Photosynthesis 113 



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0= 15 



10- 



5- 



300 



-2 -l 



Irradionce, J m s 



20 

 Temperature, °C 



30 



FIGURE 4-4. Relationships of the resistance to CO2 

 transfer to irradiance and temperature. The resistances 

 include leaf (stomatal and cuticular) ( — ), cell wall ( — ) 

 and carboxylation (...). The plants were grown in 

 growth chambers at 5°C. (Tieszen, unpubl.) 



1976, Oechel and CoUins 1976). Rates are only slightly decreased at 5°C. 

 Pogonatum photosynthesizes at 55% of the maximum rate at 0°C. The 

 high rates of photosynthesis at low temperatures are of obvious adaptive 

 significance, since tissue temperatures frequently drop to between 5 and 

 0°C during the growing season. Continuous sunlight results in positive 

 net photosynthesis during these periods. 



Temperature optima from 10 to 19°C seem high; however, during 

 periods of high irradiance, moss tissue temperatures exceed air tempera- 

 ture. In 1973, midday tissue temperatures were above 5°C 87% of the 

 time and above 10° 44% of the time. In 1972, which was warmer and 



