Interactive effect of salt (NaCl) and nitrogen form on growth, water relations and photosynthetic capacity of sunflower (Helianthus annum L.)

The effects of the ammonium (NH₄⁺) and nitrate (NO₃^-) forms of nitrogen and NaCl on the growth, water relations and photosynthesis performance of sunflower (Helianthus annuus L.) were examined under glasshouse conditions. Eight-day-old plants of cv. Hisun 33 were subjected for 21 days to Hoagland's nutrient solution containing 8 mol m^-3N as NH₄⁺or NO₃^-, and salinised with 0, 60, or 120 mol m^-3NaCl. Fresh weights of shoots and roots, and leaf area of NO₃^-supplied non-salinised plants were significantly greater than those of NH₄⁺-supplied non-salinised plants. But addition of NaCl to the rooting medium of these plants had more inhibitory effect on the growth of NO₃^--supplied plants than on NH₄⁺-supplied plants. Both leaf water and osmotic potentials of plants grown with NH₄⁺were lower than those of plants given NO₃^-under both non-saline and saline conditions. Chlorophylls a and b concentrations were higher in plants grown with NH₄⁺than N0₃^--supplied plants at the lower two levels of salinisation. The rate of photosynthesis in plants was considerably higher in non-salinised plants grown with NO₃^-than with NH₄⁺, but with increase in salinisation the photosynthesis rate decreased in NO₃^--supplied plants, but not in those given NH₄⁺. The rate of transpiration was increased significantly by salinisation in NO₃^--supplied plants, but not consistently so in NH₄⁺-supplied plants. The stomatal conductances were much higher in plants given NO₃^-than with NH₄⁺when grown under non-saline conditions, but not when salinised. As a consequence, water-use efficiency in NO₃^--supplied control plants was better than in NH₄⁺-supplied under non-saline conditions, but worse under saline conditions. The different forms of nitrogen and the addition of NaCl to the growing medium did not affect the relative intercellular concentrations of CO₂ (C_i/C_a). Overall, the NH₄⁺form of nitrogen inhibited the growth of sunflowers under non-saline conditions, but NO₃^-and NaCl interacted to inhibit growth more than did NH₄⁺under saline conditions.     

Nitrate reductase activity of two leafy vegetables as affected by nickel and different nitrogen forms

The author studied the effect of different nickel concentrations (0, 0.4, 40 and 80 μM Ni) on the nitrate reductase (NR) activity of New Zealand spinach (Tetragonia expansa Murr.) and lettuce (Lactuca sativa L. cv. Justyna) plants supplied with different nitrogen forms (NO₃ ⁻–N, NH₄ ⁺–N, NH₄NO₃). A low concentration of Ni (0.4 μM) did not cause statistically significant changes of the nitrate reductase activity in lettuce plants supplied with nitrate nitrogen (NO₃ ⁻–N) or mixed (NH₄NO₃) nitrogen form, but in New Zealand spinach leaves the enzyme activity decreased and increased, respectively. The introduction of 0.4 μM Ni in the medium containing ammonium ions as a sole source of nitrogen resulted in significantly increased NR activity in lettuce roots, and did not cause statistically significant changes of the enzyme activity in New Zealand spinach plants. At a high nickel level (Ni 40 or 80 μM), a significant decrease in the NR activity was observed in New Zealand spinach plants treated with nitrate or mixed nitrogen form, but it was much more marked in leaves than in roots. An exception was lack of significant changes of the enzyme activity in spinach leaves when plants were treated with 40 μM Ni and supplied with mixed nitrogen form, which resulted in the stronger reduction of the enzyme activity in roots than in leaves. The statistically significant drop in the NR activity was recorded in the aboveground parts of nickel-stressed lettuce plants supplied with NO₃ ⁻–N or NH₄NO₃. At the same time, there were no statistically significant changes recorded in lettuce roots, except for the drop of the enzyme activity in the roots of NO₃ ⁻-fed plants grown in the nutrient solution containing 80 μM Ni. An addition of high nickel doses to the nutrient solution contained ammonium nitrogen (NH₄ ⁺–N) did not affect the NR activity in New Zealand spinach plants and caused a high increase of this enzyme in lettuce organs, especially in roots. It should be stressed that, independently of nickel dose in New Zealand spinach plants supplied with ammonium form, NR activity in roots was dramatically higher than that in leaves. Moreover, in New Zealand spinach plants treated with NH₄ ⁺–N the enzyme activity in roots was even higher than in those supplied with NO₃ ⁻–N.     

