Kinetin application to roots and its effect on uptake, translocation and distribution of nitrogen in wheat (Triticum aestivum) grown with a split root system

The root systems of wheat seedlings ( Triticum aestivum L. cv. SUN 9E) were pruned to two seminal roots. One of the roots was supplied with a suboptimal level of NO₃, the other was deprived of N. Different levels of kinetin were supplied to the NO₃-deprived roots. Root respiration and the increment of C and N in the roots were measured to determine the C/N ratio of the phloem sap feeding the NO₃-deprived roots. Thus, it was possible to determine retranslocation of N from the shoots to the roots, as affected by the rate of kinetin application. It was calculated that the C/N ratio of phloem sap feeding roots growing without kinetin was ca 61. Kinetin application increased this ratio to ca 75, partly due to decreased translocation of N from the shoots back to the roots. Kinetin application decreased the proportion of N that was retranslocated to the roots after translocation to the shoots. Kinetin increased the rate of NO₃ uptake per root and the rate of N incorporation in both roots and shoots by ca 60%, but had no effect on shoot dry matter production. In control plants at most 70% of the N incorporated in the NO₃-fed roots could have been imported from the shoots, whilst kinetin application reduced this value to ca 40%. Thus root growth was not fully dependent on a supply of N via the phloem.
It is concluded that cytokinins affect the pattern of N-translocation in wheat plants by increasing incorporation of N in dry matter of the shoot, thus leaving less for export. Cytokinins did not play a major role in the regulation of shoot growth and the shoot to root ratio of the present plants.     

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 of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N₂ fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO₃⁻ may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO₂ levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO₃⁻. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO₃⁻ was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO₃⁻, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).     

Regulation of nitrogenase activity in Bradyrhizobium japonicum/soybean symbiosis by plant N status as determined by shoot C:N ratio

Regulation of nitrogenase is not sufficiently understood to engineer symbioses that achieve a high N₂ fixation rate under high levels of soil N. In the present hydroponic growth chamber study we evaluated the hypothesis that nitrogenase activity and the extent of its inhibition by NO₃⁻ may be related to both N and carbohydrate levels in plant tissues. A wide range of C:N ratios in various plant tissues (8.5 to 41.0, 1.9 to 3.7, and 0.8 to 1.8, respectively, in shoots, roots, and nodules) was generated through a combination of light and CO₂ levels, using two soybean genotypes differing in C and N acquisition rates. For both genotypes, N concentration in shoots was negatively correlated to nitrogenase activity and positively correlated to the extent of nitrogenase inhibition by NO₃⁻. Furthermore, nitrogenase activity was positively correlated to total nonstructural carbohydrates (TNC) and C:N ratio in shoot and nodules for both genotypes. Nitrogenase inhibition by NO₃⁻ was negatively correlated to TNC and C:N ratio in shoots, but not in nodules for both genotypes. At the onset of nitrogenase inhibition by NO₃⁻, C:N ratio declined in shoots but not in nodules. These results indicate that both C and N levels in plant tissues are involved in regulation of nitrogenase activity. We suggest that the level of nitrogenase activity may be determined by (1) N needs (as determined by shoot C:N) and (2) availability of carbohydrates in nodules. Modulation of the nitrogenase activity may occur through sensing changes in plant N, i.e. changes in shoot C:N ratio, possibly through some phloem translocatable compound(s).     

Translocation of NH4+ in oilseed rape plants in relation to glutamine synthetase isogene expression and activity

Translocation of NH₄⁺ was studied in relation to the expression of three glutamine synthetase (GS, EC 6.3.1.2) isogenes and total GS activity in roots and leaves of hydroponically grown oilseed rape ( Brassica napus ). The concentration of NH₄⁺ in the stem xylem sap of NO₃⁻-fed plants was 0.55–0.70 m M , which was ≈60% higher than that in plants deprived of external nitrogen for 2 days. In NH₄⁺-fed plants, xylem NH₄⁺ concentrations increased linearly both with time of exposure to NH₄⁺ and with increasing external NH₄⁺ concentration. The maximum xylem NH₄⁺ concentration was 8 m M , corresponding to 11% of the nitrogen translocated in the xylem. In the leaf apoplastic solution, the NH₄⁺ concentration increased from 0.03 m M in N-deprived plants to 0.20 m M in N-replete plants. The corresponding values for leaf tissue water were 0.33 and 1.24 m M , respectively. The addition of either NO₃⁻ or NH₄⁺ to N-starved plants induced both cytosolic gs isogene expression and GS activity in the roots. In N-replete plants, gs isogene expression and GS activity were repressed, probably due to carbon limitations, thereby protecting the roots against the excessive drainage of photosynthates. Repressed gs isogene expression and GS activity under N-replete conditions caused enhanced NH₄⁺ translocation to the shoots.     

