Effect of Salinity and Potassium on the Uptake of Nitrogen and on Nitrogen Metabolism in Young Barley Plants

In a solution culture experiment with 4-week-old barley plants (variety Villa) the influence of NaCl salinization and of KCl additions on the uptake and turnover of labelled N (¹⁵NH₄¹⁵NO₃) was studied. Labelled N was applied for 24 h at the end of the growth period. Salinization impaired growth and uptake of labelled N. The incorporation of labelled N into the protein fraction, however, was improved by NaCl salinization. Additions of KCl to the nutrient solution diminished the negative effect of NaCl salinization on growth. At both NaCl salinization levels (60 and 120 mM) K additions favoured the uptake of labelled N and particularly its incorporation into the protein fraction. It is suggested that the negative influence of the NaCl stress is not primarily due to an impaired protein synthesis, but is possibly caused by a deterimental effect of Na on other metabolic processes.     

Effects of aluminium on growth and root reactions of phosphorus stressed Betula pendula seedlings

The impact of two constant non-toxic levels of Al stress (0.2 and 0.4 mM) on growth and ³²P uptake capacity on sub-optimal (P-limited) Betula pendula seedlings grown in sand culture was examined.Seedling growth was under optimum controlled conditions in a growth chamber where nutrient additions were made at a predetermined relative addition rate (R_A) of 10% day^-1. Three treatment groups of seedlings 0, 0.2 and 0.4 mM Al were harvested at 15, 29 and 42 days. The excised roots were exposed to a ³²P-labelled solution for 15 minutes to measure their capacity for P uptake. Growth was determined by weighing the roots, stems and leaves of the seedlings.Growth data showed that relative growth rate (R_G) should equal the R_A of P the most limiting nutrient, which was supplied at P/N 3% instead of an optimal 15%. Therefore, Ingestad's theory can also be used succesfully in sand culture and this may be particularly important for future studies of root and rhizosphere exudates. Low levels of Al (< 0.2 mM) in combination with low P supply significantly lowered the R_G of the birch seedlings by further reducing P supply. However, previous studies of birch seedling growth and nutrient uptake using Ingestad's solution culture technique with optimumal P supply did not show any effect of Al on growth untill the Al was in excess of 3 mM. Aluminium was not directly toxic to the plants and therefore roots could respond to the ³²P bioassay.     

The physiological basis of increased biomass partitioning to roots upon nitrogen deprivation in young clonal tea (Camellia sinensis (L.) O. Kuntz)

Deprivation of nitrogen (N) increases assimilate partitioning towards roots at the expense of that to shoots. This study was done to determine the physiological basis of increased root growth of tea (sCammellia sinensis L.) under N shortage. Nine-month-old clonal tea (clone TRI2025) was grown in quartz sand under naturally lit glasshouse conditions. Three levels of N (0, 3.75 and 7.5 mM N) were incorporated in to the nutrient solution and applied daily. Plant growth, photosynthesis, root respiration and plant N contents were measured at 10-day intervals over a 45-day period. Root dry weight showed a sharp increase during the first 15 days after the plants were transferred to 0 mM N, whereas no such increase was shown in plants transferred to 7.5 mM N. In contrast, shoot dry weight increased at 7.5 mM N and was significantly greater than at 0 mM N, where no increase was observed. Due to the above changes, root weight ratio increased and leaf weight ratio decreased during the first 15 days of N deprivation. Leaf photosynthetic rates did not vary between N levels during the initial 15-day period. Thereafter, photosynthetic rates were greater at 7.5 mM and 3.75 mM N than at 0 mM N. Root respiration rate decreased at 0 mM N, whereas it increased at 3.75 and 7.5 mM N, probably because of the greater respiratory cost for nitrate uptake. Root respiratory costs associated with maintenance (R _m) and nitrate uptake (R _u) were calculated to investigate whether the sharp increase of root growth observed upon nitrogen deprivation was solely due to the reduced respiratory costs for nitrate uptake. The estimated values for R _m and R _u were 3.241 × 10^–4 mol CO₂ g^–1 (root dry matter) s^–1 and 0.64 mol CO₂ (mol N)^–1, respectively. Calculations showed that decreased respiratory costs for nitrate uptake could not solely account for the significant increase of root biomass upon N deprivation. Therefore, it is concluded that a significant shift in assimilate partitioning towards roots occurs immediately following N deprivation in tea.     