The nitrogen balance of Raphanus sativus x raphanistrum plants. II. Growth, nitrogen redistribution and photosynthesis under NO3− deprivation

Abstract. Wild radish plants deprived of, and continuously supplied with solution NO^?₃ for 7 d following 3 weeks growth at high NO^?₃ supply were compared in terms of changes in dry weight, leaf area, photosynthesis and the partitioning of carbon and nitrogen (NH₂-N and NO^?₃-N) among individual organs. Initial levels of NO^?₃-N accounted for 25% of total plant N. Following termination of NO^?₃ supply, whole plant dry weight growth was not significantly reduced for 3 d, during which time plant NH₂-N concentration declined by about 25% relative to NO^?₃-supplied plants, and endogenous NO^?₃-N content was reduced to nearly zero. Older leaves lost NO^?₃ and NH₂-N, and roots and young leaves gained NH₂-N in response to N stress. Relative growth rate declined due both to decreased net assimilation rate and a decrease in leaf area ratio. A rapid increase in specific leaf weight was indicative of a greater sensitivity to N stress of leaf expansion compared to carbon gain. In response to N stress, photosynthesis per unit leaf area was more severely inhibited in older leaves, whereas weight-based rates were equally inhibited among all leaf ages. Net photosynthesis was strongly correlated with leaf NH₂-N concentration, and the relationship was not significantly different for leaves of NO₃^?-supplied compared to NO^?₃-deprived plants. Simulations of the time course of NO^?₃ depletion for plants of various NH₂-N and NO^?₃ compositions and relative growth rates indicated that environmental conditions may influence the importance of NO^?₃ accumulation as a buffer against fluctuations in the N supply to demand ratio.     

Influence of inorganic nitrogen sources on K+/Na+ homeostasis and salt tolerance in sorghum plants

This work aimed to study the regulation of K⁺/Na⁺ homeostasis and the physiological responses of salt-treated sorghum plants [Sorghum bicolor (L.) Moench] grown with different inorganic nitrogen (N) sources. Four days after sowing (DAS), the plants were transferred to complete nutrient solutions containing 0.75 mM K⁺ and 5 mM N, supplied as either NO₃ ^? or NH₄ ⁺. Twelve DAS, the plants were subjected to salt stress with 75 mM NaCl, which was applied in two doses of 37.5 mM. The plants were harvested on the third and seventh days after the exposure to NaCl. Under the salt stress conditions, the reduction of K⁺ concentrations in the shoot and roots was higher in the culture with NO₃ ^? than with NH₄ ⁺. However, the more conspicuous effect of N was on the Na⁺ accumulation, which was severely limited in the presence of NH₄ ⁺. This ionic regulation had a positive influence on the K⁺/Na⁺ ratio and the selective absorption and transport of K⁺ in the plants grown with NH₄ ⁺. Under control and salt stress conditions, higher accumulation of free amino acids and soluble proteins was promoted in NH₄ ⁺ grown roots than NO₃ ^? grown roots at both harvesting time, whereas higher accumulation of soluble sugars was observed only at 7 days of salt stress exposure. Unlike the NH₄ ⁺ grown plants, the gas exchanges of the NO₃ ^? grown plants were reduced after 7 days of salt stress. These results suggest that external NH₄ ⁺ may limit Na⁺ accumulation in sorghum, which could contribute to improving its physiological and metabolic responses to salt stress.     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

