Glucose-6-phosphate dehydrogenase in barley roots: kinetic properties and localisation of the isoforms

Two different isoforms of glucose-6-phosphate dehydrogenase (Glc6PDH; EC 1.1.1.49) have been partially purified from barley (Hordeum vulgare L., cv. Alfeo) roots. The procedure included an ammonium sulfate step, Q-Sepharose and Reactive Blue agarose chromatography, and led to 60-fold and 150-fold purification for the two enzymes, respectively. The Glc6PDH 1 isoform accounts for 17% of total activity of the enzyme in roots, and is very sensitive to the effects of NADP⁺/NADPH ratio and dithiothreitol; the Glc6PDH 2 isoform is less affected by reducing power and represents 83% of the total activity. The isoforms showed distinct pH optima, isoelectric points, K _m for glucose-6-phosphate and a different electrophoretic mobility. The kinetic properties for the two enzymes were affected by ATP and metabolites. Both enzymes are inhibited to different extents by ATP when magnesium is omitted from the assay mixture, whereas the addition of ATP-Mg²⁺ had no effect on Glc6PDH activities. The Glc6PDH isoforms are usually present in the plastids and cytosol of plant cells. To verify the intracellular locations of the enzymes purified from barley roots, Glc6PDH was purified from isolated barley root plastids; this isoform showed kinetic parameters coincident with those found for Glc6PDH 1, suggesting a plastid location; the enzyme purified from the soluble fraction had kinetic parameters resembling those of Glc6PDH 2, confirming that this isoform is present in the cytosol of barley roots. Received: 21 June 2000 / Accepted: 28 July 2000     

Glucose-6P dehydrogenase in Chlorella sorokiniana (211/8k): an enzyme with unusual characteristics

In Chlorella sorokiniana (211/8k), glucose-6 phosphate dehydrogenase (G6PDH—EC 1.1.1.49) activity is similar in both N-starved cells and nitrate-grown algae when expressed on a PCV basis. A single G6PDH isoform was purified from Chlorella cells grown under different nutrient conditions; the presence of a single G6PDH was confirmed by native gels stained for enzyme activity and by Western blots. The algal G6PDH is recognised only by antibodies raised against higher plants plastidic protein, but not by chloroplastic and cytosolic isoform-specific antisera. Purified G6PDH showed kinetic parameters similar to plastidic isoforms of higher plants, suggesting a different biochemical structure which would confer peculiar regulative properties to the algal G6PDH with respect to higher plants enzymes. The most remarkable property of algal G6PDH is represented by the response to NADPH inhibition. The algal enzyme is less sensitive to NADPH effects compared to higher plants G6PDH: Ki_NADPH is 103 μM for G6PDH from nitrogen-starved C. sorokiniana, similarly to root plastidic P2-G6PDH. In nitrate-grown C. sorokiniana the Ki_NADPH decreased to 48 μM, whereas other kinetic parameters remained unchanged. These results will allow further investigations in order to rule out possible modifications of the enzyme, and/or the expression of a different G6PDH isoform during nitrate assimilation.     

Glutamate synthase activities and protein changes in relation to nitrogen nutrition in barley: the dependence on different plastidic glucose-6P dehydrogenase isoforms

In barley (Hordeum vulgare L. var. Nure), glutamate synthesis and the production of reducing power by the oxidative pentose phosphate pathway (OPPP) are strictly correlated biochemical processes. NADH-GOGAT was the major root isoform, whose activity increased on a medium supplied with NH4+ or NO3-; by contrast, no noticeable variations could be observed in the leaves of plants supplied with nitrogen. In the leaves, the major isoform is Fd-GOGAT, whose activity increased under nitrogen feeding. G6PDH activity increased in the roots supplied with nitrogen; no variations were observed in the leaves. Moreover, an increase of the P2 isoform in the roots was measured, giving 13.6% G6PDH activity localized in the plastids under ammonium, and 25.2% under nitrate feeding conditions. Western blots confirmed that P2-G6PDH protein was induced in the roots by nitrogen. P1-G6PDH protein was absent in the roots and increased in the leaves by nitrogen supply to the plants. The changes measured in cytosolic G6PDH seem correlated to more general cell growth processes, and do not appear to be directly involved in glutamate synthesis. The effects of light on Fd-GOGAT is discussed, together with the possibility for P2-G6PDH to sustain nitrogen assimilation upon illumination.     

