Zinc sources for rice in soil at different moisture regimes and organic matter levels

Summary Laboratory incubation experiment was conducted with a clay loam alluvial lowland rice soil to study the relative effectiveness of two sources of Zn (ZnEDTA and ZnSO₄) in maintaining Zn availability in soil under two moisture regimes (saturated and waterlogged) both in presence as well as absence of added organic matter. The results showed that ZnEDTA was always more effective than ZnSO₄ in maintaining higher amount of zinc in available form in soil for a longer perid. Results of greenhouse experiment conducted with rice showed that concentration and uptake of Zn by roots were generally higher with ZnEDTA than with ZnSO₄ both in presence and absence of added organic matter, whereas in respect of shoot this was true only in absence of added organic matter.     

Response of flooded rice grown on a vertisol from northern nigeria to zinc sources and methods of application

Summary Greenhouse and laboratory studies were conducted to study the effect of zinc sources and methods of application on correcting zinc deficiency in flooded rice grown on Vertisol from Ngala, northern Nigeria, using the variety IR-20.Plant dry matter yield was similar for ZnSO₄, ZnEDTA, metallic Zn and fritted Zn with mixed soil application. Zinc uptake was affected in the following order; ZnSO₄ > ZnEDTA > metallic Zn > fritted Zn. Comparable dry matter yield and zinc uptake were obtained with mixing, surface broadcasting and banding of ZnEDTA. Mixing the fritted Zn gave higher dry matter yield and zinc uptake than broadcasting or banding.Seed soaking with a suspension of fritted Zn resulted in higher dry matter yield and zinc uptake than with ZnEDTA solution. Seed soaking for 24 hours with fritted Zn suspension at a concentration of about 0.5 per cent Zn appeared to be a suitable method for applying zinc with direct seeded rice.     

Options for mitigating methane emission from a permanently flooded rice field

Permanently flooded rice fields, widely distributed in south and south‐west China, emit more CH₄ than those drained in the winter crop season. For understanding CH₄ emissions from permanently flooded rice fields and developing mitigation options, CH₄ emission was measured year‐round for 6 years from 1995 to 2000, in a permanently flooded rice field in Chongqing, China, where two cultivations with four treatments were prepared as follows: plain‐cultivation, summer rice crop and winter fallow with floodwater layer annually (convention, Ch‐FF), and winter upland crop under drained conditions (Ch‐Wheat); ridge‐cultivation without tillage, summer rice and winter fallow with floodwater layer annually (Ch‐FFR), and winter upland crop under drained conditions (Ch‐RW), respectively. On a 6‐year average, compared to the treatments with floodwater in the winter crop season, the CH₄ flux during rice‐growing period from the treatments draining floodwater and planting winter crop was reduced by 42% in plain‐cultivation and by 13% in ridge‐cultivation (P < 0.05), respectively. The reduction of annual CH₄ emission reached 68 and 48%, respectively. Compared to plain‐cultivation (Ch‐FF), ridge‐cultivation (Ch‐FFR) reduced annual CH₄ emission by 33%, and which was mainly occurred in the winter crop season. These results indicate that draining floodwater layer for winter upland crop growth was not only able to prevent CH₄ emission from permanently flooded paddy soils directly in the winter crop season, but also to reduce CH₄ emission substantially during the following rice‐growing period. As an alternative to the completely drainage of floodwater layer in the winter crop season, ridge‐cultivation could also significantly mitigate CH₄ emissions from permanently flooded rice fields.     

Biofortification and Bioavailability of Rice Grain Zinc as Affected by Different Forms of Foliar Zinc Fertilization

Background

Zinc (Zn) biofortification through foliar Zn application is an attractive strategy to reduce human Zn deficiency. However, little is known about the biofortification efficiency and bioavailability of rice grain from different forms of foliar Zn fertilizers.

Methodology/Principal Findings

Four different Zn forms were applied as a foliar treatment among three rice cultivars under field trial. Zinc bioavailability was assessed by in vitro digestion/Caco-2 cell model. Foliar Zn fertilization was an effective agronomic practice to promote grain Zn concentration and Zn bioavailability among three rice cultivars, especially, in case of Zn-amino acid and ZnSO₄. On average, Zn-amino acid and ZnSO₄ increased Zn concentration in polished rice up to 24.04% and 22.47%, respectively. On average, Zn-amino acid and ZnSO₄ increased Zn bioavailability in polished rice up to 68.37% and 64.43%, respectively. The effectiveness of foliar applied Zn-amino acid and ZnSO₄ were higher than Zn-EDTA and Zn-Citrate on improvement of Zn concentration, and reduction of phytic acid, as a results higher accumulation of bioavailable Zn in polished rice. Moreover, foliar Zn application could maintain grain yield, the protein and minerals (Fe and Ca) quality of the polished rice.

