Photosynthesis 127 



max •> 



200 

 % of control 



FIGURE 4-14. Simulations of the effect of the 

 light-saturated rate of photosynthesis (Pmax) on 

 daily photosynthesis and maximum simulated 

 rates using conditions for the standard day. 



tion to 3297o of the control rate. As was indicated earher, one of the com- 

 pensatory adjustments of these vascular plants is the maintenance of 

 photosynthetic rates comparable to temperate region plants. The model 

 assumes maximum competency in all photosynthetic leaves, an assump- 

 tion which leads to at least a sHght overestimate of uptake. The justifica- 

 tion for this assumption was that developing leaves with incomplete 

 competency are located near the base of the canopy where irradiance is 

 reduced; the error resulting from light-saturated responses is therefore 

 minimized. 



Irradiance 



The sensitivity of daily photosynthesis to daily irradiance (Figure 

 4-15) illustrates that the mean daily photosynthetic rate approaches satu- 

 ration near 25 MJ m"^ day"'. Dupontia, therefore, approaches maximum 

 daily photosynthesis rates at the upper range of daily intensities received. 

 For major portions of the season the leaves are light-limited and would 



