Glutamine synthetase activity and free amino acid pools of eelgrass (Zostera marina L.) roots

Activity of the enzyme glutamine synthetase (GS, EC 6.3.1.2) was determined in vitro for roots of the marine angiosperm Zostera marina L. (eelgrass) collected from a population in Great Harbor, Woods Hole, Massachusetts, U.S.A. The GS synthetase activity was lowest in roots of plants collected from the shallow region of the eelgrass bed (12.0 μmol·g^?1 (fresh wt)· h^?1) and increased in the mid (3.0 m, 40.3 μmol·g^?1 (fresh wt)·h^?1) and deep (5.0 m, 72.3 μmol·g^?1 (fresh wt)·h^?1) plant collection depths. GS transferase activity increased with collection depth in a similar manner: shallow, 28.6 μmol·g^?1 (fresh wt)·h^?1; mid, 52.0 μmol·g^?1 (fresh wt)·h^?1; deep, 92.8 μmol·g^?1 (fresh wt)·h^?1. When sediment-embedded plants were held in continuous darkness for 2 days to create extended root anoxia, root GS activities nearly doubled. In contrast, in vivo incorporation of ¹⁴C-glutamate into glutamine and protein residue remained constant or declined under short-term hypoxia and anoxia. During aerobic recovery from anoxia, root labelling of glutamine and protein increased markedly. Free amino acid patterns of eelgrass roots growing in situ were determined over a diurnal cycle. Total free amino acid content was maximal at dawn and decreased 50% by noon. In contrast, the proportion of glutamine was lowest at dawn and maximal at noon for both shallow and deep-growing plants. Despite differences in depth-specific plant sizes, root/rhizome/shoot ratios, and relative growth rates, the daily whole plant nitrogen demand of shallow and deep growing plants were equivalent. When corrected for assay temperature response, the enzyme synthetase activities measured in vitro suggest that all of the plant nitrogen assimilation requirements can be met within daylight hours during the period of peak summer biomass.