The potential of NO3--N utilization by a woody shrub species Lindera triloba: a cultivation test to estimate the saturation point of soil NO3--N for plants.

Responses of seedlings of a shrub species, Lindera triloba, grown in perlite culture medium, to nitrate (NO3--N) supply were investigated to estimate the saturating point of available NO3--N for plant utilization. NO3--N concentration and nitrate reductase activity (NRA) in leaves and roots were used as indicators of NO3--N uptake and assimilation by L. triloba. Root NRA increased with NO3--N supply when concentrations were low and reached a plateau at high NO3--N concentrations. On the other hand, root NO3--N concentration increased linearly with NO3--N supply; therefore, it is suggested that NO3--N uptake did not limit NO3--N assimilation by L. triloba. In contrast, leaf NRA and leaf NO3--N concentration were low and were not influenced by NO3--N supply. This may be caused by the lack of transport of NO3--N from roots to leaves. The NO3--N retained in perlite was compared with NO3--N pool sizes in soils from a forest where L. triloba occurs naturally to estimate the level of NO3--N availability to plants in the forest soil. The maximum NO3--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 NO3--N availability is below the saturation point for NO3--N uptake by L. triloba, and it is the limiting factor of NO3--N utilization by L. triloba under field conditions in which this species naturally occurs.