In Vivo and in Vitro Studies of Glucose-6-Phosphate Dehydrogenase from Barley Root Plastids in Relation to Reductant Supply for NO2- Assimilation

Pyridine nucleotide pools were measured in intact plastids from roots of barley (Hordeum vulgare L.) during the onset of NO2- assimilation and compared with the in vitro effect of the NADPH/NADP ratio on the activity of plastidic glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) from N-sufficient or N-starved roots. The NADPH/NADP ratio increased from 0.9 to 2.0 when 10 mM glucose-6-phosphate was supplied to intact plastids. The subsequent addition of 1 mM NaNO2 caused a rapid decline in this ratio to 1.5. In vitro, a ratio of 1.5 inactivated barley root plastid G6PDH by approximately 50%, suggesting that G6PDH could remain active during NO2- assimilation even at the high NADPH/NADP ratios that would favor a reduction of ferredoxin, the electron donor of NO2- reductase. Root plastid G6PDH was sensitive to reductive inhibition by dithiothreitol (DTT), but even at 50 mM DTT the enzyme remained more than 35% active. In root plastids from barley starved of N for 3 d, G6PDH had a substantially reduced specific activity, had a lower Km for NADP, and was less inhibited by DTT than the enzyme from N-sufficient root plastids, indicating that there was some effect of N starvation on the G6PDH activity in barley root plastids.     

Nitrite reduction in barley-root plastids: Dependence on NADPH coupled with glucose-6-phosphate and 6-phosphogluconate dehydrogenases,and possible involvement of an electron carrier and a diaphorase

Plastids from roots of barley (Hordeum vulgare L.) seedlings were isolated by discontinuous Percoll-gradient centrifugation. Coinciding with the peak of nitrite reductase (NiR; EC 1.7.7.1, a marker enzyme for plastids) in the gradients was a peak of a glucose-6-phosphate (Glc6P) and NADP⁺-linked nitrite-reductase system. High activities of phosphohexose isomerase (EC 5.3.1.9) and phosphoglucomutase (EC 2.7.5.1) as well as glucose-6-phosphate dehydrogenase (Glc6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) were also present in the isolated plastids. Thus, the plastids contained an overall electron-transport system from NADPH coupled with Glc6PDH and 6PGDH to nitrite, from which ammonium is formed stoichiometrically. However, NADPH alone did not serve as an electron donor for nitrite reduction, although NADPH with Glc6P added was effective. Benzyl and methyl viologens were enzymatically reduced by plastid extract in the presence of Glc6P+ NADP⁺. When the plastids were incubated with dithionite, nitrite reduction took place, and ammonium was formed stoichiometrically. The results indicate that both an electron carrier and a diaphorase having ferredoxin-NADP⁺ reductase activity are involved in the electron-transport system of root plastids from NADPH, coupled with Glc6PDH and 6PGDH, to nitrite.Abbreviations Cyt cytochrome - Glc6P glucose-6-phosphate - Glc6PDH glucose-6-phosphate dehydrogenase - MVH reduced methyl viologen - NiR nitrite reductase - 6PG 6-phosphogluconate - 6PGDH 6-phosphogluconate dehydrogenase     

