Abstract
Nitrogen is an essential macronutrient for plants, which take up inorganic N from soils. There are two major N sources in soils for plants; ammonium and nitrate. Plant assimilation processes of two forms of N are different, since nitrate requires reduction processes to ammonium by enzyme activity before being assimilated to organic N. The first and rate limiting step of nitrate assimilation is the reduction of nitrate to nitrite catalyzed by nitrate reductase. Nitrate reductase is a substrate inducible enzyme, and the ability to induce this enzyme and to use nitrate as N source varies among species. Nitrate is an anion, and readily leached out of ecosystems. Therefore, species characteristics about N use are important in N cycling, since nitrate use by plants prevents N leaching from ecosystems. Experiments supplying various amount of nitrate to plants showed that 1) plant nitrate reductase activity (NRA) increased with increasing nitrate supply, 2) plant NRA reached a plateau with species specific amount of nitrate supply, and 3) maximally induced NRA also varied among species. These results indicate that species dependence and responsiveness to nitrate as N source are the important characteristics of N use. Furthermore, experimental N supply should be quantitatively estimated, since N status in experimental condition has rarely been compared with natural condition.
