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Title: NITRATE REDUCTASE ACTIVITY IN LEAVES, STEMS, AND ROOTS OF HYBRID POPLAR.

Author
item Black, Brent
item FUCHIGAMI, LESLIE - HORT. DEPT. OREGON ST. U.
item COLEMAN, GARY - NRSL, UNIV. OF MARYLAND

Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: Plant species differ in how they obtain nitrogen from the environment. In non-legume plants, nitrogen is usually obtained from the soil either as nitrate or ammonium ions. Nitrate taken up by the roots must be reduced to ammonium before being incorporated into amino acids and proteins. Reduction of nitrate is an energy-intensive process which is carefully regulated by the plant. The first step in this process is the conversion of nitrate to nitrite by the nitrate reductase enzyme. Some plants concentrate nitrate reductase activity in leaves near the source of energy, while others reduce nitrate in the roots near the nitrate source. The location of nitrate reductase in a plant appears to be related to ecological adaptation. Fast growing pioneer species tend to assimilate nitrogen in the leaves, while slow growing late-succession species assimilate nitrogen in the roots. In addition to their use in agroforestry, poplar trees (aspens and cottonwoods, Genus Populus) are used as a model for studying nitrogen metabolism of trees. This research was conducted to determine if nitrate reduction occurs in the leaves or roots of poplar. Nitrate reductase enzyme activity and protein levels were 10- to 50-times higher in leaves than in stems or roots. With increased nitrate supply to the roots, there was a greater increase in enzyme activity in leaves than in roots. The localization of nitrogen assimilation in the leaves of poplar is consistent with its classification as a pioneer species. This information is important to plant physiology researchers in defining poplar nitrogen metabolism in an effort to develop improved nitrogen management practices for the agroforestry industry.

Technical Abstract: Plants differ in the tissue localization of nitrate reduction and assimilation. Some species reduce nitrate primarily in the leaves whereas others localize nitrate reduction and assimilation in the roots. To determine the site of nitrate assimilation in poplar, nitrate reductase activity (NRA) was measured in leaves, stems and roots of hydroponically-grown hybrid poplar (Populus tremula x P. alba) by in vivo assay. Compared to stems or roots, NRA was greatest in leaves and declined with leaf age. NRA increased in leaves and roots with increased nitrate supply while stem NRA remained constant. NRA in leaves was 10-fold greater than roots at all levels of nitrate availability. Protein gel blot analysis demonstrated that the higher level of activity in leaves was correlated with increased levels of nitrate reductase protein. Tissue nitrate content increased with nitrate supply and was higher in roots compared to stems and leaves. NRA declined in leaves of plants exposed to short-day photoperiods (8 hours of light and 16 hours of dark) relative to plants grown in a short day with a night break. These results show that leaves are the major site of nitrate reduction in hybrid poplar. Although significant levels of nitrate were measured in stems and roots, NRA in these tissues was relatively low.