Genome-wide analysis of glucose-6-phosphate dehydrogenases in Arabidopsis

In green tissues of plants under illumination, photosynthesis is the primary source of reduced nicotinamide adenine dinucleotide phosphate (NADPH), which is utilized in reductive reactions such as carbon fixation and nitrogen assimilation. In non-photosynthetic tissues or under non-photosynthetic conditions, the oxidative pentose phosphate pathway contributes to basic metabolism as one of the major sources of NADPH. The first and committed reaction is catalyzed by glucose-6-phosphate dehydrogenase (G6PDH). We characterized the six members of the G6PDH gene family in Arabidopsis. Transit peptide analysis predicted two cytosolic and four plastidic isoforms. Five of the six genes encode active G6PDHs. The recombinant isoforms showed differences in substrate requirements and sensitivities to feedback inhibition. Plastidic isoforms were redox sensitive. One cytosolic isoform was insensitive to redox changes, while the other was inactivated by oxidation. The respective genes had distinct expression patterns that did not correlate with the activity of the proteins, implying a regulatory mechanism beyond the control of mRNA abundance. Two cytosolic and one plastidic isoform were detected in vivo using zymograms, and the respective genes were identified using T-DNA insertion lines. The activity of a plastidic isoform was detected in all tissues including photosynthetic tissues despite its sensitivity to reduction observed in vitro. Genomic data, gene expression, and in vivo enzyme activity data were integrated with in vitro biochemical data to propose in vivo roles for individual G6PDH isoforms in Arabidopsis.     

Effects of nitrogen forms on dry matter partitioning and nitrogen metabolism in two contrasting genotypes of Catasetum fimbriatum (Orchidaceae)

The effects of either organic (urea and glutamine) or inorganic nitrogen forms (nitrate and ammonium) on dry matter accumulation in shoots and roots and on nitrogen assimilatory enzyme activities were studied in two Catasetum fimbriatum genotypes. Both genotypes, which had inverse patterns of dry matter partitioning between shoots and roots, were aseptically incubated in gelled culture media containing 6 mol m⁻³ of nitrogen and incubated in growth chamber for 30 and 60 days. In vivo nitrate reductase, glutamine synthetase, glutamate dehydrogenase activities as well as free ammonium contents were determined in shoots and roots of plants grown in four different nitrogen sources. Nitrogen assimilatory enzyme activities showed the highest values in the genotype that accumulated dry matter predominantly in the shoots. The nitrogen sources supplied affected dry matter accumulation in shoots and roots of both C. fimbriatum genotypes; however, they were not enough to change the characteristic pattern of dry matter partitioning of each genotype. On the other hand, the differences in the root/shoot ratio found among nitrogen treatments were relatively higher in the genotype that directed dry matter mainly to roots than in the genotype that allocates biomass to shoots. Our results suggest that NADH-dependent glutamate dehydrogenase plays an important role in ammonium assimilation in C. fimbriatum plants, particularly in the root system. Nitrogen metabolism and the dry matter partitioning of the two genotypes are discussed.     

Oxalate Accumulation as Regulated by Nitrogen Forms and Its Relationship to Photosynthesis in Rice (Oryza sativa L.)

Four-leaf rice seedlings (Oryza sativa L.), which had been cultivated in Kimura B complete nutrient solution, were treated with two nitrogen forms by replacing the nitrogen element in the complete solution with sole nitrate or ammonium (2.86 mmol/L). Nitrate-N nutrition tended to increase oxalate content in all parts of the plant, including the leaves, stems, roots, and root exudates, whereas ammonium had the opposite effect. Consequently, marked differences in oxalate content were observed between the two treatments throughout the time tested (0--12 d), with maximal differences of approximately 12-fold at 6 d after treatment. Photosynthetic/respiratory parameters were examined over time simultaneously with changes in oxalate content. Net photosynthetic rate, chlorophyll fluorescence parameters (i.e. maximal photochemical efficiency (Fv/Fm) and photochemical quantum yields of photosystem (PS)Ⅱ (φ PSⅡ)), and respiratory rate were not significantly different between plants treated with the two nitrogen forms, although ammonium-fed plants had apparently higher leaf chlorophyll content than nitrate-fed plants. Leaf glucose content was altered little, but the content of fructose, sucrose, and total soluble sugar was significantly higher in the leaves of ammonium-fed plants than nitrate-fed plants, The results indicate that nitrate/ammonium may serve as efficient regulators of oxalate accumulation owing to regulation of metabolism in rice leaves rather than oxalate downward transfer and root excretion, and that photosynthetic metabolism is not directly correlated with the regulation of oxalate accumulation in rice plants.     