Nitrogen and phosphorus cycling in relation to stand age of Eucalyptus regnans F. Muell

Lupins, canola, ryegrass and wheat fertilized with Na₂ ³⁵SO₄ and either ¹⁵NH₄Cl or K¹⁵NO₃(N:S=10:1), were grown in the field in unconfined microplots, and the sources of N and S (fertilizer, soil, atmosphere, seed) in plant tops during crop development were estimated. Modelled estimates of the proportion of lupin N derived from the atmosphere, which were obtained independently of reference plants, were used to calculate the proportion of lupin N derived from the soil. Total uptake of N and S and uptake of labelled N and S increased during crop development. Total uptake of S by canola was higher than lupins, but labelled S uptake by lupins exceeded uptake by canola. The form of N applied had no effect on uptake of labelled and unlabelled forms of N or S. Ratios of labelled to unlabelled S and ratios of labelled to unlabelled N derived from soil sources decreased during growth, and were less for S than for N for each crop at each sampling time. Although ratios of labelled to unlabelled soil-derived N were similar between crops at 155, 176 and 190 days after sowing, ratios of labelled to unlabelled S for lupins were higher than for the reference crops and declined during this period. The ratios of labelled to unlabelled S in lupins and the reference plants therefore bore no relationship either to ratios of labelled to unlabelled soil-derived N in the plants, or to total S uptake by the plants. Therefore the hypothesis that equal ratios of labelled N to unlabelled soil-derived N in legumes (Rleg) and reference plants (Rref) would be indicated by equal ratios of labelled to unlabelled S was not supported by the data. The results therefore show that the accuracy of reference plant-derived values of Rleg cannot be evaluated by labelling with ³⁵S.     

S‐deficiency responsive accumulation of amino acids is mainly due to hydrolysis of the previously synthesized proteins – not to de novo synthesis in Brassica napus

To characterize the mechanisms of amino acid accumulation under sulphur (S)‐deficiency and its physiological significance in Brassica napus, stable isotopes ¹⁵N and ³⁴S were employed. The plants were exposed for 9 days to S‐deficient conditions (0.05 mM vs 1.5 mM sulphate). After 9 days of S‐deficiency, leaf‐osmotic potential and total chlorophyll content significantly decreased. S uptake decreased by 94%, whereas N uptake and biomass were not significantly changed. Using ¹⁵N and ³⁴S labelling, de novo synthesis of amino acids and proteins derived from newly absorbed NO₃^? and SO₄^2? and the content of N and S in the previously synthesized amino acids and proteins were quantified. At the whole plant level, S‐deficiency increased the pool of amino acids but resulted in strong decrease of incorporation of newly absorbed NO₃^? and SO₄^2? into amino acids by 22.2 and 76.6%, respectively, compared to the controls. Total amount of N and S incorporated into proteins also decreased by 28.8 and 62.1%, respectively. The levels of ¹⁴N‐ and ³²S‐proteins (previously synthesized proteins) strongly decreased, mainly in mature leaves. The data thus indicate that amino acid accumulation under short‐term S‐deficiency results from the degradation of previously synthesized proteins rather than from de novo synthesis.     

