MAXIMUM QUANTUM YIELD IN RELATION TO LIGHT CURVES 1133 



the increment of oxygen production and the increment of absorption, 

 in tlie region above the compensation point, and noticed no systematic 

 difference between the vakies obtained in this way, and these deteimined 

 in low Hght (in other words, the hght curve appeared, in these experi- 

 ments, as a straight hue passing through the zero point). Kok (1948, 

 1949), on the other hand, found for P = f{I), a straight hne passing above 

 the zero point, and concluded that the quantum yield of true photosyn- 

 thesis is lower than that of a photochemical process ("photorespiration") 

 which predominates in low light, is light-saturated in the neighborhood of 

 the compensation point, and runs at the same saturation speed at all the 

 higher intensities. Finally, French, and Wassink et at., working with purple 

 bacteria, found, in moderate light, approximately straight light curves, 

 whose linear extrapolation passed helow the zero point. He used the slope 

 in medium light to calculate the "true" quantum yield of bacterial photo- 

 synthesis — on the assumption that in weak light a photochemical process 

 occurs which either causes no consumption of hydrogen and carbon dioxide 

 at all, or does it with a much higher quantum requirement than bacterial 

 photosynthesis in stronger hght. In this case, too, the "low light process" 

 is supposed to continue at the same rate at all intensities above its light 

 saturation. 



The upward cur\'ature of the light curves of bacterial photosynthesis 

 seems to be a generally encountered phenomenon, but the occurrence of an 

 accentuated downward curvature (or even of a sharp turn) of the light 

 curves of ordinary photosynthesis near the compensation point, remains 

 controversial. Equally open to doubt are assertions that the light curves of 

 photosynthesis (with or Avithout a break in the compensation region) re- 

 main linear, above this region, almost up to saturation. At least, the 

 most precise experiments in this field — e.g., those by Emerson and Lewis — 

 showed neither of the two phenomena, but indicated that the light curves 

 gradually bend downward with increasing light intensity, the first signs of 

 curvature being noticeable even at light fluxes of the order of only 1 kerg/ 

 cm.- sec. 



Such curves are most easily interpreted: we recall that all kinetic 

 mechanisms analyzed in chapter 28 lead to hyperbolic lighl curves. More 

 complicated kinetic mechanisms may lead to light curves of a higher order; 

 \)\\i, in any case, these curves will approach the limiting slope asymptoti- 

 cally, and are unlikely to become straight lines at any finite value of light 

 intensity. The most precise method for determining the maximiun quan- 

 tum yield may well be to measure the yields systematically as a function 

 of light intensity' in the region where deviations from linearity are small, 

 then to find an equation representing 7 adeciuatelj' as a function of /, and 

 use it for extrapolation to / = 0. 



