Nitrate Uptake and Partitioning by Corn {Zea mays L.) Root Systems and Associated Morphological Differences among Genotypes and Stages of Root Development

Pan, W. L., Jackson, W. A. and Moll, R. H. 1985. Nitrate uptakeand partitioning by corn (Zea mays L.) root systems and associatedmorphological differences among genotypes and stages of rootdevelopment.—J. exp. Bot, 36: 1341–1351 Nitrate uptake and partitioning by root systems of corn inbredlines were examined. Six-day-old root systems of decapitatedseedlings of seven corn inbred lines were shown to differ markedlyin their capacity for nitrate uptake and partitioning. The magnitudeof nitrate uptake ranged from 44–86 µmol NO^–₃g ^–1 fr. wt. during an 8 h period. Relative nitrate translocation(% of total uptake) also varied among the seven genotypes from4–25%, and differences in the proportions accumulated(28–73%) and reduced (22–58%) were observed. Threeof these genotypes were then examined at 5,6, and 8 d aftergermination to determine the effect of lateral root proliferationon the previously observed differences in nitrate uptake andpartitioning. Nitrate translocation per unit mass increasedwith root elongation and lateral root proliferation, and genotypicdifferences in this partitioning process were associated withdifferences in these morphological parameters. In contrast,differences among genotypes in their capability to accumulatenitrate were not correlated with these differences in morphology.Evaluations of genotypic differences in nitrate uptake and partitioningat the seedling stage should include the rate and characteristicsof morphological development Key words: Lateral root, root morphology, nitrate translocation     

Nitrogen Enhancement of Phosphate Transport in Roots of Zea mays L. : I. Effects of Ammonium and Nitrate Pretreatment

The effect of nitrogen status on phosphorous uptake and translocation was examined in 6-day-old dark-grown decapitated maize seedlings exposed to 25 micromolar phosphorous. Transfer to complete solutions containing 1 millimolar ammonium resulted in an increase in phosphorous uptake rate after 6 to 8 hours. The stimulus remained effective for at least 5.5 hours upon subsequent transfer to nitrogen-free solutions. Pretreatments for 16 hours with either nitrate or ammonium resulted in enhanced rates of subsequent phosphorous uptake and in enhanced translocation to the xylem of the exogenously supplied phosphorous. Both processes reached a plateau following pretreatment with 0.1 to 1.0 millimolar concentrations of either nitrogen ion. Further enhancement occurred with 10 millimolar nitrate, but not with 10 millimolar ammonium pretreatment. Although nitrogen pretreatments slightly increased the quantity of exogenous phosphorous retained in the root tissue, most of the extra phosphorous taken up by the nitrogen-pretreated seedlings was translocated to the xylem. The enhanced translocation, however, did not totally account for the increase in uptake implying a specific stimulation of the uptake process.     

Nitrate utilization in barley: relations to nitrate supply and light/dark cycles

Uptake and utilization of nitrate were investigated in Hordeum vulgare L. cvs Mette and Golf in the vegetative stage, 2 and 4 weeks after sowing. The plants were subjected to a light/dark cycle of 16/8 h (18/12°C). Results obtained with the two genotypes were essentially similar. In the light, xylem nitrate transport and shoot nitrate reduction approximately equalled the amount of nitrate absorbed by the root. A drastic decline in translocation to the shoot in darkness was entirely attributable to decreased transpiration since no major changes in xylem nitrate concentration were observed. Darkening caused only a slight decrease in nitrate uptake, while root nitrate reduction was enhanced. Nitrate starvation for 2 days did not significantlly affect dry matter increment, but resulted in a drastic drop in previously accumulated nitrate, indicating that the stored nitrate is accessible and can sustain unrestricted growth. Uptake increased upon re-addition of nitrate and after 8 h it was about twice that of non-starved plants. During recovery, restoration of root nitrate pools and root nitrate reduction took precedence over shoot nitrate accumulation and reduction. Net nitrate uptake and removal of nitrate from the root to the transpiration stream seem to be decisive for the rate of root nitrate reduction.     

Modelling of the Partitioning, Assimilation and Storage of Nitrate within Root and Shoot Organs of Castor Bean (Ricinus communis L.)

