The Effects of Nitrogen Supply on the Absorption and Distribution of Copper in Red Clover (Trifolium pratense L.) Grown in Flowing Solution Culture with a Low, Maintained Concentration of Copper

The absorption and distribution of Cu in red clover (Trifoliumpratense L.) were measured in plants grown in flowing solutionculture with Cu maintained throughout at 0.5 µg 1^–1and N supplied either as nitrate or through symbiotic fixation.Although there was a decrease in Cu absorption, both with time,and with a depleted nitrate supply, it increased to its formerrate when nitrate was adjusted to 10 mg N 1^–1 after aperiod of depletion. Differences in absorption between plantsdependent upon fixation and those supplied with nitrate wererelated to the slower initial growth of the plants fixing N.Considerable proportions (> 30 per cent) of the absorbedCu were retained by the roots. At the final harvest, and withthe exception of plants grown with nitrate adjusted to 0.1 mgN 1^–1 after a period of depletion, the proportion of theCu retained was related to the concentration of N in the roots.The different N treatments produced differences in Cu concentrationin the shoots, and the effects were greater in the youngestfully expanded leaves than in older leaves. Trifolium pratense L., red clover, absorption, copper, flowing solution culture, nitrogen     

The Uptake and Distribution of Copper in Some Forage Grasses as Affected by Nitrate-nitrogen Supply in Flowing Solution Culture

The effect of changes in nitrate-nitrogen supply on the absorptionand distribution of copper was examined in grasses grown inflowing solution culture with a maintained concentration ofcopper. Absorption by roots (µg Cu g^–1 dry root)decreased markedly when nitrogen had been depleted or was maintainedat 0.1 mg l^–1 N, but there was an immediate increase whennitrogen was maintained at 1.0 or 10.0 mg l^–1. There werealso large increases in the concentration of copper in the shootsof plants grown with 1.0 and 10.0 mg 1^–1 N. The rootsof plants grown with 0.1 or 1.0 mg 1^–1 N retained similarproportions of uptake, but a lower proportion was retained whenthe plants were grown with 10.0 mg 1^–1. Although a lowerproportion of the copper was associated with cell walls in theplants grown at 10.0 mg 1^–1 N this was the result of alower content of cell walls rather than an effect on copperitself. In a longer-term experiment in conventional solutionculture with a range of nitrogen concentration, the concentrationof copper in shoots was largely determined by shoot growth. Dactylis glomerata, Festuca arundinacea, Lolium perenne, cell walls, copper absorption, copper distribution, flowing solution culture, nitrate-nitrogen     

The Uptake of Phosphate by Plants from Flowing Nutrient Solution: IV. EFFECT OF PHOSPHATE CONCENTRATION ON THE GROWTH OF TRIFOLIUM REPENS L. SUPPLIED WITH NITRATE, OR DEPENDENT UPON SYMBIOTICALLY FIXED NITROGEN

Breeze, V. G. and Hopper, M. J. 1987. The uptake of phosphateby plants from flowing nutrient solution. IV. Effect of phosphateconcentration on the growth of Trifolium repens L. suppliedwith nitrate, or dependent upon symbiotically fixed nitrogen.—J.exp. Bot. 38: 618–630. Nodulated white clover plants were subjected to a range of phosphateconcentrations in flowing solution culture (0.32 to 8.0 mmolm^–3 P) at 41 d from sowing, either supplied with nitrateor dependent on symbiotically-fixed nitrogen. No effect of phosphateconcentration in solution on dry matter production, relativegrowth rate, root/shoot ratio, or water soluble carbohydrateconcentration of the plant tissue was observed after 24 d fromthe start of the experiment, although the plants supplied withnitrate yielded more than the others. Phosphate uptake throughoutthe experimental period was related to the solution concentration,but the source of nitrogen did not affect the phosphorus concentrationsof the shoots. However, the roots of the plants dependent onsymbiotically-fixed nitrogen had higher concentrations of phosphorusthan those supplied with nitrate, but this did not appear tobe due to an increased phosphorus requirement for nitrogen fixation,because the amount fixed was unaffected by the phosphate concentrationin solution. The cation-anion balance showed that plants dependenton nitrogen fixation had no larger requirement for calcium thanplants supplied with nitrate, but a requirement for hydroxylions equivalent to over 130 kg lime per tonne of dry shoot.It is suggested that the enhanced phosphate uptake by plantsdependent on nitrogen fixation is due to this need for a cation-chargebalancing anion. Key words: Phosphate uptake, nitrogen fixation, Trifolium repens L., repens L., cation-anion balance, flowing solution culture     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

Influx and Efflux of Nitrate and Ammonium in Italian Ryegrass and White Clover Roots: Comparisons Between Effects of Darkness and Defoliation