Amino acid transporter mutants of Arabidopsis provides evidence that a non‐mycorrhizal plant acquires organic nitrogen from agricultural soil

Although organic nitrogen (N) compounds are ubiquitous in soil solutions, their potential role in plant N nutrition has been questioned. We performed a range of experiments on Arabidopsis thaliana genetically modified to enhance or reduce root uptake of amino acids. Plants lacking expression of the Lysine Histidine Transporter 1 (LHT1) displayed significantly lower contents of ¹³C and ¹⁵N label and of U‐¹³C₅,¹⁵N₂ L‐glutamine, as determined by liquid chromatography–mass spectrometry when growing in pots and supplied with dually labelled L‐glutamine compared to wild type plants and LHT1‐overexpressing plants. Slopes of regressions between accumulation of ¹³C‐labelled carbon and ¹⁵N‐labelled N were higher for LHT1‐overexpressing plants than wild type plants, while plants lacking expression of LHT1 did not display a significant regression between the two isotopes. Uptake of labelled organic N from soil tallied with that of labelled ammonium for wild type plants and LHT1‐overexpressing plants but was significantly lower for plants lacking expression of LHT1. When grown on agricultural soil plants lacking expression of LHT1 had the lowest, and plants overexpressing LHT1 the highest C/N ratios and natural δ¹⁵N abundance suggesting their dependence on different N pools. Our data show that LHT1 expression is crucial for plant uptake of organic N from soil.     

Nitrate and ammonium absorption by plants growing at a sufficient or insufficient level of phosphorus in nutrient solutions

Summary Absorption of nitrate and ammonium was studied in water culture experiments with 4 to 6 weeks old plants of barley (Hordeum vulgare L.), buckwheat (Fagopyrum esculentum L. Moench) and rape (Brassica napus L.). The plants were grown in a complete nutrient solution with nitrate (5.7±0.2 mM) or nitrate (5.6±0.2 mM) + ammonium (0.04±0.02 mM). The pH of the nutrient solution was kept at 5.0 using a pH-stat. It was found that phosphorus deficiency reduced the rate of nitrate uptake by 58±3% when nitrate was the sole N source and by 83±1% when both nitrate and ammonium were present. The reduction occurred even before growth was significantly impeded by P deficiency. The inhibition of the uptake of ammonium was less,i.e. ammonium constituted 10±1% of the total N uptake in the P sufficient plants and 30±5% in the P deficient plants. The reduction of nitrate absorption greatly decreased the difference between the uptake of anions and cations. It is suggested that P deficiency reduced the assimilation of NO ₃ ^– into the proteins, which might cause a negative feedback on NO ₃ ^– influx and/or stimulate NO ₃ ^– efflux.     

Effects of aluminium on growth and nutrient uptake of small Picea abies and Pinus sylvestris plants

Summary The effects of aluminium concentrations between 0.2 and 30 mM at pH 3.8 ±0.2 on small plants of Norway spruce [(Picea abies (L.) Karst], Scots pine (Pinus sylvestris L.), and Scots pine infected with the ectomycorrhizal fungus Suillus bovinus (L. ex Fr.) O. Kuntze were investigated. The plants were grown at maximum relative growth rate (R_G % day^–1) with free access but very low external concentrations of nutrients. Steady-state conditions with respect to relative growth rate (R_G) and internal nutrient concentrations were achieved before addition of aluminium, which was added as AlCl₃ and/or Al(NO₃)₃. There were reductions in rg at aluminium concentrations of 0.3 mM in spruce, 6 mM in pine and 10 mM in ectomycorrhizal pine, i. e. at aluminium concentrations considerably higher than those normally occurring in the top layer of the mineral soil where most fine roots are found. Nutrient uptake rate per unit root growth rate was calculated for different nutrient elements. The uptake rate of calcium and magnesium was reduced at aluminium concentrations of 0.2 mM (spruce), 1 mM (pine) and 3 mM (ectomycorrhizal pine), without influencing R_g. The results question the validity of the hypothesis of aluminium toxicity to forest tree species at low external concentrations.     