An experimentally-based modelling technique was developed todescribe quantitatively the uptake, flow, storage and utilizationof NO₃-N over a 9 d period in mid-vegetative growth of sandcultured castor bean (Ricinus communis L.) fed 12 mol m^–3nitrate and exposed to a mean salinity stress of 128 mol m^–3NaCl. Model construction used information on increments or lossesof NO₃-N or total reduced N in plant parts over the study periodand concentration data for NO₃-N and reduced (amino acid) Nin phloem sap and pressure-induced xylem exudates obtained fromstem, petiole and leaf lamina tissue at various levels up ashoot. The resulting models indicated that the bulk (87%) of incomingnitrate was reduced, 51% of this in the root, the remainderprincipally in the laminae of leaves. The shoot was 60% autotrophicfor N through its own nitrate assimilation, but was oversuppliedwith surplus reduced N generated by the root and fed to theshoot through the xylem. The equivalent of over half (53%) ofthis N returned to the root as phloem translocate and, mostly,then cycled back to the shoot via xylem. Nitrate comprised almosthalf of the N of most xylem samples, but less than 1% of phloemsap N. Laminae of leaves of different age varied greatly inN balance. The fully grown lower three leaves generated a surplusof reduced N by nitrate assimilation and this, accompanied byreduced N cycling by xylem to phloem exchange, was exportedfrom the leaf. Leaf 4 was gauged to be just self-sufficientin terms of nitrate reduction, while also cycling reduced N.The three upper leaves (5–7) met their N balance to varyingextents by xylem import, phloem import (leaves 6 and 7 only)and assimilation of nitrate. Petioles and stem tissue generallyshowed low reductase activities, but obtained most of theirN by abstraction from xylem and phloem streams. The models predictedthat nodal tissue of lower parts of the stem abstracted reducedN from the departing leaf traces and transferred this, but notnitrate, to xylem streams passing further up the shoot. As aresult, xylem sap was predicted to become more concentratedin N as it passed up the shoot, and to decrease the ratio ofNO₃-N to reduced N from 0·45 to 0·21 from thebase to the top of the shoot. These changes were reflected inthe measured N values for pressure-induced xylem exudates fromdifferent sites on the shoot. Transfer cells, observed in thexylem of leaf traces exiting from nodal tissue, were suggestedto be involved in the abstraction process. Key words: Ricinus communis, nitrogen, nitrate, nitrate reduction, partitioning, phloem, xylem, flow models     

Nitrate Uptake during Recovery from Nitrogen Deficiency

Two-week-old nitrogen-deficient wheat plants attained a high rate of nitrate uptake on the first day of exposure to nutrient solutions supplemented with KNO₃. Ammonium uptake from similar solutions supplemented with NH₄NO₃ was also high during the first day of exposure, but nitrate uptake from this solution was lower than from the KNO₃ treatment. During the next two to three days there was a progressive decrease in uptake of both nitrogen ions. A steady increase in uptake then occurred as the plants fully recovered from the nitrogen-deficient state. The transient low nitrate uptake after three or four days of exposure to KNO₃ was not due to an excessive accumulation of nitrate in the tissue, nor to a failure in nitrate reduction as indicated by the rate of nitrate accumulation relative to the uptake rate. Nitrogen supplied as ¹⁵N-nitrite during the low uptake period was effectively incorporated into organic forms and effectively translocated to the shoots. Failure of the root tissue to increase in soluble carbohydrates during illumination was characteristic of the low uptake period. This contrasted with an increase in root soluble carbohydrates in the light during rapid uptake associated with full recovery from the nitrogen-deficient state. It is concluded that carbohydrate translocation to the root system was insufficient during the intermediate recovery period for optimal nitrate uptake, although it was sufficient for effective reduction and translocation of nitrate and reduced nitrogen. Ammonium uptake from NH₄NO₃ was restricted during darkness by the third day whereas there was little difference between light and dark periods in nitrate uptake from KNO₃ until about the sixth day of recovery. The extent to which ammonium restricted nitrate uptake increased progressively for two or three days following which a lessening influence seemed evident, and the effects were not directly associated with the rate of ammonium uptake.     

