At the 4- to 6-week growth stage, rhizome development and tillering 

 are very active processes in the natural growth of Spartina altern-i flora- 

 Some plants tended to respond more favorably to nitrate nutrition at 

 this stage of growth. However, it should be noted that with nitrate 

 cultures, 60 percent of the plants had difficulty recovering and con- 

 tinued to show symptoms prevalent in earlier growth. One problem en- 

 countered throughout this study was high inherent variability in in- 

 dividual plant growth rates, presumably because of genetic factors 

 associated with a wild species. Variability was usually more pronoun- 

 ced in nitrate-cultured plants. 



Aeration of nutrient solutions adversely affected growth. With 

 both ammonium and nitrate culture, aeration caused yellowing of the 

 leaves and reduced growth rates. Although fresh weights of ammonium- 

 cultured plants (Table 11) did not reflect adverse effects of aeration, 

 the leaves were yellowish and noticeably affected. The nitrate-cultured 

 plants were severely affected by aeration. To obtain normal plants 6 

 weeks old or more for further experiments, all nutrient solutions had 

 to be nonaerated. Whether the effects of aeration were directly re- 

 lated to oxygen toxicity or indirectly related to problems such as 

 carbon dioxide accumulation or nutrient availability has not been 

 determined. 



Plants grown in nitrate cultures contained nitrate reductase (NR) 

 indicating the capability to reduce nitrate (Table 12). Roots of the 

 nitrate plants contained a much higher NR activity than the shoots 

 did, although these shoots contained higher activity than the shoots 

 from ammonium- cultured plants. Aeration restricted NR in both tissues. 

 The presence of ammonium and nitrate in the external solution dramati- 

 cally decreased NR in root tissue and decreased a lesser extent in 

 shoot tissue. Nitrate accumulation in roots and shoots was decreased 

 by Ihe presence of ammonium under both aerated and nonaerated conditions. 

 Relatively little nitrate accumulated in shoots which, together with 

 the low NR of the shoots, suggested that little nitrate was translo- 

 cated from root tissue. Whether the decrease in NR of the root tis- 

 sue resulting from presence of ammonium was due to an inhibition in 

 the uptake of the inducer (nitrate) or was due to a specific effect 

 of ammonium (or a product of ammonium assimilation) on net enzyme 

 synthesis is unknown. There was little treatment effect on the ammon- 

 ium or nitrate concentrations in the tissue (Table 12) . 



Glutamic dehydrogenase (GDH) catalyzes the formation of glutamate 

 from ammonium and a-ketoglutarate . This reaction is the primary pro- 

 cess for ammonium assimilation. Activity of GDH was highest in the 

 root system, suggesting that most ammonium assimilation took place in 

 the roots (Table 12) . The presence of nitrate and ammonium decreased 

 the GDH activity of roots and shoots under nonaerated conditions. 

 With aeration, the effect was noted only in the roots. Soluble pro- 

 tein content was highest in tissue grown with ammonium. In general, 

 the data indicate that poor growth with nitrate as the nitrogen source 

 was associated with an inability to utilize nitrate effectively. 



47 



