Major Ion Chemistry of Soil Solution of Mountainous Soils,Alvand, Hamedan,Western Iran

Soil solution chemistry reflects the most dynamic processes occurring in soils and is responsible for their current status. This study was undertaken to determine the soil solution status in 25 mountainous soils. The major cations in the studied soil solutions are in the decreasing order of Ca²⁺ > Mg²⁺ > Na⁺ > K⁺. The anions are also arranged in decreasing order as HCO^? ₃ > Cl^? > NO^? ₃ > SO ^2? ₄ . Concentrations of NO^? ₃ , P, and K⁺ in soil solutions were in the range of 12–364 mg l^?1, 1.75–34.8 mg l^?1, and 0.78– 198 mg l^?1, respectively. Results suggest that the concentration of P in the soil solutions could be primarily controlled by of the solubility of octacalcium phosphate and ß-tricalcium phosphate. In general, the greater the dissolved P concentration in the soil solution, the closer the solution was to equilibrium with respect to the more soluble Ca²⁺ phosphate minerals. Surface soil accumulations of P, NO^? ₃ , and K⁺ have occurred in these soils to such an extent that loss of these nutrients in surface runoff and the high risk for nutrient transfer into groundwater in concentrations exceeding the groundwater quality standard has become a priority management concern.     

Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T-5)

Tomato growth was examined in solution culture under constant pH and low levels of NH₄⁺ or NO₃^?. There were five nitrogen treatments: 20 mmoles m^?3 NH₄⁺, 50 mmoles m^?3 NO₃^?, 100 mmoles m^?3 NH₄⁺ 200 mmoles m^?3 NO₃^?, and 20 mmoles m^?3 NH₄₊+ 50 mmoles m^?3 NO₃^?. The lower concentrations (20 mmoles m^?3 NH₄⁺ and 50 mmoles m^?3 NO₃^?) were near the apparent K_m for net NH₄⁺ and NO₃^? uptake; the higher concentrations (100 mmoles m^?3 NH₄⁺ and 200 mmoles m^?3 NO₃^?) were near levels at which the net uptake of NH₄⁺ or NO₃^? saturate. Although organic nitrogen contents for the higher NO₃^? and the NH₄⁺+ NO₃^? treatments were 22.2–30.3% greater than those for the lower NO₃^? treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH₄⁺ or NO₃^? concentrations are held constant over the root surface at near the apparent K_m concentration. Relative growth rates for the two NH₄⁺ treatments were much higher than have been previously reported for tomatoes growing with NH₄⁺ as the sole nitrogen source. Initial growth rates under NH₄⁺ nutrition did not differ significantly (P≥ 0.05) from those under NO₃^? or under combined NH₄⁺+ NO₃^?. Growth rates slowed after 10–15 d for the NH₄⁺ treatments, whereas they remained more constant for the NO₃^? and mixed NH₄⁺+ NO₃^? treatments over the entire observation period of 24–33 d. The decline in growth rate under NH₄⁺ nutrition may have resulted from a reduction in Ca²⁺, K⁺, and/or Mg²⁺ absorption.     

Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L. Karst) seedlings

The effect of NO₂ fumigation on root N uptake and metabolism was investigated in 3-month-old spruce (Picea abics L. Karst) seedlings. In a first experiment, the contribution of NO₂ to the plant N budget was measured during a 48 h fumigation with 100mm³m^?3 NO₂. Plants were pre-treated with various nutrient solutions containing NO₂ and NH₄⁺, NO₃^? only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH₄⁺ in the solution for 6d led to an increased capacity for NO₃^? uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO₃^? uptake. In fumigated plants, NO₂ uptake accounted for 20–40% of NO₃^? uptake. Root NO₃^? uptake in plants supplied with NH₄⁺plus NO₃^? solutions was decreased by NO₂ fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO₂ and NH₄⁺ and were fumigated or not with 100mm³m^?3 NO₂ for 7 weeks. Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and ¹⁵N labelling. Root NH₄⁺ uptake was approximately three times higher than NO₃^? uptake. Nitrogen dioxide uptake represented 10–15% of the total N budget of the plants. In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot. Phloem transport of organic nitrogen accounted for 17% of its xylem transport. In fumigated plants, neither NO₃^? nor NH₄⁺ accumulated in the shoot, showing that all the absorbed NO₂ was assimilated. Root NO₃^? reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO₂-fimugated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is discussed.     

