Photosynthesis 1 1 1 



similar (Tieszen 1973) and tend to approach saturation at 280 to 350 J 

 m"^ s"' (400 to 700 nm). These saturation requirements are sufficiently 

 high that leaves are rarely light-saturated in situ, which suggests that the 

 entire canopy might be responsive to increased irradiance. 



Individual leaves of tundra plants require very low light for carbon 

 dioxide compensation, 5.6 to 7 J m"^ s"' (400 to 700 nm) (Tieszen 1973, 

 Mayo et al. 1977). Under field conditions, whole shoots possessed simi- 

 larly low compensation requirements (Table 4-4), especially at low tem- 

 peratures. Although the respiratory capacities of these tundra plants are 

 high, the combination of efficient photosynthesis and low daily tempera- 

 tures often resulted in the maintenance of a positive carbon budget for 24 

 hours (Tieszen 1975). The close coupling of the daily course of carbon di- 

 oxide uptake to irradiance implies a direct dependence even during mid- 

 day hours. This is further documented by the significant positive regres- 

 sion between daily carbon dioxide uptake and daily irradiance which is 

 discussed later. 



This light dependence may be mainly a vascular plant phenomenon 



100 



200 



300 



Irradiance, J m s ' 



100 



200 



-? -I 

 Irradiance, J m s 



FIGURE 4-3. The response of photosynthesis to irradiance (400-700 nm) 

 in Dupontia fisheri (D.f.), Pogonatum alpinum (P. a.), and Calliergon 

 sarmentosum (C.s.). The curves for D. fisheri are from the field (o) and 

 from plants grown in the laboratory at 5°C (%). The curves for the 

 mosses are from field-collected samples measured in the laboratory. 

 Note the different vertical scales. Standard errors are shown by the ver- 

 tical bars. (After Tieszen 1974, 1975, Oechel and Collins 1976.) 



