Improved Method for 13 N‐Application in Short‐term Studies on NO 3 − Fluxes in Barley and Squash Plants

An improved experimental technique for routine application of 13 NO 3 − in short‐term flux studies on plant roots is described permitting the monitoring of NO 3 − influx and simultaneously NO 3 − net uptake at high time resolution and with quick data output. First results show that in the presence of NH 4 + (0.5 mM) in the nutrient solution the NO 3 − influx is depressed by more than 30% in barley. The effect already seems to start during the early step of NO 3 − uptake. In contrast no impairing effect of NH 4 + was observed in squash plants not even after 24 h of NH 4 + pretreatment (0.5 mM). The percentage of the simultaneously and continuously determined NO 3 − efflux was small at low NO 3 − 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 NO 3 − uptake, particularly under stress conditions.