EFFECT OF NATURAL VARIATIONS OF CHLOROPHYLL CONTENT 1261 



From this point of view, and in consideration of the participation of the "accessory" 

 pigments in the sensitization of jjhotosynthesis, made probable by several studies de- 

 scribed in chapter 30, it would be interesting to investigate the effect of changes in 

 chlorophyll concentration on the rate of photosynthesis in the light absorbed by carot- 

 enoids or phycobilins (assuming their concentration could be kept constant while that 

 of chlorophyll is changed). Experiments of this kind could perliaps be carried out with 

 aurea leaves, in which practically all absorption in the blue-vi(jlet part of the spectrum 

 must be due to the carotenoids. 



2. Influence of Natural Variations of Chlorophyll Content 



on Photosynthesis 



Lubimenko (1905, 1907, 1908, 1910) first pointed out that umbrophilic 

 plants have a comparatively high chlorophyll content. Their high photo- 

 synthetic efficiency in weak light (cf. chapter 28, page 986) can be interpreted 

 as a direct consequence of the fact that they absorb light more efficiently 

 than the less strongly pigmented heliophilic plants. 



Gabrielsen (1948) measured the photosynthesis of seventeen types of 

 leaves, with chlorophyll contents between 0.2 and 8.7 mg. per 100 cm. 2, 

 in weak incandescent light (up to 9000 lux); this series included aurea, 

 lutescens, chlorina and normal varieties of twelve species. When the yield 

 of photosynthesis at about 1500 lux was plotted as a function of chlorophyll 

 concentration, a saturation type curve was obtained, showing saturation 

 at about 6 mg. Chi (a + b) per 100 sq. cm. of leaf area. Comparison with 

 the absorption data of Seybold and Weissweiler (pp. 678, 684) indicated 

 general parallehsm between absorption and rate of photosynthesis in such 

 weak light. The chlorophyll-poor aurea varieties show, however, in 

 Gabrielsen's table, a rate of photosynthesis even lower than could be 

 anticipated from their chlorophyll content; Gabrielsen attributed this to 

 the greater relative significance, in such pigment-poor leaves, of "inactive" 

 light absorption by cell walls, nuclei and cell sap. 



Whether this is true absorption by weakly colored components or by highly diluted 

 pigments, or "false" absorption, simulated by scattering (not sufficiently taken into ac- 

 count in the evaluation of the measurements) cannot be said; plant physiologists often 

 speak of "absorption of (visible) light by colorless cell components" as if this were not a 

 contradiction in itself; cf. chapter 22, section A3. 



Gabrielsen's observations on aurea leaves in weak light seem to contra- 

 dict to some extent those of Willstatter and Stoll (cf. below) . 



In experiments to be described in detail in Chap. 37D (section 4), 

 Brackett and co-workers (1953) noted a parallelism between the maximum 

 quantum yield of photosynthesis in different Chlorella cultures, and their 

 chlorophyll content; in other words, here, too, the effect of low chlorophyll 

 content on the rate in weak light appeared stronger than could be explained 

 by less effective absorption of light. 



