Effect of nitrogen form on maize response to drought stress

Two hybrids of maize (Zea mays L.) differing in resistance to drought, were grown in chernozem soil in a greenhouse and were fertilized with two different forms of nitrogen: Ca(NO₃)₂ and (NH₄)₂SO₄ in concentrations corresponding to 100 kg of N ha^-1. After emergence of the 4th leaf, plants were exposed to drought. During the drought period, the parameters of plant water status (water potential, osmotic potential, turgor pressure and relative water content) and chlorophyll a+b concentration were monitored every two days. N and K concentration and accumulation over the drought period were also monitored.Next to differences in adaptability of the two hybrids to drought, the results demonstrate different adaptability of NH₄ and NO₃-treated plants within each hybrid. NH₄-plants of each hybrid maintain higher turgor pressure during the drought by better osmotic adaptation. Especially significant differences appear between chlorophyll (a+b) values of NH₄ and NO₃-treated plants and as affected by drought. Chlorophyll concentrations of NH₄-plants are higher than those of NO₃-plants both in control and droughted plants. NH₄ plants show a characteristic initial chlorophyll increase at the beginning of the drought period while in NO₃ plants chlorophyll constantly decreases throughout the whole drought period. The influence of the nitrogen form on chlorophyll concentration changes during drought does not appear to be affected by regulation of the K concentration.     

Effect of primary leaves on 59Fe uptake by roots and 59Fe distribution in the shoot of iron sufficient and iron deficient bean (Phaseolus vulgaris L.) plants

A study has been made on the effect of primary leaves on iron (Fe) distribution in the shoot. Bean (Phaseolus vulgaris L.) seedlings were precultured in nutrient solution with 8×10^-5 M FeEDTA for 4 days, and then grown further with either 8×10^-5 M FeEDTA (+Fe) or without Fe supply (-Fe) for another 5 days. Thereafter, both +Fe and -Fe plants were treated in three different ways: undisturbed; one primary leaf removed; or one primary leaf shaded, starting two hours before supply ⁵⁹FeEDTA to the roots. The +Fe plants were supplied with 8×10^-5 M ⁵⁹FeEDTA, and the -Fe plants with only 1×10^-6 M ⁵⁹FeEDTA. After 1 to 8 hour uptake periods, plants were harvested and ⁵⁹Fe in different organs was determined. Removal or shading of one primary leaf did not affect ⁵⁹Fe uptake by roots and ⁵⁹Fe translocation to the shoot in +Fe plants. In the -Fe plants, however, removal of one primary leaf decreased ⁵⁹Fe uptake by roots, whereas shading of one primary leaf had no effect on ⁵⁹Fe uptake but slightly enhanced ⁵⁹Fe translocation from roots to the shoot. The quantity of ⁵⁹Fe in primary leaves was positively correlated with quantity of ⁵⁹Fe in the stem in the -Fepplants, but not in the +Fe plants. In both, the +Fe and -Fe plants, the quantity of ⁵⁹Fe in the shoot apex was positively correlated with ⁵⁹Fe in primary leaves. The results suggest that irrespective of the Fe nutritional status of plants, the source of Fe for the shoot apex is Fe retranslocated from primary leaves.     

The effect of oxygen deficiency on uptake and distribution of nutrients in maize plants

Young maize (Zea mays L.) plants, 7 days after germination were exposed to nutrient solutions which were either aerated or not aerated for 14 days. Nutrients were supplied as 50% strength Hoagland’s solution or, in the case of the four ‘low nutrient’ treatments, N, P, K or Ca were supplied at the equivalent of 10% strength Hoagland’s solution. Shoot fresh weight was decreased by 25% due to lack of aeration; O₂ deficiency also impaired leaf elongation but not dry weights, suggesting that lack of O₂ in the roots impaired cell expansion in shoots more than dry weight accumulation. The distribution of N, P, K and Ca within shoots was consistent with their relative mobilities in the phloem; at least 7% of Ca in plants after 14 days of treatments was found in the oldest leaf whereas N, P and K were rapidly remobilised to younger tissues. Between 33 and 49% of the total N, P and K in the shoot was found in the 40 mm of tissue at the base of the growing leaves in plants grown for 14 days at low nutrient concentrations. Concentrations (dry weight basis) of phloem-mobile nutrients were also greatest in the growing zones of the leaves, especially in the case of N and P. Calcium, on the other hand, was found in relatively low concentrations in the youngest tissue and as with the other nutrients, concentrations declined due to low external supply, non-aeration or a combination of both. In spite of the failure of Ca to move from old to young leaves, the effect of the deficiencies of N, P and K was probably as severe as that of Ca in the youngest tissues of treated plants. Calcium uptake by the whole shoot appeared to be slightly less sensitive to O₂ deficits than that of N, P and K. This compensated for the failure of Ca to move to growing tissues during periods of low external Ca supply.     

