Photosynthesis 115 



Photosynthesis in vascular and nonvascular plants appears well 

 adapted to the low temperatures, although the mechanism for this is not 

 known. Low temperatures, therefore, must exert relatively greater ef- 

 fects on growth and developmental processes, possibly including respira- 

 tion. At low temperatures all processes function in an integrated manner, 

 suggesting a general adaptation to low temperatures rather than specific 

 changes at the enzyme level. 



Water 



The water relations of the wet meadow are particularly conducive to 

 bryophyte growth. Low temperatures, low radiation, and precipitation 

 greater than evapotranspiration maintain hydrated tissues, resulting in 

 high photosynthetic rates and avoiding tissue damage due to desiccation. 

 The photosynthetic responses of mosses to water content reflect the 

 water relations of the microtopographic units (Figure 4-6). Pogonatum 

 alpinum, which occurs in drier areas and is a more drought-resistant spe- 

 cies than Calliergon sarmentosum, reached photosynthetic compensation 

 at 60% water content, and optimal rates were observed at 200% to 350% 

 water content. Calliergon, which occurs in wetter areas including poly- 

 gon troughs and the wet meadows, appears to require higher moisture 

 contents to reach compensation (about 75% w.c.) and maximum photo- 

 synthesis (about 400 to 500% w.c.) than does Pogonatum. Both species 

 generally remain hydrated, allowing photosynthesis to proceed to near 

 maximal rates. Polygon troughs that have drained free of standing water 



^ -0.1 



200 400 



Wafer Content, % 



200 400 600 



Water Content, % 



800 



FIGURE 4-6. Response of photosynthesis to water content (per- 

 centage of dry weight) in Pogonatum alpinum (P. a.) awe/ Callier- 

 gon sarmentosum (C.s.). (After Oechel and Collins 1976.) 