Effect of NH4 +:NO3 - ratios on growth and nitrogen uptake by onions

The modelling of ion uptake by plants requires the measurement of kinetic and growth parameters under specific conditions. The objective of this study was to evaluate the effect of nine NH _inf4 ^sup+ :NO _inf3 ^sup− ratios on onions (Allium cepa L.). Twenty-eight to 84 day-old onion plants were treated with NH _inf4 ^sup+ :NO_f3/sup− ratios ranging from 0 to 100% of each ionic species in one mM solutions in a growth chamber. Maximum N influx (I_max) was assessed using the N depletion method. Except at an early stage, ionic species did not influence significantly I_max, the Michaelis constant (K_m) and the minimum concentration for net uptake (C_min). I_max for ammonium decreased from 101 to 59 pmole cm^-2 s^-1 while I_max for nitrate increased from 26 to 54 pmole cm^-2 s^-1 as the plant matured. On average, K_m and C_min values were 14.29 μM, and 5.06 μM for ammonium, and 11.90 μM and 4.54 μM for nitrate, respectively. In general, the effect of NH₄ ⁺:NO₃ ^- ratios on root weight, shoot weight and total weight depended on plant age. At an early stage, maximum plant growth and N uptake were obtained with ammonium as the sole source of N. At later stages, maximum plant growth and N uptake were obtained as the proportion of nitrate increased in the nutrient solution. The was no apparent nutrient deficiency whatever NH₄ ⁺:NO₃ ^- ratio was applied, although ammonium reduced the uptake of cations and increased the uptake of phosphorus. The research was supported by the Natural Sciences and Engineering Research Council of Canada.     

3Hthymidine incorporation of rhizosphere bacteria influenced by plant N-status

The effect of plant-root N-status on bacterial growth in the rhizosphere was studied with 5-week-old wheat plants grown in soil with low N content obtained by mixing 9:1 gravel:sandy loam. As a consequence of N limitation, significant increase in³Hthymidine (Tdr) incorporation rate occured 3 days after addition of 30 mM ammonium compared to controls without ammonium. Plants were grown with split-roots to separate the effect of soil N from effect of plant root derived organic matter-N on bacterial activity. The increase in nitrate concentration from 10 mM to 30 mM at one part of the root system led to significant increased³HT dr incorporation in the rhizosphere at the other part of root system after 4 days showing that the composition of root exudates became more favourable for bacterial growth when plants were fertilized with the higher level of nitrate.     

Mineralization-immobilization and plant uptake of nitrogen as influenced by the spatial distribution of cattle slurry in soils of different texture

The effect of incorporating cattle slurry in soil, either by mixing or by simulated injection into a hollow in soil, on the ryegrass uptake of total N and ¹⁵NH₄ ⁺-N was determined in three soils of different texture. The N accumulation in Italian ryegrass (Lolium multiflorum L.) from slurry N and from an equivalent amount of NH₄ ⁺-N in (¹⁵NH₄) SO₄ (control) was measured during 6 months of growth in pots. After this period the total recovery of labelled N in the top soil plus herbage was similar in the slurry and the control treatments. This indicated that gaseous losses from slurry NH₄ ⁺-N were insignificant. Consequently, the availability of slurry N to plants was mainly influenced by the mineralization-immobilization processes. The apparent utilization of slurry NH₄ ⁺-N mixed into soil was 7%, 14% and 24% lower than the utilization of (NH₄)₂SO₄-N in a sand soil, a sandy loam soil and a loam soil, respectively. Thus, the net immobilization of N due to slurry application increased with increasing soil clay content, whereas the recovery in plants of ¹⁵N-labelled NH₄ ⁺-N from slurry was similar on the three soils. A parallel incubation experiment showed that the immobilization of slurry N occurred within the first week after slurry application. The incorporation of slurry N by simulated injection increased the plant uptake of both total and labelled N compared to mixing the slurry into the soil. The apparent utilization of injected slurry NH₄ ⁺-N was 7% higher, 8% lower and 4% higher than the utilization of (NH₄)₂SO₄-N in the sand, the sandy loam and the loam soil, respectively. It is concluded that the spatial distribution of slurry in soil influenced the net mineralization of N to the same degree as did the soil type.     

