N-15-ALLOCATION OF (NH4)-N-15-N AND (NO3)-N-15-N TO NITROGENOUS COMPOUNDS AT THE VEGETATIVE GROWTH STAGE OF POTATO PLANTS

Effect of NH4+ and NO3- on N-15-allocation to organs and nitrogenous compounds in potato plants (Solanum tuberosum L.) was examined in hydroponic culture using an automatic pH control system, and the results obtained were as follows. 1) Growth of upper leaves was stimulated by NH4+ because (NH4)-N-15-N was preferentially distributed into upper leaves compared to (NO3)-N-15-N which was mainly distributed to lower leaves. 2) At the vegetative growth stage, (NH4)-N-15-N and (NO3)-N-15-N distribution among organs was almost similar except for roots, whereas at the tuber swelling stage, (NH4)-N-15-N was distributed to stems and roots, and (NO3)-N-15-N was distributed to leaves. 3) N-15 amino acid metabolism was affected by NH4+ or NO3-, for example, in leaves grown in (NH4)-N-15-N, the contents of aspartic acid and asparagine (Asx) increased with growth, while those of glutamic acid and glutamine (Glx) decreased with growth. In leaves grown in (NO3)-N-15-N, the Asx content remained constant, but the Glx content increased with growth. In tubers, (NH4)-N-15-N was distributed to Asx at first, but (NO3)-N-15-N was distributed to Glx. Thus, the metabolism and translocation pattern of Asx and Glx were quite different depending on the nitrogen forms.