Influence of nitrate and ammonium on the growth and 2,4-diaminobutyric acid composition of flatpea (Lathyrus sylvestris L.)

Abstract. Presence of 2.4-diaminobutyric acid (A₂bu), a neurotoxin, in tissues of flatpea ( Lathyrus sylvestris L.) necessitates a thorough understanding of the regulation of this nonprotein amino acid before the species can be recommended to livestock producers for forage applications. To determine how different concentrations and ratios of NO₃ and NH⁺₄ in growth media influence the levels of A₂bu and other free amino acids in the 'Lathco'flatpea cultivar, plants were grown hydroponically in controlled environments. The concentration of A₂bu was highest in tissues when the NO₃ to NH⁺₄ ratio in the nutrient solution was low. Responses of amides and other nonprotein amino acids, especially in the roots, followed a similar trend. Free protein amino acids in leaves and stems were generally unaffected by changes in NO₃ to NH⁺₄ ratios. In roots, protein amino acids increased as the NO₃ to NH⁺₄ ratio in the growth medium increased. Ammonium inhibited shoot and root growth; NO₃ alleviated the toxic effects of NH⁺₄. Soluble protein concentrations were higher in the shoots of NO₃-fed plants and in the roots of plants supplied with NH⁺₄. These results suggest that accumulation of A₂bu and other nonprotein amino acids, as well as asparagine and glutamine, plays a role in detoxification of NH⁺₄ and storage of N.     

Improvement of the 15N dilution method for estimation of absorption of NOx by plants supplied with 15N-labelled fertilizer

To develop further the methods for estimation of NOx absorption by plants supplied with ¹⁵N-labelled fertilizer, we proposed a new calculation method, total N fixed method (TNF), and compared with the ¹⁵N dilution method and the classical mass balance method (MB).
Hydroponically grown soybean plants were supplied with ¹⁵N-labelled nitrate and exposed to 200–250 nl l⁻¹ NO₂ for 7 d. The proportions of the N derived from NO₂ to total N in exposed plants were estimated by the three methods.
The reported rates of NO₂ absorption by several plant species, estimated by the ¹⁵N dilution method, were recalculated using the TNF method. The results of the two methods were compared and showed that: (1) The ¹⁵N dilution method overestimated the content of NO₂-N in exposed plants compared with the MB method whilst the TNF method produced estimations of NO₂-N closer to those by the MB method when the plants were supplied with 5 m M nitrate. (2) The differences in estimations between the MB method and either the ¹⁵N dilution method or the TNF method increased with decreasing supply of ¹⁵N-labelled nitrate to roots.     

Symbiotic properties of Lotus pedunculitis root nodules induced by Rhizobium loti and Bradyrhizobium sp. (Lotus)

Vance, C. P., Reibach, P. H. and Pankhurst, C. E. 1987. Symbiotic properties of Lotus pedunculatus root nodules induced by Rhizobium loti and Bradyrhizobium sp. ( Lotus ).
Symbiotic properties of root nodules were evaluated in glasshouse-grown Lotus pedunculatus Cav. cv. Maku inoculated with either a fast-growing Rhizobium loti strain NZP2037 or a slow-growing Bradyrhizobium sp. ( Lotus ) strain CC814s. Although the nodule mass of plants inoculated with NZP2037 was twice that of plants inoculated with CC814s, the yield of NZP2037 shoots and roots was 50% that of CC814s shoots and roots. Nodules induced by Bradyrhizobium fixed substantially more N than nodules induced by R. loti. Glucose requirements [mol glucose (mol N₂ fixed)^-1] of nodules induced by CC814s and NZP2037 were 7.1 and 16.6, respectively. Nodule enzymes of carbon and nitrogen assimilation reflected the disparity of the two sym-bioses. Xylem sap of the symbiosis with the higher yield contained a higher concentration of asparagine [9.86 μmol (ml xylem sap)'] than did the lower yielding symbiosis [5.80 umol (ml xylem sap)"']. Nodule CO₂ fixation was directly linked to nodule N assimilation in both symbioses. The results indicate that the difference between the two symbioses extend to nodule N and C assimilation and whole plant N transport. The data support a role for host plant modulation of bacterial efficiency and assimilation of fixed N.     

