1158 THE LIGHT FACTOR. III. COLOR CHAP. 30 



urement of absorption have verj^ little significance because of the well- 

 established rapid drop in the absorbing power of leaves in this region. 

 The only real problem is whether the yield of photosynthesis drops 'pro- 

 portionally with absorption, or more rapidly than the latter. 



Hoover (1937) was able to observe the photosynthesis of wheat up to 

 750 m/i and Burns (1933, 1934), that of different conifers up to 740 rati. 



4. Monochromatic Light Curves, and the Action Spectrum of 

 Photosynthesis in Strong Light 



When the intensity of monochromatic light is raised, one soon reaches 

 the region in which the shape of the action spectrum becomes variable. 

 The light curves bend earlier or later, and come to saturation more or less 

 suddenly, depending on the value of the absorption coefficient, and on the 

 optical density of the sample, respectively. It was postulated above 

 (page 1145) that, when all curves reach saturation, the rate must become 

 independent of wave length, and the action spectrum must lose all structure. 

 The theoretical and experimental foundations of this postulate will be con- 

 sidered later (pp. 1162, 1165). At present, we will assume it to be valid, 

 and consider only the effect of wave length on the shape of the transition 

 from the linearly ascending part of the light curves (the slope of which at a 

 given wave length is determined by the product of absorption coef- 

 ficient and maximum quantum yield) to the "saturation plateau," the 

 height of which we assume to be independent of wave length. 



The effect of optical density on light curves was discussed in chapter 28, 

 and the results were illustrated by the schematic figure 28.20. A change in 

 wave length is equivalent to a change in optical density; a cell suspension 

 that is "thin" in green light becomes "dense" in red or violet light. How- 

 ever, as far as the rate of photosynthesis is concerned, transition from 

 green to red light is not in all respects equivalent to an increase in cell 

 concentration, since the saturation level remains unchanged in the first 

 case, but increases proportionately with the number of cells in the second 

 case. Thus, the result of a change from strongly absorbed to weakly ab- 

 sorbed light is likely to be more similar to that of the change from green to 

 aurea leaves {cf. fig. 32.2), where the maximum rate is approximately the 

 same for both varieties. 



As mentioned before, the comparison of light curves at different wave 

 lengths should be carried out by plotting the rate against N^p (number of 

 incident quanta/cm. ^ sec.,) rather than against the energy flux, 7, in erg 

 (or cal)/cm.^ sec. Otherwise, the light curves for the shorter waves will 

 remain below those for the longer waves, even if the photochemical ef- 

 ficiency of both kinds of quanta are identical (fig. 30. 6C). An example of 



