NITROGEN ASSIMILATION AND PARTITIONING IN THE MEDITERRANEAN SEAGRASS POSIDONIA-OCEANICA

The supply of nitrogen (N) often limits the productivity of marine mac rophytes. In vitro and in vivo assays of glutamine synthetase (GS) act ivity were employed to investigate patterns of N assimilation by the M editerranean seagrass Posidonia oceanica (L.) DELILE. Biomass-specific GS activity was measured in root tissue, in leaves within a shoot, in shoots collected at two sites during two seasons and over a depth ran ge of 5-33 m. Root tissue was less important than shoot tissue in assi milating inorganic N in P. oceanica, due both to the small roots' biom ass (ca. 3% of total plant biomass) and greatly lower (10- to 50-fold) GS activities. While the GS activity and N assimilatory potential (bi omass x GS activity; mu mol N.h(-1)) were greatest in leaf 2, leaves 1 and 3-5 assimilated N at significant rates. Shoots from a site charac terized by elevated N availability in the winter water column and no s ignificant sediment N reservoir exhibited GS activities that were 9-ti mes higher than shoots from a more oligotrophic site. Shoot GS activit ies in July increased linearly from 5 to 33 m and were correlated with light availability as defined by H-sat (daily period during which pho tosynthetic reactions are light-saturated). This may represent metabol ic compensation by P. oceanica to maintain N influx. Factors contribut ing to the ecological success of P. oceanica include the ability to as similate N under conditions of severe light limitation (<35 mu mol pho tons.m(-2).s(-1)), and metabolic plasticity to ensure the de novo gene ration of N-containing organic compounds.