Regulation of aldehyde oxidase and nitrate reductase in roots of barley (Hordeum vulgare L.) by nitrogen source and salinity

The molybdenum cofactor (MoCo) is a component of aldehyde oxidase (AO EC 1.2.3.1), xanthine dehydrogenase (XDH EC 1.2.1.37) and nitrate reductase (NR, EC 1.6.6.1). The activity of AO, which catalyses the last step of the synthesis of abscisic acid (ABA), was studied in leaves and roots of barley (Hordeum vulgare L.) plants grown on nitrate or ammonia with or without salinity. The activity of AO in roots was enhanced in plants grown with ammonium while nitrate-grown plants exhibited only traces. Root AO in barley was enhanced by salinity in the presence of nitrate or ammonia in the nutrient medium while leaf AO was not significantly affected by the nitrogen source or salinity of the medium.Salinity and ammonium decreased NR activity in roots while increasing the overall MoCo content of the tissue. The highest level of AO in barley roots was observed in plants grown with ammonium and NaCl, treatments that had only a marginal effect on leaf AO. ABA concentration in leaves of plants increased with salinity and ammonium.Keywords: ABA, aldehyde oxidase, ammonium, nitrate, salinity.      

Nitrogen Assimilation in Barley (Hordeum vulgare L. cv. Mazurka) in Response to Nitrate and Ammonium Nutrition

Barley plants (Hordeum vulgare L. cv. Mazurka) were grown inaerated solution cultures with 2 mM or 8 mM inorganic nitrogensupplied as nitrate alone, ammonium alone or 1:1 nitrate+ammonium.Activities of the principal inorganic nitrogen assimilatoryenzymes and nitrogen transport were measured. Activities ofnitrate and nitrite reductases, glutamine synthetase and glutamatesynthase were greater in leaves than in roots but glutamatedehydrogenase was most active in roots. Only nitrate and nitritereductases changed notably (4–10 times) in response tothe different nitrogen treatments. Nitrate reductase appearedto be rate-limiting for nitrate assimilation to glutamate inroots and also in leaves, where its total in vitro activitywas closely related to nitrate flux in the xylem sap and wasslightly in excess of that needed to reduce the transportednitrate. Xylem nitrate concentration was 13 times greater thanthat in the nutrient solution. Ammonium nitrogen was assimilatedalmost completely in the roots and the small amount releasedinto the xylem sap was similar for the nitrate and the ammoniumtreatments. The presence of ammonium in the nutrient decreasedboth export of nitrate to the xylem and its accumulation inleaves and roots. Nitrate was stored in stem bases and was releasedto the xylem and thence to the leaves during nitrogen starvation.In these experiments, ammonium was assimilated principally inthe roots and nitrate in the leaves. Any advantage of this divisionof function may depend partly on total conversion of inorganicnitrogen to amino acids when nitrate and ammonium are givenin optimal concentrations. Hordeum vulgare L., barley, nitrate, ammonium, nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase, nitrogen transport     

Nitrogen assimilation and cysteine biosynthesis in barley: Evidence for root sulphur assimilation upon recovery from N deprivation