Growth and Major Nutrient Concentrations in Brassica campestris Supplied with Different NH4^＋/NO3 Ratios

In the present study, we investigated whether growth and main nutrient ion concentrations of cabbage （Brassica campestris L.） could be increased when plants were subjected to different NH4^＋/NO3- ratios. Cabbage seedlings were grown in a greenhouse in nutrient solutions with five NH4^＋/NO3- ratios （1：0; 0.75：0.25; 0.5：0.5; 0.25：0.75; and 0：1）. The results showed that cabbage growth was reduced by 87% when the proportion of NH4^＋-N in the nutrient solution was more than 75% compared with a ratio NH4^＋/NO3- of 0.5：0.5 35 d after transplanting, suggesting a possible toxicity due to the accumulation of a large amount of free ammonia in the leaves. When the NH4＋/NO3- ratio was 0.5：0.5, fresh seedling weight, root length, and H2PO4- （P）, K^＋, Ca^2＋, and Mg^2＋ concentrations were all higher than those in plants grown under other NH4^＋/NO3- ratios. The nitrate concentration in the leaves was the lowest in plants grown at 0.5： 0.5 NH4^＋/NO3-. The present results indicate that an appropriate NH4^＋/NO3- ratio improves the absorption of other nutrients and maintains a suitable proportion of N assimilation and storage that should benefit plant growth and the quality of cabbage as a vegetable.     

A novel approach and a fully automated microcomputer-based system to study kinetics of NO−3, NO−2, and NH+4 transport simultaneously by intact wheat seedlings

Abstract A 16-channel fully automated microcomputer-based system was designed to measure the disappearance of NO^?₃ NO^?₂ and NH⁺₄ simultaneously from uptake solutions. The analyses were done using high-performance liquid chromatography. Statistical procedures were used to generate transport kinetics and interactions amongst NO^?₃, NO^?₂ and NH⁺₄ by intact wheat seedlings. The simultaneous analysis of NO^?₃, NO^?₂ and NH⁺₄ at real-time; the accommodation of varying sampling intervals; the capability to study up to 16 experimental units in synchrony; and the analysis of the data with a microcomputer, make this a powerful system for studying transport kinetics and interactions.     

Changes in NO3− and K+ uptake by four species in flowing solution culture in response to increased irradiance

Net rates of NO₃^? and K⁺ uptake were compared for oilseed rape (Brassica napus L. cv. Jet neuf), perennial ryegrass (Lolium perenne L. cv. S23), Italian ryegrass (Lolium multiflorum Lam. cv. Augusta) and wheat (Triticum aestivum L. cv. Fen-man) in flowing solution culture during a 4-day sequence of low-low-high-high natural irradiance. Concentrations of NO₃^? (10 μM) and K⁺ (2.5 μM) in solutions were maintained automatically and hourly variation in net uptake of these ions was measured. During the 2 days of low irradiance (<1 MJ m^?2 day^?1) the uptake rates of both ions by all species were low at <1 mmol NO₃^?, m^?2 h^?1 and <0.4 mmol K⁺ m^?2 h^?1. Uptake increased in each species during the first day of high irradiance (7.90 MJ m^?2 day^?1) to >4 mmol NO₃^? m^?2 h^?1 and >1.4 mmol K⁺ m^?1 h^?1. These higher rates were maintained throughout the following night. The lag-time between maximum irradiance and the onset of the highest acceleration in uptake was greater for NO₃^? (5–8 h) than for K⁺ (≤1 h) in rape, wheat and Italian ryegrass. Uptake of NO₃^?, by perennial ryegrass showed an almost constant acceleration for 18 h following maximum irradiance. In all species the measured maximum inflows (uptake rate per unit root length) of both ions were greater than theoretical maximum potential inflows to a non-competing infinite-sink root in soil, by factors of 7 and 36, respectively, for NO₃^? and K⁺, averaged over all species.     

Kinetics of nitrate and ammonium absorption and accompanying H+ fluxes in roots of Lolium perenne L. and N2-fixing Trifolium repens L.