Nitrite in the root zone and its effects on ion uptake and growth of wheat seedlings

The immediate and posteffects of various concentrations of NaNO₂ on ion uptake of wheat ( Triticum aestivum L. cv. GK Öthalom) seedlings were studied at different pH values. Without pretreatment, the higher the concentration of NaNO₂ the greater was the decrease in uptake of K⁺ into the roots, both at pH 4 and pH 6. At pH 6 but not at pH 4 the reverse was true when the seedlings were pretreated with NaNO₂. Due to the high Na⁺ content of the roots, an effect of Na⁺ in this process cannot be excluded. Nitrite was taken up by the roots more rapidly than nitrate. Nitrite at 0.1 m M in the medium induced the development of an uptake system for both NO⁻₂ and NO⁻₃ in wheat roots. At higher concentrations pretreatment with NO⁻₂ decreased NO⁻₃ uptake by the roots, but NO₃ did not inhibit the uptake of NO₂. The toxic effect of NO⁻₂ was strongly pH dependent. Lower pH of the external solution led to an increased inhibition by NO⁻₂ of both ion uptake and growth of seedlings. The inhibitory effect of NO⁻₂ differed considerably for roots and shoots. The roots and especially the root hairs were particularly sensitive to NO⁻₂ treatment.     

Nitrate assimilation and nitrogen circulation in Austrian pine

Nitrate uptake, reduction and translocation were examined in 5-week-old Austrian pines ( Pinus nigra Arnold var. nigricans Host.) during exposure to 5 m M NaNO₃. The rate of nitrate uptake was linear during the 7 h light period. ¹⁵N-NO₃⁻ was detected in all parts of the pine except in the needles. By the 7th hour, 43% of the absorbed nitrate had been reduced, and this increased to 64% by the 24th hour. The major part of the total reduction occurred in the roots at this growth stage. Accumulation of ¹⁵N in reduced soluble and insoluble fractions was more prevalent in roots than in shoots. In the needles, the translocated nitrogen was mainly incorporated into the insoluble fraction. It is likely that most of the nitrogen from nitrate was transported from the roots to the aerial organs as organic nitrogen; however part of the upward nitrogen flux took place as nitrate ions.
An experiment in which an exposure for 24 h to 5 m M Na¹⁵NO₃ was followed by 13 days exposure to Na¹⁴NO₃ (pulse chase experiment) revealed a half time of about 1 day for depletion of root nitrate. A large part of this depletion was due to the loss of ¹⁵N-NO₃⁻ to the nutrient solution. The remaining pool of ¹⁵N-nitrate was partitioned between a metabolically inactive and an active pool. During the chase period, the simultaneous decrease of ¹⁵N-incorporation in the soluble N fraction and increase in the insoluble N fraction in different pine parts, particularly in the needles, suggested that protein synthesis occurred mostly in young tissues of the shoot and was the major sink of the newly absorbed ¹⁵N-NO₃.     

Nitrate and nitrite reduction in the plant fraction of alfalfa root nodules

The plant fraction of alfalfa ( Medicago sativa L. cv. Aragon) nodules contained both nitrate reductase (NR) and nitrite reductase (NiR). Specific activity of NADH-NR from the cytosol of nodules not treated with NO₃- was about 30 nmol (mg protein)^-1-h^-1 and was not basically affected by NO₃ addition. In contrast, typical specific activity for cytosolic NiR was 1.5 umol (mg protein)^-1h^-1 using methyl viologen as electron donor. This activity strongly increased with NO₃ concentration, probably due to substrate induction. Maximal activity was 3.5 μmol (mg protein)^-1h^-1 at 50 to 200 mM NO₃.
Estimates indicate that the contribution of cytosol to the overall NR and NiR activities of alfalfa nodules is distinctly different: less than 10% and about 70%, respectively. The increasing amounts of NO₂ accumulating in the cytosol upon NO₃, supply, and the different response to NO₃ of bacteroid and cytosolic NRs support the concept that most of this NO₂ comes from the bacteroids.     

