Chapter 28 

 THE LIGHT FACTOR. I. INTENSITY * 



A. Light Curves of Photosynthesis 



The relation between photosynthesis and the quantity of Hght available 

 to the plants was investigated for the first time in 1866, when the Russian 

 botanist Volkov (Wolkoff) counted the oxygen bubbles evolved by sub- 

 merged aquatic plants at different distances from a sun-illuminated, frosted 

 glass Avindow. He found that the rate of gas evolution was proportional 

 to the intensity of illumination. In 1883, Reinke in Germany extended 

 similar measurements to stronger illuminations and observed that, when the 

 light intensity approached that of full simlight, the "light curves" (i. e., 

 curves in which the rate of photosynthesis was plotted against the intensity 

 of incident light) bent, and finally became horizontal. He had thus dis- 

 covered the phenomenon of ligJit saturation. 



Ewart (1896, 1897, 1898) showed that, if the illumination is increased 

 still further, far beyond the saturating value, the rate begins to decrease 

 again, and photosynthesis may even be completely inhibited. 



The initial increase of the rate with light intensity, the "light saturation" 

 that follows and the ultimate "light inhibition" of photosynthesis were 

 again observed by Pantanelli in 1903; at that time, it was natural to in- 

 terpret these results in terms of the theory of "cardinal points" {cf. fig. 

 26.1). Blackman and Matthaei (1905) and Blackman and Smith (1911) 

 pointed out, however, that photosynthesis requires no minimum light 

 intensity (at least, if one considers the true rate, corrected for respira- 

 tion, rather than the rate of net gas exchange). Furthermore, it shows a 

 broad "saturation plateau" instead of a sharp optimum. Therefore, they 

 argued, light curves can better be explained by means of Blackman's theory 

 of "limiting factors" {cj. fig. 26.2) than by reference to the three "cardinal 

 points." 



Singh and Kumar (1935) and Lubimeuku (193G) observed that the light curves of 

 some land plants are sigmoid in shape, and Lubimenko saw in this the proof of the exist- 

 ence of a "light threshold" of photosynthesis; but this conclusion runs contrary to the 

 results of all the other observers. On the other hand, sigmoid-shaped light curves ap- 

 pear to be the rule with purple liartcria {rf. French 1937; Wassink, Katz and Dorrestein 

 1942). 



* Bibliography, page 1078. 



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