TEMPERATURE CURVES WITH SEVERAL OPTIMA 1243 



In figure 31.9, the middle curve referred to the temperature dependence 

 of photosjTithesis in a red alga (Gigartina) in high light and comparatively- 

 low carbon dioxide concentration (initial concentration, [C02]o = 2.5 X 

 10~^ mole/1., about twice that in air). The rate is about doubled between 

 4 and 14° C, corresponding to Qio = 2; but the influence of temperature 

 still is very much weaker than in the presence of abundant carbon dioxide 

 (upper curve), where the rate is increased, in the same temperature inter- 

 val, by more than a factor of six. 



Tamiya, Huzisige and Mii (1948) found that, at 1 X 10 -« mole/1. CO2, 

 the temperature curves of Chlorella ellipsoidea could be interpreted by a 

 sequence of three reactions, with Ea = 0, 6, and 27 kcal, respectively 

 (c/. above their results obtained at CO2 saturation). Only the first reac- 

 tion depends on CO2 concentration. 



The factor that may be important for the temperature dependence of 

 photosynthesis at low temperatures, in the carbon dioxide-limited state, 

 is the viscosity of the protoplasm (with the stomata fully open, the main 

 diffusion resistance between air and chloroplasts may be in the aqueous 

 phase within the cell; cf. chapter 27, page 916). According to Weber 

 (1916) the temperature coefficient of the viscosity of the protoplasm is 

 about 1.4 (between 10° and 20° C). 



Miller and Burr (1935) noted (in experiments described on page 898) that, at ap- 

 proximately 20,000 lux, potted plants of Pelargonium, Tolmica, Coleus, Bryophyllum, 

 Eichhornia, Primula, Saxifraga, Zebrina and Begonia, as well as an unidentified Cras- 

 sulacea, reached a balance between respiration and photosynthesis at approximately 

 0.01% CO2 in the air, at all temperatures between 4° and 37° C. (This balanced state 

 was maintained for many hours at temperatures <30°, but was soon disturbed, ap- 

 parently by a progressive heat inhibition of photosynthesis, at 35-37° C.) Since, at 

 0.01% CO2 and 20,000 lux, carbon dioxide supply probably is the main rate-limiting 

 factor in photosynthesis, one could consider these results as proof of a practical equality 

 of the temperature coefficients of respiration and carbon dioxide supply. This would be 

 remarkable, since the temperature coefficient of respiration is high (Qw ^^ 2). However, 

 it must be taken into account that, in the state of compensation, a large part of the 

 carbon dioxide used for photosynthesis is supplied internally, by respiration, and there- 

 fore has only a very short diffusion path (or is even used directly in situ). The tempera- 

 ture coefficient of photosynthesis may thus be that of the carbon dioxide production by 

 respiration. 



4. Temperature Curves with Several Optima 



It was mentioned on page 1220 that temperature curves with two or 

 more optima have been observed with some plants. 



The first such observation was made by Henrici (1921) on alpine lichens. Lunde- 

 gardh (1924) found, in the study of tomato, potato and cucumber leaves, a main maxi- 

 mum at 30-35° C, which was most prominent in strong light and with an abundant 



