Nitrogen uptake and allocation in the seagrass Amphibolis antarctica

Ammonium acquisition and internal allocation of nitrogen in the seagrass Amphibolis antarctica ((Labill.) Sender ex. Aschers.) were studied on freshly collected plants in laboratory experiments. The uptake kinetics were studied from the depletion of ammonium in split chamber experiments, while N uptake by entire plants and internal allocation patterns were studied using N-15 techniques on culture plants. The uptake of ammonium was concentration dependent and followed Michaelis-Menten kinetics. Maximum uptake rates for leaves were 5-38-fold higher than for the root-rhizome complex and the ammonium uptake by leaves was transiently enhanced when plants were suddenly exposed to ammonium. Transiently elevated uptake rates were relatively short-lived and only significant at very high substrate concentrations. Amphibolis rarely, or never, experience nitrogen concentrations that high, and so, surge uptake has only little ecological relevance. The uptake of ammonium at low and ecologically relevant substrate concentrations could only supply about 70% of the nitrogen demand of rapid Amphibolis growth during summer and the remaining 30% had to be met from internal sources. The N-15 experiments showed that both young and old leaves took up nitrogen but most of the nitrogen taken up by old leaves was immediately exported to young actively growing plant parts. Also, nitrogen was re-mobilized and subsequently exported from old to young plant parts. This export of re-mobilized nitrogen could supply about 36% of the nitrogen incorporated into young actively growing tissues, thus lowering the demand for external nitrogen by an equivalent amount. Re-mobilization and subsequent allocation of nitrogen from old plant tissues seem to be an important way to reduce the demand for external nitrogen in Amphibolis and, therefore, this seagrass seems well adapted to sustain rapid growth in nutrient-poor environments.