The effect of exogenous NH4' on NO3-uptake and in vivo NO3-reductase activity (NRA) in roots of Phascolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO3-uptake. Pretreatment with NH41 (0.15-50 milli-molar) affected neither the time pattern nor the steady state rate of N03-uptake. When NH4' was given at the start of NO3-nutrition, the time pattern of NO3-uptake was the same as in plants receiving no NH,4. After 6 hours, however, the NO3-uptake rate (NUR) and root NRA were inhibited by NHI4 to a maximum of 45% and 60%, respectively. The response of the NUR of NO3-induced plants depended on the NH4C1 concentration. Below I millimolar NH4, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH44 at 2 millimolar and above caused a rapid and transient stimulation of N03-uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO3-induced plants, root NRA responded less and more slowly to NI1-4' than did NUR. Methionine sulfoximine and azaserine, inhibitors of glutamine synthe-tase and glutamate synthase, respectively, relieved the NH.4 inhibition of the NUR of NO3-induced plants. We conclude that repression of the NUR by NH@4 depends on NH,4' assimilation. The repression by NH4' was least at the lowest and highest NHI4 levels tested (0.04 and 25 millimolar). N03-is the main source of N for crop plants, including legumes (23). At present there is much concern about the ecological and nutritional impact of NO3-fertilizers. A better understanding of the physiological processes by which the plant responds to the presence of N03 in its environment may permit a more intelligent use of NO3-. Despite its biological and agricultural significance, detailed studies on the process of N03-acquisition by plants are fairly recent and insight into the regulation of NO3-uptake is still in its infancy (12). The process of NO3-uptake by higher plants differs from that of other inorganic nutrients. For example, the fate of NO3-in the plant may be accumulation as well as conversion and both uptake and reduction have characteristics of induction. Against this background, we started an investigation into the initial events of NO3-utilization in dwarf bean, a species which is very flexible in the choice of its N source. Our previous work has identified NO3-uptake and reduction of NO3-to NO2-in the root system ' M. S. was a student from the Department of Soil Science and Plant Nutrition of the Agricultural University at Wageningen. as two key steps in NO3-utilization by N-depleted dwarf bean (3,6). Utilization of NO3-encompasses all processes that deliver exogenous N03-N to its final destination which may range from deposition in the root to biosynthesis in the shoot. Assimilation of NO3-has a number of obligatory chemical intermediates such as NO2, NH4', amides, and amino acids. Each inte...

The effect of the exogenous and endogenous N03-concentration on net uptake, influx, and efflux of N03-and on nitrate reductase activity (NRA) in roots was studied in Phaseolus vulgars L. cv. Witte Krombek.After exposure to an Beyond 100 micromoles per cubic decimeter, root NRA was not affected by exogenous NO3-indicating that N03-uptake was not coupled to root NRA, at least not at high concentrations.Nitrate utilization in higher plants is essentially the integration of three processes: acquisition, conversion, and translocation of nitrate-nitrogen. The amount of N03-taken up by the roots represents the upper limit of a plant's N03-utilization capacity. Despite its great biological and agricultural significance, the uptake of NO3 has been studied much less extensively than the uptake ofother important ions. It is noteworthy that recent reviews on the concentration kinetics of ion transport do not give data on NO3 (14,26). Although kinetic parameters of NO3 uptake have been reviewed (7,18) sufficiently rigorous analytical procedures.In this study, several techniques were combined, e.g. measurement of net uptake, influx, and efflux of N03-and the assay of nitrate reductase activity, to construct a kinetic picture of the uptake and reduction of N03 by bean roots. Additionally, a comparison of characteristics of net N03 uptake with those of unidirectional fluxes was made and the kinetic behavior of plants during environmental depletion of NO3 was studied. Short reports of our findings have appeared (4, 5). MATERIALS AND METHODSPlant Cultivation. Phaseolus vulgaris L. cv. Witte Krombek was germinated in Perlite and transferred to N-free basal medium (3), the pH of which was kept between 4.2 and 6.5 with dilute KOH or at 5.0 ± 0.1 with a pH stat. Plants were grown for 10 d in the former and 7 d in the latter treatment. After these periods, the roots were unnodulated, the primary leaves had expanded, and the contribution of trifoliate leaves to plant weight or plant NRA2 was negligible (3). No major differences in plant weight, development, rate of N03 uptake, and NRA occurred between plants grown with manual or automatic pH control. Barley (Hordeum vulgare L. cv. Herta) was grown for 10 d with manual pH control.Nitrate utilization was initiated by adding Ca(NO3)2 to plants grown in basal medium. Germination, cultivation, and experiments were performed at 20 ± 1VC, 65 ± 10%1o RH, and a 16-h photoperiod at 30 w m-2. Dwarf bean was grown for 10 d on CaCl2 solutions (0.5 mmol dm-3) to produce low-salt roots. We refer to previous papers for detailed conditions of cultivation and experimentation (2, 3).Experiments. Net uptake of N03 and H2PO4 was assayed by following nutrient depletion in the ambient solution for several hours. In experiments with various starting concentrations, N03 was regularly replenished to the initial levels in long-term experiments, while in short-term experiments the depletion in a certain interval was plotted against the average concentration in that interval. In experiments on the time ...