Kinetic parameters for NH₄⁺ and NO₃^? uptake were measured in intact roots of Lolium perenne and actively N₂-fixing Trifolium repens. Simultaneously, net H⁺ fluxes between the roots and the root medium were recorded, as were the net photosynthetic rate and transpiration of the leaves. A Michaelis–Menten-type high-affinity system operated in the concentration range up to about 500 mmol m^?3 NO₃^? or NH₄⁺. In L. perenne, the V_max of this system was 9–11 and 13–14 μmol g^?1 root FW h^?1 for NO₃^? and NH₄⁺, respectively. The corresponding values in T. repens were 5–7 and 2 μmol g^?1 root FW h^?1. The K_m for NH₄⁺ uptake was much lower in L. perenne than in T. repens (c. 40 compared with 170 mmol m^?3), while K_m values for NO₃^? absorption were roughly similar (around 130 mmol m^?3) in the two species. There were no indications of a significant efflux component in the net uptake of the two ions. The translocation rate to the shoots of nitrogen derived from absorbed NO₃^?-N was higher in T. repens than in L. perenne, while the opposite was the case for nitrogen absorbed as NH₄⁺. Trifolium repens had higher rates of transpiration and net photosynthesis than L. perenne. Measurements of net H⁺ fluxes between roots and nutrient solution showed that L. perenne absorbing NO₃^? had a net uptake of H⁺, while L. perenne with access to NH₄⁺ and T. repens, with access to NO₃^? or NH₄⁺, in all cases acidified the nutrient solution. Within the individual combinations of plant species and inorganic N form, the net H⁺ fluxes varied only a little with external N concentration and, hence, with the absorption rate of inorganic N. Based on assessment of the net H⁺ fluxes in T. repens, nitrogen absorption rate via N₂ fixation was similar to that of inorganic N and was not down-regulated by exposure to inorganic N for 2 h. It is concluded that L. perenne will have a competitive advantage over T. repens with respect to inorganic N acquisition.     

THE INABILITY OF THE TEXAS “BROWN TIDE” ALGA TO USE NITRATE AND THE ROLE OF NITROGEN IN THE INITIATION OF A PERSISTENT BLOOM OF THIS ORGANISM1

A planktonic alga similar in general morphology and pigments to Aureococcus anophagefferens Hargraves and Sieburth has caused persistent and ecologically damaging blooms along the south Texas coast. Experiments using 100 μM NO₃^?, NO₂^?, and NH₄⁺ demonstrated that the alga could not use NO₃^? for growth but could use NO₂^? and NH₄⁺. Doubling iron or trace metal concentrations did not permit growth on NO₃^?. Chemical composition data for cultures grown in excess NO₃^? or NH₄⁺, respectively, were as follows: N·cell^?1 (0.88 vs. 1.3 pg), C:N ratio (25:1 vs. 6.4:1), C:chlorophyll a (chl a) (560:1 vs. 44:1), and chl a·cell^?1 (0.033 vs. 0.16 pg). These data imply that cells supplied with NO₃^? were N-starved. Culture addition of 10 mM final concentration chlorate (a nitrate analog) did not affect the Texas isolate while NO₃^? utilizing A. anophagefferens was lysed, suggesting that the NO₃^? reductase of the Texas isolate is nonfunctional. Rates of primary productivity determined during a dense bloom indicated that light-saturated growth rates were ca. 0.45 d^?1, which is similar to maximum rates determined in laboratory experiments (0.58 d^?1± 0.16). However, chemical composition data were consistent with the growth rate of these cells being limited by N availability (C:N 28, C:chl a 176, chl a·cell^?1 0.019). Calculations based on a mass balance for nitrogen suggest that the bloom was triggered by an input of ca. 69 μM NH₄⁺ that resulted from an extensive die-off of benthos and fish.     

