The influence of cytokinins in nitrate regulation of nitrate reductase activity and expression in barley

The responses of nitrate reductase (NR) activity and levels of NR-mRNA to environmental nitrate and exogenous cytokinins are characterised in roots and shoots of barley ( Hordeum vulgare L., cv. Golf), using a chemostate-like culture system for controlling nitrate nutrition. Experiments were mainly performed with split root cultures where nitrate-N was supplied at a constant relative addition rate of 0.09 day⁻¹, and distributed between the subroots in a ratio of 20%:80%. The subroot NR-mRNA level and NR activity, as well as the endogenous level of zeatin riboside (ZR), increased when the local nitrate supply to one of the subroots was increased 4-fold by reversing the nitrate addition ratio (i.e. from 20%:80% to 80%:20%). Also shoot levels of ZR, NR-mRNA and NR activity increased in response to this treatment, even though the total nitrate supply remained unaltered. External supply of ZR at 0.1 μ M caused an approximately 3-fold increase in root ZR levels within 6 h. which is comparable to the nitrate-induced increase in root ZR. External application of ZR. zeatin. isopentenyl adenine or isopentenyl adenosine at 0.1 μ M caused from insignificant to 25% increases in NR-mRNA and activity in roots and up to 100% stimulation in shoots, whereas adenine or adenosine had no effect. No synergistic effects of perturbed nitrate supply and cytokinin application were detected in either roots or shoots. The translocation of nitrate from the root to the shoot was unaffected by application of ZR or switching the nitrate distribution ratio between subroots. The data give arguments for a physiological role of cytokinins in the response of root and shoot NR to environmental nitrate availability. The nature and limitations of the physiological role of cytokinins are discussed.     

Accumulation of nitrate in the shoot acts as a signal to regulate shoot-root allocation in tobacco†

Mutants and transformants of tobacco (Nicotiania tabacum L. cv Gatersleben 1) with decreased expression of nitrate reductase have been used to investigate whether nitrate accumulation in the shoot acts as a signal to alter allocation between shoot and root growth. (a) Transformants with very low (1–3% of wild-type levels) nitrate reductase activity had growth rates, and protein, amino acid and glutamine levels similar to or slightly lower than a nitrate-limited wild-type, but accumulated large amounts of nitrate. These plants should resemble a nitrate-limited wild-type, except in responses where nitrate acts as a signal. (b) Whereas the shoot:root ratio decreases from about 3.5 in a well-fertilized wild-type to about 2 in a nitrate-limited wild-type, the transformants had a very high shoot:root ratio (8–10) when they were grown on high nitrate. When they were grown on lower nitrate concentrations their shoot:root ratio declined progressively to a value similar to that in nitrate-limited wild-types. Mutants with a moderate (30–50%) decrease of nitrate reductase also had a small but highly significant increase of their shoot:root ratio, compared to the wild-type. The increased shoot:root ratio in the mutants and transformants was due to a stimulation of shoot growth and an inhibition of root growth. (c) There was a highly significant correlation between leaf nitrate content and the shoot:root ratio for eight genotypes growing at a wide range of nitrate supply. (d) A similar increase of the shoot:root ratio in nitrate reductase-deficient plants, and correlation between leaf nitrate content and the shoot:root ratio, was found in plants growing on ammonium nitrate. (f) Split-root experiments, in which the transformants were grown with part of their root system in high nitrate and the other part in low nitrate, showed that root growth is inhibited by the accumulation of nitrate in the shoot. High concentrations of nitrate in the rooting medium actually stimulate local root growth. (g) The inhibition of root growth in the transformants was relieved when the transformants were grown on limiting phosphate, even though the nitrate content of the root remained high. This shows that the nitrate-dependent changes in allocation can be overridden by other signals that increase allocation to root growth. (h) The reasons for the changed allocation were investigated in transformants growing normally, and in split-root culture. Accumulation of nitrate in the shoot did not lead to decreased levels of amino acids or protein in the roots. However, it did lead to a strong inhibition of starch synthesis and turnover in the leaves, and to decreased levels of sugars in the root. The rate of root growth was correlated with the root sugar content. It is concluded that these changes of carbon allocation could contribute to the changes in shoot and root growth.     

