ABSENCE OF TEMPERATURE INFLUENCE IN WEAK LIGHT 



1233 



When it was generally assumed that 4 quanta are sufficient to reduce 

 one molecule of carbon dioxide, the scarcity of energy available in 4 

 quanta of red light caused Franck and Herzfeld (1937) to examine the 

 possibility that light energy might be supplemented by thermal energy, 

 available in the many degrees of freedom of the chlorophyll molecule. 

 However, recourse to such hypotheses becomes unnecessary if the lati- 

 tude of at least 6 quanta per molecule of carbon dioxide is allowed to the 

 theorists. Another potential source of temperature effects in the light- 

 limited state (?". e., of a temperature dependence of the maximum quantum 



e 



" 60 



High intensity, high [CO2] 



High intensity, low [003] 

 -o- 



Low intensity, high [CO2] 



8 12 16 



TEMPERATURE, "C. 



Fig. 31.9. Rate of Gigartina photosynthesis plotted against temperature (after 

 Emerson and Green 1934). 60 w. incandescent lamps 8 cm. below vessel; 10% 

 transmission filter used for low light curve. Artificial sea water; 35 X 10~^ M 

 CO2/I. for "high CO2," and 2.5 X 10-^ M CO2/I. for "low CO2." 



yield of photosynthesis) also was discussed before (page 1138) — the possible 

 influence of temperature on the relative probability of the forward and the 

 back reaction of the primary photochemical products. This effect, if it 

 exists, must be determined by the difference of two activation energies, 

 and therefore could be comparatively small. 



Experimental results speak against any theory requiring a strong tem- 

 perature dependence of the rate in low light. Wlierever the illumination 

 was weak and the supply of carbon dioxide adequate, the rate of photo- 

 synthesis was found to be more or less exactly constant over a considerable 

 range of temperatures. Thus, Matthaei (1904) found that the rate of 

 production of oxygen by cherry laurel leaves in weak light is practically 

 constant between 0° and 20° C. Warburg (1919) observed that the 

 temperature coefficient of photosynthesis of Chlorella, Q ^ P (20° C.)/P 

 (10° C), roughly equal to 2 in strong light (/ = 16-45 relative units), de- 



