Photosynthesis 103 



capacity is realized is a function of the microenvironment within the can- 

 opy (see Chapter 3.)- At the cell and leaf level, responses to light, temper- 

 ature and water are most crucial in determining the rate of photosynthe- 

 sis. Nutrients can limit leaf carbon dioxide uptake at the cell level by 

 affecting the internal capacity and at the plant and canopy level by 

 affecting the allocation for the production of more photosynthetic tis- 

 sues. Similarly, grazing will alter this pattern directly by removing estab- 

 lished tissues at various developmental stages. Most of these interactions 

 are discussed in this chapter and have been incorporated into a canopy 

 photosynthesis model (Miller et al. 1976). 



INTRINSIC FACTORS AFFECTING 

 CARBON DIOXIDE EXCHANGE 



Maximum Rates and Growth Forms 



The maximum photosynthetic rates for expanded blades range be- 

 tween 7 and 31 mg CO2 dm"^ hr"' (Table 4-1) or nearly as widely as those 

 of vascular plants in other Biomes (Tieszen and Wieland 1975). Within 

 the tundra, however, similar species, e.g. Carex spp. at Barrow and 

 Devon Island, Canada (Mayo et al. 1977), show very comparable rates. 

 Furthermore, photosynthetic rates show a distinct relationship to growth 

 forms. The rates are highest in graminoid types and forbs (~30 mg CO2 

 dm"^ hr"'), sHghtly lower in some of the deciduous dwarf shrubs ('^^20 mg 

 CO2 dm"^ hr"'), except for Salix species which tend to be higher, and still 

 lower among the evergreen dwarf shrubs (7 mg CO2 dm"^ hr"') such as 

 Cassiope tetragona, Ledum decumbens, and Vaccinium vitis-idaea 

 (Johnson and Tieszen 1976). Photosynthesis rates in vascular plants are 

 equivalent to rates of similar growth forms in more temperate zones 

 (Table 4-2), suggesting that these species are adapted genetically or physi- 

 ologically to the low ambient temperatures. As expected, photosynthetic 

 rates are much lower in mosses than in vascular plants (Table 4-2), rang- 

 ing between 1.0 and 4.4 mg CO2 gdw"' hr"' (Oechel 1976, Oechel and 

 Collins 1976, Oechel and Sveinbjornsson 1978). Although the rates of 

 different moss species vary widely, they are similar to those of temperate, 



Photosynthesis, mg COg dm' ^hr~' 



8 10 12 14 16 FIGURE 4-1. Photosynthetic 



~\ — ' — I — ' — 1 — I — I — ' — I 



rates of various plant parts of 



Leaf 



Sheath 



Exposed Culm 



Dupontia fisheri near mid-sea- 

 son. The rates were determined 



^infiorescence ^'^^^ ^he '^C system. N = 8. 



Enclosed Culm (After Tieszen and Johnson 



1975.) 