Conclusions

Foliar application of Zn in rice offers a practical and useful approach to improve bioavailable Zn in polished rice. According to current study, Zn-amino acid and ZnSO₄ are recommended as excellent foliar Zn forms to ongoing agronomic biofortification.     

Mechanism of nitrogen effect on zinc nutrition of flooded rice

Summary Nitrogen application increased Zn contents of flooded rice on two calcareous soils. Urea and (NH₄)₂SO₄ being better N carriers than NH₄NO₃ resulted in higher increase. Nitrogen enhanced Zn contents partly through growth promotion but mainly by increasing soil Zn solubility and root efficiency for Zn absorption. Zinc solubility rose by an enigmatic mechanism and not from pH reduction or soluble Zn-HN₃ complex formation as occurs for upland plants. Nitrogen aggravated Zn retention in upland plant roots as immobile Zn-protein complex was not important for rice. Bicarbonate inhibition of Zn uptake by rice from CO(NH₂)₂ application or its stimulation by lower redox potential from NH₄NO₃ addition were not involved.No. V in the series Micronutrient availability to cereals from calcareous soils.     

Reaction of labelled65ZnCl2,65ZnEDTA and65ZnDTPA with different clay-systems and some alluvial Egyptian soils

Summary The clay fraction separated from an alluvial Egyptian soil and montmorillonite clay mineral were equilibrated with CaCl₂ or NaCl solution then treated with humic acid isolated from composted clover straw to obtain different clay systems: Ca-clay, Ca-clay-HA, Na-clay, Na-clay-HA, Ca-mont and Ca-mont-HA. These clays as well as seven soil samples were reacted with different amounts of labelled⁶⁵ZnCl₂,⁶⁵ZnEDTA and⁶⁵ZnDTPA. The effectiveness of these Zn-sources for maintaining soluble Zn²⁺ ions in the equilibrium solution was the greatest for ZnDTPA and the lowest for ZnCl₂. Ca-clay provided greater Zn sorption capacity than Na-clay, and complexing the clay with humic acid depressed its capacity for Zn sorption. At the pH of the clay-systems (pH=6.5), the possibilities of Zn(OH)₂ formation were reduced especially in the presence of Zn-chelates. Reactions of⁶⁵ZnE DTA and⁶⁵ZnDTPA with the seven soils produced higher levels of soluble Zn²⁺ ions in the equilibrium solution rather than⁶⁵ZnCl₂ meanwhile ZnDTPA was more effective than ZnEDTA. The calculated Zn(OH)₂ ion product in the solution of ZnCl₂-soil systems indicated the precipitation of Zn as Zn(OH)₂. However, this was not valid in the Zn-chelates-soil systems. The results also revealed the role of soil carbonate, organic matter and soil texture as soil variables affecting Zn sorption by natural soils.     

Soil temperature and nitrogen effects on response of flooded and nonflooded rice to zinc

Summary Response of direct seeded rice (cv. Bluebelle) to Zn was studied in flooded and nonflooded (field capacity) Crowley soil (pH 7.6) maintained at soil temperatures of 18 and 30°C. Urea and (NH₄)₂SO₄ were compared as sources of N to determine their effect on plant uptake of Zn from ZnSO₄ either mixed or surface applied to the soil. Grain yields were slightly higher from nonflooded than from flooded soil. Higher dry matter production at 30 than at 18°C was not related to Zn nutrition. Urea and (NH₄)₂SO₄ resulted in similar yields and Zn uptake by flooded rice, but (NH₄)₂SO₄ was superior for nonflooded rice in the absence of applied Zn. More fixation of mixed Zn by the limed Crowley soil probably caused its lower effectiveness, as compared to surface-applied Zn.     

