Photosynthesis 119 



diurnal ampHtude and of decreasing photosynthetic input. However, 

 photosynthetic efficiency computed on a land area basis remains high 

 and attains its maximum between the middle of July and the first week in 

 August, which agrees with tiller carboxylation data (Figures 4-2 and 

 5-16). Maximum efficiencies are above \% for the graminoids and attain 

 2.3% for Salix pulchra. 



Early in the season, while light intensity is high and often above 

 saturation, mosses under simulated canopies show rates of carbon diox- 

 ide incorporation similar to those of mosses growing in open areas. Dur- 

 ing this period, protection from photoinhibition and higher rates at mid- 

 day under reduced sunlight offset the effects of reduced levels of carbon 

 dioxide incorporation during the evening. However, as the light intensity 

 decreases, especially during the period around solar midnight, and as 

 dark respiration increases, relative rates of carbon dioxide incorporation 

 by mosses under the canopy decrease. When midday radiation values are 

 high, advantage is conferred through shading, but if midday radiation 

 values are below saturation, there is a lowering of photosynthetic rates at 

 midday in response to shading. During evening hours photosynthesis is 

 also lowered by shading, often below the compensation point, and the 

 period of dark respiration is increased as a result. 



The mosses differ from vascular plants in their levels of energy cap- 

 ture. The mosses are much lower in overall efficiencies than are vascular 

 plants, except under periods of low radiation when the percentage of 

 energy capture increases. They also differ from vascular plants in that 

 mosses show no decreasing efficiencies at the end of the season resulting 

 from senescence. Moss photosynthesis shows less seasonal variation than 

 does vascular plant photosynthesis, but both have equally marked diur- 

 nal changes. Light intensity as well as water status are important control- 

 ling factors in moss photosynthesis. 



Other Factors 



Other factors could potentially influence photosynthesis and alter 

 the daily and seasonal courses just described. Plant pathogens, for exam- 

 ple, commonly inhibit photosynthesis by damaging chloroplasts and/or 

 by destroying proteins. Pathogens are not obvious on plant species of the 

 tundra at Barrow. One of the most striking impressions given by the veg- 

 etation is the absence of leaf lesions. Root nematodes are present and 

 fungi become active after the leaf senesces, but neither of these relation- 

 ships affects photosynthesis directly. Grazing may also influence the 

 photosynthetic response patterns, but mainly by altering the relative 

 number of young, mature and senescent leaves. These phenomena and 

 the role of accUmation are discussed in the following sections. 



