Utilization of Previously Accumulated and Concurrently Absorbed Nitrogen during Reproductive Growth in Maize : Influence of Prolificacy and Nitrogen Source

A prolific maize (Zea mays L.) genotype was grown to physiological maturity under greenhouse conditions to examine the effects of reproductive sink demand on (a) the remobilization of N accumulated during vegetative growth, and (b) the partitioning of N accumulated concurrent with ear development. One- and two-eared plants were treated with either a NO₃⁻ or NH₄⁺ source of ¹⁵N-labeled N during reproductive growth. Plants with two ears enhanced grain production, N remobilization from the stalk and roots, and N translocation to the grain from concurrently assimilated N. But, remobilization of leaf-N was unaffected by ear number. In addition, N uptake and total dry matter accumulation during the reproductive period were also unaffected, although P uptake was greater in the two-eared plants. Less than 15% of the total K⁺ uptake was accumulated after silking while during this time more than 40% of the total N and more than 50% of the total P were absorbed. The data also indicate that with NO₃⁻ nutrition, internal recirculation of K⁺ between shoots and roots may play a prominent role in the transport of nitrogenous solutes during grain development. N source had no effect on dry matter production and N uptake of both one- and two-eared plants. However, slightly greater partitioning of labeled-N from the NH₄⁺ source to the grain was observed in the two-eared plants.     

Nitrate Reduction in Roots as Affected by the Presence of Potassium and by Flux of Nitrate through the Roots

Dark-grown, detopped corn seedlings (cv. Pioneer 3369A) were exposed to treatment solutions containing Ca(NO₃)₂, NaNO₃, or KNO₃; KNO₃ plus 50 or 100 millimolar sorbitol; and KNO₃ at root temperatures of 30, 22, or 16 C. In all experiments, the accelerated phase of NO₃⁻ transport had previously been induced by prior exposure to NO₃⁻ for 10 hours. The experimental system allowed direct measurements of net NO₃⁻ uptake and translocation, and calculation of NO₃⁻ reduction in the root. The presence of K⁺ resulted in small increases in NO₃⁻ uptake, but appreciably stimulated NO₃⁻ translocation out of the root. Enhanced translocation was associated with a marked decrease in the proportion of absorbed NO₃⁻ that was reduced in the root. When translocation was slowed by osmoticum or by low root temperatures, a greater proportion of absorbed NO₃⁻ was reduced in the presence of K⁺. Results support the proposition that NO₃⁻ reduction in the root is reciprocally related to the rate of NO₃⁻ transport through the root symplasm.     

Role of cytokinin in enhanced productivity of maize supplied with NH4 + and NO3 −

Supplying both N forms (NH₄ ⁺+NO₃ ⁻) to the maize (Zea mays L.) plant can optimize productivity by enhancing reproductive development. However, the physiological factors responsible for this enhancement have not been elucidated, and may include the supply of cytokinin, a growth-regulating substance. Therefore, field and gravel hydroponic studies were conducted to examine the effect of N form (NH₄ ⁺+NO₃ ⁻ versus predominantly NO₃ ⁻) and exogenous cytokinin treatment (six foliar applications of 22 μM 6-benzylaminopurine (BAP) during vegetative growth versus untreated) on productivity and yield of maize. For untreated plants, NH₄ ⁺+NO₃ ⁻ nutrition increased grain yield by 11% and whole shoot N content by 6% compared with predominantly NO₃ ⁻. Cytokinin application to NO₃ ⁻-grown field plants increased grain yield to that of NH₄ ⁺+NO₃ ⁻-grown plants, which was the result of enhanced dry matter partitioning to the grain and decreased kernel abortion. Likewise, hydroponically grown maize supplied with NH₄ ⁺+NO₃ ⁻ doubled anthesis earshoot weight, and enhanced the partitioning of dry matter to the shoot. NH₄ ⁺+NO₃ ⁻ nutrition also increased earshoot N content by 200%, and whole shoot N accumulation by 25%. During vegetative growth, NH₄ ⁺+NO₃ ⁻ plants had higher concentrations of endogenous cytokinins zeatin and zeatin riboside in root tips than NO₃ ⁻-grown plants. Based on these data, we suggest that the enhanced earshoot and grain production of plants supplied with NH₄ ⁺+NO₃ ⁻ may be partly associated with an increased endogenous cytokinin supply.     

