684 LIGHT ABSORPTION BY PIGMENTS JN VIVO CHAP. 22 



3. Absorption by Nonplastid Pigments 



When the total absorption of hght by a leaf, thallus or cell suspension 

 has been determined, the question arises as to what part of this absorption 

 is due to the chloroplast pigments. It has been assumed by many authors 

 — ^from Reinke (1886) to Noddack and Eichhoff (1939) — that a certain 

 part of the absorption of white light in plants is due to the "colorless" 

 parts of the tissue, the cytoplasm, cell sap, nuclei, starch and cellulose. 

 Seybold arbitrarily ascribed one eighth of the total absorption to these 

 components, and seven eights to the chloroplast pigments. An absorption 

 curve of a white Pelargonium leaf, given by Seybold and Weissweiler (1942), 

 shows considerable absorption near the blue-violet end of the visible spec- 

 trum. 



Of course, no really colorless substance can absorb visible light. But the 

 plant cells contain coloring materials associated with the cell walls or with 

 the cell sap rather than with the plastids, such as flavones, tannins, etc. 

 Some of these substances are only weakly colored — ^usually yellow; others, 

 although intensely colored, are present only in small quantities, as com- 

 pared to the plastid pigments. In some species, however, flavones and 

 anthocyanines are so abundant as to give the leaves a striking red color 

 (leaves of the purpurea variety, and many young leaves in the spring). 

 The color of these leaves advertises the fact that much of the light energy 

 they absorb goes to nonplastid pigments. 



Figure 22.10 shows, as an example, the spectral transmission and re- 

 flection curves of three varieties of Corylus avellana, normal, aurea and 

 purpurea. Curves 1 and 2 illustrate the effect on light absorption of ex- 

 treme variations in the concentration of chlorophyll (c/. page 678), while 

 curve 3 shows the considerable increase in absorption, particularly in the 

 green, caused by the presence of anthocyanines in purpurea leaves. (Light 

 absorption in aurea and purpurea leaves will again be discussed in chap- 

 ters 28 and 30, in relation to the yield of photosynthesis.) 



As far as green leaves and algae are concerned, the participation of non- 

 plastid pigments in light absorption remains a moot question. It probably 

 varies widely from species to species. For example, according to Thimann 

 (unpublished), leaves of Phaseolus vulgaris contain a large quantity of yel- 

 low, water-soluble pigments. The same is true of the needles of the coni- 

 fers (Burns 1942). 



It was mentioned above (page 675) that "white" leaves may absorb 

 10 or 20% of incident white light (Seybold 1933S 1942, cf. fig. 22.12). This 

 absorption, too, is probably due to nonplastid pigments. 



Most investigators who measured the yield of photosynthesis in rela- 

 tion to the amount of absorbed light silently assumed that the absorption 



