THE LIGHT FACTOR IN PHOTOSYNTHESIS 1027 



Of great significance for the absorption of light by the leaf is the 

 phototaxis of the chloroplasts, whereby the amount of light absorbed 

 by the leaf is modified. The factors affecting these movements are of a 

 very complex nature; earUer work in this field has been well summarized 

 by Senn (103) and recently Voerkel (131) has studied the effect of different 

 wave-lengths. Schanderl and Kaempfert (97) conclude that, due to the 

 phototactic movements of the chloroplasts, the transmission of light 

 by a leaf may be altered very materially within a short period; they 

 observed increases in transmission of about 40 per cent within 10 to 

 40 min. This is particularly noticeable in the blue end of the spectrum. 

 With the accumulation of the photosynthate, notably starch, the trans- 

 mission of the leaf for light may also be considerably altered. 



All of these facts serve to emphasize that leaves or other chlorophyll 

 bearing organs present an exceedingly complex optical system and that, 

 although the absorption of light by the pigments is of primary importance 

 for photosynthesis, there are many factors which make quantitative 

 measurements of this very difficult. Consequently, attempts to estimate 

 the amount of light absorbed by the pigments in the leaf through methods 

 based upon the extraction of the pigments and the determination of the 

 absorption of these solutions, can give only approximations which are 

 usually low (104c). 



Much attention has been given to the question as to which spectral 

 regions are effective in photosynthesis. The results depend to a con- 

 siderable degree upon the methods which have been used.^ However, 

 there is general agreement that the greatest photosynthetic activity in 

 green plants is in the red portion of the spectrum, corresponding to the 

 greater absorption of light in that region. The existence of a second 

 maximum in the blue-violet region, corresponding to the absorption of 

 the pigments in this region (26, 26a, 51, 53) has not been found by some 

 other workers (21, 124, 134). The lower activity in the blue-violet 

 region has been ascribed to the increased scattering of the light and to 

 the absorption thereof by the yellow pigments. The problem is in 

 need of accurate quantitative work with monochromatic light of various 

 wave-lengths and controlled incident intensity. 



OPTICAL PROPERTIES OF THE CHLOROPLAST PIGMENTS 



The utilization of light in photosynthesis is dependent upon its 

 absorption by the pigments in the plant. In most plants the pigments 

 which are directly involved in this process are located in the chloroplasts, 

 although the exact structure of these organs and the relationships of the 

 pigments and colorless portions thereof are not clearly understood as yet. 



'^ Englemann (26, 26a) used the motile bacteria method; Kniep and Minder (53) 

 the bubble counting method; Ursprung (124) the formation of starch; Warburg (134) 

 the manometric method; Ehrke (21) the Winkler method for dissolved oxygen; 

 Klugh (51) the indicator method for changes in alkalinity. 



