AbstractAn improved experimental technique for routine application of13NO3−in short‐term flux studies on plant roots is described permitting the monitoring of NO3−influx and simultaneously NO3−net uptake at high time resolution and with quick data output. First results show that in the presence of NH4+(0.5 mM) in the nutrient solution the NO3−influx is depressed by more than 30 in barley. The effect already seems to start during the early step of NO3−uptake. In contrast no impairing effect of NH4+was observed in squash plants not even after 24 h of NH4+pretreatment (0.5 mM). The percentage of the simultaneously and continuously determined NO3−efflux was small at low NO3−concentrations in the solution but can rise to more than 25 of the influx rate.Further research is needed to elucidate the role of efflux as an element in the regulation of NO3−uptake, particularly under
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