Phosphorus stress effects on assimilation of nitrate

An experiment was conducted to investigate alterations in uptake and assimilation of NO₃⁻ by phosphorus-stressed plants. Young tobacco plants (Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were deprived of an external phosphorus (P) supply for 12 days. On selected days, plants were exposed to ¹⁵NO₃⁻ during the 12 hour light period to determine changes in NO₃⁻ assimilation as the P deficiency progressed. Decreased whole-plant growth was evident after 3 days of P deprivation and became more pronounced with time, but root growth was unaffected until after day 6. Uptake of ¹⁵NO₃⁻ per gram root dry weight and translocation of absorbed ¹⁵NO₃⁻ out of the root were noticeably restricted in −P plants by day 3, and effects on both increased in severity with time. Whole-plant reduction of ¹⁵NO₃⁻ and ¹⁵N incorporation into insoluble reduced-N in the shoot decreased after day 3. Although the P limitation was associated with a substantial accumulation of amino acids in the shoot, there was no indication of excessive accumulation of soluble reduced-¹⁵N in the shoot during the 12 hour ¹⁵NO₃⁻ exposure periods. The results indicate that alterations in NO₃⁻ transport processes in the root system are the primary initial responses limiting synthesis of shoot protein in P-stressed plants. Elevated amino acid levels evidently are associated with enhanced degradation of protein rather than inhibition of concurrent protein synthesis.     

Nitrate-ammonium synergism in rice. A subcellular flux analysis

Many reports have shown that plant growth and yield is superior on mixtures of NO₃⁻ and NH₄⁺ compared with provision of either N source alone. Despite its clear practical importance, the nature of this N-source synergism at the cellular level is poorly understood. In the present study we have used the technique of compartmental analysis by efflux and the radiotracer ¹³N to measure cellular turnover kinetics, patterns of flux partitioning, and cytosolic pool sizes of both NO₃⁻ and NH₄⁺ in seedling roots of rice (Oryza sativa L. cv IR72), supplied simultaneously with the two N sources. We show that plasma membrane fluxes for NH₄⁺, cytosolic NH₄⁺ accumulation, and NH₄⁺ metabolism are enhanced by the presence of NO₃⁻, whereas NO₃⁻ fluxes, accumulation, and metabolism are strongly repressed by NH₄⁺. However, net N acquisition and N translocation to the shoot with dual N-source provision are substantially larger than when NO₃⁻ or NH₄⁺ is provided alone at identical N concentrations.     

Uptake and Assimilation of NO(3) and NH(4) by Nitrogen-Deficient Perennial Ryegrass Turf

Assimilation of NO₃⁻ and NH₄⁺ by perennial ryegrass (Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH₄⁺, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO₃⁻ had been reduced, whereas 97% of the NH₄⁺ had been assimilated. During the early stages (0 to 8 hours) of NO₃⁻ uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO₃⁻, NH₄⁺ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO₃⁻ uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate.     

Effect of N-source on soybean leaf sucrose phosphate synthase, starch formation, and whole plant growth

Soybeans (Glycine max L. Merr. cv Tracy and Ransom) were grown under N₂-dependent or NO₃⁻-supplied conditions, and the partitioning of photosynthate and dry matter was characterized. Although no treatment effects on photosynthetic rates were observed, NO₃⁻-supplied plants in both cultivars had lower starch accumulation rates than N₂-dependent plants. Leaf extracts of NO₃⁻-supplied plants had higher activities of sucrose phosphate synthase (SPS) and cytoplasmic fructose-1,6-bisphosphatase (FBPase) than N₂-dependent plants. The variation in starch accumulation was correlated negatively with the activity of SPS, but not the activity of FBPase, UDP-glucose pyrophosphorylase, or ADP-glucose pyrophosphorylase. These results suggested that starch accumulation is biochemically controlled, in part, by the activity of SPS. Leaf starch content at the beginning of the photoperiod was lower in NO₃⁻-supplied plants than N₂-dependent plants in both cultivars which suggested that net starch utilization as well as accumulation was affected by N source.

Total dry matter accumulation and dry matter distribution was affected by N source in both cultivars, but the cultivars differed in how dry matter was partitioned between the shoot and root as well as within the shoot. The activity of SPS was correlated positively with total dry matter accumulation which suggested that SPS activity is related to plant growth rate. The results suggested that photosynthate partitioning is an important but not an exclusive factor which determines whole plant dry matter distribution.