Nitrate assimilatory properties of barley grown under long-term N limitation: Effects of local nitrate supply in split-root cultures

The effect of nitrate availability on characteristics of the nitrate assimilatory system was investigated in N-limited barley (Hordeum valgare L. cv. Golf), grown with the seminal root system split into initially equal-sized halves. The cultures were continuously supplied with nitrate-N at a relative addition rate (RA) of 0.09 day^?1, which resulted in relative growth rates (RG) that were ca 85% of those observed under surplus nitrate nutrition. The total N addition was divided between the subroots in ratios of 100:0, 80:20, 70:30, 60:40, and 50:50. For comparison, standard cultures were grown at RAs ranging from 0.03 to 0.18 day^?1. Initially, biomass and N partitioning to the subroots responded strongly and proportionally to the nitrate distribution ratio. After 12-14 days no further effect was observed. The V_max for net nitrate uptake and in vitro nitrate reductase (NR) activity were measured in acclimated plants, i.e., after > 14 days under a certain nitrate regime. In subroots fed from 20 to 100% of the total N addition, V_max for net nitrate uptake increased slightly, whereas NR activity was unaffected. Uptake and NR activities were insignificant in the 0%-subroot. Uneven nitrate supply to individual subroots had negligible effect on the whole-plant ability for nitrate uptake, and the relative V_max (unit N taken up per unit N in whole plant tissue and time) remained about 7-fold in excess of the demand set by growth. Balancing nitrate concentrations (the resulting external nitrate concentrations at a certain RA) generally ranged between 2 and 10 μM at growth-limiting RA, both when predicted from uptake kinetics and when actually measured. When comparing split root and standard cultures when acclimated, it appears that uptake and NR activities in roots respond more strongly to over-all nitrate availability than to nitrate availability to individual subroots.     

Nitrate-regulated growth and cytokinin responses in seminal roots of barley

The influence of nitrate availability on growth of seminal roots, and root cytokinin levels, was studied in barley (Hordeum vulgare L. cv Golf). Nitrate was continuously supplied to initially N-starved seedlings at relative addition rates (RA) of 0.03 to 0.21 per day (standard cultures) or at RA 0.09 per day in split root cultures with the nitrate additions distributed in ratios of 100:0 or 80:20 to the two subroots. Data were collected both during a phase of acclimation (first 10 days of N additions) and in the acclimated stage (>10 days after onset of N additions). Limitation of whole-plant growth was observed at RA <0.15 per day. The lateral root frequency increased with RA in plants of equal chronological age. However, the lateral root frequency was related to root size rather than to RA; roots of uneven age but having comparable total root lengths also had comparable lateral root frequencies. Growth of individual subroots in split root systems during acclimation was proportional to the fraction of the total N addition that was fed to the root. All subroots had comparable relative growth rates in acclimated plants, and their lateral root frequency correlated with total root length in the same manner as in standard cultures. Onset of N additions in a 80:20 split root culture resulted in doublings of zeatin riboside (ZR) levels in shoots and in the “80” root, whereas the response of the “20” root was small. No effect of perturbed nitrate availability on xylem translocation of ZR was observed. The ZR levels remained higher in the “80” root during acclimation but returned to the level of the “20” root after acclimation. Root cytokinin levels and xylem translocation in acclimated standard cultures were unaffected by RA in the lower range but increased at high RA. Arguments for involvement of cytokinins in the nitrate-regulated growth response are discussed.     

Interactions Between Nitrogen and Manganese Nutrition of Barley Genotypes Differing in Manganese Efficiency