Zinc Mobility in Wheat: Uptake and Distribution of Zinc Applied to Leaves or Roots

Little is known about transport of Zn from leaves to other plantorgans. The present study tested a range of Zn forms appliedfoliarly for their suitability to provide adequate Zn nutritionto wheat (Triticum aestivum L.). Transport of⁶⁵Zn applied eitherto leaves or to one side of the root system was also studied.Inorganic (ZnO, ZnSO₄) and chelated sources of Zn (ZnEDTA, glycine-chelatedBiomin Zn) applied foliarly provided sufficient Zn for vigorousgrowth. Zinc concentrations in roots and shoots were in thesufficiency range, except in the -Zn control. Foliar treatmentswith ZnSO₄and chelated Zn forms resulted in shoot Zn concentrationsin 7-week-old plants being about two-fold greater than thosein plants supplied with Zn in the root environment or via foliarspray of ZnO. Adding surfactant to foliar sprays containingchelated forms of Zn did not cause negative growth effects,but surfactant added to ZnO or ZnSO₄foliar sprays decreasedshoot growth. Adding urea to the ZnO foliar spray had no effecton shoot growth. Foliarly-applied⁶⁵Zn was translocated to leavesabove and below the treated leaf as well as to the root tips.Stem girdling confirmed that⁶⁵Zn transport toward lower leavesand roots was via the phloem. Split-root experiments showedintensive accumulation of⁶⁵Zn in the stem and transport to allleaves as well as to the root tips in the non-labelled sideof the root system. Foliar application of Zn in inorganic ororganic form is equally suitable for providing adequate Zn nutritionto wheat. Phloem transport of Zn from leaves to roots was demonstrated.Copyright 2001 Annals of Botany Company Foliar spraying, phloem, surfactant, urea, xylem, wheat, zinc     

Mobilization of iron by phytosiderophores as affected by other micronutrients

It has been shown previously (Treeby et al., 1989) that phytosiderophores, released by roots of iron deficient grasses (Gramineae), mobilize from calcareous soils not only iron (Fe) but also zinc (Zn), manganese (Mn) and copper (Cu). Mobilization of Fe may therefore be impaired by other micronutrient cations. This has been studied in both, model experiments with Fe hydroxide and with a calcareous soil (15% CaCO₃, pH 8.6) amended with micronutrients as sulfate salts.Mobilization of Fe from Fe hydroxide by phytosiderophores (epi-3-hydroxymugineic acid) was not affected by the addition of CaCl₂, MgSO₄ and MnSO₄, slightly inhibited by ZnSO₄ and strongly inhibited by CuSO₄. In a calcareous soil amended with increasing levels of ZnSO₄, MnSO₄ and CuSO₄, mobilization of Fe by phytosiderophores remained uneffected by Zn and Mn amendments but was progressively impaired by increasing levels of Cu amendment, correlated with corresponding enhancement of Cu mobilization.High concentrations of ZnSO₄ and MnSO₄ and relatively high concentrations of CuSO₄ were required for inhibition of Fe mobilization by phytosiderophores. It is therefore concluded that in most calcareous soils phytosiderophores efficiently mobilize Fe, and that phytosiderophores play an important role in Fe acquisition by grasses grown on calcareous soils.     

Uptake and Distribution of Soil Applied Zinc by Citrus Trees—Addressing Fertilizer Use Efficiency with 68Zn Labeling

The zinc (Zn) supply increases the fruit yield of Citrus trees that are grown, especially in the highly weathered soils of the tropics due to the inherently low nutrient availability in the soil solution. Leaf sprays containing micronutrients are commonly applied to orchards, even though the nutrient supply via soil could be of practical value. This study aimed to evaluate the effect of Zn fertilizers that are applied to the soil surface on absorption and partitioning of the nutrient by citrus trees. A greenhouse experiment was conducted with one-year-old sweet orange trees. The plants were grown in soils with different textures (18.1 or 64.4% clay) that received 1.8 g Zn per plant, in the form of either ZnO or ZnSO₄ enriched with the stable isotope ⁶⁸Zn. Zinc fertilization increased the availability of the nutrient in the soil and the content in the orange trees. Greater responses were obtained when ZnSO₄ was applied to the sandy loam soil due to its lower specific metal adsorption compared to that of the clay soil. The trunk and branches accumulated the most fertilizer-derived Zn (Zn_dff) and thus represent the major reserve organ for this nutrient in the plant. The trees recovered up to 4% of the applied Zn_dff. Despite this relative low recovery, the Zn requirement of the trees was met with the selected treatment based on the total leaf nutrient content and increased Cu/Zn-SOD activity in the leaves. We conclude that the efficiency of Zn fertilizers depends on the fertilizer source and the soil texture, which must be taken into account by guidelines for fruit crop fertilization via soil, in substitution or complementation of traditional foliar sprays.     