Growth,chemical composition,and nitrate reductase activity of Rumex species in relation to form and level of N supply

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH₄ ⁺, NO₃ ^-, or both) and level of N supply (4 mM N, and zero-N following a period of 4mM N). A distinct preference for either NH₄ ⁺ or NO₃ ^- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO₃ ^- and free NH₄ ⁺. Especially the accumulation of free NH₄ ⁺ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH₄ ⁺ and NO₃ ^-. Compared to mixed N supply, RGR of NO₃ ^-- and NH₄ ⁺-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH₄ ⁺ supply probably resulted from direct or indirect toxicity effects associated with high NH₄ ⁺ and/or low Ca²⁺ contents of tissues. NRA in NO₃ ^- and NH₄NO₃ plants was very similar with maxima in the leaves of ca 40 μmol NO₂ ^- g^-1 DW h^-1. ‘Basal’ NRA levels in shoot tissues of NH₄ ⁺ plants appeared relatively high with maxima in the leaves of ca 20 μmol NO₂ ^- g^-1 DW h^-1. Carboxylate to organic N ratios, (C-A)/N_org, on a whole plant basis varied from 0.2 in NH₄ ⁺ plants to 0.9 in NO₃ ^- plants. After withdrawal of N, all accumulated NO₃ ^- and NH₄ ⁺ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50–80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH₄ ⁺ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N_org ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N_org ratio and NRA of plants in their natural habitats are briefly discussed.     

叶施NO对NaCl胁迫下红砂幼苗叶片和根系中氮代谢酶及营养物质的影响

以当年生红砂(Reaumuria soongorica)幼苗为材料,采用盆栽实验,考察叶面喷施不同浓度(0、0.01、0.10、0.25、0.50、1.00 mmol·L^-1)NO供体硝普钠 (SNP) 对NaCl(300 mmol·L^-1)胁迫下红砂根、叶中可溶性蛋白、游离氨基酸和硝态氮含量,以及谷氨酰胺合成酶(GS)、谷氨酸合酶(GOGAT)、硝酸还原酶(NR)活性的影响,并采用主成分分析和隶属函数法筛选NO对NaCl胁迫缓解效应的氮代谢指标和最佳NO浓度,以探讨外源NO对NaCl 胁迫下红砂缓解效应的氮代谢响应机制。结果表明：(1)在300 mmol·L^-1 NaCl胁迫处理下,红砂幼苗根、叶中可溶性蛋白、硝态氮含量以及GS、GOGAT、NR活性均比对照显著下降。(2)外源NO能显著提高盐胁迫下红砂叶、根中GS、GOGAT、NR活性和硝态氮含量,增加根中可溶性蛋白和游离氨基酸含量。(3)NR和GOGAT活性可用于评价NO对NaCl胁迫下红砂幼苗的缓解作用,外源NO(SNP)对红砂幼苗在NaCl胁迫下的缓解效果强弱表现为0.25 mmol·L^-1> 0.50 mmol·L^-1> 0.10 mmol·L^-1> 1.00 mmol·L^-1> 0.01 mmol·L^-1。研究发现,300 mmol·L^-1 NaCl胁迫显著抑制了红砂幼苗氮代谢,外源NO(SNP)有助于提高盐胁迫下红砂NR活性,加快硝态氮转化为铵态氮,促进红砂叶片和根中GS/GOGAT对转化物的同化,从而增强红砂幼苗的耐盐性,并以0.25 mmol·L^-1SNP处理时缓解作用最佳;NR和GOGAT活性可作为NO缓解盐胁迫的评价指标。     

Nitrogen source and water regime effects on durum wheat photosynthesis and stable carbon and nitrogen isotope composition

