Effect of Exogenous and Endogenous Nitrate Concentration on Nitrate Utilization by Dwarf Bean

Abstract

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 ...

The Uptake of NO3−, NO2−, and NH4+ by Intact Wheat (Triticum aestivum) Seedlings

“…This is in general agreement with a K,, of 0.023 mm for maize (29), 0.033 mm for perennial rye-grass (19), 0.040 mm for mechanism I in Arabidopsis (10), and 0.001 to 0.025 mm for barley and 0.005 to 0.015 mm for beans (5). However, K,n values higher than found in this study have also been reported; 0.1 mm for mechanism I in barley (23), 0.25 mm for barley (7), and 7.5 mm for excised corn roots (22).…”

“…The reported induction periods required to achieve a steady state uptake rate differ: 1 to 2 h, corn roots (22); 3 to 4 h, barley seedlings (23); 4 h, wheat seedlings (16); 6 to 7 h, dwarf bean seedlings (5). Much less work has been done on the induction of a NO; transporter; its apparent induction by NO;2, requiring about 4 h, has been reported (17).…”

“…For example, NO3 uptake has been described as resembling a single (19,29), dual (10,23), single multiphasic (5) isotherm or the additive result of one saturable plus one nonsaturable, linear component (14). Similarly, NH4+ transport was mediated by a single (2,19,28,30), two different (3), and a single multiphasic transport system (18).…”

Comparative Kinetics and Reciprocal Inhibition of Nitrate and Nitrite Uptake in Roots of Uninduced and Induced Barley (Hordeum vulgare L.) Seedlings

“…To our knowledge, however, a constitutive NO2-transporter has not been reported. The presence of multiple transporters for NO3-is supported by reports of two kinetic systems in the range of 0 to 0.2 mm substrate in induced seedlings (7,31). Conversely, only a single system for NO2-transport has been reported (1,14).…”

Kinetics of nitrate and ammonium absorption and accompanying H+fluxes in roots ofLolium perenneL. and N2‐fixingTrifolium repensL.

“…Deviation between net uptake and influx would only be of signifieanee under eonditions with high N efflux. A sigtiifieant efflux has been shown under high N eonditions (Morgan, Volk & Jaekson 1973;Breteler & Nissen 1982) or by maintaining low N eoneentrations in the external solution when using the relative N addition approach (Larsson et al 1992). The good agreement between uptake kinetie paratneters, estimated on the basis of initial solution NOj^ and NH4^ depletion rates or ' "^N aecumulation over 2 h, suggests that efflux only constituted a small proportion of N absorption in the ptesent study, whieh is in agreement with Maeduff & Jackson (1992).…”