1226 



THE TEMPERATURE FACTOR 



CHAP. 31 



synthesis. While higher plants and algae adapted to moderate climates 

 attain the maximum rate of photosynthesis at 30-35° C. and suffer rapid 

 injury above 40°, cold-adapted (cryophilic) plants often show a maximum 

 of true photosynthesis below 10°, and a maximum of net oxygen evolution 

 near the freezing point. Some of them have been found to suffer thermal 

 injury at temperatures as low as 12° {Phaeocystes poucheti). 



Arctic land plants, polar sea algae and the flora of snow fields and 

 glaciers ("red snow") are examples of such cold-adapted species. Plants 



3- 



2- 





I 



o 

 o 



I 



ce 



I 



3 5 7 9 II 2 6 10 14 18 22 



LIGHT INTENSITY, klux LIGHT INTENSITY, klux 



Fig. 31.7. Light curves of two arctic plants — Salix glauca (left) and Chamaenerium 

 latifolium (right) at different temperatures (after Miiller 1928). 



adapted to high temperatures do not reach the maximum rate of net photo- 

 synthesis before 40° C; they are capable of organic synthesis even at 50° 

 and survive without permanent injury at 80° or even 90°. Algae that 

 live in hot springs and tropical desert plants are extreme examples of such 

 "thermophilic" plants. 



Among studies of the temperature dependence of photosynthesis of 

 cold-adapted species, we can mention that of Miiller (1928) with Salix 

 glauca and Chamaenerium latifolium at Disco, Greenland. Figure 31.7 

 shows the relative positions of light curves of net oxygen evolution by the 

 two plants at 0°, 10° and 20°. In weak light (ordinate a) the largest 

 oxygen production is obtained at 0° ; at 2000 lux (ordinate h), the optimum 

 of net synthesis is shifted to 10°, while in stronger light (ordinate c), it 

 lies at (or above) 20° C. Similar behavior is shown by cryophilic marine 

 algae. Thus, Harder (1915) found that Fucus serratus liberates, in weak 

 light, twenty times as much oxygen at as at 17° C. It must be pointed 

 out, however, that within the linear range of the light curves, where true 