Water stress and nitrogen (N) availability are the main constraints limiting yield in durum wheat (Triticum turgidum L. var. durum). This work investigates the combined effects of N source (ammonium–NH₄⁺, nitrate–NO₃^– or a mixture of both–NH₄⁺:NO₃^–) and water availability (well‐watered vs. moderate water stress) on photosynthesis and water‐use efficiency in durum wheat (cv. Korifla) flag leaves grown under controlled conditions, using gas exchange, chlorophyll fluorescence and stable carbon isotope composition (δ¹³C). Under well‐watered conditions, NH₄⁺‐grown plants had lower net assimilation rates (A) than those grown with the other two N forms. This effect was mainly due to lower stomatal conductance (g_s). Under moderate water stress, differences among N forms were not significant, because water regime (WR) had a stronger effect on g_s and A than did N source. Consistent with lower g_s, δ¹³C and transpiration efficiency (TE) were the highest in NH₄⁺ leaves in both water treatments. These results indicate higher water‐use efficiency in plants fertilized with NH₄⁺ due to stomatal limitation on photosynthesis. Moreover, leaf δ¹³C is an adequate trait to assess differences in photosynthetic activity and water‐use efficiency caused by different N sources. Further, the effect of these growing conditions on the nitrogen isotope composition (δ¹⁵N) of flag leaves and roots was examined. Water stress increased leaf δ¹⁵N in all N forms. In addition, leaf δ¹⁵N increased as root N decreased and as leaf δ¹³C became less negative. Regardless of WR, the leaf δ¹⁵N of NO₃^–‐grown plants was lowest. Based on stepwise and canonical discriminant analyses, we conclude that plant δ¹⁵N together with δ¹³C and other variables may reflect the conditions of N nutrition and water availability where the plants were grown. Thus well‐watered plants grown with NH₄⁺:NO₃^– resembled those grown with NO3^–, whereas under water stress they were closer to plants grown with NH₄⁺.     

Changing nitrogen source on the yield and nitrogen content of soybeans

Summary Soybean plants were grown in nutrient culture solutions containing 150 ppm of N either as an equal concentration of NH₄ ⁺ or NO₃ ^–, or all NO₃ ^–. At the R2 stage of growth for some plants, the N form was changed to either all NO₃ ^– or all NH₄ ⁺, but at the same total N concentration as before. Highest seed yield was obtained with all NO₃ ^– over the entire growth period, the poorest when the N form was switched from an equal ratio of NH₄ ⁺ and NO₃ ^– to all NH₄ ⁺ at the R2 stage. Kjeldahl N concentrations in the plant leaves and seed were highest when NH₄ ⁺ was part or all of the N source in the nutrient solution. These results may partially explain why the literature is inconsistent on the effect of added fertilizer N on soybean seed yield, and may pose a problem in using leaf Kjeldahl N concentration to determine plant N sufficiency.     

Characterization of ammonium and nitrate uptake and assimilation in roots of tea plants

It has been pointed out that tea (Camellia sinensis (L.) O. Kuntze) prefers ammonium (NH ₄ ⁺ ) over nitrate (NO ₃ ^? ) as an inorganic nitrogen (N) source. ¹⁵N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH ₄ ⁺ and NO ₃ ^? by tea roots. The total ¹⁵N was detected, and kinetic parameters were calculated after feeding ¹⁵NH ₄ ⁺ or ¹⁵NO ₃ ^? to tea plants. The process of N assimilation was studied by monitoring the dynamic ¹⁵N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with ¹⁵NH ₄ ⁺ absorbed significantly more ¹⁵N than those supplied with ¹⁵NO ₃ ^? . The kinetics of ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^? influx into tea plants followed a classic biphasic pattern, demonstrating the action of a high affinity transport system (HATS) and a low affinity transport system (LATS). The V _max value for NH ₄ ⁺ uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO ₃ ^? (39.3 nmol/(g dry wt min)). K_M estimates were approximately 0.06 mM for NH ₄ ⁺ and 0.16 mM for NO ₃ ^? , indicating a higher rate of NH ₄ ⁺ absorption by tea plant roots. Tea plants fed with ¹⁵NH ₄ ⁺ accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with ¹⁵NO ₃ ^? . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with ¹⁵N under both conditions. The rate of N assimilation into Thea in the roots of NO ₃ ^? -supplied tea plants was quicker than in NH ₄ ⁺ -supplied tea plants. NO ₃ ^? uptake by roots, rather than reduction or transport within the plant, seems to be the main factor limiting the growth of tea plants supplied with NO ₃ ^? as the sole N source. The NH ₄ ⁺ absorbed by tea plants directly, as well as that produced by NO ₃ ^? reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The ¹⁵N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH ₄ ⁺ .     

