106 L. L. Tieszen et al. 



dioxide incorporated. Mosses and lichens represent an extreme develop- 

 ment of this trend since nearly all tissues are photosynthetic. Thus at high 

 latitudes plants are selected which either have little nonphotosynthetic 

 tissue or are highly opportunistic in their CO2 uptake. 



Enzyme Levels and Component Resistances 



The maximum rates of carbon dioxide uptake among all vascular 

 species are highly correlated with specific leaf density or thickness (r = 

 + 0.83) and with carboxylation activity {r = +0.76, N = 54) (Tieszen 

 1973). Chabot et al. (1972) noted an acclimation response of Oxyria 

 digyna that resulted in higher carboxylation levels at low temperatures, 

 and Treharne (1972) suggested a causal relationship between carboxyla- 

 tion activity and photosynthesis. Their data suggest that the range in car- 

 bon dioxide uptake potential is determined by differences in carboxyla- 

 tion activity. Further support is provided by data from the Biome 

 research area, which showed high correlations between photosynthesis 

 near light saturation and carboxylation activity among all leaves 

 throughout the season {Dupontia, r = +0.74, p > 0.97; Carex, r = 

 + 0.81, /7 > 0.99; Eriophorum, r = +0.75, p > 0.99). Therefore, species 

 differences and seasonal patterns are directly related to carboxylation ac- 

 tivity. Since ribulose-l,5-diphosphate carboxylase is a substantial por- 

 tion of total cell protein (Huf faker and Peterson 1974), this enzyme also 

 accounts for the major changes of nitrogen content through the season. 



The high correlation of maximum photosynthesis with carboxyla- 

 tion activity further suggests that differences in photosynthetic rates are 

 related more to differences in some component of the mesophyll resis- 

 tance than to leaf resistance. In the field, minimum leaf resistances for 

 Dupontia are generally less than 2 to 3 s cm"', whereas minimum meso- 

 phyll resistances are rarely below 7 s cm"' and are often well above 12 s 

 cm"'. Similar values for Carex, Eriophorum angustifolium, Salix pulchra 

 and Petasites frigidus support this contention. This trend is even more 

 pronounced in mosses, where leaf resistances are generally less than 1 s 

 cm'' but mesophyll resistances are large (Oechel and Sveinbjdrnsson 

 1978). 



Growth Rate and Developmental Stage 



Photosynthetic competence is a function of leaf development, in- 

 creasing as the leaf elongates or expands until a mature stage is attained. 

 The leaf usually remains at full competence until senescence occurs and 

 carbon dioxide uptake ability decreases as proteins and other materials 



