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Nitrogen Assimilation Pathways in Leaf Mesophyll and Bundle Sheath Cells of C₄ Photosynthesis Plants Formulated from Comparative Studies with Digitaria sanguinalis (L.) Scop. ¹

^a Departments of Botany and Biochemistry, University of Georgia, Athens, Georgia 30602

Nitrogen assimilation in crabgrass Digitaria sanguinalis (L.) Scop., was studied by comparing leaf extracts with isolated mesophyll cell and bundle sheath strand extracts. The results show that both nitrate and nitrate reductase are localized in mesophyll cells; glutamine synthetase is nearly equally distributed in the mesophyll and bundle sheath; approximately 67% of the glutamate synthase activity is in the bundle sheath and 33% is in the mesophyll; and 80% of the glutamate dehydrogenase activity is in the bundle sheath, with the NADH-dependent form exhibiting a 2.5-fold higher activity than the NADPH-dependent form.

Isolated crabgrass mesophyll cells reduce NO₂^– coupled to the photochemical production of O₂ but are inactive with NO₃^–. The NO₂^– -dependent O₂ evolution is light-dependent; inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea; stimulated by photophosphorylation uncouplers; and exhibits a stoichiometry of O₂ evolved to NO₂^– reduced of 1.45 and 0.67 in coupled and uncoupled experiments, respectively. Isolated bundle sheath strands are inactive in O₂ evolution with NO₃^– or NO₂^–.

Based on these results, plus literature data, two schemes for crabgrass leaf nitrogen assimilation are presented, depending on whether the plant is using ammonium or nitrate as its nitrogen source. It is proposed that the increased nitrogen use efficiency in crabgrass and other C₄ plants is due partially to a "division of labor" between mesophyll and bundle sheath cells, where NO₃^– and NO₂^– reductase in mesophyll cells act as nitrogen reduction traps in an analogous fashion to phosphoenolpyruvate carboxylase acting as a CO₂ trap during C₄ photosynthesis.

¹ This research was partially supported by National Science Foundation Grant PCM 7708548.

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