Seedlings of Italian ryegrass (Lolium multiflorum Lam. cv. RVP)and clonal stolon cuttings of white clover (Trifolium repensL. cv. Blanca) were grown for 19 d in flowing solution culture,with N supplied as either 250 mmol m^–3 NO₃^– or NH₃⁺.Rates of net uptake, influx and translocation of NO₃^–and NH₄⁺ were then determined using ¹⁵N and ¹³N labelling techniques:between 3–5 h into the photoperiod following 8 h darknessfor white clover (CL), and for ryegrass plants that were eitherentire (IL) or with shoots excised 90 min prior to ¹³N influx(IC); and 75 min into the photoperiod following 37–39h darkness for ryegrass (ID). Rates of net uptake, influx andefflux of NH₄⁺ exceeded those of NO₃^– in IL and IC ryegrassplants: the opposite occurred in white clover (CL). The decreasein net uptake following defoliation of ryegrass was greaterfor NH₄⁺ (62%) than NO₃^– (40%). For NH₄⁺ this was associatedwith a large decrease in influx from 110 to 6.0µmol h^–1g^–1 root fr. wt; but for NO₃^–, influx only decreasedfrom 42 to 37 µmol h^–1 g^–1. Prolonged exposureto darkness (ID plants) also lowered net uptake of NO₃^–and NH₄⁺ by, respectively, 86% and 95% of IL levels. For NH₄⁺this was characterized by a large decrease in influx and a smalldecrease in efflux; whilst for NO₃^– the effect of a largedecrease in influx was reinforced by a smaller increase in efflux. The data were used to estimate the translocatory fluxes of NO₃^–(03–20µmol h^–1 g^–1) and NH₄⁺ (003–0.4µmolh^–1 g^–1), assimilation in the roots of NO₃^–(02–26µmol h^–1 g^–1) and NH⁺⁴ (05–89 µmolh^–1 g^–1), and the concentrations of NO₃^– (9–15mol m^–3) in the cytoplasmic compartment of the roots.The relevance of variable influx and efflux to models for theregulation of N uptake is discussed. Key words: Lolium multiflorum, Trifolium repens, influx, efflux, nitrate, ammonium, ¹³N     

Salt Tolerance in the Succulent, Coastal Halophyte, Sarcocornia natalensis

The effects of 0, 50, 100, 200, 300, 400 and 500 mol m^–3NaCl on growth and ion accumulation in the succulent, coastalhalophyte Sarcocornia natalensis (Bunge ex Ung.-Sternb.) A.J. Scott were investigated. Increase in salinity from 0 to 300 mol m^–3 NaCl stimulatedproduction of fresh, dry, and organic dry mass, increased succulenceand shifted resource allocation from roots to shoots. Growthwas optimal at 300 mol m^–3 and decreased with furtherincrease in salinity. Water contributed to a large proportion of the increase in freshmass. Inorganic ions, especially Na⁺ and Cl– contributedsubstantially to the dry mass. At 300 mol m^–3 NaCl inorganicions contributed to 37% of total dry mass and NaCl concentrationin the shoots was 482 mol m^–3. Expressed sap osmotic potentialsdecreased from –2.10 to –3.95 MPa as salinity increasedfrom 0 to 300 mol m^–3 NaCl. Massive accumulation of inorganicions, especially Na⁺ and Cl^–, accounted for 86% of theosmotic adjustment at 300 mol m^–3 NaCl. Salinity treatments decreased the concentrations of K+ in shoots.Plant Na+ :K+ ratios increased steadily with salinity and reacheda maximum of 16.6 at 400 mol m3 NaCl. It is suggested that the exceptional salt tolerance of S. natalensisis achieved by massive inorganic ion accumulation which providessufficient solutes for osmoregulation, increased water fluxand turgor-induced growth. Key words: Sarcocornia natalensis, salt tolerance, halophyte     

The Combined Effects of Salinity and Root Anoxia on Growth and Net Na+ and K+-accumulation in Zea mays Grown in Solution Culture

