PHOTOSYNTHESIS RATE UJJDER NATURAL CONDITIONS 



1003 



In addition to vagaries of carbon dioxide supply, other external factors, 

 such as variations in temperature and humidit}'-, and internal changes re- 

 sponsible for permanent "aging" and temporary "resting" of plants, also 

 affect the average rate of photosynthesis under natural conditions. And 

 lastly, variations in the brightness of the daylight, even though less im- 

 portant than one is at first inchned to believe, certainly affect the yield of 

 photosynthesis, particularly in the case of species adapted to strong hght. 



We estimated in chapter 1 the average conversion yield of incident solar 

 energy as 1.5 to 6% (assuming 20 klux as the average intensity of illumina- 

 tion). We will now compare this estimate with the results of several inves- 

 tigations in which the determination of the yield of photosynthesis under 

 natural conditions was combined with the measurement (or estimation) of 

 the solar radiation that fell upon the plants during the same period. These 

 investigations can be divided into two groups : experiments of short dura- 

 tion (several hours), and studies lasting several weeks or months. Table 

 28. VII contains the results of three short-time experiments. The figures 

 of Brown and Escombe were obtained in the same investigation that gave 

 the low absolute yields listed in Table 28. VI; the conversion yields were 

 correspondingly low (of the order of 1% of incident energy, or an estimated 

 2.5% of the visible radiation absorbed by the plants). The figures of Pure- 

 vich and Bose are considerably higher, and can be placed alongside the 

 higher absolute reduction rates found by Willstatter and StoU in 5% carbon 

 dioxide (Table 28. V), and by many recent investigators in ordinary air 

 (Table 28. VI). 



Table 28.VII 

 Energy Conversion under Natural Conditions 



" This average conversion of absorbed energy was obtained by multiplying the aver- 

 age conversion of incident energ}^ by 2.5 (factor 2 to account for far red and infrared radia- 

 tion, and factor 1.25 to account for reflection and transmission of visible light). 



We decided above that the larger reduction rates are likely to be the 

 correct ones. (We mean values of the order of 20-25 mg./hr. 100 cm.' 