A test system for root regeneration was developed that consists of stem slices of apple shoots (ca. 0.5 mm thick; fresh weight ca. 1 mg). Roots regenerated synchronously without intervening callus formation and without interference with compounds originating from other parts of the plant. Supply of indolebutyric acid (IBA) or indoleacetic acid (IAA) induced maximally an average of 8 or 4.5 roots per slice, respectively. After uptake of IBA, a high degree of conjugation resulted in a recovery of 2.5% as internal free IBA (ca. 2 times the medium concentration). Due to conversion of absorbed IBA into IAA a fraction of 0.4% was recovered as (physiologically active) free IAA. After incubation on medium with IAA, 0.5% of the absorbed hormone was recovered in the free acid form. No conversion of IAA into IBA was observed. Equimolar contents of internal free IAA after incubation on IBA or IAA resulted for IBA in a higher number of roots than for IAA. This means that IBA may also act via internal free IBA or may synergistically modify the action or endogenous synthesis of IAA.

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1984, Gas and ion exchanges in wheat roots after nitrogen supply, -Physiol. Plant. 61: 357-362.Wheat {Tiilicuin aestivum L. cv, Sicco) was grown for ]0 days on CaSO^ (O.S mmol dm') and then exposed for 2 days to various nitrogenous salts in an apparatus designed to measure the exchange of O, and CO,, at constant pH and pNOj, Nitrate salts (KNO, at 0,S and Ca(NO3)2 at 0,25 and 1 mmol dnr •") caused a transient increase (40-50%) in both O, uptake and CO, release by the roots. The rate of gas exchange was nearly doubled by (NH4)2SO4 (0,25 mmol dm '), Respiration was constant in roots kept on CaSOj or given KCl, In CaSO4 the content of water-soluble sugars in roots fell by about 15% day'. The pletion of soluble sugars was higher with NO5 and NH4+ (40 and 30% day-', respectively). At most 10 to 20% of the released CO; is involved in HCO5-NO5 exchange and this fraction represents at most 10% of the total carbon imported or 30% of the net carbon gain by the roots. The contribution of the non-phosphorylating "alternative" route to total root respiration was 15% in CaSO^ and over 40% with NH4+ In NO^ the roots respired exclusively via the cytochrome route. Increased respiration at decreased efficiency in roots of NH,; plants may be due 10 an overproduction of NADH. Our data support the contention thai excess NADH as a "by-producf of the formation of carboxylates in the citrate cycle can be disposed of in an alternative respiratory pathway during NH4n utrition.