Exogenous 15 NO 3 − Influx and Endogenous 14 NO 3 − Efflux by Two Maize ( Zea mays L.) Inbreds during Nitrogen Deprivation

Abstract

The influence of nitrogen stress on net nitrate uptake resulting from concomitant '5NO3 influx and 14NO3 efflux was examined in two 12-day-old inbred lines of maize. Plants grown on '4NO 3 were deprived of nitrogen for up to 72 hours prior to the 12th day and then exposed for 0.5 hour to 0.15 millimolar nitrate containing 98.7 atom % '5N. The nitrate concentration of the roots declined from approximately 100 to 5 microm-olar per gram fresh weight during deprivation, and '4NO3 efflux was linearly related to root nitrate concentration. Influx of 15NOs was suppressed in nitrogen-replete plants and increased with nitrogen deprivation up to 24 hours, indicating a dissipation of factors suppressing influx. Longer periods of nitrogen-deprivation resulted in a decline in '5NO3 influx from its maximal rate. The two inbreds differed significantly in the onset and extent of this decline, although their patterns during initial release from influx suppression were similar. Except for plants of high endogenous nitrogen status, net nitrate uptake was largely attributable to influx, and genetic variation in the regulation of this process is implied. ward movement of previously accumulated endogenous '4NO-3 (22). With wheat plants (12) and decapitated maize roots (19, 20) such experiments have shown that exposure to high nitrate concentrations can suppress net 15NO3 influx and enhance '4NO0 efflux, although the relative effects on the two processes may be dissimilar. However, the nitrate influx system also is subject to substrate induction (3, 11, 13, 23), and the induced activity declines upon exposure to nitrate-free media (21). Hence expression of nitrate influx activity during nitrate deprivation initially may reflect the lifting of feedback suppression (derepression or deinhibition), whereas more prolonged deprivation may indicate a decay of the induced system from its fully derepressed or dein-hibited state (3). Roots of maize inbreds have been shown to differ in nitrate uptake and assimilation processes (22, 25) and it is possible that the pattern of relief from suppression (15, 17) and/or decay of the induced transport system (21) may exhibit genotypic diversity. Accordingly, the present investigation was initiated to examine whether (a) increases in nitrate influx and decreases in nitrate efflux occur in parallel as autotrophic corn plants suppressed in net uptake undergo nitrate-deprivation, (b) decay of influx from an initial stimulated rate occurs during prolonged deprivation, and (c) intraspecific differences exist in the nature of those responses. Depletion of specific nutrients in higher plants is commonly accompanied by an enhanced capacity for uptake of those ions. This response may be viewed as a release from suppression of uptake which occurs in the presence of high endogenous concentrations of the ion and its assimilation products. Influx of phosphate, sulfate, and chloride into roots is subject to this kind of regulation (e.g., Ref. 14). Some experiments with barley, involving pretreatments with various nitrate concentrations, indicate little effect on subsequent nitrate influx, as measured with 36CIO3 or '3NO (5-7, 9). In these instances, net uptake was largely influenced by changes in nitrate efflux. In other experiments with barley, however, the stimulation in net nitrate uptake resulting from nitrogen deprivation (15) was accompanied by a stimulation in influx (17). Hence both restricted influx and high efflux (16) were characteristic of plants of high nitrogen status (17). In Pisum, both influx and net uptake of nitrate were enhanced by moderate nitrogen deprivation; more severe deprivation was required to restrict efflux (24). Exposure of roots to highly enriched (-98-99 atom %) '5NO3 permits direct, simultaneous measurement of the net inward movement of the exogenously supplied '5NO3 and the net out-'Paper maize (Zea mays L.) lines derived from the open-pollinated population 'Jarvis Golden Prolific,' two lines were identified which differed appreciably in net nitrate uptake and partitioning (27). These two lines, designated as 53 and 71, were selected for comparison in the experiment reported herein. Plant Culture. Fungicide-treated seed of each genotype were germinated in the dark in paper rolls moistened with 0.1 mM CaSO4 at 30°C and 98% relative humidity. After 84 h, seedlings were selected for uniformity within genotype, seminal roots excised , and the primary roots of two seedlings (a culture) were threaded through holes in hollow polyethylene stoppers (culture cups). The seedlings were grown in aerated nutrient solution containing 1.25 mm K2SO4, 1.0 mM MgSO4, 0.25 mm Ca(H2P04)2, 3.72 mm (CaNO3)2 and micronutrients at two-fifths of the concentration of Hoagland's solution (10). Iron was supplied as ferric diethylenetriaminepentaacetate (3.5 mg Fe L-1) and FeCl3(1.6 mg Fe L-1), and the acidity was adjusted to pH 4.6 with H2SO4. Endosperms were packed in cotton and moistened daily with 0.1 mM CaSO4. Plants were maintained in the laboratory under a metal halide light bank which provided a photon flux density of 1000 ,umol m-2S-1 for 16 h daily. Ambient temperature was 25± +2C, and air exchange was continuously maintained by an electric fan. Solutions were replaced 5 times at progressively shorter intervals as the seedlings grew. 778