Tillage effects on nitrogen uptake by maize from fine textured soils in the northwestern Corn Belt,USA

Summary Nitrogen (N) accumulation data from a replicated field study were fitted to a tanh (time) function and the derivate obtained to determine relative maximum rates of accumulation by maize. Both positive and negative effects of tillage on N accumulation rates were observed. Most of the N accumulation occurred over a 30-day period and time of N accumulation was not affected by tillage. Tilled profiles tend to contain greater NO₃–N, greater aeration, and lower moisture contents than untilled profiles, and these characteristics interact to affect plant N accumulation.     

Relationships between dynamics of nitrogen uptake and dry matter accumulation in maize crops. Determination of critical N concentration

The concept of critical nitrogen concentration(%N _c) has been proposed as the minimum%N in shoots required to produce the maximum aerial biomassat a given time. Several authors have shown that%N _c declines as a function of aerial biomassaccumulation (W) and the %N _c –W relationship has been proposed as a diagnostic tool of N statusin different crops, excluding maize. From data obtained in five nitrogenfertilisation experiments in irrigated maize crops, 26 critical data-pointswere selected with a precise statistical procedure. An allometric relationwas fitted and a critical %N−W relationshipmodel is proposed in maize as: If W < 1 t ha^-1%N _c = 3.40 If 1 t ha^-1≤ W ≤ 22 t ha^-1%N _c = 3.40(W)^−0.37 The model is applicable to maize crop development between emergenceand silking + 25 days. The model was tested and validated with dataobtained in a network of 17 N fertilisation experiments conducted inFrance under contrasting pedoclimatic conditions. In only nineout of 280 data-points (3.2%), the plant N status was mispredictedwhen ±5% error around %N _c wasallowed. A critical N uptake model (Nu_c, kg Nha^-1) is proposed as Nu_c = 34 (W)^0.63 A comparison between Nu_c and N uptake observedin N treatments giving the maximal grain yields has shown that maizecrops assimilate at least 30 kg N ha^-1 in a storage N poolat the silking stage. The significance of the critical%N−W and Nu−W relationships is discussed in relation to theoretical models proposed inwhole plant ecophysiology. Different relationships calculated betweenleaf area index and aerial biomass accumulation, and between N uptakeand leaf area were consistent with previous results for other crops.This strengthens the interest of the critical%N−W relationship for use as diagnostictool of nitrogen status in maize crops. This revised version was published online in June 2006 with corrections to the Cover Date.     

Regulation of nitrate uptake by amino acids in maize cell suspension culture and intact roots

The effect of amino acids on nitrate transport was studied in Zea mays cell suspension cultures and in Zea mays excised roots. The inclusion of aspartic acid, arginine, glutamine and glycine (15mM total amino acids) in a complete cell-culture media containing 1.0 mM NO₃ ^- strongly inhibited nitrate uptake and the induction of accelerated uptake rates. The nitrate uptake rate increased sharply once solution amino acid levels fell below detection limits. Glutamine alone inhibited induction in the cell suspension culture. Maize seedlings germinated and grown for 7 days in a 15 mM mixture of amino acids also had lower nitrate uptake rates than seedlings grown in 0.5 mM Ca(NO₃)₂ or 1 mM CaCl₂. As amino acids are the end product of nitrate assimilation, the results suggest an end-product feed-back mechanism for the regulation of nitrate uptake.     

