994 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



max. 

 ma.^. 



Since inhibition by excess light is a time effect {cf. chapter 19), the P 

 values of shade-adapted plants change with the duration of illumination. 

 We recall in this connection the time curves that Harder (1933) found for 

 Fonfinalis antipyretica (fig. 26.8). The general impression made by these 

 complex curves was that photosynthesis declined with time {i. e., the plants 

 suffered light injury) whenever the illuminating light was more intense 

 than the light to which the specimens were accustomed during the growth 

 period. 



It was noted on page 987 that in the shade-adapted plants the apparent 

 lower content of the enzyme responsible for the absolute saturation of 

 photosynthesis is coupled with a higher content of chlorophyll. We will 

 encounter, in chapter 32, other cases in which the content of the rate- 

 limiting enzyme appears to be independent of that of chlorophyll (Chlorella 

 cells grown in strong or weak light, cf. Tables 25.1 and 28. V; green and 

 aurea varieties of land plants which were mentioned above, cf. Table 28.V 

 and fig. 32.2) as well as cases in which these two concentrations change in 

 the same direction {Chlorella cells made chlorotic by iron deficiency, cf. 

 figs. 32.3 and 32.4). 



The shape of the light curves of shade-adapted plants has been much discussed in the 

 ecological literature, particularly in relation to the photosynthetic production of aquatic 

 plants at different levels under the surface. Even green algae, or submerged higher 

 plants, found only a few meters under the surface, which should not have acquired ex- 

 treme umbrophilic characteristics, were observed to produce a maximum of oxygen when 

 placed at a certain depth, and to show light inhibition when exposed to direct sunlight. 

 This, however, might have been, at least in part, a thermal effect. Much more pro- 

 nounced optima on yield vs. depth curves were reported for the photosynthetic efficiency 

 of colored (brown or red) algae at different levels under the sea. 



Ruttner (1926) and Schomer (1934) observed that several aquatic higher plants 

 {Elodea, Myriophyllum, Cerathopyllum) had a maximum efficiency 1-5 meters under 

 the surface. Curtis and Juday (1937) found similar optima for the green algae Ana- 

 boena and Gloethea (in 9-10 meter depth). Van der Paauw (1932) found that Hormi- 

 dium grown in a light of 2000 lux suffered light inhibition at 5000 lux. On the other 

 hand, Gessner (1938) found no "optimum" in the light curves of shadow-grown or sun- 

 grown Elodea plants in lamp light up to 30,000 lux. He tried ultraviolet light (360-400 

 m.y.) to imitate sunlight, but this, too, produced no inhibition. He suggested that the 

 reported depth optimum of Elodea may be caused by chromatic adaptation (to bluish- 

 green light) rather than by intensity adaptation. However, this explanation is im- 

 plausible since it implies that photosynthesis can be inhibited by the addition of red and 

 blue-violet light to green light, which has never been observed. Perhaps, carbon dioxide 

 supply conditions are more favorable at a certain depth than on the surface, and this 

 causes the rate to increase with increasing depth, as long as illumination remains suf- 

 ficient for light saturation. 



Particular attention has been paid to the maximum efficiency and light 

 inhibition of colored algae in relation to their vertical distribution in the sea. 

 Engelmann suggested (see chapter 15, page 420) that the color of brown. 