Impact of nitrogen form on iron uptake and distribution in maize seedlings in solution culture

Comparative studies on the effect of nitrogen (N) form on iron (Fe) uptake and distribution in maize (Zea mays L. cv Yellow 417) were carried out through three related experiments with different pretreatments. Experiment 1: plants were precultured in nutrient solution with 1.0×10^–4 M FeEDTA for 6 d and then exposed to NO₃–N or NH₄–N solution with 1.0×10^–4 M FeEDTA or without for 7 d. Experiment 2: plants were precultured with ⁵⁹FeEDTA for 6 d and were then transferred to the solution with different N forms, and 0 and 1.0×10^–4 M FeEDTA for 8 d. Experiment 3: half of roots were supplied with ⁵⁹FeEDTA for 5 d and then cut off, with further culturing in treatment concentrations for 7 d. In comparison to the NH₄-fed plants, young leaves of the NO₃-fed plants showed severe chlorosis under Fe deficiency. Nitrate supply caused Fe accumulation in roots, while NH₄–N supply resulted in a higher Fe concentration in young leaves and a lower Fe concentration in roots. HCl-extractable (active) Fe was a good indicator reflecting Fe nutrition status in maize plants. Compared with NO₃-fed plants, a higher proportion of ⁵⁹Fe was observed in young leaves of the Fe-deficient plants fed with NH₄–N. Ammonium supply greatly improved ⁵⁹Fe retranslocation from primary leaves and stem to young leaves. Under Fe deficiency, about 25% of Fe in primary leaves of the NH₄-fed plants was mobilized and retranslocated to young leaves. Exogenous Fe supply decreased the efficiency of such ⁵⁹Fe retranslocation. The results suggest that Fe can be remobilized from old to young tissues in maize plants but the remobilization depends on the form of N supply as well as supply of exogenous Fe.     

Absorption of nitrate and ammonium ions by Lolium perenne from flowing solution cultures at low root temperatures

Lolium perenne L. cv. 23 (perennial ryegrass) plants were grown in flowing solution culture and acclimatized over 49 d to low root temperature (5°C) prior to treatment at root temperatures of 3, 5, 7 and 9°C for 41 d with common air temperature of 20/15°C day/night and solution pH 5·0. The effects of root temperature on growth, uptake and assimilation of N were compared with N supplied as either NH₄ or NO3 at 10 mmol m^?3. At any given temperature, the relative growth rate (RGR) of roots exceeded that of shoots, thus the root fraction (Rf) increased with time. These effects were found in plants grown with the two N sources. Plants grown at 3 and 5°C had very high dry matter contents as reflected by the fresh weight: freeze-dried weight ratio. This ratio increased sharply, especially in roots at 7 and 9°C. Expressed on a fresh weight basis, there was no major effect of root temperature on the [N] of plants receiving NHJ but at any given temperature, the [N] in plants grown with NHJ was significantly greater than in those grown with NO₃. The specific absorption rate (SAR) of NH⁺₄ was greater at all temperatures than SAR-NO₃. In plants grown with NH⁺, 3–5% of the total N was recovered as NH⁺₄, whereas in those grown with NO^?₃ the unassimilated NO^?₃ rose sharply between 7 and 9°C to become 14 and 28% of the total N in shoots and roots, respectively. The greater assimilation of NH⁺₄ lead to concentrations of insoluble reduced N (= protein) which were 125 and 20% greater, in roots and shoots, respectively, than in NO^?₃-grown plants. Plants grown with NH⁺₄ had very much greater glutamine and asparagine concentrations in both roots and shoots, although other amino acids were more similar in Concentration to those in NO^?₃ grown plants. It is concluded that slow growth at low root temperature is not caused by restriction of the absorption or assimilation of either NH⁺₄ or NO^?₃. The additional residual N (protein) in NH⁺₄ grown plants may serve as a labile store of N which could support growth when external N supply becomes deficient.     