Stimulation of nodulation in field peas (Pisum sativum) by low concentrations of ammonium in hydroponic culture

Although the inhibitory effects of high concentrations of mineral N (> 1.0 mM) on nodule development and function have often been studied, the effects of low, static concentrations of NH₄⁺ (< 1.0 mM) on nodulation are unknown. In the present experiments we examine the effects of static concentrations of NH₄⁺ at 0, 0.1 and 0.5 mM in flowing, hydroponic culture on nodule establishment and nitrogenase activity in field peas [Pisum sativum L. cv. Express (Svalöf AB)] for the initial 28 days after planting (DAP). Peas grown in the presence of low concentrations of NH₄⁺ had significantly greater nodule numbers (up to 4-fold) than plants grown without NH₄⁺. Nodule dry weight per plant was significantly higher at 14, 21 and 28 DAP in plants grown in the presence of NH₄⁺, but individual nodule mass was lower than in plants grown without NH₄⁺. The nodulation pattern of the plants supplied with NH₄⁺ was similar to that often reported for supernodulating mutants, however the plants did not express other growth habits associated with supernodulation. Estimates of N₂ fixation indicate that the plus-NH₄⁺ peas fixed as much or more N₂ than the plants supplied with minus-NH₄⁺ nutrient solution. There were no significant differences in nodule numbers, nodule mass or NH₄⁺ uptake between the plants grown at the two concentrations of NH₄⁺. Nodulation appeared to autoregulate by 14 DAP in the minus-NH₄⁺ treatment. Plant growth and N accumulation in the minus-NH₄⁺ plants lagged behind those of the plus-NH₄⁺ treatments prior to N₂ fixation becoming well established in the final week of the experiment. The plus-NH₄⁺ treatments appeared not to elicit autoregulation and plants continued to initiate nodules throughout the experiment.     

Nitrite Uptake by Nitrogen-Depleted Wheat Seedlings

Intact, 14-day-old nitrogen-depleted wheat (Triticum vulgare cv. Blueboy) seedlings were exposed to solutions of 0.5 mM KNO₂, 0.05 mM CaSO₄ and 1 mM sodium 2-[N-morpholino]-ethanesulfonate, pH 6.1. Nitrite uptake was determined from depletion of the ambient solution or from incorporation of ¹⁵N in the tissue. An initial nitrite uptake shoulder was followed by a relatively slow uptake rate which subsequently increased to a substantially greater rate. This accelerated phase was maintained through 24 h. Nitrite accumulated to a slight extent in the root tissues during the first few hours but declined to low values when the accelerated rate was fully developed, indicating an increase in nitrite reductase activity paralleling the increase in nitrite uptake capacity. About 50% of the nitrogen absorbed as nitrite was translocated to the shoots by 9–12 h. Development of the accelerated nitrite uptake rate was restricted in excised roots, in intact plants kept in darkness, by 400 μg puromycin ml^?1 and by 1 mM L-ethionine. When puromycin and L-ethionine were added after the accelerated phase had been initiated, their effects were not as detrimental as when they were added at first exposure to KNO₂. The two inhibitors restricted translocation more than uptake. The data indicate an involvement of protein synthesis and a requirement for movement of a substance from shoots to roots for maximal development of the accelerated nitrite uptake phase. A requirement for protein synthesis in the transport of soluble organic nitrogen from roots to shoots is also suggested.     

Cyanobacterial inoculation and nitrogen fertilization in rice

During three rice-growing seasons in Uruguay, field experiments were conducted to study the contribution of cyanobacterial inoculation and chemical N fertilization to rice production. Neither grain yield nor fertilizer recovery by the plant were affected by inoculation with native cyanobacterial isolates. A low fertilizer use efficiency (around 20%) was observed when labelled (NH₄)₂SO₄ was applied at sowing. Recovery of applied ¹⁵N by the soil–plant system was 50%. Inoculation did not modify ¹⁵N uptake by the plant when the fertilizer was three-split applied either. The total N-fertilizer recovery was higher when the fertilizer was split than when applied in a single dose. Plant N-fertilizer uptake was higher when the fertilizer was applied at tillering. Uptake of ¹⁵N from cyanobacteria by rice was studied in a greenhouse pots experiment without chemical nitrogen addition. Recovery of ¹⁵N from labelled cyanobacteria by rice in greenhouse growth conditions was similar to that of partial recovery of (NH₄)₂SO₄ applied at sowing in the field. Cyanobacterial N mineralization under controlled conditions was fast as cyanobacterial N was detected in plants after 25 days. Moreover 40 days after inoculation non-planted and inoculated soil had more inorganic N than the non-inoculated one.     