Effects of nitrate on influx, efflux and translocation of potassium in young sunflower plants

Influx, efflux and translocation of K⁺(⁸⁶Rb) were studied in the roots of sunflower seedlings ( Helianthus annuus L. cv. Uniflorus) treated with 0–4.0 m M NO₃⁻ during a 9 day growth period or a 24 h pretreatment period. Roots treated with high levels of NO₃⁻ absorbed and translocated more K⁺(⁸⁶Rb) than seedlings treated with low levels of NO₃⁻. The content of K⁺ in the shoots was, however, higher in seedlings treated with low levels of NO₃⁻, indicating a low rate of retranslocation of K⁺ in those plants. K⁺(⁸⁶Rb) efflux was highest into the low-NO₃⁻ solutions. All effects on K⁺(⁸⁶Rb)-fluxes were more obvious in high-K plants than in low-K plants. The results are discussed in relation to the Dijkshoorn-Ben Zioni hypothesis for K⁺+ NO₃⁻-uptake and translocation in plants.     

Contribution of current photosynthates to root respiration of non-nodulated Medicago sativa: effects of light and nitrogen supply

Abstract: The effects of light (PFD) and nitrogen (N) supply on root respiration of new C (currently assimilated carbon, R _new) and old C ( R _old) were analysed in non-nodulated Medicago sativa . Plants were pre-treated with high/low PFD and high/low N supply with a regular 16/8 h light/dark cycle. Five to eight weeks after planting current photosynthates were labelled with ¹³C and their contribution to root respiration was continuously measured during a 24 h day/night cycle. PFD conditions during labelling were either those of the pre-treatments (control, 25 or 6 mol m^-2 d^-1) or, for high PFD plants, 6 mol m^-2 d^-1 by shortening the photoperiod or reducing irradiance. The fraction of new C in the respiratory CO₂ increased during the light period, but remained constant in the dark period. In control plants, R _new contributed 40 % to the daily root respiration in high PFD/high N conditions. Continuously low PFD increased (50 %) and low N decreased (26 %) the contribution of R _new. Exposing plants from high PFD pre-treatments to a short photoperiod or to low PFD stimulated R _old, indicating mobilisation of reserve C. This stimulation was more pronounced in plants with high N supply than in those with low N supply. Comparison with other legumes suggested that R _new in root respiration was mainly defined by the ratio between the assimilatory capacity of the shoots and the maintenance costs of roots with a short-term capacity of buffering respiratory demand by mobilisation of reserves in situations of fluctuating PFD.     

Maximal biomass production can occur in corn (Zea mays) in the absence of NO3 accumulation in either leaves or roots

In order to investigate effects of limited NO₃ availability in corn ( Zea mays L. cv. Brulouis) 17-day-old plants were grown for a further 25 days on sand in a growth chamber. The plants received frequent irrigation with a complete nutrient solution containing 0.2, 0.6, 1.5 or 3.0 mM NO₃. With 0.2 mM NO; nitrate levels in both roots and leaves diminished rapidly and were almost zero after 10 days treatment. Concurrently, as signs of nitrogen deficiency appeared, shoot growth was restricted, whereas root growth was enhanced. In addition, the concentration of reduced nitrogen and malate in the leaves declined, and in vitro nitrate reductase activity (NRA. EC 1.6.6.1), soluble protein and chlorophyll levels of leaf tissue were depressed and starch concentration was enhanced. With 0.6 mM NO₃ in the nutrient solution, the decrease in NO₃ levels in the tissues and the increase in root development were similar to those observed with 0.2 mM NO₃. However, shoot growth, reduced nitrogen concentration in leaves, and the above-mentioned biochemical characteristics were almost identical to those obtained at 1.5 and 3.0 mM NO₃. This indicates that when supplied with 0.6 mM NO₃, corn plants were able to absorb sufficient NO3 to support maximal biomass production without appreciable NO₃ accumulation in roots or shoot. It is, thus, suggested that the plants responded to low NO₃, availability in medium by enhancing root growth and by maximizing NO₃ reduction relative to NO₃ accumulation.     