The mixed effects of nitrogen nutrition and sulphate assimilation were investigated in barley plants (Hordeum vulgare var. Alfeo) that were subjected to long-term sulphur and/or nitrogen starvation, by measuring the O-acetylserine(thio)lyase (OASTL-EC 4.2.99.8) activity, changes in -SH compounds and amino acid levels.The growth of barley plants cultured in the hydroponic vessels was severely affected by altered nutrient levels. The barley plants grown in medium deprived of nitrogen and/or sulphur sources for 21 days showed increase in both root length and weight. In contrast, the shoot growth was reduced in nitrogen-starved plants and was unaffected by sulphur deprivation. Sulphur starvation affected the level of proteins in barley plants more than nitrogen deprivation. The decline in the protein levels observed under sulphur-deficient conditions was coupled with the accumulation of glutamine, asparagine and serine, mainly in the roots; additionally, a nitrogen deficiency in the roots promoted a decrease in both glutathione and cysteine levels.The simultaneous deprivation of nitrogen and sulphur in plants leads to an alteration in their metabolism; high levels of glutathione (GSH) in the shoots could signify the induction of a mechanism intended for coping with stressful conditions.Sulphate deprivation enhanced OASTL activity, mainly in the roots; on the other hand, OASTL increases observed under S deprivation were clearly dependent on the nitrogen availability in the culture medium. In fact, the nitrate supply to the N and S starved plants that showed OASTL activity very low, rapidly recovered the OASTL activities to the levels typical of control plants. Nevertheless, the ammonium supply had negligible effects on the OASTL activity only observed after three days in the roots.Our results support the hypothesis that in barley plants, a portion of S assimilation (up to cysteine biosynthesis) occurs in the roots, and a reciprocal influence of nitrogen assimilation on cysteine synthesis occurs.     

Early responses to cadmium exposure in barley plants: effects on biometric and physiological parameters

Cadmium represents one of the most toxic pollutants in plant ecosystems: at high concentrations it can cause severe effects, such as plant growth inhibition, decrease in photosynthesis and changes in plant basal metabolism. Changes in pigments’ content, RubisCO large subunit, and D1 protein indicated a severe reduction in photosynthetic efficiency. Furthermore, the decrease of nitrate reductase activity and changes in free amino acids levels show a general stress condition of nitrogen assimilation. Cadmium increased the activities of ROS scavenging enzymes; among these, ascorbate peroxidase rate was the most noticeably increased. It is worth noting that glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.64), showed changes in both activities and occurrence during cadmium stress. Interestingly, our data suggest that G6PDH would modulate redox homeostasis under metal exposure, and possibly satisfy the increased request of reductants to counteract the oxidative burst induced by cadmium. Therefore, the results suggest that APX and G6PDH may play a pivotal role to counteract the oxidative stress induced by cadmium in young barley plants.     

Oxalate Accumulation as Regulated by Nitrogen Forms and Its Relationship to Photosynthesis in Rice （Oryza sativa L.）

Four-leaf rice seedlings (Oryza sativa L.), which had been cultivated in Kimura B complete nutrient solution, were treated with two nitrogen forms by replacing the nitrogen element in the complete solution with sole nitrate or ammonium (2.86 mmol/L). Nitrate-N nutrition tended to increase oxalate content in all parts of the plant, including the leaves, stems, roots, and root exudates, whereas ammonium had the opposite effect. Consequently, marked differences in oxalate content were observed between the two treatments throughout the time tested (0-12 d), with maximal differences of approximately 12-fold at 6d after treatment. Photosynthetic/respiratory parameters were examined over time simultaneously with changes in oxalate content. Net photosynthetic rate, chlorophyll fluorescence parameters (i.e. maximal photochemical efficiency (Fv/Fm) and photochemical quantum yields of photosystem (PS)II (ΦPSⅡ)), and respiratory rate were not significantly different between plants treated with the two nitrogen forms, although ammonium-fed plants had apparently higher leaf chlorophyll content than nitrate-fed plants. Leaf glucose content was altered little, but the content of fructose, sucrose, and total soluble sugar was significantly higher in the leaves of ammonium-fed plants than nitrate-fed plants. The results indicate that nitrate/ammonium may serve as efficient regulators of oxalate accumulation owing to regulation of metabolism in rice leaves rather than oxalate downward transfer and root excretion, and that photosynthetic metabolism is not directly correlated with the regulation of oxalate accumulation in rice plants.     