Total nitrogen uptake by maize with Azotobacter inoculation

Summary The economic feasibility of using cultures of nitrogen fixing microorganisms in programmes to increase crop production, as a selfgenerating source of nitrogen, has been proved beyond doubtviz. Legume-Rhizobium symbiosis and blue green algal ‘fertilizer’ for rice. The extent to which the free living, N-fixing, aerobic, heterotropicAzotobacter chroococcum could replace the application of nitrogenous fertilizer to maize was investigatedin vivo. Total nitrogen uptake (kg ha⁻¹) by maize after inoculation with Azotobacter combined with moderate applications of nitrogen fertilizer and farmyard manure was influenced significantly and resulted in a higher nitrogen concentration in grain and stover along with a higher yield. Part of Ph.D. Thesis     

Effect of nitrate on water transfer across roots of nitrogen pre-starved maize seedlings

The addition of 10 mM KNO₃ to the solution bathing the roots of young nitrogen-starved seedlings of Zea mays L. enhanced root water transfer within 15 h, compared with 10 mM KCl addition. The free exudation flux was 2.2–3.9 times higher in excised KNO₃-treated roots than in KCl-treated ones. Cryo-osmometry data for xylem sap suggested that, compared with chloride, nitrate treatment increased the steady solute flux into the xylem, but did not modify the osmotic concentration of sap. Root growth was not significantly modified by nitrate within 15 h. Root hydraulic conductances were measured by using either hydrostatic-pressure or osmotic-gradient methods. During hydrostatic experiments, the conductance (k_p), which is thought to refer mainly to the apoplasmic pathway, was 1.6 times larger in KNO₃-than in KCl-treated plants. From experiments in which polyethylene glycol (PEG) 8000 was used as external osmolyte, osmotic conductances (k_s) were found to be smaller by 5–20 times than k_p for the two kinds of plants. The KCl-treated roots were characterized by a low k_s which was the same for influx or efflux of water. By contrast, KNO₃-treated roots exhibited two distinct conductances k_s1 and k_s2, indicating that influx of water was easier than efflux when the water flow was driven by the osmotic pressure gradient. Infiltration of roots with KNO₃ solution supported the idea that nitrate might enhance the efficiency of the cell-to-cell pathway. The low k_s value of KCl-treated roots and the existence of two contrasting k_s values (k_s1 and k_s2) for KNO₃-treated roots are discussed in terms of reversible closing of water channels.     

Combining ability estimates of sulfate uptake efficiency in maize

Summary Plant root nutrient uptake efficiency may be expressed by the kinetic parameters, V_max and K_m, as well as by normal enzymatic reactions. These parameters are apparently useful indices of the level of adaptation of genotypes to the nutrient conditions in the soil. Moreover, sulfate uptake capacity has been considered a valuable index for selecting superior hybrid characterized by both high grain yield and efficiency in nutrient uptake. Therefore, the purpose of this research was to determine combining ability for sulfate uptake, in a diallel series of maize hybrids among five inbreds. Wide differences among the 20 single crosses were obtained for V_max and K_m. The general and specific combining ability mean squares were significant and important for each trait, indicating the presence of considerable amount of both additive and nonadditive gene effects in the control of sulfate uptake. In addition, maternal and nonmaternal components of F1 reciprocal variation showed sizeable effects on all the traits considered. A relatively high correlation was also detected between V_max and K_m. However, both traits displayed enough variation to suggest that simultaneous improvement of both V_max and K_m should be feasible. A further noteworthy finding in this study was the identification of one inbred line, which was the best overall parent for improving both affinity and velocity strategies of sulfate uptake.     

Quantitative trait loci for yield and correlated traits under high and low soil nitrogen conditions in tropical maize

The first objective of this study was to map and characterize quantitative trait loci (QTL) for grain yield (GY) and for secondary traits under varying nitrogen (N) supply. To achieve this objective, a segregating F_2:3 population previously developed for QTL mapping under water-limited conditions was used. The population was evaluated in Mexico under low N conditions in the dry winter season and under low and high N conditions in the wet summer season. From eight QTLs identified for GY under low N conditions, two were also detected under high N conditions. Five QTLs were stable across the two low N environments and five co-localized with QTLs identified for the anthesis-silking interval (ASI) or for the number of ears per plant (ENO) under low N conditions. The percentage of the phenotypic variance expressed by all QTLs for ASI and ENO was quite different when evaluated under low N conditions during the dry winter (40% for ASI and 22% for ENO) and the wet summer seasons (22% for ASI and 46% for ENO). The results suggest optimizing different breeding strategies based on selection index depending on the growing season. Good QTL colocalization was observed for ASI (four QTLs) and ENO (three QTLs) when looking at QTL identified under low N and water-limited conditions in the same population. The results suggest that that both secondary traits can be used in breeding programs for simultaneous improvement of maize against low N and drought stresses.     

