Nitrate fluxes in squash seedlings measured with13N

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Abstract
Influx of 13N"‐labelled NO3” was studied in roots of intact squash seedlings (Cucurbita max. Duch.) previously grown in nutrient solution without (uninduced) or with NO3” (induced). At the uninduced constitutive level NO3” influx was saturable (Vmax=6.2 μmol (g FW)‐1‐h"1; Km=37.0 μM), but it increased again, probably passively, at high external NO3” concentrations. Initial NO3” efflux started during induction of the transport system when the influx rate increased and conceivably the NO3” concentration in the cytoplasm was raised. Likewise, this tendency was observed after a period of NO3” starvation. The kinetic parameters of NO3” influx of the substrate‐induced high capacity transport system accounted for a Vmax of 14.0 μmol (gFW)‐1 h‐1 and a Km of 15 μM. Both the uninduced and the induced uptake reacted in a similar pattern to the treatment of inhibitors affecting the protein synthesis (cycloheximide, fluorophenylalanine or puromycin) suggesting a single membrane protein population carrying NO3” across the plasmalemma. A rapid onset of a strong inhibition of the NO3” influx by the treatment with 1 mM phenyglyoxal was revealed. The influx recovered, but only slowly and was not much enhanced after 5h of recovery at increasing NO3” concentrations (100 to 8000 μM) in the ambient solution. Compared with barley, the relative tolerance of squash NO3" influx to the presence of up to 5 mM NH4 + in the solution was confirmed. Only after pretreatment with 5 mM NH4 + was the influx depressed, presumably because of feedback signals resulting from metabolite accumulation from the NH4 + assimilation.