Effect of salt stress on antioxidant system of plants as related to nitrogen nutrition

In plants of wheat (Triticum aestivum L.) grown in the media with nitrate (NO ₃ ^? plants), ammonium (NH ₄ ⁺ plants), and without nitrogen (N-deficient plants), the response to oxidative stress induced by the addition of 300 mM NaCl to the nutrient solution was investigated. Three-day-long salinization induced chlorophyll degradation and accumulation of malondialdehyde (MDA) in the leaves. These signs of oxidative stress were clearly expressed in NO ₃ ^? and N-deficient plants and weakly manifested in NH ₄ ⁺ plants. In none of the treatments, salinization induced the accumulation of MDA in the roots. Depending on the conditions of N nutrition, salt stress was accompanied by diverse changes in the activity of antioxidant enzymes in the leaves and roots. Resistance of leaves of NH ₄ ⁺ plants to oxidative stress correlated with a considerable increase in the activities of ascorbate peroxidase and glutathione reductase. Thus, wheat plants grown on the NH ₄ ⁺ -containing medium were more resistant to the development of oxidative stress in the leaves than those supplied with nitrate.     

Nitrogen isotope composition of tomato (Lycopersicon esculentum Mill. cv. T-5) grown under ammonium or nitrate nutrition

Studies that quantify plant δ¹⁵N often assume that fractionation during nitrogen uptake and intra-plant variation in δ¹⁵N are minimal. We tested both assumptions by growing tomato (Lycopersicon esculetum Mill. cv. T-5) at NH₄⁺ or NO^?₃ concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole-plant δ¹⁵N was not significantly different from source δ¹⁵ N for plants grown on either nitrogen form. No intra-plant variation in δ¹⁵N was observed for plants grown with NH⁺₄. In contrast. δ¹⁵N of leaves was as much as 5.8% greater than that of roots for plants grown with NO^?₃. The contrasting patterns of intra-plant variation are probably caused by different assimilation patterns. NH⁺₄ is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO^?₃ assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ¹⁵N of NO^?₃ to become enriched relative to organic nitrogen; the δ¹⁵N of NO^?₃ was 11.1 and 12.9% greater than the δ¹⁵N of organic nitrogen in leaves and roots, respectively. Leaf δ¹⁵N may therefore be greater than that of roots because the NO^?₃ available for assimilation in leaves originates from a NO^?₃ pool that was previously exposed to nitrate assimilation in the root.     

The influence of NO3 - and NH4 + nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO₃ ^- or NH₄ ⁺ nutrition were investigated using either ¹⁴C or ¹⁵N techniques. Greater allocation of ¹⁴C to amino-N fractions occurred at the expense of allocation of ¹⁴C to carbohydrate fractions in NH₄ ⁺-compared to NO₃ ^--fed plants. The [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO₃ ^--compared to 12 mM NH₄ ⁺-fed wheat. In both 4 mM and 12 mM N-fed maize the [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO₃ ^--compared to NH₄ ⁺-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO₃ ^--and the other in NH₄ ⁺-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of ¹⁴N and ¹⁵N within separate sets of plants confirmed that NH₄ ^--fed plants accumulated more amino-N compounds than NO₃ ^--fed plants. Wheat roots supplied with ¹⁵NH₄ ⁺ for 8 h were found to accumulate ¹⁵NH₄ ⁺ (8.5 g ¹⁵N g^-1 h^-1) whereas in maize roots very little ¹⁵NH₄ ⁺ accumulated (1.5 g ¹⁵N g^-1 h^-1)It is proposed that the observed accumulation of ¹⁵NH₄ ⁺ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH₄ ⁺, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C₄ maize plants ensure greater carbon availability to the root than in the C₃ wheat plants. These differences in carbon and nitrogen partitioning between NO₃ ^--and NH₄ ⁺-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO₃ ^- and NH₄ ⁺ nutrition.     