Involvement of ethylene in reversal of salt‐inhibited photosynthesis by sulfur in mustard

Sulfur (S) assimilation results in the synthesis of cysteine (Cys), a common metabolite for the formation of both reduced glutathione (GSH) and ethylene. Thus, ethylene may have regulatory interaction with GSH in the alleviation of salt stress. The involvement of ethylene in the alleviation of salt stress by S application was studied in mustard (Brassica juncea cv. Pusa Jai Kisan). First, the effects of 0, 0.5, 1.0 and 2.0 mM SO₄^2? were studied on photosynthetic and growth parameters to ascertain the S requirement as sufficient‐S and excess‐S for the plant. In further experiments, the effects of sufficient‐S (1 mM SO₄^2?) and excess‐S (2 mM SO₄^2?) were studied on the alleviation of salt stress‐induced by 100 mM NaCl, and ethylene involvement in the alleviation of salt stress by S. Under non‐saline condition, excess‐S increased ethylene with less content of Cys and GSH and adversely affected photosynthesis and growth. In contrast, excess‐S maximally alleviated salt stress due to high demand for S and optimal ethylene formation, which maximally increased GSH and promoted photosynthesis and growth. The involvement of ethylene in S‐mediated alleviation of salt stress was further substantiated by the reversal of the effects of excess‐S on photosynthesis by aminoethoxyvinylglycine (AVG), ethylene biosynthesis inhibitor. The studies suggest that plants respond differentially to the S availability under non‐saline and salt stress and excess‐S was more potential in the alleviation of salt stress. Further, ethylene regulates plants' response and excess S‐induced alleviation of salt stress and promotion of photosynthesis.     

Cross-talk between salicylic acid and NaCl-generated reactive oxygen species and nitric oxide in tomato during acclimation to high salinity

Hydrogen peroxide (H₂O₂) and nitric oxide (NO) generated by salicylic acid (SA) are considered to be functional links of cross‐tolerance to various stressors. SA‐stimulated pre‐adaptation state was beneficial in the acclimation to subsequent salt stress in tomato (Solanum lycopersicum cv. Rio Fuego). At the whole‐plant level, SA‐induced massive H₂O₂ accumulation only at high concentrations (10^?3–10^?2M), which later caused the death of plants. The excess accumulation of H₂O₂ as compared with plants exposed to 100 mM NaCl was not associated with salt stress response after SA pre‐treatments. In the root tips, 10^?3–10^?2M SA triggered the production of reactive oxygen species (ROS) and NO with a concomitant decline in the cell viability. Sublethal concentrations of SA, however, decreased the effect of salt stress on ROS and NO production in the root apex. The attenuation of oxidative stress because of high salinity occurred not only in pre‐adapted plants but also at cell level. When protoplasts prepared from control leaves were exposed to SA in the presence of 100 mM NaCl, the production of NO and ROS was much lower and the viability of the cells was higher than in salt‐treated samples. This suggests that, the cross‐talk of signalling pathways induced by SA and high salinity may occur at the level of ROS and NO production. Abscisic acid (ABA), polyamines and 1‐aminocyclopropane‐1‐carboxylic acid, the compounds accumulating in pre‐treated plants, enhanced the diphenylene iodonium‐sensitive ROS and NO levels, but, in contrast to others, ABA and putrescine preserved the viability of protoplasts.     

