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Comparison of the Uptake of Nitrate and Ammonium by Rice Seedlings

Influences of Light, Temperature, Oxygen Concentration, Exogenous Sucrose, and Metabolic Inhibitors ¹

Laboratory of Plant Nutrition, Department of Agricultural Chemistry, Faculty of Agriculture, Nagoya University, Nagoya 464, Japan

The uptake of nitrate and ammonium by rice (Oryza sativa) seedlings was compared under various conditions. Nitrate uptake showed a 1-hour lag phase and then a rapid absorption phase, whereas ammonium uptake showed passive absorption during the first hour, then a shoulder of absorption, followed by a rapid metabolism-dependent absorption phase. Light did not affect the uptake of nitrate or ammonium. The uptake of nitrate and ammonium was markedly suppressed by removal of the endosperm. After removal of the endosperm, the uptake was restored by exogenous supply of 30 mM sucrose. No appreciable nitrate uptake was detected at temperatures below 15 C, whereas appreciable ammonium uptake occurred at 15 C, although lowered passive absorption and complete inhibition of the rapid metabolism-dependent absorption phase were observed at 5 C.

Nitrate uptake was decreased by aeration and also by bubbling the mixture with O₂. It was greatly depressed by bubbling the mixture with N₂. These results suggest that there is a low optimal O₂ concentration for nitrate uptake. In contrast, ammonium uptake was increased by aeration, not influenced by bubbling with O₂, and slightly decreased by bubbling the mixture with N₂.

Nitrate uptake was almost entirely inhibited by addition of cycloheximide, KCN, or arsenate. These compounds had scarcely any effect on the passive absorption of ammonium, but entirely depressed its successive metabolism-dependent absorption. Ammonium uptake occurred before nitrate uptake in solution containing both nitrate and ammonium.

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