Accelerated Nitrate Uptake in Wheat Seedlings: Effects of Ammonium and Nitrite Pretreatments and of 6-Methylpurine and Puromycin

The experiments reported herein had two objectives. One was to determine if the slow rate of nitrate uptake which occurs upon initial exposure of nitrogen-depleted wheat (Triticum vulgare cv. Knox) plants to nitrate was the result of insufficient reduced nitrogen. The second was to determine the impact of restrictions in ribonucleic acid or protein synthesis on both nitrate uptake and nitrate reduction. Pretreatments of 14-day-old seedlings for a few hours in ammonium or nitrite did not result in an enhancement of the initial slow rate of nitrate uptake. Growth for two additional weeks in ammonium also failed to eliminate the induction period. The evidence indicates that the presence of nitrate, rather than a product of its reduction, was required to initiate development of the accelerated rate of nitrate uptake. Puromycin (400 μg ml^?1) and 6-methylpurine (0.5 mM) prevented development of the accelerated phase of nitrate uptake. With both compounds, the relative restriction of nitrate uptake was greater than that of nitrate reduction as revealed by incorporation of ¹⁵N from labeled nitrate into reduced forms. The proportion of reduction which occurred in the root system under the imposed treatments could not be delineated precisely, preventing an unequivocal determination of the extent to which the two processes are coupled in the root system. The data nevertheless indicate nitrate reduction was closely associated with nitrate uptake. Accumulation of nitrate in the shoots was markedly restricted in presence of 6 methylpurine. This effect most likely was a result of a severe restriction in the translocation of nitrate into the xylem, rather than an increase in the reduction rate in the shoots.     

Effects of rhizospheric bicarbonate on net nitrate uptake and partitioning between the main nitrate utilising processes in Populus canescens and Sambucus nigra

Increased levels of rhizospheric dissolved inorganic carbon have repeatedly been demonstrated to enhance plant growth by up to 80%, although carbon from dark fixation accounts for only 1–3% of total plant carbon gain. This study, therefore, aimed at investigating the effects of bicarbonate on nitrate uptake, assimilation and translocation to shoots. Clonal saplings of poplar (Populus canescens(Ait.) Sm.) and elder (Sambucus nigraL.) were grown hydroponically for 35 days in a nutrient solution containing 0, 0.5 and 1 mM bicarbonate and 2 mM nitrate as the sole nitrogen source at pH 7.0. Net nitrate uptake, root nitrate accumulation and reduction, and export of nitrogenous solutes to shoots were measured after incubating plants with ¹⁵N-labelled nitrate for 24 h. Net nitrate uptake increased non-significantly in plant species (19–61% compared to control plants) in response to 1 mM bicarbonate. Root nitrate reduction and nitrogen export to shoots increased by 80 and 95% and 15 and 44% in poplar and elder, respectively. With enhanced root zone bicarbonate, both species also exhibited a marked shift between the main nitrate utilising processes. Poplar plants increasingly utilised nitrate via nitrate reduction (73–88% of net nitrate uptake), whereas the proportions of export (20–9%) and storage in roots (7–3%) declined as plants were exposed to 1 mM external bicarbonate. On the other hand, elder plants exhibited a significant increase of root nitrate reduction (44–66%) and root nitrate accumulation (6–25%). Nitrate translocation to elder shoots decreased from 50 to 8% of net nitrate uptake. The improved supply of nitrogen to shoots did not translate into a significant stimulation of growth, relative growth rates increased by only 16% in poplar saplings and by 7% in elder plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

The partitioning of nitrate assimilation between root and shoot of higher plants

Abstract The partitioning of nitrate assimilation between root and shoot of higher plant species is indicated by the relative proportions of total plant nitrate reductase activity (NRA) in the two plant parts and the relative concentrations of nitrate and reduced N in the xylem sap. These have been collated here from the literature and temperate and tropical species compared. Both the distribution of NRA and xylem sap nitrate: reduced N indicate that the following four generalizations can be made.

• 1 Temperate, perennial species growing in low external nitrate concentrations (about 1 mol m^?3) carry out most of their nitrate assimilation in the root. As external nitrate concentration increases (in the range found in agricultural soils, 1–20 mol m^?3), shoot nitrate assimilation becomes increasingly important.
• 2 Temperate, annual legume species growing in low external nitrate concentrations carry out most of their nitrate assimilation in the root. Shoot nitrate assimilation increases in importance as external nitrate concentration is increased.
• 3 Temperate, annual non-legume species vary greatly in their partitioning of nitrate assimilation between root and shoot when growing in low external nitrate concentrations. Regardless of the proportion carried out in the root at low external nitrate concentrations, nitrate assimilation in the shoot becomes increasingly important as external nitrate concentration is increased.
• 4 Tropical and subtropical species, annual and perennial, carry out a substantial proportion of their nitrate assimilation in the shoot when growing in low external nitrate concentrations. The partitioning of nitrate assimilation between root and shoot remains constant as external nitrate concentration increases.