大气CO2浓度倍增对油菜韧皮部汁液成分及根部氮素积累的影响

以甘蓝型欧洲油菜(Brassica napus) ‘814’和‘湘油15’两个品种为研究对象, 探寻在两个CO₂浓度(自然CO₂浓度: 400 μmol·mol^-1和高CO₂浓度: (800 ± 20) μmol·mol^-1)和两个氮素水平(低氮和常氮)处理下, 欧洲油菜韧皮部汁液中可溶性糖和游离氨基酸含量, 以及根部干物质量和氮素累积量的变化。结果表明: 1) CO₂浓度升高提高欧洲油菜韧皮部汁液可溶性糖含量, 施氮条件下‘814’在抽薹期达到0.29%, ‘湘油15’在盛花期达到0.25%, 其含量均显著高于自然CO₂浓度处理。2) CO₂浓度对韧皮部汁液游离氨基酸含量的影响因品种而异, 无论施氮与否, CO₂浓度升高使‘814’的游离氨基含量降低; 而‘湘油15’ CO₂浓度升高促使其在低氮条件下含量升高, 在常氮条件下则降低。3)根部干物质量和氮素累积量均随CO₂浓度升高而增加, 且欧洲油菜韧皮部汁液中可溶性糖含量与游离氨基酸含量与根部干物质量和氮素累积量呈显著正相关。     

Effect of high external NaCl concentration on ion transport within the shoot of Lupinus albus. II. Ions in phloem sap

Abstract. Phloem sap was collected from petioles of growing and fully expanded leaves of lupins exposed to 0–150 mol m⁻³ [NaCl]_ext, for various periods of time. Sap bled from growing leaves only after the turgor of the shoot was raised by applying pneumatic pressure to the root. Increased pressure was also needed to obtain sap from fully expanded leaves of plants at high [NaCl]_ext. Exposure to NaCl caused a rapid rise in the Na⁺ concentration in phloem sap to high levels. The Na⁺ concentration reached 20 mol m⁻³ within a day of exposure and reached a plateau of about 60 mol m⁻³ in plants at 50–150 mol m⁻³ [NaCl]_ext, after a week. There was a slower, smaller increase in the Cl⁻ concentration. K⁺ concentrations in phloem sap were not affected by [NaCl]_ext. Cl⁻ concentrations in phloem sap collected from growing leaves were similar to those from old leaves while Na⁺ concentrations were somewhat increased, suggesting that there was no reduction in the salt content of the phloem sap while it flowed within the shoot to the apex. Calculations of ion fluxes in xylem and phloem sap indicated that Na⁺ and Cl⁻ fluxes in the phloem from leaves of plants at high NaCl could be equal to those in the xylem. This prediction was borne out by observations that Na⁺ and Cl⁻ concentrations in recently expanded leaves remained constant.     

Does down‐regulation of photosynthetic capacity by elevated CO2 depend on N supply in Dactylis glomerata?

Dactylis glomerata was grown hydroponically in a controlled environment at ambient (360 μl l⁻¹) or elevated (680 μl l⁻¹) CO₂ and four concentrations of nitrogen (0.15, 0.6, 1.5 and 6.0 m M NO₃⁻), to test the hypothesis that reduction of photosynthetic capacity at elevated [CO₂] is dependent on N availability and mediated by a build-up of non-structural carbohydrates. Photosynthetic capacity of the youngest fully expanded leaf (leaf 5, 2 days after full expansion) was reduced in CO₂-enriched plants at low, but not high N supply and so the stimulation of net photosynthesis by CO₂ enhancement was less at low than at high N supply. CO₂ enrichment resulted in a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content on a leaf area basis at 0.6 and 1.5 m M NO₃⁻, but not at 0.15 and 6.0 m M NO₃⁻, and had no effect on the total N content of the leaf on an area basis. However, decreases in Rubisco content could be primarily accounted for by a decrease in total N content of leaves, independent of [CO₂]. A doubling of the Rubisco content by increasing the N supply beyond 0.6 m M had only a marginal effect on the maximum carboxylation velocity in vivo, suggesting that the fraction of inactive Rubisco increased with increasing N supply. Although CO₂-enriched plants accumulated more non-structural carbohydrates in the leaf, the reduction of photosynthetic capacity at low N supply was not mediated simply by a build-up of carbohydrates. In D . glomerata , the photosynthetic capacity was mainly determined by the total N content of the leaf.     

