(Table 15] . Nitrate reductase activity was induced by exposure to ni- 

 trate, and it was higher in the roots than in shoots. Aeration de- 

 pressed nitrate reductase by approximately 40 percent in root tissue 

 suggesting a greater aeration effect on nitrate metabolism than on the 

 uptake process itself (Fig. 26). However, endogenous root nitrate 

 seemed slightly enhanced by aeration. If aeration were affecting ni- 

 trate reduction rather than uptake, a buildup of endogenous nitrate 

 would be expected. 



d. Experiment 4 . Plants were germinated and grown in a nonaerated 

 ammonium culture (Table 1) for 6 weeks (described for experiment 1) in 

 a greenhouse environment (September and October 1973) instead of growth 

 chamber conditions. At the end of 6 weeks' growth, plants were trans- 

 ferred to minus nitrogen solutions for 2 days in preparation for a 

 24-hour uptake period. The same treatments, except with four replica- 

 tions were used as in experiment 3. Nitrogen was labeled with 98- 

 percent ■'■^N; the initial concentration of ammonium and nitrate was 1 

 mM. Four plants were placed in 880-milliliter containers and allowed 

 to deplete the concentration without renewal. The solutions were not 

 depleted of nitrate or ammonium by the end of the experiment. In 

 contrast to experiment 3, all other nutrients were provided as given 



in Table 1 and the uptake period occurred under greenhouse conditions. 

 The experiment was initiated at 0900 hours and had a 10-hour dark 

 period from 2200 hours until 0600 hours the following morning. After 

 harvesting, tissue was separated into an insoluble and soluble fraction. 



The results of this experiment (Fig. 29) agree with the results of 

 experiment 3 in showing a low initial nitrate uptake which slowly in- 

 creased. The ammonium uptake rate again slowed substantially after 

 about 6 hours. Aeration suppressed the uptake of both NHt^ and NO3, 

 the effect being somewhat greater than in experiment 3. Uptake of both 

 ions (based on root weight) was greater than in experiment 3, perhaps 

 due to their greater initial weight (approximately threefold) . Dif- 

 ferences in environmental conditions during the experiment and the 

 presence of other ions in the uptake media could have contributed to 

 these differences between the two experiments. The data are consis- 

 tent overall in showing that aeration restricted the uptake of both 

 forms of nitrogen, and that Spartina altemiflora can develop the 

 capacity to absorb nitrate. However, ammonium continued to be ab- 

 sorbed more effectively than nitrate during the course of these two 

 experiments . 



e. Experiment 5 , One hypothesis relating to the relative growth 

 rates of ammonium versus nitrate-cultured plants is that newly ger- 

 minated plants cannot utilize nitrate effectively and are primarily 

 dependent on an ammonium source to promote growth. At later stages of 

 growth (5 to 6 weeks) , plants may begin to develop the necessary meta- 

 bolic machinery to utilize nitrate. Experiment 5 was designed to 

 test this hypothesis. 



56 