Effect of zinc sources and levels on the growth and Zn nutrition of soybean (Glycine max. L.) in the presence of chloride and sulphate salinity

A study was conducted in a screen house in pots on a sandy loam soil deficient in Zn. Salinity was induced by adding 44, 88 and 132 me/l of chloride and sulphate salts in the saturation extract. To these treatments, 0, 5 and 10 ppm Zn were added as ZnSO₄·7H₂O or Zn-EDTA. The results indicated that the yield of soybean shoot was lowest at the highest salinity level and highest at the lowest level. Shoot yield improved markedly with Zn application. Both sources of Zn were equally effective in augmenting crop yields. Yields were low in Cl-salinity when compared with equivalent levels of SO₄-salinity. Application of ZnSO₄·7H₂O produced higher yields in SO₄-dominant salinity. Zinc content increased and Zn uptake decreased with increase in Cl-salinity regardless of Zn sources. In SO₄-salinity, ZnSO₄·7H₂O did not influence the Zn content, but uptake was suppressed with increase in SO₄-salinity. Increasing rates of SO₄-salinity enhanced Zn content in the presence of Zn-EDTA.     

Effect of zinc-amended poultry manure and zinc sulphate on the growth and uptake of zinc by corn (Zea mays L.)

Summary A greenhouse experiment conducted to study the effect of different levels of Zn supplied through Zn-amended poultry manure and ZnSO₄ on corn (Zea mays L.) indicated that both the sources significantly increased the dry matter yield and uptake of zinc. The zinc amended poultry manure at all levels of zinc application was more effective than ZnSO₄ in this respect. The percent zinc in corn derived from the fertilizer and percent utilization of applied zinc from the Zn-amended poultry manure was higher than that from ZnSO₄ when applied at equivalent levels.     

Contribution of inoculation with azolla combined with nitrogen,phosphorus and zinc to rice in Nile Delta

Rice performance, in terms of plant height, productive tillering, yield and N-contents of grain and straw, harvest index (grain yield as a percentage of grain plus straw yields) and relative fertilizer N-use efficiency (kg grain yield/kg fertilizer-N) was enhanced by urea, ZnSO₄ and green manuring withAzolla caroliniana. Unlike urea fertilizer, calcium superphosphate increased the rate of azolla field colonization and promoted a thick, healthy dark-green mat of the fern. Response to ZnSO₄ was higher in the azolla-free sub-subplots. Application of ZnSO₄ corrected symptoms of Zn deficiency in rice but the addition of calcium superphosphate in the absence of Zn intensified the symptoms of Zn deficiency.     

Availability of iron,manganese, zinc and phosphorus in submerged sodic soil as affected by amendments during the growth period of rice crop

Summary Effect of amendments, gypsum (12.5 tonnes/ha), farmyard manure (30 tonnes/ha), rice husk (30 tonnes/ha) and also no amendment (control) on the availability of native Fe, Mn and P and applied Zn in a highly sodic soil during the growth period of rice crop under submerged conditions was studied in a field experiment. Soil samples were collected at 0, 30, 60 and 90 days of crop growth. Results showed that extractable Fe (1N NH₄OAC pH 3) and Mn (1N NH₄OAC pH 7) increased with submergence upto 60 days of crop growth but thereafter remained either constant or declined slightly. Application of farmyard manure and rice husk resulted in marked improvement of these elements over gypsum and control. Increases in extractable Mn (water soluble plus exchangeable) as a result of submergence and crop growth under different amendments were accompanied by corresponding decreases in easily reducible Mn content of the soil. Application of 40 kg zinc sulphate per hectare to rice crop could substantially raise the available Zn status (DTPA extractable) of the soil in gypsum and farmyard manure treated plots while the increase was only marginal in rice husk and control plots indicating greater fixation of applied Zn. Available P (0.5M NaHCO₃ pH 8.5) behaved quite differently and decreased in the following order with crop growth: gypsum>rice husk>farmyard manure>control.     

