1132 THE LIGHT FACTOR. II. QUANTUM YIELD CHAP. 29 



5. Maximum Quantum Yield in Relation to Light Curves as a Whole 



While much time and ingenuity have been invested in measuring the 

 yield of photosynthesis in very weak light in order to determine directly 

 the maximum quantum yield, no comparable effort has been made to ex- 

 tend these measurements to higher light intensities and to connect them 

 with the determination of the general shape of the light curve, described 

 in chapter 28. Tliis hiatus is worth filling in. 



Pitfalls and corrections that loom large in the interpretation of experi- 

 mental results obtained in very weak light gradually fade into unimpor- 

 tance as light intensity increases. If the light curves, P = f(I), are smooth 

 curves of a comparatively simple and analytically expressible form, it 

 should be possible to determine the initial slope of these curves (i. e., I/to) 

 by extrapolation from reliable observations in comparatively strong light. 

 At least, it should be possible to use, as an additional criterion of reliability 

 of measurements in very weak light, the requirement that they should be 

 compatible with the results of measurements further up the light curve. 



Reversing the above argument, the reliability of results obtained in 

 strong light, can sometime be judged by inquiring into their compatibility 

 with the quantum yield measurements in weak light. 



(a) Extrapolation of Maxwium Quantum Yield from Measurements 

 at Higher Light Intensities 



Many empirical light curves appear to be practically straight lines up 

 to comparatively high light intensities. If this straight line passes through 

 the zero point of coordinates, it seems safe to assume that its slope actually 

 represents the maximum quantum yield of photosynthesis under the condi- 

 tions to which the light curve refers. Probably, a more reliable value of 

 7n can be derived from this slope than from single points measured, with all 

 possible accuracy, near the origin of the coordinates, where the per cent 

 error of measurements is high and the respiration correction is larger than 

 the total measured gas exchange. 



Whether the slope of light curves which appear as straight lines not 

 passing through the zero point, can be used to determine the quantum 

 yield, is less certain. Such a determination implies the assumption that 

 an increment of absorbed light energy produces an increment of true photo- 

 synthesis, while the photochemical process (or processes) responsible for 

 the curvature of the light curve near the zero point continue at the same 

 rate at all light intensities above the turning point- 

 Warburg, Burk and co-workers (1949, 1950), and Moore and Duggar 

 (1949) detemiined the quantum yield of photosynthesis from the ratio of 



