Functional Transport of Nitrogen Compounds in the Sporophyte of Laminaria japonica

Abstract

The nitrogen transportation within the thallus of Laminaria japonica Areschoug was investigated both in laboratory and field experiments. In the laboratory, it was observed by using 15N as a tracer that the thallus allocated labelled soluble organic N to the younger basal tissues and labelled ammonium to the mature apical tissues. In addition, the younger basal tissues had a higher NH+4 uptake rate than mature apical tissues, and the assimilation of NH+4 was more active in the mature tissues. These results suggest that this transportation is based on two source-sink relationships. One is the transport of inorganic N from basal to mature tissues for assimilation. Another is the transport of soluble organic N assimilated in mature tissues to basal meristematic tissues for growth. To understand the transportation in the natural environment, tissue removal experiments were conducted during N-rich (Jan-Feb) and N-poor (Apr-May) periods. During N-rich periods, the removal of the apical parts led to a decrease of total soluble N content and an increase of inorganic N content in basal tissues and was associated with reduced growth rate. On the other hand, the removal of the basal part resulted in the reverse phenomenon to the removal of the apical part. Thus, the defined transport mechanism is closely connected with the younger basal tissues and mature apical tissues. It is suggested that the mechanism plays an important role in the growth and survival of the blade in the natural environment. The characteristics of the N transport mechanism are also discussed.