PHYSIOLOGICAL ACCLIMATION OF MARINE PHYTOPLANKTON TO DIFFERENT NITROGEN SOURCES1

We examined the energetic dependency of the biochemical and physiological responses of Thalassiosira pseudonana Hasle and Heimdal. Chaetoceros gracilis Schütt, Dunaliella tertiolecta Butcher, and Gymnodinium sanguineum Hirasaka to NH₄⁺, NO₃^?, and urea by growing them at subsaturating and saturating photon flux (PF). At subsaturating PF, when energy was limiting, NO₃^? and NH₄⁺ grown cells had similar growth rates and C and X quotas. Therefore, NO₃^? grown cells used up to 48% more energy than NH₄⁺ grown cells to assimilate carbon and nitrogen. Based on our measurements of pigments, chlorophyll-a-specific in vivo absorption cross-section, and fluorescence-chlorophyll a^?1, we suggest that NO₃^?, grown cells do not compensate for the greater energy requirements of NO₃^? reduction by trapping more light energy. At saturating PF, when energy is not limiting, the utilization of NO₃^?, compared to NH₄⁺ resulted in lower growth rates and N quotas in Thalassiosira pseudonana and lower N quotas in Chaetoceros gracilis, suggesting enzymatic rather than energetic limitations to growth. The utilization of urea compared to Nh₄⁺ resulted in lower growth rates in Chaetoceros gracilis and Gymnodinium sanguineum (saturating PF) and in lower N quotas in all species tested at both subsaturating and saturating PF. The high C:N ratios observed in all urea-grown species suggest that nitrogen assimilation may be limited by urea uptake or deamination and that symptoms of N limitation in microalgae may be induced by the nature of the N source in addition to the N supply rate. Our results provide new eridence that the maximum growth rates of microalgae may be limited by enzymatic processes associated with the assimilation of NO₃^?, or urea.     

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.     

Photosynthetic energy supply for NO3− assimilation in Scenedesmus

NO₃^?-dependent O₂ in synchronous Scenedesmus obtusiusculus Chod. in the absence of CO₂ is stoichiometric with NH₄⁺ excretion, indicating a close coupling of NO₃^? reduction to non-cyclic electron flow. Also in the presence of CO₂, NO₃^? stimulates O₂ evolution as manifested by an increase in the O₂/CO₂ ratio from 0.96 to 1.11. This quotient was increased to 1.36 by addition of NO₂^?, without competitive interaction with CO₂ fixation, indicating that the capacity for non-cyclic electron transport at saturating light is non-limiting for simultaneous reduction of NO₃^? and CO₂ at high rates. During incubation with NO₃^?+ CO₂, no NH₄⁺ is released to the outer medium, whereas during incubation with NO₂^?+ CO₂, excess NH₄⁺ is formed and excreted. NO₃^? uptake is stimulated by CO₂, and this stimulation is also significant when the cellular energy metabolism is restricted by moderate concentrations of carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, whereas NO₃^? uptake in the absence of CO₂ is severely inhibited by the uncoupler. Also under energy-restricted conditions NO₃^? uptake is not competitive with CO₂ fixation. Antimycin A is inhibitory for NO₃^? uptake in the absence of CO₂, and there is no enhancement of NO₃^? uptake by CO₂ in the presence of antimycin A. It is assumed that the energy demand for NO₃^? uptake is met by energy fixed as triosephosphates in the Calvin cycle. Antimycin A possibly affects the transfer of reduced triose phosphates from the chloroplast to the cytoplasm. Active carbon metabolism also seems to exert a control effect on NO₃^? assimilation, inducing complete incorporation of all NO₃^? taken up into amino acids. This control effect is not functional when NO₂^? is the nitrogen source. Active carbon metabolism thus seems to be essential both for provision of energy for NO₃^? uptake and for regulation of the process.     

Ionic processes underlying the decrease in osmotic potential of Chara L-cell fragments in dilute electrolyte solutions