     

Relative Content of NO(3) and Reduced N in Xylem Exudate as an Indicator of Root Reduction of Concurrently Absorbed NO(3)

It is unclear if the relative content of NO₃⁻ and reduced N in xylem exudate provides an accurate estimate of the percentage reduction of concurrently absorbed NO₃⁻ in the root. Experiments were conducted to determine whether NO₃⁻ and reduced N in xylem exudate of vegetative, nonnodulated soybean plants (Glycine max [L.] Merr., `Ransom') originated from exogenous recently absorbed ¹⁵NO₃⁻ or from endogenous ¹⁴N pools. Plants either were decapitated and exposed to ¹⁵NO₃⁻ solutions for 2 hours or were decapitated for the final 20 minutes of a 50-minute exposure to ¹⁵NO₃⁻ in the dark and in the light. Considerable amounts of ¹⁴NO₃⁻ and reduced ¹⁴N were transported into the xylem, but almost all of the ¹⁵N was present as ¹⁵NO₃⁻. Dissimilar changes in transport of ¹⁴NO₃⁻, reduced ¹⁴N and ¹⁵NO₃⁻ during the 2 hours of sap collection resulted in large variability over time in the percentage of total N in the exudate which was reduced N. Over a 20-minute period the rate of ¹⁵N transport into the xylem of decapitated plants was only 21 to 36% of the ¹⁵N delivered to the shoot of intact plants. Based on the proportion of total ¹⁵N which was found as reduced ¹⁵N in exudate and in intact plants in the dark, it was estimated that 5 to 17% of concurrently absorbed ¹⁵NO₃⁻ was reduced in the root. This was much less than the 38 to 59% which would have been predicted from the relative content of total NO₃⁻ and total reduced N in the xylem exudate.     

In Situ Nitrogen Mineralization,Nitrification, and Ammonia Volatilization in Maize Field Fertilized with Urea in Huanghuaihai Region of Northern China

Nitrogen (N) fertilization potentially affects soil N mineralization and leaching, and can enhance NH₃ volatilization, thus impacting crop production. A fertilizer experiment with five levels of N addition (0, 79, 147, 215 and 375 kg N ha^-1) was performed in 2009 and 2010 in a maize field in Huanghuaihai region, China, where > 300 kg N ha^-1 has been routinely applied to soil during maize growth period of 120 days. Responses of net N mineralization, inorganic N flux (0–10cm), NH₃ volatilization, and maize yield to N fertilization were measured. During the growth period, net N mineralization and nitrification varied seasonally, with higher rates occurring in August and coinciding with the R1 stage of maize growth. Soil NO₃ ⁻-N contributed to more than 60% of inorganic N flux during maize growth. Cumulative NH₃ volatilization increased with N additions, with total NH₃ volatilization during maize growth accounting for about 4% of added N. Relative to the control, mean maize yield in the fertilizer treatments increased by 17% and 20% in 2009 and 2010, respectively. However, grain yield, aboveground biomass, and plant N accumulation did not increase with added N at levels > 215 kg N ha^-1. These results suggest that the current N rate of 300 kg N ha^-1 is not only excessive, but also reduces fertilizer efficacy and may contribute to environmental problems such as global warming and eutrophication of ground water and streams.     

Effects of Altered Carbohydrate Availability on Whole-Plant Assimilation of NO(3)

An experiment was conducted to investigate the relative changes in NO₃⁻ assimilatory processes which occurred in response to decreasing carbohydrate availability. Young tobacco plants (Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were exposed to 1.0 millimolar ¹⁵NO₃⁻ for 6 hour intervals during a normal 12 hour light period and a subsequent period of darkness lasting 42 hours. Uptake of ¹⁵NO₃⁻ decreased to 71 to 83% of the uptake rate in the light during the initial 18 hours of darkness; uptake then decreased sharply over the next 12 hours of darkness to 11 to 17% of the light rate, coincident with depletion of tissue carbohydrate reserves and a marked decline in root respiration. Changes also occurred in endogenous ¹⁵NO₃⁻ assimilation processes, which were distinctly different than those in ¹⁵NO₃⁻ uptake. During the extended dark period, translocation of absorbed ¹⁵N out of the root to the shoot varied rhythmically. The adjustments were independent of ¹⁵NO₃⁻ uptake rate and carbohydrate status, but were reciprocally related to rhythmic adjustments in stomatal resistance and, presumably, water movement through the root system. Whole plant reduction of ¹⁵NO₃⁻ always was limited more than uptake. The assimilation of ¹⁵N into insoluble reduced-N in roots remained a constant proportion of uptake throughout, while assimilation in the shoot declined markedly in the first 18 hours of darkness before stabilizing at a low level. The plants clearly retained a capacity for ¹⁵NO₃⁻ reduction and synthesis of insoluble reduced-¹⁵N even when ¹⁵NO₃⁻ uptake was severely restricted and minimal carbohydrate reserves remained in the tissue.     