Ammonium-fed plants may acidify the rhizosphere and thus increaseavailability of Mn in calcareous alkaline soils. The importanceof N nutrition in the differential expression of tolerance toMn deficiency among cereal genotypes is not yet clear. Two factorialexperiments testing effects of the NH₄-N/NO₃-N ratio and Mnfertilization on growth of barley genotypes differing in toleranceto Mn deficiency were conducted in two calcareous alkaline soilsin pots in a controlled environment. In the soil containing80% CaCO₃at pH 8.5, better root and shoot growth and highershoot Mn concentrations were achieved with nitrate supply, especiallyat lower rates of Mn fertilization. The Mn-efficient genotypeWeeah (tolerant of Mn deficiency) achieved better root and shootgrowth than Mn-inefficient Galleon barley (sensitive to Mn deficiency)regardless of experimental treatment. Fertilization with Mndid not influence total N concentration in barley roots andshoots. In the soil containing 5% CaCO₃at pH 7.8, ammonium-fedplants had better root and shoot growth and, at shoot Mn concentrationsabove the critical level, Mn-inefficient Galleon performed betterthan Mn-efficient Weeah barley. It appears that differentialexpression of Mn efficiency among barley genotypes is not associatedwith differences in Mn availability expected to be producedby differential rhizosphere acidification as a response to differentforms of N supply. There is an apparent preference of locallyselected barley genotypes for nitrate nutrition when grown onthe highly calcareous alkaline soils of southern Australia. Ammonium; calcareous soil; Hordeum vulgare ; manganese; nitrate; nitrogen form; nutrient efficiency; rhizosphere     

Abscisic acid accumulation in the roots of nutrient-limited plants: its impact on the differential growth of roots and shoots

We describe the involvement of abscisic acid (ABA) in the control of differential growth of roots and shoots of nutrient limited durum wheat plants. A ten-fold dilution of the optimal concentration of nutrient solution inhibited shoot growth, while root growth remained unchanged, resulting in a decreased shoot/root ratio. Addition of fluridone (inhibitor of ABA synthesis) prevented growth allocation in favour of the roots. This suggests the involvement of ABA in the redirecting of growth in favour of roots under limited nutrient supply. The ABA content was greater in shoots and growing apical root parts of starved plants than in nutrient sufficient plants. Accumulation of ABA in shoots of nutrient deficient plants was linked to a decrease in leaf turgor. Increased flow of ABA in the phloem apparently contributed to the accumulation of ABA in the apical part of the roots. Thus, partitioning of growth between roots and shoots of wheat plants limited in mineral nutrients appears to be modulated by accumulation of ABA in roots. This ABA may originate in the shoots, where its synthesis is stimulated by the loss of leaf turgor.     

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     

Root Cluster Formation and Citrate Exudation of White Lupin (Lupinus albus L.) as Related to Phosphorus Availability

A split-root system was used to investigate whether the external or internal P concentration controls root cluster formation and citrate exudation in white lupin (Lupinus albus L.) grown under controlled conditions. In spite of low P concentrations in the shoots and roots of the -P plant, its dry weight was not reduced compared with the P plant. Supplying external P (0.25 mmol/L) to one root halfresulted in an increase in P concentration not only in the shoot, but also in the P-deprived root half, indicating P cycling within the plants. Omitting P from both split-root pots stimulated root cluster formation in both root halves,whereas P supply to one root halfstimulated root cluster formation at the beginning of the treatment. Neither P supply to just one root half continuously nor resupply of P to one root half after 19 d of P starvation inhibited root cluster formation on the P-deprived side, although the concentration of P in this root half and shoot increased markedly. The results indicate that root cluster formation in L. albus is controlled by both shoot and root P concentrations. The rates of citrate exudation by both root halves with P deficiency were higher than those of the one root half supplied with P only. In the treatment with one root half supplied with P, the rates of citrate exudation by either the P-supplied or -deprived root halves were almost the same,regardless of P concentration in the roots. The results suggest that internal P concentration controls root cluster formation and citrate exudation in white lupin, but these processes may be regulated by different mechanisms.     

Biosensor reporting of root exudation from Hordeum vulgare in relation to shoot nitrate concentration