The effects of excess salinity and oxygen deficiency on growthand solute relations in Zea mays L. cv. Pioneer 3906 were examinedin greenhouse experiments. The roots of plants 14 d old growingin nutrient solution containing additions of NaCl in the range1.0–200 mol m^–3 were either exposed to a severedeficiency of O₂ by bubbling with nitrogen gas (N₂ treatment),or maintained with a supply of air (controls), for a periodof 1–7 d. The threshold NaCl concentration resulting inappreciable inhibition of leaf extension, and shoot f. wt gainin controls was between 10 and 25 mol m^–3. At 25 mol m^–3NaCl the ratio of Na+/K+ transported to shoots was about 20times greater than in plants in 1.0 mol m^–3 NaCl. Theeffect of addition of NaCl to the nutrient solution was to enhanceNa+ movement but simultaneously depress the rate of K+ transportto shoots (per g f. wt roots). Interactions between NaCl levels and aeration treatment wereshown by analyses of variance to be statistically significantfor leaf extension, shoot and root f. wt gains, Na+ and K+ concentrationsin shoots and roots. When roots were N₂-treated, shoot and rootgrowth were depressed, the effect of aeration treatment beinggreatest at NaCl concentrations of 50 mol m^–3 or less.Additionally, N₂-treatment greatly accelerated Na- transportto shoots while depressing K+ transport still further, so thatat 10 mol m^–3 NaCl the ratio Na+/K+ acquired by the shootswas 230 times greater than in controls. Over the concentrationrange 1.0 to 50 mol m^–3 NaCl, the ratio Na+/K+ transportedto shoots by anoxic roots increased by a factor of 860. Mechanisms controlling changes in solute flux to the shoot,and the significance in relation to plant tolerance of excesssalts or oxygen deficiency are discussed. Anaerobic, corn, flooding, maize, oxygen-deficiency, salinity     

Role of Copper Binding, Absorption, and Translocation in Copper Tolerance of Agrostis gigantea Roth

The capacity of roots to accumulate and retain copper was examinedin two clones of Agrostis gigantea which differ in their toleranceto excess copper. Root elongation growth in the non-tolerantclone was completely inhibited by 16 mmol m^–3 Cu whereas40 mmol m^–3 was required for inhibition in the tolerantclone. The amount of readily exchangeable copper was greaterin roots of the tolerant clone than in the non-tolerant clone.The higher capacity for binding copper did not prevent the entryof copper into the cells of intact or excised roots of the tolerantclone. Roots of both clones contained similar amounts of copperafter removal of the readily exchangeable fraction. More copperwas translocated to the shoots of the tolerant than the non-tolerantclone. The explanation of copper tolerance in Agrostis giganteamust be sought in areas other than those of differences in grosscopper absorption and retention by roots.     

Fe-deficiency stress response in Fe-deficiency resistant and susceptible subterranean clover: importance of induced H+ release

Plants can exhibit Fe-deficiency stress response when they areexposed to Fe-deficiency conditions. The relative importanceof the individual Fe-deficiency stress-response reactions, forexample, increased release of H⁺ from roots, enhanced root plasmamembrane-bound Fe³⁺ -reductase activity, and release of reductant,in Fe-deficiency resistance is not understood. To address thisproblem, the Fe-deficiency stress response of two cultivarsof subterranean clover (subclover), Koala (Trifolium brachycalycinumKatzn. and Morley) (Fe-deficiency resistant) and Karridale (T.subterraneum L.) (Fe-deficiency susceptible), were evaluated.The plants were cultured hydroponically at 0 (–Fe) and30 (+Fe) µM Fe³⁺ EDTA conditions. After 6 d Fe treatment,the –Fe Koala and Karridale decreased the pH of the nutrientsolution by 1.83 and 0.79 units, respectively, while the +Feplants increased the pH of the nutrient solution. The H⁺ -releaserate of the –Fe Koala determined 7 d after Fe treatmentinitiation was more than three times higher than that of the–Fe Karridale. The –Fe plants had a significantlyenhanced Fe³⁺ -reduction rate compared with the +Fe plants foreach cultivar, but the resistant cultivar did not exhibit ahigher root Fe³⁺ -reduction rate than the susceptible cultivarat each Fe treatment. Reductant release from the roots of subcloverwas negligible. These results indicate that Fe-deficiency-inducedH⁺ release may be the predominant factor influencing Fe-deficiencyresistance in subclover. Key words: Fe-deficiency, Fe³⁺ reduction, H⁺ release, stress response, Trifolium     

Uptake and Distribution of Copper in Chrysanthemum morifolium

The effects of various levels of copper on the uptake and distributionof copper in Chrysanthemum morifolium grown in solution cultureand peat-sand have been examined. Whole plants growing in shortdays were sampled at regular intervals, divided into roots,stem, leaves and lateral shoots, and analysed for copper. Thepartitioning of copper between these tissues showed that a relativelylarge proportion (30–40 per cent) of the total plant copperwas accumulated in the roots of normal plants during the harvestingperiod, compared with approximately 10 per cent in the rootsof copper deficient plants. Whilst the copper content (ug g^–1) of leaves and stemfrom normal plants was negatively correlated with the amountof dry matter produced (P < 0·001), the correspondingcopper deficient tissues showed little variation in copper contentwith increases in tissue dry weight. A more detailed investigationof the copper content of leaves from normal plants showed thatgradients existed within the plant with respect to both leafposition and time of harvest which could be described by a singlecubic surface equation (P < 0·001).     

