The Occurrence of Nitrate Reduction in the Leaves of Woody Plants

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Abstract
Nitrate reductase activities greater than 0·2 μmol h−1 g−1 f. wt, measured by an in vivo assay, occurred in 41 per cent of a large sample (555 species) of woody plants. If several taxonomic groups (Gymnosperms, Ericaceae and Proteaceae) with consistently low activities were discounted activities greater than 0·2 μmol h−1 g−1 f. wt occurred in 73 per cent of the species. This compares with 93 per cent in herbaceous species, suggesting that leaf nitrate reduction is of common occurrence in woody plants. In a small sample of species leaf nitrate reductase activity correlated with nitrate concentration in the xylem sap. Low activities occurred consistently in the Gymnosperms, Ericaceae and Proteaceae. Feeding cut shoots of representatives of these groups with nitrate caused induction of leaf nitrate reductase activity in the Gymnosperms and Proteaceae, but only limited induction in the Ericaceae. The Ericaceae, with the exception of two species, had low activities and low nitrate reductase inducibility. Root assimilation may predominate in the Gymnosperms and Proteaceae. It is suggested that nitrate reduction generally occurs in the leaves of trees from a variety of plant communities and that this may be related to the lower energy cost of leaf, as opposed to root, nitrate assimilation.