Uptake and distribution of cadmium in maize inbred lines

Genotypic variation in uptake and distribution of cadmium (Cd) was studied in 19 inbred lines of maize (Zea mays L.). The inbred lines were grown for 27 days on an in situ Cd-contaminated sandy soil or for 20 days on nutrient solution culture with 10 µg Cd L^-1. The Cd concentrations in the shoots showed large genotypic variation, ranging from 0.9 to 9.9 µg g^-1 dry wt. for the Cd-contaminated soil and from 2.5 to 56.9 µg g^-1 dry wt. for the nutrient solution culture. The inbred lines showed a similar ranking for the Cd concentrations in the shoots for both growth media (r²=0.89). Two main groups of inbreds were distinguished: a group with low shoot, but high root Cd concentrations (shoot: 7.4±5.3 µg g^-1 dry wt.; root: 206.0±71.2 µg g^-1 dry wt.; shoot Cd excluder) and a group with similar shoot and root Cd concentrations (shoot: 54.2±3.4 µg g^-1 dry wt.; root: 75.6±11.2 µg g^-1 dry wt.; non-shoot Cd excluder). The classification of the maize inbred lines and the near equal whole-plant Cd uptake between the two groups demonstrates that internal distribution rather than uptake is causing the genotypic differences in shoot Cd concentration of maize inbred lines. Zinc (Zn), a micronutrient chemically related to Cd, showed an almost similar distribution pattern for all maize inbred lines. The discrepancy in the internal distribution between Cd and Zn emphasizes the specificity of the Cd distribution in maize inbred lines.     

Evaluation of strategy I mechanisms in iron efficient and inefficient maize cultivars

The iron (Fe) efficient maize cultivar WF9 and the Fe inefficient maize mutant ys1 were grown in nutrient solutions with varied Fe supply. Changes in pH, reducing capacity of the roots and the release of Fe(III) reducing compounds were monitored over a period of 11 days. In both cultivars under Fe deficiency, there was no increased release of protons or reducing compounds and no increased reducing capacity of the roots. Indeed, the Fe(III) reduction of the roots of both cultivars tended to be higher in Fe sufficient plants. In contrast to some recent reports, these results demonstrate that the Fe efficient maize cultivar WF9 does not respond to Fe deficiency by strategy I mechanisms. This suggests that differences in Fe efficiency between the two cultivars are probably due to use of strategy II mechanisms for Fe acquisition.     

Partitioning of reduced nitrogen derived from exogenous nitrate in maize roots: Initial priority for protein synthesis

Summary When roots of five day-old maize seedlings were exposed to¹⁵N-nitrate, a constant (25–29%) proportion of the reduced¹⁵N derived from the entering¹⁵N-nitrate accumulated as insoluble¹⁵N nitrogen. Constancy was established by two hours and lasted through 12 hours at ambient¹⁵N-nitrate concentrations of 0.05 mM to 20.0 mM. Even when little¹⁵N nitrate had been reduced (<2 moles), there was a linear relationship between accumulation of insoluble¹⁵N (but not accumulation or translocation of soluble reduced¹⁵N) and total reduced¹⁵N. It is proposed that protein synthesis from the entering nitrate occurs in close association with nitrate reduction.Paper No. 9764 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC, 27695-7619, USA. This research was supported by Grant No. PCM-8118661 from the National Science Foundation.Use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the product's name or criticisms of similar ones not mentioned.     

Heterogeneity of Maize Root Mitochondria from Plants Grown in the Presence of Ammonium

Mitochondria isolated from root tissue of maize plants grown on a modified Knop solution containing 10.9 mM nitrate ± 7.2 mM ammonium were purified on the discontinuous Percoll density gradient with polyvinylpyrrolidone (PVP) added. The presence of PVP allowed separation of several mitochondrial fractions of a different density. Contrary to mitochondria isolated from plants grown in the presence of nitrate alone, revealing only two fractions, the mitochondria from NH₄ ⁺/NO₃ ^–-plants were distributed in four fractions. Total amount of mitochondria, as well as specific activities of some nitrogen metabolism enzymes and tricarboxylic acid (TCA) cycle enzymes of all mitochondrial fractions, and respiratory activities of two lower density fractions isolated from plants grown on mixed nitrogen were higher in comparison to mitochondria from nitrate-grown plants.     