Movements of ions are considered to be governed by the electroneutrality rule. Therefore, a cation moving across the cell membrane into the cell either passively or actively should move together with its counterion, an anion, in equal amounts of charge or in exchange for another cation inside the cell. This means that the net influx of the cation in question should be affected by the permeability of its counterion and/or another cation inside the cell. To examine osmotic and ionic regulation in Chara cells, cell fragments of Chara having a lower osmotic pressure than normal (L-cell fragments) were prepared. The L-cell fragments were individually put into various dilute electrolyte solutions and their osmotic potentials were measured with a turgor balance. Concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^?, NO^?₃. and SO^2?₄. in the external electrolyte solutions in which L-cells had been incubated were also analysed by ion chromatography. The results showed that in 0.5 mM KCL + 0.1 mM CaCl₂ solution, Chara L-cell fragments absorbed K⁺ and Cl^? to maintain electroneutrality and then regained their osmotic potential very rapidly. When the anion was Cl, the cation absorbed at the highest rate was K⁺ On the other hand, when the cation was K, the anion absorbed at the highest rate was Cl, Other ions Ca²⁺, SO^2?₄ and NO^?₃ showed much less permeability than K⁺ and Cl ^?for the Chara plasma membrane. The conclusion from these findings was that due to the constraint of electroneutral transport, the uptake rate of a salt into L-cells is limited by the permeability of the least permeable ion.     

Oxygen stress in Salvinia natans: Interactive effects of oxygen availability and nitrogen source

The ability of Salvinia natans (L.) All. to tolerate growth in oxic, hypoxic and anoxic nutrient solutions when supplied with either NH₄⁺ or NO₃^? were studied in the laboratory to test the hypothesis that inorganic N-source affects the response of the plants to O₂ deprivation. The relative growth rate (RGR) was significantly reduced in the anoxic treatment, but in the hypoxic treatment RGR was only slightly affected. The NH₄⁺ fed plants generally had a higher shoot to root ratio than the NO₃^? fed plants, and highest in the anoxic treatment. Plants had more roots and larger leaves when supplied with NH₄⁺ as compared with NO₃^?, particularly in the oxic treatment, and root length was most affected by O₂ deprivation for NO₃^? fed plants. Cell walls in the endodermis, the bundle sheath and the cortex adjacent to endodermis developed thickened sclerenchymatous walls when deprived of O₂, and more so in plants supplied with NO₃^?. Plants lost chlorophylls, had lower rates of photosynthetic electron transport (ETR_max) and lower quantum yields (Fv/Fm ratios) when grown in anoxic solutions, and the negative effects were mildest for NO₃^? fed plants suggesting that NO₃^? may be used as an alternative e^?-acceptor in non-cyclic electron transport in the chloroplasts. Overall S. natans grew best on NH₄⁺, but it also grew well on NO₃^?, and the O₂ stress symptoms differed somewhat between NH₄⁺ fed and NO₃^? fed plants. However, because N-form itself significantly influenced morphology and cell metabolism, it was impossible to conclusively identify the role of N-form for the O₂ stress reactions. S. natans is not well-adapted to grow in O₂ deficient waters and will not tolerate completely anoxic conditions as will prevail in waters receiving high loadings of organic pollutants such as livestock wastewater.     

AMMONIUN AND NITRATE UPTAKE BY THE MARINE MACROPHYTES HYPNEA MUSVUFORMIS (RHODOPHYTA) AND MACROCYSTIS PYRIFERA (PHAEOPHYTA)1, 2

NH₄⁺ and NO₃^? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO₃^?up take followed saturable kinetics (K₂=4.9 μg-at N t^?1, V_max= 2.85 μg- at N, g(wet)^?1. h^?1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO₃^? uptake was reduced one-half in the presence of NH₄⁺, but presence of NO₃^? had no effect on NH₄⁺ uptake. Darkness reduced both NO₃^? and NH₄⁺ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO₃^? uptake followed saturable kinetices: K₂=13.1 μg-at N. l^?1. V_max=3.05 μg-at N. g(wet)^?1. h^?1.NH₄⁺ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K₂=5.3 μg-at N.1–1, V_max= 2.38 μg-at N G (wet)^?1.h^?1: at higher concentration uptake increased lincarly with concentrations. NO₃^?and NH₄⁺ were taken up simulataneously: presence of one form did not affect uptake of the other.     

