Throughout the past three decades, plant physiologists have repeatedly proposed that plants conducting nitrogen assimilation in aboveground tissue should require less photosynthate per unit N assimilated than those that conduct N-assimilation in their roots. A review of research on agricultural and transgenic plants reveals that the Leaf:Root (L:R) partitioning of one enzyme in particular, glutamine synthetase (GS, E.C. 184.108.40.206) has a greater effect on
stress tolerance and growth rate than the other enzymes of N metabolism. The rate limiting enzyme in N-assimilation, GS catalyzes the assimilation of ammonium to glutamine, producing the first organic molecule in the N-assimilation pathway. While not universally accepted, studies in agricultural and model plants have generated strong evidence supporting the advantages of a high L:R GS ratio, yet the ecological implications remain unexplored. In this thesis, I explore GS partitioning in two genera of salt marsh grasses through field and growth chamber studies. The first studies are on congeneric Spartina alterniflora and S. patens that exist adjacently in the salt marsh, along the natural stress gradients of salinity and waterlogging. I correctly hypothesized that the faster growing, more stress tolerant S. alterniflora would have a higher L:R GS activity ratio than S. patens and that the L:R GS activity in both species would be altered by N-source. To corroborate these findings, I investigated GS partitioning in native and invasive subspecies of Phragmites australis, finding that the invasive subspecies appears to maintain a higher L:R
GS activity than the less stress tolerant, slower growing native subspecies. I discuss my findings in both the Spartina spp. and P. australis ssp. from an ecological perspective.