(13)NO(3) (-) was used to investigate patterns of NO(3) (-) influx into roots of barley plants (Hordeum vulgare L. cv Klondike) previously grown with (;induced') or without (;uninduced') a source of external NO(3) (-) ([NO(3) (-)](0)). In both induced and uninduced plants, (13)NO(3) (-) influx was biphasic in the range from 0.005 to 50 moles per cubic meter [NO(3) (-)](0). In the low concentration range (<1 mole per cubic meter for induced plants and <0.3 mole per cubic meter for uninduced plants), influx was saturable and V(max) and K(m) values for influx either increased or decreased according to NO(3) (-) pretreatment. By contrast, (13)NO(3) (-) influx in the high concentration range revealed a strictly linear concentration dependence. These fluxes appeared to be mediated by a constitutive, rather than an inducible, transport system.

Using (13)NO(3) (-), effects of various NO(3) (-) pretreatments upon NO(3) (-) influx were studied in intact roots of barley (Hordeum vulgare L. cv Klondike). Prior exposure of roots to NO(3) (-) increased NO(3) (-) influx and net NO(3) (-) uptake. This ;induction' of NO(3) (-) uptake was dependent both on time and external NO(3) (-) concentration ([NO(3) (-)]). During induction influx was positively correlated with root [NO(3) (-)]. In the postinduction period, however, NO(3) (-) influx declined as root [NO(3) (-)] increased. It is suggested that induction and negative feedback regulation are independent processes: Induction appears to depend upon some critical cytoplasmic [NO(3) (-)]; removal of external NO(3) (-) caused a reduction of (13)NO(3) (-) influx even though mean root [NO(3) (-)] remained high. It is proposed that cytoplasmic [NO(3) (-)] is depleted rapidly under these conditions resulting in ;deinduction' of the NO(3) (-) transport system. Beyond 50 micromoles per gram [NO(3) (-)], (13)NO(3) (-) influx was negatively correlated with root [NO(3) (-)]. However, it is unclear whether root [NO(3) (-)] per se or some product(s) of NO(3) (-) assimilation are responsible for the negative feedback effects.

A computer-controlled multichannel data acquisition system was employed to obtain continuous measurements of net nitrate or chlorate uptake by roots of intact barley plants (Hordeum vulgare cv Betzes) using nitrate-specific electrodes. Plants, previously grown in solutions maintained at 10 or 200 micromolar NO(3) (-) (low N or high N conditions, respectively), were provided with 200 micromolar NO(3) (-) or ClO(3) (-) during the uptake period. Initial rates of NO(3) (-) uptake were several times higher in low N plants than in high N plants. Within 10 min, uptake in the former plants declined to a new steady rate which was sustained for the remainder of the experiment. No such time-dependent changes were evident in the high N plants. Rates and patterns of net chlorate uptake exhibited almost identical dependence upon previous nitrate provision. NO(3) (-) ((36)ClO(3) (-)) influx, by contrast, appeared to be independent of NO(3) (-) pretreatment prior to influx determination. Nitrate efflux, estimated by several different methods, was strongly correlated with internal nitrate concentration of the roots.

The influence of NH(4) (+), in the external medium, on fluxes of NO(3) (-) and K(+) were investigated using barley (Hordeum vulgare cv Betzes) plants. NH(4) (+) was without effect on NO(3) (-) ((36)ClO(3) (-)) influx whereas inhibition of net uptake appeared to be a function of previous NO(3) (-) provision. Plants grown at 10 micromolar NO(3) (-) were sensitive to external NH(4) (+) when uptake was measured in 100 micromolar NO(3) (-). By contrast, NO(3) (-) uptake (from 100 micromolar NO(3) (-)) by plants previously grown at this concentration was not reduced by NH(4) (+) treatment. Plants pretreated for 2 days with 5 millimolar NO(3) (-) showed net efflux of NO(3) (-) when roots were transferred to 100 micromolar NO(3) (-). This efflux was stimulated in the presence of NH(4) (+). NH(4) (+) also stimulated NO(3) (-) efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO(3) (-) occur via effects upon efflux. By contrast to the situation for NO(3) (-), net K(+) uptake and influx of (36)Rb(+)-labeled K(+) was inhibited by NH(4) (+) regardless of the nutrient history of the plants. Inhibition of net K(+) uptake reached its maximum value within 2 minutes of NH(4) (+) addition. It is concluded that the latter ion exerts a direct effect upon K(+) influx.

Evidence is presented that chlorate is an extremely good analog for nitrate during nitrate uptake by intact barley (Hordeum vulgare cv. Fergus) roots. The depletion of ClO(3) (-) or NO(3) (-) from uptake media over 2 to 6 hours by seedlings was found to be dependent on combined NO(3) (-) plus ClO(3) (-) concentrations, and total anion uptake was equivalent at different NO(3) (-)/ClO(3) (-) ratios. After loading barley seedlings with (36)ClO(3) (-) for 6 hours, kinetic parameters were derived from the analysis of efflux of [(36)Cl] chlorate into unlabeled solution. On the basis of this analysis, the half times for exchange for the cytoplasmic and vacuolar phases were 17 minutes and 20 hours, respectively.Data pooled from a number of different experiments were used to calculate kinetic constants (K(m) and V(max)) for (36)ClO(3) (-) influx into barley roots at different external ClO(3) (-)/NO(3) (-) ratios, using short (10 minutes) influx times. There appeared to be no discrimination by the root cells between ClO(3) (-) and NO(3) (-). Lineweaver-Burk analysis of the interaction between nitrate and chlorate were characteristic of competitive inhibition at low nitrate concentrations (0-0.5 mm). At higher concentrations, in the range of >1 mm, similar interactions between these ions were evident.