Source and Magnitude of Ammonium Generation in Maize Roots

Abstract

Short-term studies with 13 NH 4 ϩ have provided estimates of NH 4 ϩ influx, efflux, and cytoplasmic concentration in spruce (Kronzuker et al., 1995a(Kronzuker et al., , 1995b. As the NH 4 ϩ concentration in the external solution increased from 10 to 1500 m, cytosolic NH 4 ϩ levels increased from 2 to 33 mm and efflux increased from 10% to 35% of influx. Similar rates were reported for rice: as external NH 4 ϩ levels increased from 2 to 1000 m, cytosolic NH 4 ϩ levels increased from 3 to 38 mm and efflux rose from 11% to 29% of influx (Wang et al., 1993 ϩ generation in maize roots was estimated to be 50% faster than concurrent NH 4 ϩ uptake (Jackson, et al., 1993). The potential for substantial generation, recycling, and efflux of endogenous NH 4 ϩ in roots is thus indicated.A crucial question raised by these observations is whether protein turnover is the source of endogenous NH 4 ϩ generation, or if recycling of intermediates of the NH 4 ϩ assimilation pathway, such as Gln, is the source. To address this question, maize (Zea mays) seedlings that had been grown on 14 NH 4 ϩ were exposed to highly labeled 15 NH 4 ϩ for 3 d. It was hypothesized that if protein turnover is the source of NH 4 ϩ and if part of this NH 4 ϩ is subject to efflux and translocation to the shoot, a decline in endogenous 14 N protein in the root should occur as new protein is synthesized from the entering 15 NH 4 ϩ . The fact that 15 NH 4 ϩ was applied during six diurnal periods also permitted us to (a) directly measure of the diurnal pattern of NH 4 ϩ fluxes into and out of roots, (b) compare 15 NH 4 ϩ uptake and assimilation by roots during successive light and dark periods, and (c) determine the relationship of the latter processes to carbohydrate levels in shoots and roots. MATERIALS AND METHODS Plant CultureMaize (Zea mays L. cv Pioneer 3320) caryopses were germinated at 30°C in contact with 0.1 mm CaSO 4 . After 30 h, uniform seedlings were selected and their seminal roots excised. Cultures of eight seedlings each were transplanted into 160 L of basal nutrient solution, pH 6.0, containing 0.125 mm (NH 4 ) 2 SO 4 , 1.25 mm K 2 SO 4 , 0.25 mm Ca(H 2 PO 4 ) 2 , 1 mm CaSO 4 , 46 m B, 9 m Mn, 0.8 m Zn, 0.3 m Cu, 0.1 m Mo, and 54 m Fe as ferric diethylenetriamine pentaacetate. The solution was aerated with compressed air that had been washed with H 2 SO 4 and water to remove ambient NH 4 ϩ . A combination of sodium vapor and metal halide lamps provided 1140 E m Ϫ2 s Ϫ1 illumination at canopy height during a 14-h photoperiod (7 am to 9 pm). The average air temperature during the experiment was 23.4°C Ϯ 1.6°C. Both pH and [NH 4 ϩ ] of the nutrient

Specification of Cortical Parenchyma and Stele of Maize Primary Roots by Asymmetric Levels of Auxin, Cytokinin, and Cytokinin-Regulated Proteins

“…Asn acts as a nitrogen storage compound synthesized under high concentrations of ammonium and thus acts in ammonium detoxification (Givan, 1979;Sieciechowicz et al, 1988). Higher levels of ammonium cannot only be due to uptake from the environment but also result from the catabolism of soluble organic nitrogen in maize roots (Feng et al, 1998). This emphasizes a role of Asn synthetase in ammonium assimilation in the cortex.…”

Acute Biochemical Responses of a Submersed Macrophyte,Potamogeton crispusL., to High Ammonium in an Aquarium Experiment

“…Similarly, high ammonium in the culture media caused decreased sugar in the terrestrial Phaseolus vulgaris (Mehrer et al 1989) and wheat (Cramer et al 1993) and increased amino acids in the aquatic macrophyte Stratiotes aloides (Smolders et al 1996). It was assumed that ammonium assimilation may result in depletion of soluble sugar (Feng et al 1998) and thus may retard plant growth (Cramer et al 1993). In our study, there was enhanced amino acid synthesis at higher ammonium levels, with 5 mg/L external N&-N possibly being a threshold of assimilation for f!…”

“…crispus reminded us to be more prudential before postulating that the depletion of soluble sugar was most the result of ammonium assimilation (Cramer et al 1993). Based on the assumption that glutamine is the main product of ammonium assimilation and that synthesis of 1 mole glutamine costs 0.505 mole glucose (Schubert 1980, Feng et al 1998, in our study the loss of soluble sugar at 20 mg/L N&-N compared with the control should be equivalent approximately to a synthesis of 15 mg glutamine, which is much greater than the maximum free amino acid content of 2 mg found in P: crispus. Therefore, processes other than amino acid synthesis were operating to reduce sugar in our study.…”

Cytosolic Concentrations and Transmembrane Fluxes of NH4 +/NH3. An Evaluation of Recent Proposals

“…Physiological studies have demonstrated that NH 4 ϩ fluxes commonly exceed those of NO 3 Ϫ from equimolar solutions, and when both ions are present, NO 3 Ϫ influx is reduced within minutes of exposure to NH 4 ϩ (Lee and Drew, 1989;Gazzarini et al, 1999;Kronzucker et al, 1999). As a byproduct of photorespiration and the shikimic acid pathway, as well as through the turnover of various nitrogen pools, large quantities of NH 4 ϩ are also generated internally (Joy, 1988;Feng et al, 1998). Although much of this NH 4 ϩ is re-assimilated through the activity of Gln synthetase (GS), many plants show high values of NH 4 ϩ efflux, and some develop symptoms of toxicity when exposed to millimolar concentrations of NH 4 ϩ in nutrient media, particularly when NH 4 ϩ is the sole source of nitrogen.…”