Responses of soybean to ammonium and nitrate supplied in combination to the whole root system or separately in a split-root system

To address the questions of whether allocation of carbohydrates to roots is influenced by ionic form of nitrogen absorbed and whether allocation of carbohydrates to roots in turn influences proportionality between NH₄⁺ and NO₃^? uptake from mixed sources, NH₄⁺ and NO₃^? were supplied separately to halves of a split-root hydroponic system and were supplied in combination to a whole-root system. Dry matter accumulation in the split-root system was 18% less in the NH₄⁺-fed axis than in the NO₃^?-fed axis. This, however, does not indicate that partitioning of carbohydrate between the two axes was different. Most of the reduction in dry matter accumulation in the NH₄⁺-fed axis can be accounted for by the retransport of CH₂O equivalents from the root back to the shoot with amino acids produced by NH₄⁺ assimilation. Uptake of NH₄⁺ or NO₃^? by the respective halves of the split-root system was proportional to the estimated allocation of carbohydrate to that half. When NH₄⁺ and NO₃^? were supplied to separate halves of the split-root system, the cumulative NH₄⁺ to NO₃^? uptake ratio was 0.81. When supplied in combination to the whole-root system, the cumulative NH₄⁺ to NO₃^? uptake ratio was 1.67. Thus, while the shoot may affect total nitrogen uptake through the export of carbohydrates to roots, the shoot (common for halves of the split-root system) apparently does not exert a direct effect on proportionality of NH₄⁺ and NO₃^? uptake by roots. For whole roots supplied with both NH₄⁺ and NO₃^?, the restriction in uptake of NO₃^? may involve a stimulation of NO₃^? efflux rather than an inhibition of NO₃^? influx. While only the net uptake of NH₄⁺ and NO₃^? was measured by ion chromatography, monitoring at approximately hourly intervals during the first 3 days of treatment revealed irregularly occurring intervals of both depletion (net influx) and enrichment (net efflux) in solutions. In the case of NH₄⁺, numbers of net efflux events were similar (21 to 24 out of 65 sequential sampling intervals) whether NH₄⁺ was supplied with NO₃^? to whole-root systems or separately to an axis of the split-root system. In the case of NO₃^?, however, the number of net efflux events increased from 8 when NO₃^? was supplied to a separate axis of the split-root system to between 19 and 24 when NO₃^? was supplied with NH₄⁺ to whole-root systems.     

OSMOTIC ADJUSTMENT IN LAMINARIA DIGITATA (PHAEOPHYTA) WITH PARTICULAR REFERENCE TO SEASONAL CHANGES IN INTERNAL SOLUTE CONCENTRATIONS1

Laboratory studies have indicated that Na⁺, K⁺ (together with Cl^? the presumed counter-ion to these cations), NO₃^? and mannitol represent the major cellular osmotica in Laminaria digitata (Huds.) Lamour. The cellular content of NO₃^? (together with a fraction of the K⁺ pool which acts as the counter-ion to NO₃^?) was found to be inversely proportional to that of mannitol, suggesting that L. digitata maintains a constant turgor by means of an isotonic substitution between these compounds. An analysis of the seasonal changes in solute content in an Arbroath (Scotland) population of L. digitata confirmed this hypothesis and indicated that the total pool of stored photosynthate was partitioned between the interconvertible carbohydrates mannitol and laminaran (which has a much lower osmotic potential than mannitol) depending on the size of the cellular pool of NO₃^?.     

Distribution of NO3− reduction between roots and shoots of peachtree seedlings as affected by NO3− uptake rate

The distribution of NO₃^? reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO₃^?, together with transport through xylem and phloem of the newly reduced N were estimated, using ¹⁵N labellings, in intact plants supplied for 90 h with 0.5 mM NH₄⁺ and 0.5, 1.5 or 10 mM NO₃^?. Xylem transport of NO₃^? was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO₃^? reduction at all 3 levels of NO₃^? nutrition. However, the contribution of the shoots to the whole plant NO₃^? reduction increased with increasing external NO₃^? availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO₃^?, respectively. Both ¹⁵N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO₃^? translocation from roots to shoots. It is proposed that the lack of NO₃^? export to the shoots at low NO₃^? uptake rate resulted from a competition between NO₃^? reduction in the root epidermis/cortex and NO₃^? diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO₃^? nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO₃^? assimilation, principally occurring in the roots, was mainly incorporated in the shoots.     