It is proposed that a greater proportion of nitrate assimilation occurs in the shoot when an increase in the rate of nitrate uptake does not induce an increase in NR level in the root. Thus, a greater proportion of the nitrate taken up remains unassimilated and is passed into the xylem. A constant partitioning of nitrate assimilation between root and shoot is achieved by balancing NR levels in the root with rates of nitrate uptake. The advantages and disadvantages of assimilating nitrate in either the root or shoot are discussed in relation to temperate and tropical habitats.     

Influence of Nitrogen Supply on Uptake and Translocation of Strontium and Calcium in Wheat Seedlings

Supplying nitrate to N-depleted wheat seedlings (Triticum vulgare cv. Knox) stimulated the uptake and translocation of both ⁸³Sr and ⁴⁵Ca. Since the increase in ⁴⁵Ca accumulation was greater, the ⁸⁵Sr/⁴⁵Ca ratio in the plant tissue was decreased. Nitrate had relatively little influence on the amount of the divalent cations and ⁸⁵Sr/⁴⁵Ca ratio in the exchangeable fraction on the root surfaces, whereas it greatly increased the uptake into root tissue and translocation to shoots. The increase in percent transported to shoots occurred largely in the period of most rapid nitrate uptake. A split root study indicated that nitrate was ineffective when it was supplied to a different portion of the root system than that exposed to ⁸⁵Sr and ⁴⁵Ca. Nevertheless, ammonium and urea also increased the translocation of the two cations, indicating that the effects of nitrate could not entirely be ascribed to a direct effect of the nitrate anion.     

COMPARATIVE pH DEPENDENT METAL INHIBITION OF NUTRIENT UPTAKE BY SCENEDESMUS QUADRICAUDA(CHLOROPHYCEAE)1,2

Cadmium and copper inhibition of nutrient uptake by the green alga Scenedesmus quadricauda is highly pH dependent in an inorganic medium; both metals are less toxic at low pH. The alga was grown in chemostats with both N and P approaching limiting levels; it was then possible to study metal toxicity to the nitrate, ammonium, and phosphate uptake systems of algae in an identical physiological state. When the logarithm of the Cd concentration causing 25% inhibition of nitrate, ammonium, and phosphate uptake was regressed against pH almost perfect linear relationships were obtained. This was also true at the 50% inhibition level, except for a smaller than predicted increase in Cd toxicity to ammonium uptake at pH 8, which may be due to the beginning of Cd precipitation at this pH. Cu²⁺ toxicity was linearly related to pH for ammonium and phosphate uptake and although, its toxicity for nitrate uptake also increased with pH, the increase was not perfectly linear. The toxicity of total Cu showed no linear relationship to pH. Cd²⁺ and Cu²⁺ toxicity increased by up to four orders of magnitude from pH 5 to 8. Competition between free metal and hydrogen ions for uptake sites on the cell surface is suggested as a mechanism increasing the toxicity of free metal, ions as the hydrogen ion content decreases (i.e. at higher pH).     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

Drought-induced root aerenchyma formation restricts water uptake in rice seedlings supplied with nitrate

Previous studies demonstrated that ammonium nutrition results in higher water uptake rate than does nitrate nutrition under water stress, and thus enhances the tolerance of rice plants to water stress. However, the process by which water uptake is related to nitrogen form under water stress remains unknown. A hydroponic experiment with simulated water stress induced by polyethylene glycol (PEG6000) was conducted in a greenhouse to study the relationship between root aerenchyma formation and water uptake rate, such as xylem sap flow rate and hydraulic conductance, in two different rice cultivars (cv. 'Shanyou 63' hybrid indica and cv. 'Yangdao 6' indica, China). The results showed that root aerenchyma tissue increased in water-stressed plants of both cultivars fed by nitrate. No significant difference was found in root hydraulic conductivity and/or xylem sap flow rate between the two rice cultivars fed by ammonium regardless of water status, whereas these parameters decreased significantly in water-stressed plants fed by nitrate. It was concluded that aerenchyma that formed in the root cortex impeded the radial transport of water in the root cylinder and decreased water uptake in water-stressed rice plants fed by nitrate. Water transport occurred mainly through Hg-sensitive water channels in rice roots supplied with ammonium.     