The aim of this study was to determine the relationship between shoot nitrate concentration, mediated by nitrate supply to roots, and root exudation from Hordeum vulgare. Plants were grown for 14 d in C-free sand microcosms, supplied with nutrient solution containing 2 mM nitrate. After this period, three treatments were applied for a further 14 d: (A) continued supply with 2 mM nitrate (zero boost), (B) supply with 10 mM nitrate (low boost), and (C) supply with 20 mM nitrate (high boost). At the end of the treatment period, a bacterial biosensor (Pseudomonas fluorescens 10586 pUCD607, marked with the lux CDABE genes for bioluminescence) was applied to the microcosms to report on C-substrate availability, as a consequence of root exudation. The nitrate boost treatments significantly affected shoot nitrate concentrations, in the order C>B>A. In treatments receiving a nitrate boost (B, C), increased shoot nitrate concentration was correlated with increased plant biomass, reduced root length, reduced number of root tips, and increased mean root diameter, relative to the no boost treatment (A). Imaging of biosensor bioluminescence (proportional to metabolic activity in response to availability of root exudates) indicated that root exudation increased with decreasing shoot nitrate concentration. Biosensor reporting of root C-flow indicated that exudation was greater from root tip regions than from the whole root, but that specific exudation rates for all sites were unaffected by treatments. Total root exudation across treatments was found to be closely correlated with total root length, indicating that increased root exudation, per unit root biomass, with decreasing nitrate supply was associated with altered root morphology, as a consequence of systemic plant responses to internal N-status.     

Root Cluster Formation and Citrate Exudation of White Lupin （Lupinus albus L.） as Related to Phosphorus Availability

A split-root system was used to investigate whether the external or internal P concentration controls root cluster formation and citrate exudation in white lupin (Lupinus albus L.) grown under controlled conditions. In spite of low P concentrations in the shoots and roots of the -P plant, its dry weight was not reduced compared with the P plant. Supplying external P (0.25 mmol/L) to one root half resulted in an increase in P concentration not only in the shoot, but also in the P-deprived root half, indicating P cycling within the plants. Omitting P from both split-root pots stimulated root cluster formation in both root halves,whereas P supply to one root half stimulated root cluster formation at the beginning of the treatment. Neither P supply to just one root half continuously nor resupply of P to one root half after 19 d of P starvation inhibited root cluster formation on the P-deprived side, although the concentration of P in this root half and shoot increased markedly. The results indicate that root cluster formation in L. albus is controlled by both shoot and root P concentrations. The rates of citrate exudation by both root halves with P deficiency were higher than those of the one root half supplied with P only. In the treatment with one root half supplied with P, the rates of citrate exudation by either the P-supplied or -deprived root halves were almost the same,regardless of P concentration in the roots. The results suggest that internal P concentration controls root cluster formation and citrate exudation in white lupin, but these processes may be regulated by different mechanisms.     

Control of plant growth resides in the shoot,and not in the root,in reciprocal grafts of flacca and wild-type tomato (Lysopersicon esculentum), in the presence and absence of salinity stress

Grafted plants of flacca, an ABA-deficient mutant of tomato (Lycopersicon esculentum), and the wild-type variety Rheinlands Ruhm were grown with and without salinity stress to test the roles of roots and shoots in the regulation of plant growth. Fourteen days after exposure to 200 mM NaCl, shoot and root fresh weight, endogenous ABA concentrations, nitrate concentration, activities of selected enzymes related to nitrogen assimilation, and cation accumulation were determined. Rootstock genotype had little influence on the growth of the grafted plants, whereas grafted plants having wild-type shoots (Ws) produced more biomass than those having flacca shoots (Fs), irrespective of the salinity level. Growth of flacca shoots grafted onto wild-type rootstock (Fs/Wr) was superior to that of flacca shoots grafted onto flacca rootstock (Fs/Fr). The improved growth correlated with enhanced levels of ABA in the flaccashoots of Fs/Wr. In all the graft combinations, ABA content was higher in wild-type shoots than in flacca shoots, with or without salinity. There were no significant differences in root ABA concentrations among the different grafted types. Enhanced growth correlated with higher nitrate levels and higher nitrate reductase activity in the roots and shoots of plants with wild-type shoots and with higher shoot concentrations of ABA in plants with wild-type shoots. There were no significant differences in glutamine synthetase and phosphoenol pyruvate carboxylase activities in the shoots and roots of all the grafted plants, regardless of the salinity level. While shoot genotype determined the accumulation of K⁺ and Na⁺ in grafted plants regardless of salinity, it had no influence on Ca²⁺ concentrations. Regardless of the salinity, the total concentration of cations was the same in all the plants, while salinity decreased Mg²⁺ concentration in roots and shoots of all grafts, with the exception of flacca grafted shoots. The scion genotype – and its ABA level – thus played the major role in the growth of grafted plants, regardless of the rootstock genotype and the salinity of the growth medium.     