Growth and solute composition of the salt-tolerant kallar grass [Leptochloa fusca (L.) Kunth] as affected by nitrogen source

A study was conducted to elucidate the effect of N form, either NH₄ ⁺ or NO₃ ^–, on growth and solute composition of the salt-tolerant kallar grass [Leptochloa fusca (L.) Kunth] grown under 10 mM or 100 mM NaCl in hydroponics. Shoot biomass was not affected by N form, whereas NH₄ ⁺ compared to NO₃ ^– nutrition caused an almost 4-fold reduction in the root biomass at both salinity levels. Under NH₄ ⁺ nutrition, salinity had no effect on the biomass yield, whereas under NO₃ ^– nutrition, increasing salinity from 10 mM to 100 mM caused 23% and 36% reduction in the root and shoot biomass, respectively. The reduced root growth under NH₄ ⁺ nutrition was not attributable to impaired shoot to root C allocation since N form did not affect the overall root sugar concentration and the starch concentration was even higher under NH₄ ⁺ compared to NO₃ ^– nutrition. The low NH₄ ⁺ (2 mM) and generally higher amino-N concentrations in NH₄ ⁺- compared to NO₃ ^–-fed plants indicated that the grass was able to effectively detoxify NH₄ ⁺. Salinity had no effect on Ca²⁺ and Mg²⁺ levels, whereas their concentration in shoots was lower under NH₄ ⁺ compared to NO₃ ^– nutrition (over 66% reduction in Ca²⁺; over 20% reduction in Mg²⁺), but without showing deficiency symptoms. Ammonium compared to NO₃ ^– nutrition did not inhibit K⁺ uptake, and the K⁺-Na⁺ selectivity either remained unaffected or it was higher under NH₄ ⁺ than under NO₃ ^– nutrition. Results suggested that while NH₄ ⁺ versus NO₃ ^– nutrition substantially reduced root growth, and also strongly modified anion concentrations and to a minor extent concentrations of divalent cations in shoots, it did not influence salt tolerance of kallar grass.     

Metabolic regulation and gene expression of root phosphoenolpyruvate carboxylase by different nitrogen sources

Alfalfa (Medicago sativa L.) N-sufficient plants were fed 1·5 mM N in the form of NO₃⁻, NH₄⁺ or NO₃⁻ in conjunction with NH₄⁺, or were N-deprived for 2 weeks. The specific activity of phosphoenolpyruvate carboxylase (PEPC) from the non-nodulated roots of N-sufficient plants was increased in comparison with that of N-deprived plants. The PEPC value was highest with NO₃⁻ nutrition, lowest with NH₄⁺ and intermediate in plants that were fed mixed salts. The protein was more abundant in NO₃⁻-fed plants than in either NH₄⁺- or N mixed-fed plants. Nitrogen starvation decreased the level of PEPC mRNA, and nitrate was the N form that most stimulated PEPC gene expression. The malate content was significantly lower in NO₃⁻-deprived than in NO₃⁻-sufficient plants. Root malate accumulation was high in NO₃⁻-fed plants, but decreased significantly in plants that were fed with NH₄⁺. The effect of malate on the desalted enzyme was also investigated. Root PEPC was not very sensitive to malate and PEPC activity was inhibited only by very high concentrations of malate. Asparagine and glutamine enhanced PEPC activity markedly in NO₃⁻-fed plants, but failed to affect plants that were either treated with other N types or N starved. Glutamate and citrate inhibited PEPC activity only at optimal pH. N-nutrition also influenced root nitrate and ammonium accumulation. Nitrate accumulated in the roots of NO₃⁻- and (NO₃⁻ + NH₄⁺)-fed plants, but was undetectable in those administered NH₄⁺. Both the nitrate and the ammonium contents were significantly reduced in NO₃⁻- and (NO₃⁻ + NH₄⁺)-starved plants. Root accumulation of free amino acids was strongly influenced by the type of N administered. It was highest in NH₄⁺-fed plants and the most abundant amides were asparagine and glutamine. It was concluded that root PEPC from alfalfa plants is N regulated and that nitrate exerts a strong influence on the PEPC enzyme by enhancing both PEPC gene expression and activity.     