Foliar application of nitrate or ammonium as sole nitrogen supply in Ricinus communis. II. The flows of cations, chloride and abscisic acid

Following a precultivation with pedospheric nitrogen nutrition, Ricinus plants were supplied with nitrogen solely by spraying nitrate or ammonium solution onto the leaves during the experimental period. The chemical composition of tissues, xylem and phloem exudates was determined and on the basis of the previously determined nitrogen flows (Peuke et al., New Phytologist (1998), 138 , 657–687) the flows of potassium, sodium, magnesium, calcium, chloride and ABA were modelled. These data, which permit quantification of net-uptake, transport in xylem and phloem, and utilization in shoot and root, were compared with results obtained in plants with pedospherically-supplied nitrate or ammonium and data in the literature. Although the overall effects on the chemical composition of supplying ammonium to the leaves were not as pronounced as in pedospherically supplied plants, there were some typical responses of plants fed with ammonium (ammonium syndrome). In particular, in ammonium-sprayed plants uptake and transport of magnesium decreased and chloride uptake was increased compared with nitrate-sprayed plants. Furthermore, acropetal ABA transport in the xylem in ammonium-sprayed Ricinus was threefold higher than in nitrate-sprayed plants. Additionally, concentrations of anions were more or less increased in tissues, particularly in the roots, and transport fluids. The overall signal from ammonium-sprayed leaves without a direct effect of ammonium ions on uptake and transport systems in the root is discussed.     

Root Morphology and Nitrogen Uptake of Maize Simultaneously Supplied with Ammonium and Nitrate in a Split-root System

We studied the response of maize (Zea mays L. cv. Anjou 256)to a simultaneous, but separated supply of ammonium and nitrate(localized supply, LS). A split-root system was used to supplyhalf of the roots with ammonium and the other half with nitrate.A homogeneously distributed supply of both nitrogen forms (HS)was the control treatment. Seedlings were grown for 12 d fromthe two-leaf to the three-leaf stage in hydroponics at threepH levels (4, 5·5 and 7). The total N concentration was3 mol m^-3. The split-root system was established by removingthe seminal root system and using only four nodal roots perplant. Total root length and root surface area were recordedautomatically with a modified Delta- T area meter. Other morphologicalroot traits (such as main axis length and diameter, number,density, and length of laterals) were recorded manually. Uptakeof ammonium and nitrate was measured by the depletion of thenutrient solution. As compared with LS, HS was superior in shootand root DM, total root length and root surface area, ammoniumand nitrate uptake and shoot nitrogen concentration, irrespectiveof pH level. This indicates that, also under field conditions,mixed ammonium and nitrate fertilization is only beneficialto plant growth if both N forms are evenly distributed in thesoil. At both HS and LS, ascending pH increased the ammonium:nitrateuptake ratio. At LS, declining pH induced a considerable shiftin the distribution of root DM, root length, and root surfacearea the nitrate-fed compartment.Copyright 1993, 1999 AcademicPress Maize, Zea may L., ammonium, nitrate, pH, root morphology, split-root     

Flows of elements, ions and abscisic acid in Ricinus communis and site of nitrate reduction under potassium limitation.

In a pot experiment Ricinus communis plants were cultivated in quartz sand and supplied daily with a nutrient solution which contained 4 mol m(-3) nitrate as the nitrogen source and either full strength potassium (1.3 mol m(-3), control) or 8% potassium (0.1 mol m(-3), K(+)-limitation). Although the final fresh weight of the whole plant was not affected by K(+)-limitation, the root-shoot ratio was increased due to a relatively increased root growth and inhibited development of younger shoot parts. Owing to K(+)-limitation, photosynthesis was slightly decreased, while dark respiration of the shoot markedly decreased and root respiration was nearly doubled. The transport of carbon in the phloem, and to some extent in the xylem, was greater and the root was favoured in the partitioning of carbon. This was also true for nitrogen and potassium which were both taken up at lower rates, particularly potassium. In these two cases a high remobilization and recycling from the old part of the shoot was observed. By contrast, uptake of sodium was 2.4-fold higher under K(+)-limitation and this resulted in increased flows in the plants, which was discussed generally as a means for charge balance (in combination with a slight increase in uptake of magnesium and calcium). Nitrate reduction took place in the same portion in the root and shoot. This was a shift to the root compared to the control and points to an inhibition of xylem transport caused by limitation of K(+) as an easily permeating countercation. Low K(+) supply also resulted in an increased biosynthesis of ABA in the roots (265%). This caused a slightly increased deposition of ABA in the roots (193%) and a 4.6-fold higher root-to-shoot and a doubled shoot-to-root ABA signal in the xylem or phloem, respectively. The high degradation of ABA in the shoots prevented ABA accumulation there.     