Kinetic properties and ammonium-dependent regulation of cytosolic isoenzymes of glutamine synthetase in Arabidopsis

Glutamine synthetase (GS; EC 6.3.1.2) is a key enzyme of nitrogen assimilation, catalyzing the synthesis of glutamine from ammonium and glutamate. In Arabidopsis, cytosolic GS (GS1) was accumulated in roots when plants were excessively supplied with ammonium; however, the GS activity was controlled at a constant level. The discrepancy between the protein content and enzyme activity of GS1 was attributable to the kinetic properties and expression of four distinct isoenzymes encoded by GLN1;1, GLN1;2, GLN1;3 and GLN1;4, genes that function complementary to each other in Arabidopsis roots. GLN1;2 was the only isoenzyme significantly up-regulated by ammonium, which correlated with the rapid increase in total GS1 protein. GLN1;2 was localized in the vasculature and exhibited low affinities to ammonium (Km = 2450 +/- 150 microm) and glutamate (Km = 3.8 +/- 0.2 mm). The expression of the counterpart vascular tissue-localizing low affinity isoenzyme, GLN1;3, was not stimulated by ammonium; however, the enzyme activity of GLN1;3 was significantly inhibited by a high concentration of glutamate. By contrast, the high affinity isoenzyme, GLN1;1 (Km for ammonium < 10 microm; Km for glutamate = 1.1 +/- 0.4 mm) was abundantly accumulated in the surface layers of roots during nitrogen limitation and was down-regulated by ammonium excess. GLN1;4 was another high affinity-type GS1 expressed in nitrogen-starved plants but was 10-fold less abundant than GLN1;1. These results suggested that dynamic regulations of high and low affinity GS1 isoenzymes at the levels of mRNA and enzyme activities are dependent on nitrogen availabilities and may contribute to the homeostatic control of glutamine synthesis in Arabidopsis roots.     

Glutamine synthetase regulation by energy charge in sunflower roots

Energy charge [(ATP) + ½ (ADP)]/[(ATP) + (ADP) + (AMP)] and glutamine synthetase activity (transferase reaction) of roots increase in a near congruent manner when decotyledonized sunflower plants (Helianthus annuus L. var. Mammoth Russian) are grown in nitrate for 9 days. Replacement of nitrate with ammonium for the final 2 days leads to a higher energy charge and increased enzyme activity. Similar correlations occur when nitrate plants are placed on a zero nitrogen regimen and when they are subjected to continuous darkness. A rank order correlation of 0.72 is obtained for all data. Control concepts such as adenylylation-deadenylylation and ammonium inhibition of enzyme synthesis are not supported by the data. Energy charge-enzyme activity plots support the view that glutamine synthetase of sunflower roots is subject to control by end products of glutamine metabolism. Alanine appears to exert a modulating effect on the regulation of glutamine synthetase by energy charge.     

Carbon,Nitrogen, and Nutrient Interactions in Beta vulgaris L. as Influenced by Nitrogen Source,NO3- versus NH4+

Sugar beets (Beta vulgaris L. cv F58-554H1) were grown hydroponically in a 16-h light, 8-h dark period (photosynthetic photon flux density of 0.5 mmol m-2 s-1) for 4 weeks from sowing in half-strength Hoagland nutrient solution containing 7.5 mM nitrate. Half of the plants were then transferred to 7.5 mM ammonium N; the rest remained in solution with 7.5 mM nitrate N. Upon transfer from nitrate to ammonium, the total N concentration decreased sharply in the fibrous roots and petiole/midribs and increased substantially in the leaf blades. This was because of the decreased nitrate concentrations in fibrous roots and petioles and a concomitant increase in amino acid/amide-N and protein N in leaf blades. Sugar beets acclimated to ammonium partly by a 2.5-fold increase in glutamine synthase activity in fibrous roots and a 1.7-fold increase in leaf blades. Rapid ammonium assimilation into glutamine consumed carbon skeletons, leading to a depletion of foliar starch, sucrose, and maltose. Ammonium treatment stimulated activities of some glycolytic/Krebs cycle enzymes, e.g. pyruvate dehydrogenase. Nitrate-fed leaf blades contained substantially larger concentrations of osmolytes (i.e. nitrate, cations, and sucrose), which may have contributed to the faster rates of leaf expansion in nitrate-fed compared to ammonium-fed plants.     

