THEORETICAL AND ACTUAL MAXIMUM QUANTUM YIELD 1137 



which corresponds to a quantum yield of about 0.13 (assuming 550 m/x as 

 average wave length) . In j&g. 32.2, the yellow leaves of Samhucus are shown 

 to reduce, at 3000 lux, 0.067 mg. COj/cm.^ hr., or 4 X lO-^" mole/cm.^ 

 sec. These leaves contain less than one tenth the chlorophyll present in 

 green leaves of the same species. Measurements such as those represented 

 in figure 22.10 indicate that the aurea leaves absorb, in the region above 

 500 m/i, not more than 20% of the incident energy. (Blue and violet light 

 do not contribute much to photosynthesis in artificial light; cf. page 1163.) 

 Thus, the energy conversion factor of the yellow leaves, can be estimated as: 



- ~ (4 X IQ-^" X 112 X 10^) cal = o 31 



* ~ (3 X 103 X 5 X 0.24 X 10"' X 0.4) cal. 



corresponding to a quantum yield of about 0.14. 



Similar difiiculties arise in the interpretation of some of the maximum 

 yields of photosynthesis listed in Table 28.V (0.8 or 0.9 mg. COa/cm.^ hr.). 

 As far as can be judged from the known light curves of land plants, it seems 

 safe to presume that saturation yields can be obtained in light of the order 

 of 40,000 lux. A yield of 0.9 mg. C02/cm.2 hr. at 40,000 lux means an av- 

 erage quantum yield of the order of 0.1, obtained in a region of almost com- 

 plete light saturation! 



These estimates contain too many approximations to be used as quanti- 

 tative arguments against the upper limit 0.10 ± 0.02 for the quantum yield 

 of photosynthesis; but they show that it would be well to extend future 

 investigations of the quantum yield to the leaves of higher plants— partic- 

 ularly those of the aurea varieties— and to cover the entire length of the 

 light curves. 



6. Theoretical and Actual Maximum Quantum Yield 



All kinetic theories of photosynthesis agree that the (approximately) 

 linear lower part of the light curves corresponds to the state in which the 

 primary photochemical process is so slow that the nonphotochemical reac- 

 tions—the "preparatory" as well as the "finishing" ones— can supply the 

 materials and transform the products of this process without delay. It 

 may thus seem as if the maximum quantum yield, calculated from the 

 limiting slope of the light ciu-ves, should be equal to the number of quanta 

 actually needed for photosynthesis (except for the practically negligible 

 fraction lost by fluorescence). The fact that the experimentally determined 

 maximum cjuantum yields often are much lower than 0.1 shows that, in 

 many cases, the photosynthetic apparatus, or parts of it, are in a noneffi- 

 cient state, so as to cause the loss of tiu; prodomiiinnt fraction of all the 