Nitrogen transformation processes in relation to improved cultural practices for lowland rice

Summary Inappropriate method and timing of N fertilizer application was found to result in 50–60% N losses. Recent nitrogen transformation studies indicate that NH₃ volatilization in lowland rice soils is an important loss mechanism, causing a 5–47% loss of applied fertilizer under field conditions. Estimated denitrification losses were between 28 and 33%. Ammonia volatilization losses from lowland rice can be controlled by i) placement of fertilizer in the reduced layer and proper timing of application, ii) using phenylphosphorodiamidate (PPD) to delay urease activity in flooded soils, and iii) using algicides to help stabilize changes in floodwater pH. Appropriate fertilizer placement and timing is probably the most effective technique in controlling denitrification at the farm level. The effectivity of nitrification inhibitors as another method is still being evaluated. With 60–80% of N absorbed by the crop derived from the native N pool, substantial yield gains in lowland rice are highly possible with resources already in the land. Extensive studies on soil N and its management, and an understanding of soil N dynamics will greatly facilitate the decrease in immobilization and ammonium fixation in the soil and the increase in N availability to the rice crop. Critical research needs include greater emphasis on N transformation processes in rainfed lowland rice which is grown under more harsh and variable environmental regimes than irrigated lowland rice.     

Rhizosphere pH as a result of nitrogen levels and NH4/NO3 ratio and its effect on zinc availability and on growth of rice flower (Ozothamnus diosmifolius)

The effect of modification of the rhizosphere pH, via solution-N concentration and source, on rice flower (Ozothamnus diosmifolius, Astraceae) growth was investigated in two different experiments. In order to simulate a wide range of pHs easily, the plants were grown in an inert artificial substrate (perlite). In the first the rhizosphere pH was modified through variation of N concentrations and the NH₄/NO₃-N ratio in the irrigation water. In the second the rhizosphere pH was modified solely by altering the NH₄/NO₃-N ratio while irrigation-N concentration was held at the level found to be optimal in the first experiment. Cultivation of rice flower, a new crop in Israel, is hampered by lack of knowledge on its Zn nutrition. Because availability of soil Zn largely depends on pH we investigated in the second experiment the effect of Zn foliar application. The growth of rice flower plants under low-N fertilization or low NH₄/NO₃-N ratio was poor and the plants exhibited growth disorders such as tipburn, severe chlorosis and necrosis. These growth disorders could not be ascribed to any direct effect of N nutrition therefore it was suggested that the indirect effect of the treatments, e.g., the rhizosphere pH dominates rice flower growth through its effect on nutrient availability. The only nutrient that was significantly correlated with pH and yield parameters in both experiments was Zn. All irrigation-nutrients concentrations were within the recommended range for hydroponically grown plants; however, the leaf-Mn concentration of plants grown in pH above 7.5 was in the toxic range while that of Zn was deficient. The high preferential uptake of Mn over Zn by rice flower plants and the question of whether high Mn uptake induced Zn deficiency remain open.     

Characterization of natural variation for zinc, iron and manganese accumulation and zinc exposure response in Brassica rapa L.

Brassica rapa L. is an important vegetable crop in eastern Asia. The objective of this study was to investigate the genetic variation in leaf Zn, Fe and Mn accumulation, Zn toxicity tolerance and Zn efficiency in B. rapa. In total 188 accessions were screened for their Zn-related characteristics in hydroponic culture. In experiment 1, mineral assays on 111 accessions grown under sufficient Zn supply (2 μM ZnSO₄) revealed a variation range of 23.2–155.9 μg g⁻¹ dry weight (d. wt.) for Zn, 60.3–350.1 μg g⁻¹ d. wt. for Fe and 20.9–53.3 μg g⁻¹ d. wt. for the Mn concentration in shoot. The investigation of tolerance to excessive Zn (800 μM ZnSO₄) on 158 accessions, by using visual toxicity symptom parameters (TSPs), identified different levels of tolerance in B. rapa. In experiment 2, a selected sub-set of accessions from experiment 1 was characterized in more detail for their mineral accumulation and tolerance to excessive Zn supply (100 μM and 300 μM ZnSO₄). In this experiment Zn tolerance (ZT) determined by relative root or shoot dry biomass varied about 2-fold. The same six accessions were also examined for Zn efficiency, determined as relative growth under 0 μM ZnSO₄ compared to 2 μM ZnSO₄. Zn efficiency varied 1.8-fold based on shoot dry biomass and 2.6-fold variation based on root dry biomass. Zn accumulation was strongly correlated with Mn and Fe accumulation both under sufficient and deficient Zn supply. In conclusion, there is substantial variation for Zn accumulation, Zn toxicity tolerance and Zn efficiency in Brassica rapa L., which would allow selective breeding for these traits.     

