Effects of NO3- availability on NO3- use in seedlings of three woody shrub species.

We determined the effects of short-term cultivation with various amounts of available nitrate nitrogen (NO3-) on NO3- use by woody shrub species. Nitrate concentration ([NO3-]) and nitrate reductase activity (NRA) were measured in leaves and roots of seedlings of Hydrangea hirta (Thunb.) Siebold, Lindera triloba (Sieb. et Zucc.) Blume and Pieris japonica (Thunb.) D. Don. Root [NO3-] increased with increasing NO3- supply in all species, whereas leaf [NO3-] remained low. There were significant correlations between [NO3-] in roots and leaves in all species, but no correlation was found between root NRA and leaf NRA. The low proportion of leaf NO3- assimilation to total NO3- assimilation in all species can be ascribed to the lack of NO3- transport from roots to leaves. In all species, root NRA increased with increasing NO3- supply until reaching a plateau. Species ranking based on maximum root NRA was H. hirta > L. triloba > P. japonica. Root NRA in P. japonica was low, even though root [NO3-] increased with NO3- supply, indicating that NO3- was not an effective N source for this species. The ranking also suggested that H. hirta depended more on NO3- as an N source than L. triloba. The increase in root NRA with increasing NO3- supply was greater in H. hirta than in L. triloba, possibly indicating that a change in NO3- availability has a stronger influence on NO3- use in H. hirta than in L. triloba.