

ABSOLUTE MAXIMUM RATE 993 



to be higher, .since roughly the same yield of carbon dioxide reduction is 

 obtained here with a lower light absorption. However, the average ab- 

 sorption of white light by aurca leaves can vary, depending on their actual 

 chlorophyll content, from as low as 30% or less, to as high as 75% of that 

 of normal green leaves of the same species. An estimate of the quantum 

 5'ield in the light-saturated state requires therefore that absorption de- 

 terminations and yield measurements be performed on the same specimens. 



The fact that aurea leaves may absorb only a slightly smaller propor- 

 tion of incident light than normal green leaves, caused Seybold and Weiss- 

 weiler (1942) to consider their higher "assimilation numbers" (table 28. V) 

 as irrelevant (and not — as assumed by Willstatter and Stoll — as a sign of 

 exceptionally high capacity for photosynthesis). However, the capacity 

 for photosynthesis f». the lighl-saturaled state, P"^^'., is not a function of the 

 efficiency of light absorption, but a measure of the amount of a limiting 

 enzyme present in the cells. The values of Pmax.' for aurea leaves show that 

 in these leaves an abnormally low chlorophyll content is not accompanied 

 by a proportional reduction in the content of the rate-limiting enzyme. 



Yields obtained by Noddack and Kopp (1940) with Chlorella pyren- 

 oidosa, if related to dry weight, are higher than those given for most land 

 plants in Table 28. V. However, because of the high concentration of 

 chlorophjdl in Chlorella (3-4%, instead of 0.5 to 1% in leaves), the assimila- 

 tion numbers are not higher, but somewhat lower, and the assimilation 

 times somewhat longer than those given by Willstatter and Stoll for the 

 leaves of the higher plants. 



Like the maximum quantum yield (at low light intensity) , the maximum 

 rate of photosynthesis (in strong light) is a constant of the plant, i. e., it is 

 independent of the optical density of the selected material. The only ex- 

 ternal factor that affects it (apart from the presence of poisons or inhibi- 

 tors) is temperature (as illustrated by figs. 28.G-28.8). It is difficult, if not 

 impossible, to define the absolute maximum rate of photosynthesis also 

 as a function of temperature. In short experiments, the highest rates can 

 be obtained, with plants adapted to moderate conditions, at about 35° C. ; 

 but, in prolonged experiments, "heat inhibition" is apt to occur even at 

 temperatures as low as 22-25° C. (c/. chapter 31). We have used, in 

 Table 28. V, mostly values obtained at 18-20° C, which are certainly smal- 

 ler than the highest efficiencies of which most of the investigated plants 

 were capable at higher temperatures, at least for short periods of time. 



The maximum rate of photosynthesis of a species or individual plant 

 depends on adaptation to strong or weak light. As described on p. 986, 

 shade-adapted species or individuals generally have a lower "ceiling rate," 

 indicating a decreased content of a rate-limiting catalyst. In addition, 

 they often show an early onset of light inhibition (c/. fig. 28.19). 



