TEMPERATURE RANGE OF PHOTOSYNTHESIS 



1217 



leaves exposed to artificial light of high intensity, because this light may 

 contain ten times more infrared rays than sunlight of equal visual bright- 

 ness. Although leaves are comparatively transparent above 800 m/x {cf. 

 chapter 22), they absorb enough in this region to affect their heat balance 

 seriously. In the experiments of Seybold and Brambring, illuminated 

 white leaves, which absorb only infrared light, acquired temperatures not 

 much lower than those reached by green leaves (which absorbed both infra- 

 red and visible hght). For example, the temperature difference was -1-2.7° C. 

 for the upper surface of a white Pelargonium leaf, and -}-3.0° for the 

 upper surface of a green leaf of the same species (cf. fig. 31.1) ; the corre- 

 sponding figures for Abutilon were +2.3° and ^-3.7° C, respectively. 



I 2 3 4 5 6 



TIME, min. 



Fig. 31.1. Changes in leaf temperature upon illumination and darkening (after 

 Seybold and Brambring 1933): R, Rhododendron hybridum; P«, Perlargonium 

 zonule, green leaf; Pio, same, white leaf; B, Bryophyllum, succulent leaf. 



Karmanov (1951) found that leaves exposed to a flux of 1000 kerg/ 

 (cm. 2 sec.) from a 750 watt incandescent lamp, acquired a temperature up 

 to 20° C. above that of the ambient air. 



Franck and French (1941) found the temperature of Hydrangea leaves, 

 illuminated by collimated light from a 1000 watt lamp (about 80,000 lux) 

 to be 1.3° C. above that of the medium; these leaves were cooled by rapid 

 circulation of gas in a thermostated vessel. 



Because of the effective cooling of aquatic plants by the surrounding 

 water, such plants have no opportunity to acquire internal temperatures 

 markedly different from those of the medium, even if subjected to very 

 intense illumination. They thus provide the most appropriate material 

 for the quantitative study of the influence of temperature on the rate of 

 photosynthesis. 



B. Temperature Range of Photosynthesis* 



All life processes are restricted to a certain "biokinetic" range of tem- 

 peratures. Above and below this range, the organisms suffer a more or 



* Bibliography, page 1254. 