Effects of increasing inorganic carbon supply to roots on net nitrate uptake and assimilation in tomato seedlings

We investigated the influence of an increased inorganic carbon supply in the root medium on NO^?₃ uptake and assimilation in seedlings of Lycopersicon esculentum (L.) Mill. cv. F144. The seedlings were pre-grown for 4 to 7 days with 0 or 100 mM NaCl in hydroponic culture using 0.2 mM NO^?₃ (group A) or 0.2 mM NH⁺₄ (group B) as nitrogen source. The nutrient solution for group A plants was aerated with air or with air containing 4 800 μumol mol^?1 CO₂. Nitrate uptake rate and root and leaf malate contents in these plants were determined. The plants of group B were subdivided into two sets. Plants of one set were transferred either to N-free solution containing 0 or 5 mM NaHCO₃, or to a medium containing 2 mM NO^?₃ and 5 mM NaHCO₃. Both sets of group B plants were grown for 12 h in darkness prior to 2 h of illumination, and were assayed for malate content and NO^?₃ uptake rate (only for plants grown in N-free solution). The second set of group B plants was labeled with ¹⁴C by a 1-h pulse of H¹⁴CO^?₃ which was added to a 5 mM NaHCO₃ solution containing 0 or 100 mM NaCl and 0 or 2 mM NO^?₃, and ¹⁴C-assimilates were extracted and fractionated. The roots of group B plants growing in carbonated medium accumulated twice as much malate as did control plants. This malate was accumulated only when NO^?₃ was absent from the root medium. Both a high level of root malate and aeration with CO₂-enriched air stimulated NO^?₃ uptake. Analysis of ¹⁴C-assimilates indicated that with no NO^?₃ in the medium, the ¹⁴C was present mainly in organic acids, whereas with NO^?₃, a large proportion of ¹⁴C was incorporated into amino acids. Transport of root-incorporated ¹⁴C to the shoot was enhanced by NO^?₃, while the amino acid fraction was the major ¹⁴C-assimilates in the shoot. It is concluded that inorganic carbon fixed through phosphoenolpyruvate carboxylase (EC 4.1.1.31) in roots of tomato plants may have two fates: (a) as a carbon skeleton for amino acid synthesis; and (b) to accumulate, mainly as malate, in the roots, in the absence of a demand for the carbon skeleton. Inorganic carbon fixation in the root provides carbon skeletons for the assimilation of the NH⁺₄ resulting from NO₃ reduction, and the subsequent removal of amino acids through the xylem. This ‘removal’ of NO^?₃ from the cytoplasm of the root cells may in turn increase NO^?₃ uptake.     

Daily Patterns under the Life Cycle of a Maize Crop

Together with photosynthesis, transpiration and respiration, the daily uptake of NO₃^?, NH₄⁺, H₂PO₄^?, K⁺, Ca²⁺, Mg²⁺, SO₄^2?, the root respiration, root volume increase and root excretions have been studied by daily measurements during the growth period of whole maize plants (Zea mays L. cv. INRA F7 × F2) raised until complete maturity on nutrient solution. The uptake patterns show a maximum absorption of NO₃^?, K⁺ and Ca²⁺ during the vegetative growth phase. The absorption of these ions declines during maturation while that of H₂PO₄^? reaches a maximum. Root respiration and particularly the uptake of NO₃^? and K⁺ are well correlated with the rate of root growth. Root excretion is more notable in young plants than in the old. It represents less than 0.2% of the net assimilation of adult plants.     

RAPID AMMONIUM‐ AND NITRATE‐INDUCED PERTURBATIONS TO CHL a FLUORESCENCE IN NITROGEN‐STRESSED DUNALIELLA TERTIOLECTA (CHLOROPHYTA)1

When NH₄ ⁺ or NO₃ ^? was supplied to NO₃ ^? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO₃ ^? and 10 μM for NH₄ ⁺ . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO₃ ^? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO₃ ^? versus NH₄ ⁺ and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO₃ ^? disappeared, whereas the response to NH₄ ⁺ persisted. N‐starved cells previously grown with NH₄ ⁺ alone showed fluorescence responses with NH₄ ⁺ but not NO₃ ^? additions. The distinct responses to NO₃ ^? and NH₄ ⁺ may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO₃ ^? or NH₄ ⁺ as an N source to phytoplankton populations and as a diagnostic tool for N limitation.