ACTION SPECTRUM OF GREEN PLANTS 1147 



In addition to wave length, the quality of light is characterized by its polarization. 

 Almost universally, no attention has been paid to this characteristic in the study of the 

 relationship between light and photosynthesis. Dastur and Asana (1932) and Johnson 

 (1937) found no difference in the rate of photosynthesis in ordinary and linearly polar- 

 ized light of the same intensity; but Dastur and Gunjikar (1934) observed a marked 

 deficiency in the absorption by leaves of elliptically polarized light, and later (1935) 

 also a similar deficiency in the synthesis of carbohydrates. Although we may doubt 

 the correctness of these results, it must be borne in mind that birefringence of the chlo- 

 roplasts {cf. Vol. I, p. 362) and the correlated dichroism may conceivably lead to dif- 

 ferences in the capacity to utilize light of a different state of polarization. However, 

 only a very weak dichroism has been observed in the chloroplasts in the natural state 

 (cf. Vol. I, p. 366). 



2. Quantum Yield and Wave Length in Green Plants. 

 Role of Carotenoids 



In chapter 29, we discussed the maximum quantum yield of photosyn- 

 thesis in green plants, without paying much attention to the quality of 

 light used for its determination, in the tacit assumption that it is either 

 altogether immaterial or has only a secondary influence. Quantum yield 

 measurements -with Chlorella in monochromatic light were first carried out 

 by Warburg and Negelein (1923). The results are shown in Table 30.1. 

 The quantum yield in blue light is somewhat higher than in red light, if 

 referred to the (estimated) absorption by chlorophyll alone, and somewhat 

 lower, if referred to the total absorption by all pigments. 



If we assume that the relative yields of Warburg and Negelein in dif- 

 ferent spectral regions are significant (even though their absolute values 

 had been questioned by Emerson et al. ; cf. chapter 29), the figures in Table 

 30.1 point to a participation of the carotenoids as sensitizers in photosynthe- 

 sis, but with an efficiency inferior to that of the chlorophylls. 



Table 30.1 

 Quantum Yields (after Warburg and Negelein 1923) 



Light X, m/i y e 



Red 610-690 0.23 0.59 



Yellow 578 0.23 0.54 



Green 546 0.21 0.44 



Blue 436 0.20" 0.34" 



0.28* 0.48'' 



" Referred to all pigments. ' Referred to chlorophyll alone. 



When Warburg repeated his quantum yields measurements 25 years 

 later (Warburg 1946, 1948), he again found a somewhat lower ;yield (7 = 

 0.20 and 0.16 in two experiments) in blue light (X = 436 mju) as compared 

 with yellow light. 