Localization of Nitrate Absorption and Translocation within Morphological Regions of the Corn Root

The absorption of NO₃⁻ was characterized in six regions of a 7-d-old corn root (Zea mays L. cv W64A × W182E) growing in a complete nutrient solution. Based on changing rates of ¹⁵N accumulation during 15-min time courses, translocation of the concurrently absorbed N through each region of the intact root was calculated and distinguished from direct absorption from the medium. Of the ¹⁵N accumulated in the 5-mm root tip after 15 min, less than 15 and 35% had been absorbed directly from the external solution at 0.1 and 10 mm NO₃⁻ concentration of the external solution, respectively. The characterization of the apical portion of the primary root as a sink for concurrently absorbed N was conconfirmed in a pulse-chase experiment that showed an 81% increase of ¹⁵N in the 5-mm root tip during a 12-min chase (subsequent to a 6-min labeling period). The lateral roots alone accounted for 60% of root influx and 70% of 15-min whole root ¹⁵N accumulation at either 0.1 or 10 mm. NO₃⁻ concentration of the external solution. Because relatively steady rates of ¹⁵N accumulation in the shoot were reached after 6 min, the rapidly exchanging pools in lateral roots must have been involved in supplying ¹⁵N to the shoot. The laterals and the basal primary root also showed large decreases (24 and 17%) in ¹⁵N during the chase experiment, confirming their role in rapid translocation.     

In Vivo Determination of Parameters of Nitrate Utilization in Wheat (Triticum aestivum L.) Seedlings Grown with Low Concentration of Nitrate in the Nutrient Solution

Six genotypes of winter wheat (Triticum aestivum L.) differing in grain protein concentration were grown on a nutrient solution containing low concentrations of NO₃⁻ (2 millimolar). Total NO₃⁻ uptake varied between genotypes but was not related to grain protein content. An in vivo nitrate reductase assay was used to determine the affinity of the enzyme for NO₃⁻, and large phenotypic variations were observed. In vivo estimations of the concentration and size of the metabolic pool were variable. However, the three genotypes with the higher ratios of metabolic pool size to leaf total NO₃⁻ concentration were the high protein varieties. It is proposed that a high affinity of nitrate reductase for nitrate might be a biochemical marker for the capacity of the plant to continue assimilating NO₃⁻ for a longer period during the last stage of growth.     

Influence of Nitrate and Ammonium Nutrition on the Uptake, Assimilation, and Distribution of Nutrients in Ricinus communis

Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO₃⁻ or NH₄⁺, the solutions being maintained at pH 5.5. In NO₃⁻-fed plants excess nutrient anion over cation uptake was equivalent to net OH⁻ efflux, and the total charge from NO₃⁻ and SO₄²⁻ reduction equated to the sum of organic anion accumulation plus net OH⁻ efflux. In NH₄⁺-fed plants a large H⁺ efflux was recorded in close agreement with excess cation over anion uptake. This H⁺ efflux equated to the sum of net cation (NH₄⁺ minus SO₄²⁻) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO₃⁻ that is taken up and reduced in NO₃⁻-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH₄⁺-fed plants absorbed NH₄⁺ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO₃⁻-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients.     