Maize and pigweed response to nitrogen supply and form

As nitrogen management practices change to achieve economic and environmental goals, effects on weed-crop competition must be examined. Two greenhouse experiments investigated the influence of N amount and form on growth of maize and redroot pigweed (Amaranthus retroflexus L.). In Experiment 1, maize and pigweed were grown together in a replacement series (maize: pigweed ratios of 0:4, 1:3, 2:2, 3:1, 4:0) under three NH₄NO₃-N supplies (0, 110, and 220 mg N kg^-1 soil). Maize was planted into established pigweed and plants were harvested 24 days after maize germination. Pigweed responded more to supplemental N than maize and accumulated 2.5 times as much N in shoots at the high N supply. Competition effects were not significant. Maize and pigweed were grown separately in Experiment 2 and supplied 220 mg N kg^-1 as either Ca(NO₃)₂ or (NH₄)₂SO₄ plus a nitrification inhibitor (enhanced ammonium supply, EAS). In maize, EAS treatment did not affect shoot growth and reduced root growth 25% relative to the NO₃-N treatment. In pigweed, shoot and root growth were restricted 23 and 86% by EAS treatment, respectively. Total plant N accumulation under EAS treatment was higher in maize, less in pigweed. Under EAS treatment, pigweed leaves were crinkled and chlorotic; leaf disks extracted in 70% ethanol, pH 3, contained less malate and oxalate but more NH₄ ⁺ compared to the NO₃-N treatment. Maize leaf disk malate levels were generally higher compared to pigweed but were less due to EAS treatment. Ammonium level in maize leaf disks was unaffected by N form and EAS treatment increased oxalate levels. Final bulk soil pH was generally lower in pots where pigweed were grown and tended to be lower due to EAS. Leaf disk malate levels and soil pH were positively associated. Results indicate that pigweed is more likely to compete with maize when high levels of NO₃-N are provided. Enhancing the proportion of N supplied as NH₄ ⁺ should restrict the growth of NH₄ ⁺-sensitive pigweed.     

Effects of inorganic nitrogen application on the dynamics of the soil solution composition in the root zone of maize

The effect of inorganic nitrogen (N) fertilizer on the ionic composition of the soil solution under maize (Zea mays L.) was studied. A pot experiment was carried out with two treatments combined factorially, with or without N application (Ca(NO₃)₂; +N and –N treatments, respectively), and with or without plants. Three looped hollow fiber samplers were installed in each pot to sample soil solutions nondestructively from the root zone, seven times during the 50-day growth period. Plants were harvested on the 50th day, and their nutrient contents determined.Effects of N fertilizer on the soil solutions were observed by the first sampling, 2 days after sowing. The concentrations of Ca and NO₃ ^– and electrical conductivity (EC) increased significantly in the +N treatments as direct effects of fertilizer application. In addition, the concentrations of Mg, K, Na and H⁺ also increased and that of P decreased significantly as indirect effects caused by the re-establishment of chemical equilibria. This suggested the greater supply as well as the greater possibility of leaching loss not only of NO₃ ^– but also of Ca, Mg and K. In the treatments with plants, the concentrations of NO₃ ^–, Ca, Mg and K decreased with time and pH increased significantly compared with the unplanted soil. The depletion of N in the soil solution roughly agreed with the amount of N taken up by the plant. The depletions of K from the soil solution amounted to less than 10% of the amount of the K taken up, suggesting intensive replenishment of K from exchange sites in the soil. Depletions of Ca and Mg were several times higher than the amounts taken up, indicating that the depletions resulted from the adsorption of the divalent cations by the soil rather than uptake by plants. Because NO₃ ^– is hardly absorbed by exchange sites in soil and was the dominant anion in solution, it was concluded that NO₃ ^– had a major role in controlling cation concentrations in the soil solution and, consequently, on their availability for uptake by plants as well as their possible leaching loss. ei]H Marschner