Restricted nitrate influx and reduction in corn seedlings exposed to ammonium

The effect of ambient ammonium (0.5 millimolar [¹⁴NH₄]₂SO₄) added to a nutrient solution containing 1.0 millimolar K¹⁵NO₃, 99 atom per cent ¹⁵N, upon [¹⁵N]nitrate assimilation and utilization of previously accumulated [¹⁴N]nitrate was investigated. Corn seedlings, 5-day-old dark-grown decapitated (experiment I) and 10-day-old light-grown intact (experiment II), which had previously been grown on K¹⁴NO₃ nutrient solution, were used. In both experiments, the presence of ambient ammonium decreased [¹⁵N]nitrate influx (20% after 6 hours) without significantly affecting the efflux of previously accumulated [¹⁴N]nitrate. In experiment I, relative reduction of [¹⁵N]nitrate (reduction as a percentage of influx) was inhibited more than was [¹⁵N]nitrate influx. Nevertheless, in experiment I, where all reduction could be assigned to the root system, the absolute inhibition of reduction during the 12 hours (13 micromoles/root) was less than the absolute inhibition in influx (24 micromoles/root). The data suggest that the influence of ammonium on [¹⁵N]nitrate influx could not be totally accounted for by the decrease in the potential driving force which resulted from restricted reduction; an additional impact on the influx process is indicated. Reduction of [¹⁵N]nitrate in experiment II after 6 hours accounted for 30 and 18% of the tissue excess ¹⁵N in the control and ammonium treatments, respectively. Relative distribution of ¹⁵N between roots and exudate (experiment I), or between roots and shoots (experiment II) was not affected by ammonium. On the other hand, the accumulation of [¹⁵N]nitrate in roots, shoots, and xylem exudate was enhanced by ammonium treatment compared to the control, whereas the accumulation of reduced ¹⁵N was inhibited.     

Root-shoot interactions in mineral nutrition

In this paper four classes of co-operative root-shoot interations are addressed. (I) Nitrogen concentrations in the xylem sap originating from the root and in the phloem sap as exported from source leaves are much lower than those required for growth by apices and developing organs. Enrichment of xylem sap N is achieved by xylem to xylem (X-X) transfer, by which reduced N, but not nitrate, is abstracted from the xylem of leaf traces and loaded into xylem vessels serving the shoot apex. Nitrogen enrichment of phloem sap from source leaves is enacted by transfer of reduced N from xylem to phloem (X-P transfer). Quantitative data for the extent of the contribution of X-X and X-P transfer to the nutrition of young organs of Ricinus communis L. and for their change with time are presented. (II) Shoot and root cooperate in nitrate reduction and assimilation. The partitioning of this process between shoot and root is shifted towards the root under conditions of nitrate- and K-deficiency and under salt stress, while P deficiency shifts nitrate reduction almost totally to the shoot. All four changes in partitioning can be attributed to the need for cation-anion balance during xylem transport and the change in electrical charge occurring with nitrate reduction. (III) Even maintenance of the specificity of ion uptake by the root may – in addition to its need for energy – require a shoot-root interaction. This is shown to be needed in the case of the maintenance of K/Na selectivity under the highly adverse condition of salt stress and absence of K supply from the soil. (IV) Hormonal root to shoot interactions are required in the whole plant for sensing mineral imbalances in the soil. This is shown and addressed for conditions of salt stress and of P deficiency, both of which lead to a strong ABA signalling from root to shoot but result in different patterns of response in the shoot.     

Whole Plant Studies Using Radioactive 13-Nitrogen: III. THE RELATIONSHIP BETWEEN NITRATE AND AMMONIUM ION UPTAKE AND WATER FLUX FOR ZEA MAYS

Nitrogen-13 labelled nitrate and ammonium ions were used tomeasure uptake and assimilation rates in non-destructive experimentsin which the water flux through hydroponically grown maize plantswas doubled by changing the ambient humidity. Although no immediateeffects on ion uptake were observed, longer term observationsshowed different patterns of coupling for the uptake parametersfor nitrate and for ammonium. The patterns were compared withpatterns predicted for different combinations of processes foruptake and for discharge to the xylem. Those which were compatiblewith the experimental patterns indicated that for nitrate theuptake into the root showed coupling to water flux whereas dischargeto the xylem did not. For ammonium there appeared to be couplingto the water flux of both uptake into the root and of dischargeto the xylem though it was less evident in the former when theexternal ammonium ion concentration was low. Key words: Ion-water coupling, Ammonium uptake, Nitrate uptake