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Simultaneous Influx and Efflux of Nitrate during Uptake by Perennial Ryegrass ¹

^a Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27607

Experiments with intact plants of Lolium perenne previously grown with ¹⁴NO₃^– revealed significant efflux of this isotopic species when the plants were transferred to solutions of highly enriched ¹⁵NO₃^–. The exuded ¹⁴NO₃^– was subsequently reabsorbed when the ambient solutions were not replaced. When they were frequently replaced, continual efflux of the ¹⁴NO₃^– was observed. Influx of ¹⁵NO₃^– was significantly greater than influx of ¹⁴NO₃^– from solutions of identical NO₃^– concentration. Transferring plants to ¹⁴NO₃^– solutions after a six-hour period in ¹⁵NO₃^– resulted in efflux of the latter. Presence of Mg²⁺, rather than Ca²⁺, in the ambient ¹⁵NO₃^– solution resulted in a decidedly increased rate of ¹⁴NO₃^– efflux and a slight but significant increase in ¹⁵NO₃^– influx. Accordingly, net NO₃^– influx was slightly depressed. A model in accordance with these observations is presented; its essential features include a passive bidirectional pathway, an active uptake mechanism, and a pathway for recycling of endogenous NO₃^– within unstirred layers from the passive pathway to the active uptake site.

¹ Paper No. 3794 of the Journal Series of the North Carolina State University Agricultural Experiment Station, Raleigh, N. C. These investigations were supported by the United States Atomic Energy Commission, Contract AT-(40-1)-2410.

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