Abstract
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. trilobaunder field conditions in which this species naturally occurs.