Response of Contrasting Rice Genotypes to Zinc Sources under Saline Conditions

Abiotic stresses are among the major limiting factors for plant growth and crop productivity. Among these, salinity is one of the major risk factors for plant growth and development in arid to semi-arid regions. Cultivation of salt tolerant crop genotypes is one of the imperative approaches to meet the food demand for increasing population. The current experiment was carried out to access the performance of different rice genotypes under salinity stress and Zinc (Zn) sources. Four rice genotypes were grown in a pot experiment and were exposed to salinity stress (7 dS m⁻¹), and Zn (15 mg kg⁻¹ soil) was applied from two sources, ZnSO₄ and Zn-EDTA. A control of both salinity and Zn was kept for comparison. Results showed that based on the biomass accumulation and K⁺/Na⁺ ratio, KSK-133 and BAS-198 emerged as salt tolerant and salt sensitive, respectively. Similarly, based on the Zn concentration, BAS-2000 was reported as Zn-in-efficient while IR-6 was a Zn-efficient genotype. Our results also revealed that plant growth, relative water content (RWC), physiological attributes including chlorophyll contents, ionic concentrations in straw and grains of all rice genotypes were decreased under salinity stress. However, salt tolerant and Zn-in-efficient rice genotypes showed significantly higher shoot K⁺ and Zn concentrations under saline conditions. Zinc application significantly alleviates the harmful effects of salinity by improving morpho-physiological attributes and enhancing antioxidant enzyme activities, and the uptake of K and Zn. The beneficial effect of Zn was more pronounced in salt-tolerant and Zn in-efficient rice genotypes as compared with salt-sensitive and Zn-efficient genotypes. In sum, our results confirmed that Zn application increased overall plant’s performance under saline conditions, particularly in Zn in-efficient and tolerant genotypes as compared with salt-sensitive and Zn efficient rice genotypes.     

Effect of green manure and floodwater algae on diurnal fluctuations of floodwater pH and depth of aerobic soil layer under lowland rice conditions

The present investigation was undertaken to elucidate the effect of floodwater algae and green manure on floodwater pH and depth of aerobic soil layer which are mainly responsible for the nitrogen losses in lowland rice production systems. The study was conducted in an environmental chamber using a sandy loam soil. Cylindrical plastic bottles 7 cm in diameter were used and 5 cm soil layer covered with 5 cm deep floodwater was created in each bottle to simulate submerged rice conditions. The presence of algae in the floodwater increased the floodwater pH owing to removal of CO₂ during photosynthesis during the day, the value decreasing again during the night. Addition of green manure, without algal growth, depressed the pH for the first week and after 4 weeks of incubation. With algae, the application of green manure in vitro resulted in lower diurnal pH increases especially for the first 2 weeks of incubation and a consistently higher morning pH throughout the study. The addition of green manure eliminated the aerobic soil layer below the soil-water interface, presumably because green manure cause a high O₂ demand owing to increased microbial activity which results in a lower Eh value.     

Nutrient changes in direct-seeded submerged rice soils with varying nutrio-environments

Summary Nutrient changes in submerged rice soil were studied in soil-water-plant ecosystem in direct-seeded rice crop. Continuous removal of nutrients by the crop resulted in ultimate decrease in the availability of NH₄–N, P, K, Ca, Mg, Mn, Zn and Cu. However, there was a slight increase in Fe availability in soil with increase in period of submergence and crop growth. The data was subjected to statistical function fittings to study the nature of changes. Depending on the R²% values, quadratic type was the best fit for pH, available NH₄–N, K, Mg, Fe, Mn and Cu, whereas logarithmic type was the best fit for available P, Ca and Zn. No response was noticed to the application of P and K. Highest correlation coefficient between grain yield and NH₄–N in soil was obtained at the panicle initiation stage indicating that limiting nitrogen during this period might affect grain yield to the maximum extent compared to tiller initiation and maximum tillering stages.