Pea responses to saline stress is affected by the source of nitrogen nutrition (ammonium or nitrate)

The effect of the source of nitrogen nutrition (ammonium or nitrate), onthe response of pea plants to a moderate saline stress (30 mMNaCl)was studied. Growth declined under saline stress but nitrate-fed plants wereless sensitive to salinity than ammonium-fed plants. This different sensitivitywas due mainly to a better maintenance of root growth in nitrate-fed plants.Organic nitrogen content decreased significantly in roots of ammonium-fedplants. Water relations changed slightly under saline stress leading to adecrease in stomatal conductance, which was correlated to a decline in carbonassimilation rates regardless of nitrogen source. Salinity affects the uptakeofseveral nutrients in a different way, depending on the nitrogen source. Thus,chloride was accumulated mainly in nitrate-fed plants, displacing nitrate,whereas sodium was accumulated mainly in ammonium-fed plants, especially inroots, displacing other cations such as ammonium and potassium. It is concludedthat the nitrogen source (ammonium or nitrate) is a major factor affecting pearesponses to saline stress, plants being more sensitive when ammonium is thesource used. The different sensitivity is discussed in terms of a competitionfor energy between nitrogen assimilation and sodium exclusion processes.     

Change in uptake, transport and accumulation of ions in Nerium oleander (rosebay) as affected by different nitrogen sources and salinity

Background and Aims

The source of nitrogen plays an important role in salt tolerance of plants. In this study, the effects of NaCl on net uptake, accumulation and transport of ions were investigated in Nerium oleander with ammonium or nitrate as the nitrogen source in order to analyse differences in uptake and cycling of ions within plants.

Methods

Plants were grown in a greenhouse in hydroponics under different salt treatments (control vs. 100 mm NaCl) with ammonium or nitrate as the nitrogen source, and changes in ion concentration in plants, xylem sap exuded from roots and stems, and phloem sap were determined.

Key Results

Plant weight, leaf area and photosynthetic rate showed a higher salt tolerance of nitrate-fed plants compared with that of ammonium-fed plants. The total amount of Na⁺ transported in the xylem in roots, accumulated in the shoot and retranslocated in the phloem of ammonium-fed plants under salt treatment was 1·8, 1·9 and 2·7 times more, respectively, than that of nitrate-treated plants. However, the amount of Na⁺ accumulated in roots in nitrate-fed plants was about 1·5 times higher than that in ammonium-fed plants. Similarly, Cl⁻ transport via the xylem to the shoot and its retranslocation via the phloem (Cl⁻ cycling) were far greater with ammonium treatment than with nitrate treatment under conditions of salinity. The uptake and accumulation of K⁺ in shoots decreased more due to salinity in ammonium-fed plants compared with nitrate-fed plants. In contrast, K⁺ cycling in shoots increased due to salinity, with higher rates in the ammonium-treated plants.

Conclusions

The faster growth of nitrate-fed plants under conditions of salinity was associated with a lower transport and accumulation of Na⁺ and Cl⁻ in the shoot, whereas in ammonium-fed plants accumulation and cycling of Na⁺ and Cl⁻ in shoots probably caused harmful effects and reduced growth of plants.Key words: Mineral cycling, Nerium oleander, nitrogen source, salinity, xylem and phloem transport