THE COMBINED EFFECTS OF LOW TEMPERATURE AND SO2+NO2 POLLUTION ON THE NEW SEASON'S GROWTH AND WATER RELATIONS OF PICEA SITCHENSIS

Picea sitchensis (Bong.) Carr. seedlings were exposed to SO₂, NO₂ and SO₂+ NO₂ during dormancy in controlled environments, and were taken to night temperatures of 4, 0, −5, −10 and −15 °C in a freezer. Conditions in the freezer were carefully monitored during the low–temperature treatments. In two experiments, different photoenvironments and temperature regimes were imposed prior to the cold treatments, and different effects were observed. In the first, only limited frost hardiness was achieved and night temperatures of −15 °C were lethal. Temperatures of −5 and − 10 °C led to poor survival of lateral buds, particularly in plants exposed to 45 ppb SO₂. The poor bud break in plants exposed to SO₂ and to − 5 °C resulted in a loss of the effectiveness of this temperature as a chill requirement. Pressure-volume analysis showed that the shoots of plants exposed to NO₂ had greater elasticity (lower elastic moduli, e), so that loss of turgor occurred at lower relative water contents. In contrast, a hardening period (2 weeks in night/day temperatures of 3/10 °C and 8 h days at 50 μmol m⁻² s⁻¹ PAR) gave decreased elasticity and lower solute potentials of spruce shoots. In the second experiment, exposure to 30 ppb SO₂ and SO₂+ NO₂ led to slight, but consistent, increases in frost injury to the needles of plants frozen to − 5 and − 10 °C. The results suggest that the main interaction of low temperatures and winter pollutants may be on bud survival rather than on needle damage, but that effects are subtle, only occurring with certain combinations of pollutant dose and cold treatment.     

Solubility of zinc and interactions between zinc and phosphorus in the hyperaccumulator Thlaspi caerulescens

The relationship between Zn and P in the Zn hyperaccumulator Thlaspi caerulescens J. & C. Presl was investigated using hydroponic culture. Total concentrations of Zn in the shoots increased from 0·2 to 27 g kg^–1 dry mass when solution Zn increased from 1 to 1000 mmol m^–3. Water-soluble Zn accounted for > 80% of the total Zn in the shoots containing > 5 g Zn kg^–1 dry mass. Total P was maintained at about 3 g kg^–1 dry mass in the shoots containing < 20 g Zn kg^–1 dry mass, but significantly decreased with higher Zn concentrations. Linear regression between insoluble P and insoluble Zn in the shoots produced a small slope, suggesting that co-precipitation of Zn and P was not an important detoxification mechanism in the shoots. In contrast, there was a strong correlation between insoluble P and insoluble Zn in the roots, with a linear slope of 0·3 — close to the P:Zn ratio in Zn₃(PO₄)₂. Foliar sprays of phosphate did not affect shoot dry mass significantly, but decreased root length and root dry mass significantly at Zn concentrations in solution from 10 to 3000 mmol m^–3. Foliar P was translocated to roots to enhance co-precipitation of Zn and P, although this did not enhance Zn tolerance. The results suggest that T.caerulescens possesses mechanisms which allow it to accumulate and sequester huge amounts of Zn in the shoots without causing P deficiency.     

Alteration of N nutrition in Myrica gale induces changes in nodule growth, nodule activity and amino acid composition

Changes in nodule growth and activity and in the concentrations of soluble N compounds in nodules, leaves and xylem sap under conditions of altered N nutrition in the actinorhizal plant Myrica gale L. are reported. Altering the N nutrition of symbiotic plants may alter the internal regulation of combined N which in turn may regulate nodule growth and activity. Flushing nodules daily with 100% O₂ caused a decline in amide concentration and an increase in nodule growth although plants had recovered some nitrogenase activity within 4 h of exposure to O₂. Samples of nodules, leaves and xylem sap were derivatized and amino acids identified and quantified using either reverse phase high performance liquid chromatography or gas chromatography-mass spectrometry in single ion monitoring mode. The ratio of asparagine in the nodules to that in the xylem was much higher in plants fed N (6.7 for NH⁺₄-fed and 8.3 for NO⁻₃-fed plants) than for N₂-fixing plants (2.5). Significant amounts of ¹⁵N added as ¹⁵NH⁺₄ or ¹⁵NO⁻₃ accumulated in nodules following accumulation in the shoot which is consistent with the translocation of N to the nodules via the phloem. The uptake of ¹⁵NH⁺₄ led to the synthesis and subsequent translocation of glutamine in the xylem sap. These results are discussed in terms of the feedback mechanisms that may regulate nitrogen fixation in Myrica root nodules.