depends on light intensity. (A part of what has been described by Emer- 

 son and Lewis as increase in volume may have been decrease in duration, 

 and consequent greater prominence of the gush in the first few minutes of 

 illumination.) 



Successive gushes can be produced by increasing the light intensity in 

 steps. 



Emerson and Lewis estimated that the initial quantum yield of carbon 

 dioxide liberation during the gush reaches (or even exceeds) unity; a total 

 of about 20 mm.^ CO2 can be hberated, in 3 minutes, from 100 mm.' of 

 ChloreUa cells, containing about 20 mg. dry matter and about 1 mg. chloro- 

 phyll. This corresponds to approximately one carbon dioxide molecule 

 per chlorophyll molecule, and shows that the size of the "reservoir" from 

 which the carbon dioxide is taken is approximately equivalent to the 

 amount of chlorophyll present. 



Figure 33.13 indicates that the resorption of carbon dioxide in the dark occurs much 

 more slowly than its release in light; but, since the two shaded areas are approximately 

 equal, Emerson and Lewis concluded that, after a sufficiently long dark interval, the 

 carbon dioxide loss suffered during the gush is completely recovered. 



The extent of the uptake of carbon dioxide in the dark, and thus also 

 the volume of the gush that occurs upon subsequent illumination, strongly 

 depends on the concentration of carbon dioxide. Up to 0.5% CO2, the up- 

 take is small; this may explain why McAlister found no carbon dioxide 

 gush in his induction experiments — he used only concentrations up to 

 0.24%. At 5%, the gush is very prominent (fig. 29.3A) ; it continues to 

 increase with [GO2] up to concentrations as high as 12%. 



Phenylurethan, in a concentration of 0.005% (which causes a 50% in- 

 hibition of photosynthesis), leaves the carbon dioxide gush unafl"ected, at 

 least in its initial stage. Higher urethan quantities, w^hich completely 

 inhibit photosynthesis, prevent also the carbon dioxide gush. Addition 

 of malic, maleic, citric and other plant acids had no effect on the gush. 

 (This experiment was conducted because Emerson and Lewis thought 

 that the gush might be due to the photodecarboxylation of an acid of this 

 type.) Low femperaiures slowed down the gush, and reduced its total 

 volume; below 10°, the gush required more than 10 minutes for comple- 

 tion, and the gulp in the dark was extended to an even much longer period. 