Effect of Nitrate and Ammonium Nutrition of Nonnodulated Phaseolus vulgaris L. on Phosphoenolpyruvate Carboxylase and Pyruvate Kinase Activity

Young bean plants (Phaseolus vulgaris L. var Saxa) were fed with 3.5 or 10 millimolar N in either the form of NO₃⁻ or NH₄⁺, after being grown on N-free nutrient solution for 8 days. The pH of the nutrient solutions was either 6 or 4. The cell sap pH and the extractable activities of phosphoenolpyruvate carboxylase and of pyruvate kinase from roots and primary leaves were measured over several days.

The extractable activity of phosphoenolpyruvate carboxylase (based on soluble protein) from primary leaves increased with NO₃⁻ nutrition, whereas with NH₄⁺ nutrition and on N-free nutrient solution the activity remained at a low level. Phosphoenopyruvate carboxylase activity from the roots of NH₄⁺-fed plants at pH 4 was finally somewhat higher than from the roots of plants grown on NO₃⁻ at the same pH. There was no difference in activity from the root between the N treatments when pH in the nutrient solutions was 6. The extractable activity of pyruvate kinase from roots and primary leaves seemed not to be influenced by the N nutrition of the plants.

The results are discussed in relation to the physiological function of both enzymes with special regard to the postulated functions of phosphoenolpyruvate carboxylase in C3 plants as an anaplerotic enzyme and as part of a cellular pH stat.

     

Changes in the Activity of Antioxidant Enzymes in Wheat Leaves and Roots as a Function of Nitrogen Source and Supply

The activity of enzymes participating in the systems of antioxidant protection was assayed in the second leaf and roots of 21-day-old wheat seedlings (Triticum aestivum L.) grown in a medium with nitrate (NO^– ₃ treatment), ammonium (NH⁺ ₄ treatment), or without nitrogen added (N-deficiency treatment). The activities of superoxide dismutase (SOD), peroxidase, ascorbate peroxidase, glutathione reductase, and catalase in the leaves and roots of the NH⁺ ₄ plants was significantly higher than in the plants grown in the nitrate medium. The activity of SOD decreased and ascorbate peroxidase markedly increased in leaves, whereas the activity of ascorbate peroxidase increased in the roots of N-deficient plants, as compared to the plants grown in nitrate and ammonium. Low-temperature incubation (5°, 12 h) differentially affected the antioxidant activity of the studied plants. Whereas leaf enzyme activities did not change in the NH⁺ ₄ plants, the activities of SOD, peroxidase, ascorbate peroxidase, and catalase markedly increased in the NO^– ₃ plants. In leaves of the N-deficient plant, the activity of SOD decreased; however, the activity of other enzymes increased. In response to temperature decrease, catalase activity increased in the roots of NO^– ₃ and NH⁺ ₄-plants, whereas in the N-deficient plants, the activity of peroxidase increased. Thus, in wheat, both nitrogen form and nitrogen deficiency changed the time-course of antioxidant enzyme activities in response to low temperature.