Simultaneous Influx and Efflux of Nitrate during Uptake by Perennial Ryegrass

Experiments with intact plants of Lolium perenne previously grown with (14)NO(3) (-) revealed significant efflux of this isotopic species when the plants were transferred to solutions of highly enriched (15)NO(3) (-). The exuded (14)NO(3) (-) was subsequently reabsorbed when the ambient solutions were not replaced. When they were frequently replaced, continual efflux of the (14)NO(3) (-) was observed. Influx of (15)NO(3) (-) was significantly greater than influx of (14)NO(3) (-) from solutions of identical NO(3) (-) concentration. Transferring plants to (14)NO(3) (-) solutions after a six-hour period in (15)NO(3) (-) resulted in efflux of the latter. Presence of Mg(2+), rather than Ca(2+), in the ambient (15)NO(3) (-) solution resulted in a decidedly increased rate of (14)NO(3) (-) efflux and a slight but significant increase in (15)NO(3) (-) influx. Accordingly, net NO(3) (-) 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 NO(3) (-) within unstirred layers from the passive pathway to the active uptake site.