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.     

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.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Effect of Whole Plant and Local Heating on the ABA Content in Cucumber Seedling Leaves and Roots and on Their Heat Tolerance

The effects of heating at 38°C of whole cucumber (Cucumis sativus L.) seedlings or local heating of their shoots and roots on ABA content and heat tolerance of leaves and roots were investigated. During the initial period of the high-temperature treatment of whole seedlings, the ABA concentration in leaves and roots increased considerably. Local heating of the shoot or root resulted in an increase in the ABA concentration not only in the heated organ, but also in unheated seedling parts. A high-temperature treatment of the whole seedlings and the local treatment of shoots or roots caused an increase in the heat tolerance of leaf cells. The heat tolerance of root cells virtually did not change after heating of the whole seedlings or shoots, but decreased after heating of roots. The possible role of ABA in changing the heat tolerance of leaf and root cells by local heating of the seedling is discussed.     

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.     

JA、ABA在海棠幼苗和枝条中的传输分布以及与水分的关系

采用示踪技术探索了JA和ABA在海棠幼苗和技条中的传输分布以及与水分的关系,发现从的行径主要由韧皮部自上而下运输,ABA主要通过木质部由下向上运输。两者均对土壤水分变化敏感,水分亏缺会加速JA向下运输,促进ABA向上运输,推测JA下运除影响根系生长外,可能与根系对根际环境变化的感应以及根源ABA的合成有关。     

Control of Nitrate Uptake by Phloem-Translocated Glutamine in Zea mays L. Seedlings

Abstract: The putative role of glutamine, exported from leaves to roots, as a negative feedback signal for nitrate uptake was investigated in Zea mays L. seedlings. Glutamine (Gln) was supplied by immersion of the tip-cut leaves in a concentrated solution. Nitrate (NO₃⁻) uptake was measured by its depletion in amino acid-free medium. The treatment with Gln resulted in a strong inhibition of nitrate uptake rate, accompanied by a significant enrichment of amino compounds in root tissue. The effect of N-availability on NO₃⁻ uptake was determined in split-root cultures. The plants were subjected to complete or localized N supply. Inducible NO₃⁻ uptake systems were also induced in N-deprived roots when the opposite side of the root system was supplied with KNO₃. The inhibitory effect of Gln was unaffected by localized N supply on one side of the split-root. The potential role of Gln in the shoot-to-root control of NO₃⁻ uptake is discussed.     

Regulation and manipulation of ABA biosynthesis in roots

Overexpression of 9-cis-epoxycarotenoid dioxygenase (NCED) is known to cause abscisic acid (ABA) accumulation in leaves, seeds and whole plants. Here we investigated the manipulation of ABA biosynthesis in roots. Roots from whole tomato plants that constitutively overexpress LeNCED1 had a higher ABA content than wild-type (WT) roots. This could be explained by enhanced in situ ABA biosynthesis, rather than import of ABA from the shoot, because root cultures also had higher ABA content, and because tetracycline (Tc)-induced LeNCED1 expression caused ABA accumulation in isolated tobacco roots. However, the Tc-induced expression led to greater accumulation of ABA in leaves than in roots. This demonstrates for the first time that NCED is rate-limiting in root tissues, but suggests that other steps were also restrictive to pathway flux, more so in roots than in leaves. Dehydration and NCED overexpression acted synergistically in enhancing ABA accumulation in tomato root cultures. One explanation is that xanthophyll synthesis was increased during root dehydration, and, in support of this, dehydration treatments increased beta-carotene hydroxylase mRNA levels. Whole plants overexpressing LeNCED1 exhibited greatly reduced stomatal conductance and grafting experiments from this study demonstrated that this was predominantly due to increased ABA biosynthesis in leaves rather than in roots. Genetic manipulation of both xanthophyll supply and epoxycarotenoid cleavage may be needed to enhance root ABA biosynthesis sufficiently to signal stomatal closure in the shoot.     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.