Postanthesis nitrate assimilation in winter wheat : in situ flag leaf reduction

When adequate levels of soil NO₃⁻ are available, concurrent NO₃⁻ absorption and assimilation, and mobilization of vegetative N reserves accumulated prior to anthesis, may be used to supply N to developing wheat (Triticum aestivum L.) kernels. Vegetative wheat components (stems, leaves, spike) are known to possess NO₃⁻ reductase activity, but the in situ utilization of NO₃⁻ translocated to the shoot has not been studied. Assimilation and partitioning of ¹⁵N was determined in winter wheat `Doublecrop.' At 7 days after anthesis, the stem immediately above the peduncle node was heat girdled to block phloem export from the flag leaf. Control plants were not girdled. One day later, 50 micromoles of ¹⁵NO₃⁻ (98 atom percent ¹⁵N) was injected into the penultimate internodal lacuna, after which ¹⁵NO₃⁻ utilization was determined sequentially over a 5 day period. Based on differences in spike accumulation of reduced ¹⁵N excess between treatments and the amount of reduced ¹⁵N excess remaining in the flag leaf, it was estimated that the flag leaf contributed 37% of the total reduced ¹⁵N excess in the injected shoot. The lower shoot contribution was 18% and that of the peduncle plus spike was 45%.     

Nitrogen retranslocation in plants of maize,lupin and cocklebur

Summary In a split root experiment translocation of N from shoot to root was studied using¹⁵NO ₃ ^– . The three plant species selected for this experiment differed significantly with respect to root NRA. For lupin, maize and cocklebur about 80, 50 and 6% of all absorbed NO ₃ ^– was assmilated in the roots, respectively.Although NO ₃ ^– was reduced in the roots of lupin and maize plants to a greater extent than required for the roots' demand for organic N, a significant phloem flow of N from shoot to roots was found in these plants. Unexpectedly, for cocklebur, the plant with the very low root NRA, the fraction of total N present in the root that has been imported from the shoot was only half that as found for lupin and maize.     

A dynamic whole-plant model of integrated metabolism of nitrogen and carbon. 2. Balanced growth driven by C fluxes and regulated by signals from C and N substrate

A whole-plant model of C and N metabolism is presented for the juvenile stage. It is aimed at comparing the growth performance of (wild) plant species in a range of environments with respect to irradiance and availability of nitrate (NO₃ ^-) and ammonium (NH₄ ⁺). State variables are the structural masses of leaves, stem and root, NO₃ ^- concentrations in root and shoot, non-structural carbohydrate (C) densities in leaves, stem and root and non-structural organic N concentration in the whole plant. Explicit expressions for NO₃ ^- influx, efflux, translocation and assimilation, and for NH₄ ⁺ uptake and assimilation have been formulated in an accompanying paper. Photosynthetic rate is derived from electron-transport rate which depends on irradiance and chlorophyll concentration on a leaf-area basis. The latter is proportional to non-structural organic N concentration. Photosynthetic N is considered non-structural. Unique features of the model are the use of metabolite signals and the treatment of C allocation and balanced growth. Metabolite signals are dimensionless functions of non-structural compounds (NO₃ ^-, C, organic N) and modify rate variables involved in N uptake and assimilation, C allocation and growth. Carbon allocation is driven by concentration differences of the cytosolic C pools in stem and root and is modified by the N status of the plant such that a high N status increases the apparent size of the shoot. Photosynthate is unloaded into C buffers which degrade at a constant specific rate. The sugar fluxes which arise from these buffers drive the growth rate of stem and root. No parameters are included for maximum specific growth or for activity or strength of sinks. Primary stem growth is proportional to growth of the leaf compartment: leaves arise from stems in a modular fashion. Leaves are autonomous with respect to their C balance. The model is presented as a system of differential equations which is integrated numerically. Parameter values, e.g., for uptake and assimilation capacities and costs of uptake, assimilation, maintenance and growth, are estimated for a grass species, Dactylis glomerata. Juvenile growth is simulated under optimal conditions with respect to irradiance and NO₃ ^- availability and compared with literature data. Diurnal and daily patterns of C utilisation and respiration, expressed as percentages of gross photosynthetic rate, are discussed. The model satisfactorily simulates typical responses to nutrient and light limitation and pruning, such as redirected C allocation, adjusted root and leaf weight ratios and compensatory growth. A sensitivity analysis is included for selected parameters. This revised version was published online in June 2006 with corrections to the Cover Date.     

