The Potential of NO(3)-N Utilization by a Woody Shrub Species Lindera triloba: A Cultivation Test to Estimate the Saturation Point of Soil NO(3)-N for Plants

Responses of seedlings of a shrub species, Lindera triloba, grown in perlite culture medium, to nitrate (NO(3)-N) supply were investigated to estimate the saturating point of available NO(3)-N for plant utilization. NO(3)-N concentration and nitrate reductase activity (NRA) in leaves and roots were used as indicators of NO(3)-N uptake and assimilation by L. triloba. Root NRA increased with NO(3)-N supply when concentrations were low and reached a plateau at high NO(3)-N concentrations. On the other hand, root NO(3)-N concentration increased linearly with NO(3)-N supply; therefore, it is suggested that NO(3)-N uptake did not limit NO(3)-N assimilation by L. triloba. In contrast, leaf NRA and leaf NO(3)-N concentration were low and were not influenced by NO(3)-N supply. This may be caused by the lack of transport of NO(3)-N from roots to leaves. The NO(3)-N retained in perlite was compared with NO(3)-N pool sizes in soils from a forest where L. triloba occurs naturally to estimate the level of NO(3)-N availability to plants in the forest soil. The maximum NO(3)-N pool size in the forest soil was comparable to concentrations at which root NRA reached a plateau in perlite cultures. These results indicate that soil NO(3)-N availability is below the saturation point for NO(3)-N uptake by L. triloba, and it is the limiting factor of NO(3)-N utilization by L. trilobaunder field conditions in which this species naturally occurs.