Alterations in nitrogen assimilation and partitioning in nitrogen stressed plants

Although nutrient stress is known to alter partitioning between shoots and roots, the physiological basis for the phenomenon is unresolved. Experiments were conducted to examine assimilation of ¹⁵NO₃ by N-stressed plants and to determine whether apparent changes in assimilation in the root contributed to alterations in whole-plant partitioning of reduced-N. Tobacco plants (Nicotiana tabacum L. cv. NC 2326) were exposed to a low concentration of NO₃^? in solution (80 μM) for 9 days to effect a N-stress response. Exposure of plants to 1000 μM¹⁵NO₃^? for 12 h on selected days revealed that roots of N-stressed plants developed an increased capacity to absorb NO₃^?, and accumulation of reduced-¹⁵N in the root increased to an even greater extent. When plants were exposed to 80 or 1000 μM¹⁵NO₃^? in steady-state, ¹⁵NO₃^? uptake over a 12 h period was noticeably restricted at the lower concentration, but a larger proportion of the absorbed ¹⁵N still accumulated as reduced-¹⁵N in the root. The alteration in reduced-¹⁵N partitioning was maintained in N-stressed plants during the subsequent 3-day “chase” period when formation of insoluble reduced-¹⁵N in the root was quantitatively related to the disappearance of ¹⁵NO₃^? and soluble reduced-¹⁵N. The results indicate that increased assimilation of absorbed NO₃^?, in the root may contribute significantly to the altered reduced-N partitioning which occurs in N-stressed plants.     

Humic acid differentially improves nitrate kinetics under low‐ and high‐affinity systems and alters the expression of plasma membrane H+‐ATPases and nitrate transporters in rice

Humic acids (HAs) have a major effect on nutrient uptake, metabolism, growth and development in plants. Here, we evaluated the effect of HA pretreatment applied with a nutrient solution on the uptake kinetics of nitrate nitrogen (N‐NO₃^?) and the metabolism of nitrogen (N) in rice under conditions of high and low NO₃^? supply. In addition, the kinetic parameters of NO₃^? uptake, N metabolites, and nitrate transporters (NRTs) and the plasma membrane (PM) H⁺‐ATPase gene expression were examined. The plants were grown in a growth chamber with modified Hoagland and Arnon solution until 21 days after germination (DAG), and they were then transferred to a solution without N for 48 h and then to another solution without N and with and without the addition of HAs for another 48 h. After this period of N deprivation, the plants received new nutrient solutions containing 0.2 and 2.0 mM N‐NO₃^?. Treatment of rice plants with HA promoted the induction of the genes OsNRT2.1‐2.2/OsNAR2.1 and some isoforms PM H⁺‐ATPase in roots. The application of HAs differentially modified the parameters of the uptake kinetics of NO₃^? under both concentrations. When grown with 0.2 mM NO₃^?, the plants pretreated with HA had lower K_m and C_min values as well as a higher V_max/K_m ratio. When grown with 2 mM NO₃^?, the plants pretreated with HA had a higher V_max value, a greater root and shoot mass, and a lower root/shoot ratio. The N fractions were also altered by pretreatment with HA, and a greater accumulation of NO₃^? and N‐amino was observed in the roots and shoots, respectively, of plants pretreated with HA. The results suggest that pretreatment with HA modifies root morphology and gene expression of PM H⁺‐ATPases and NO₃^? transporters, resulting in a greater efficiency of NO₃^? acquisition by high‐ and low‐affinity systems.     

Effects of CaSO4 and NaCl on growth and nitrogen fixation ofLeucaena leucocephala

Effects and interactions of varying CaSO₄ and NaCl levels on growth and nitrogen fixation ofLeucaena leucocephala K8 were examined. Leucaena was grown in nutrient solution at four levels of CaSO₄ (0.5, 1.0, 2.5 and 5.0 mM) and NaCl (1, 25, 50 and 100 mM) in randomized blocks with five replications. While NaCl significantly reduced plant growth, additions of CaSO₄ increased plant height, leaf number, and biomass of salt treated plants. For the nonsaline treatments, high CaSO₄ levels slightly depressed growth, which contradicts suggestions that Leucaena has a high calcium requirement. A significant calcium/sodium interaction was not seen for nodule number or weight. Nodule number was significantly depressed by 100 mM NaCl and nodule weight of the salt stressed plants significantly increased as CaSO₄ concentration increased from 0.5 to 2.5 mM. Effects of NaCl and CaSO₄ on nitrogen content of plant parts were inconclusive. The promotion of Leucaena salinity tolerance by addition of CaSO₄ may be attributed to the effect of calcium in maintaing the selective permeability of membranes.