Growth and organic production of eelgrass ( Zostera marina L.) in temperate waters of the Pacific coast of Japan. III. The kinetics of nitrogen uptake

Kinetics of ammonium and nitrate uptake by eelgrass, Zostera marina L., were investigated by a 15N tracer technique. The rate of ammonium uptake by leaves was linear to an ammonium concentration at least up to 20 μg atoms N l −1. The uptake of nitrate by leaves was of equal magnitude but was half-saturated at a nitrate concentration of 23 μg atoms N l −1. Ammonium uptake by roots seemed to saturate at an ammonium concentration of ca. 100 μg atoms N l −1, but, it increased when the ammonium concentration was raised to 500 μg atoms N l −1. Nitrate uptake by roots was repressed by high ammonium concentration. The overall process of nitrate uptake by roots appeared to b by roots. There was no diurnal variation in uptake of ammonium or nitrate by leaves and roots. Total nitrogen uptake by the plant agreed with the specific growth rate measured by a marking method. The translocation of nitrogen from the root—rhizome tissues to leaf tissues was more active during day than during night, suggesting involvement of a process(es) which is photosynthetically dependent. Nitrogen was translocated from other parts of the plant to the most actively growing young leaves and the leaves with flowering organs.