Assimilation of NO(3) Taken Up by Plants in the Light and in the Dark

An experiment was conducted to determine the extent that NO₃⁻ taken up in the dark was assimilated and utilized differently by plants than NO₃⁻ taken up in the light. Vegetative, nonnodulated soybean plants (Glycine max L. Merrill, `Ransom') were exposed to ¹⁵NO₃⁻ throughout light (9 hours) or dark (15 hours) phases of the photoperiod and then returned to solutions containing ¹⁴NO₃⁻, with plants sampled subsequently at each light/dark transition over 3 days. The rates of ¹⁵NO₃⁻ absorption were nearly equal in the light and dark (8.42 and 7.93 micromoles per hour, respectively); however, the whole-plant rate of ¹⁵NO₃⁻ reduction during the dark uptake period (2.58 micromoles per hour) was 46% of that in the light (5.63 micromoles per hour). The lower rate of reduction in the dark was associated with both substantial retention of absorbed ¹⁵NO₃⁻ in roots and decreased efficiency of reduction of ¹⁵NO₃⁻ in the shoot. The rate of incorporation of ¹⁵N into the insoluble reduced-N fraction of roots in darkness (1.10 micromoles per hour) was somewhat greater than that in the light (0.92 micromoles per hour), despite the lower rate of whole-plant ¹⁵NO₃⁻ reduction in darkness.

A large portion of the ¹⁵NO₃⁻ retained in the root in darkness was translocated and incorporated into insoluble reduced-N in the shoot in the following light period, at a rate which was similar to the rate of whole-plant reduction of ¹⁵NO₃⁻ acquired during the light period. Taking into account reduction of NO₃⁻ from all endogenous pools, it was apparent that plant reduction in a given light period (~13.21 micromoles per hour) exceeded considerably the rate of acquisition of exogenous NO₃⁻ (8.42 micromoles per hour) during that period. The primary source of substrate for NO₃⁻ reduction in the dark was exogenous NO₃⁻ being concurrently absorbed. In general, these data support the view that a relatively small portion (<20%) of the whole-plant reduction of NO₃⁻ in the light occurred in the root system.

     

Plant Growth-Promoting Rhizobacteria Inoculation to Enhance Vegetative Growth,Nitrogen Fixation and Nitrogen Remobilisation of Maize under Greenhouse Conditions

Plant growth-promoting rhizobacteria (PGPR) may provide a biological alternative to fix atmospheric N₂ and delay N remobilisation in maize plant to increase crop yield, based on an understanding that plant-N remobilisation is directly correlated to its plant senescence. Thus, four PGPR strains were selected from a series of bacterial strains isolated from maize roots at two locations in Malaysia. The PGPR strains were screened in vitro for their biochemical plant growth-promoting (PGP) abilities and plant growth promotion assays. These strains were identified as Klebsiella sp. Br1, Klebsiella pneumoniae Fr1, Bacillus pumilus S1r1 and Acinetobacter sp. S3r2 and a reference strain used was Bacillus subtilis UPMB10. All the PGPR strains were tested positive for N₂ fixation, phosphate solubilisation and auxin production by in vitro tests. In a greenhouse experiment with reduced fertiliser-N input (a third of recommended fertiliser-N rate), the N₂ fixation abilities of PGPR in association with maize were determined by ¹⁵N isotope dilution technique at two harvests, namely, prior to anthesis (D₅₀) and ear harvest (D₆₅). The results indicated that dry biomass of top, root and ear, total N content and bacterial colonisations in non-rhizosphere, rhizosphere and endosphere of maize roots were influenced by PGPR inoculation. In particular, the plants inoculated with B. pumilus S1r1 generally outperformed those with the other treatments. They produced the highest N₂ fixing capacity of 30.5% (262 mg N₂ fixed plant⁻¹) and 25.5% (304 mg N₂ fixed plant⁻¹) of the total N requirement of maize top at D₅₀ and D₆₅, respectively. N remobilisation and plant senescence in maize were delayed by PGPR inoculation, which is an indicative of greater grain production. This is indicated by significant interactions between PGPR strains and time of harvests for parameters on N uptake and at. % ¹⁵N_e of tassel. The phenomenon is also supported by the lower N content in tassels of maize treated with PGPR, namely, B. pumilus S1r1, K. pneumoniae Fr1, B. subtilis UPMB10 and Acinetobacter sp. S3r2 at D₆₅ harvest. This study provides evidence that PGPR inoculation, namely, B. pumilus S1r1 can biologically fix atmospheric N₂ and provide an alternative technique, besides plant breeding, to delay N remobilisation in maize plant for higher ear yield (up to 30.9%) with reduced fertiliser-N input.