Responses of nitrate assimilation and N translocation in tomato (Lycopersicon esculentum Mill) to reduced ambient air humidity

The impact of low humidity in ambient air on water relations,nitrate uptake, and translocation of recently absorbed nitrogen,was investigated in 5-week-old tomato (Lycopersicon esculentumMill cv. Ailsa Craig) plants grown hydroponically in a completenutrient solution. Plants were subjected to dry air (relativehumidity 2–4% for 6 h. The transpiration rate increasedseveral-fold and the shoot water content decreased by almost20%, whereas root water content was unaffected. No effect onin vitro nitrate reductase (NR) activity was detected when usingan EDTA-contraining assay buffer. Replacement of EDTA with Mg²⁺revealed a significant decline in shoot NR activity, which suggestsphosphorylation of the enzyme during the stress treatment. Plantswere grown in a split-root system, in which one root half wasfed ¹⁵N-nitrate during the treatment, in order to determinenitrate uptake and translocation of recently absorbed nitrogenin the plants. Uptake of nitrate was substantially inhibited,but the proportion of absorbed ¹⁵N that was translocated tothe shoots was only slightly affected. In untreated plants,71% of the ¹⁵N recovered in roots had been retranslocated fromthe shoots, whereas in plants subjected to stress the deliveryof ¹⁵N from shoots to roots appeared to be completely inhibited.The data show that lowered humidity in air has significant effectson both uptake of nitrate as well as translocation of nitrogenwithin the plants. Some of these effects appear to be commonwith those observed in plants subjected to reduced water potentialsin the root environment and point to the possibility of theshoot water relations being highly influential on nitrogen uptakeand translocation. Key words: Air humidity, nitrate assimilation, nitrate reductase activity, nitrogen translocation, tomato, water stress     

Translocation of copper and other micronutrients in tomato plants (Lycopersicon esculentum Mill.): nicotianamine-stimulated copper transport in the xylem

The influence of the endogenous micronutrient chelator, nicotianamine(NA), and of Cu nutrition on the distribution of Cu, Fe, Mn,Zn, and NA was investigated in eight different shoot organs,roots, and in xylem exudates of the NA-containing tomato wildtype Lycopersicon esculentum Mill. cv. Bonner Beste and itsNA-less mutant chloronerva. Contrary to the other heavy metals, copper transport in thexylem was inefficient in the mutant and was enhanced by an applicationof NA to the roots or leaves in proportion to the applied NAconcentration. Also, with NA application, the Cu concentrationin mutant roots decreased significantly, and increased in theshoot. Fe and Mn transport in the xylem was greater in the mutantthan in the wild type, and was decreased in the mutant by theapplication of NA to the leaves. Zn transport in the xylem wasthe same in both genotypes and was unaffected by NA application.After application of NA to leaves and roots of the mutant itwas possible to detect NA in the xylem exudate (up to 2nmolNA(g^–1 root FWh^–1). High Cu supply (3 µM) resulted in higher Cu and Mn concentrationsin all organs of the wild type as compared to mutant organs,but Fe concentrations were not influenced. Under high Cu supply(3µM) the NA concentrations of roots and the three youngestleaves of the wild type were higher than under normal Cu supply(0.3 µM). The highest concentrations were found in theshoot apex under both Cu conditions (up to 361 nmol NAg^–1FW). It is concluded from our experiments and from the high stabilityconstant of the NA-Cu-complex (log K= 18.6) that NA is involvedin Cu translocation whereas for the translocation of Fe, Mn,and Zn, NA is not essential. Key words: Copper transport, micronutrients, mobilization, nicotianamine, xylem     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions I. Carbon Assimilation and Utilization

Simulated mixed swards of Perennial Ryegrass (Lolium perenneL.) cv. S23 and White clover (Trifolium repens L.) cv. S100were grown from seed under a constant 20 °C day/15 °Cnight temperature regime and their growth and carbon economyexamined. The swards received a nutrient solution daily, whichcontained either High (220 mg l^–1) or Low (10 mg l^–1)nitrate N. Rates of canopy photosynthesis and respiration, and final drymatter yields were similar in the two treatments although theproportions of grass and clover differed greatly. The Low-Nswards were made up largely of clover. The grass plants in theseswards had high root: shoot ratios and low relative photosyntheticrates – both signs of N deficiency – and were clearlyunable to compete with the vigorously growing Low-N clover plants.These had higher relative growth rates and dry matter yieldsthan their High-N counterparts. In the High-N swards clovercontributed around 50 per cent to the sward dry weight throughoutthe measurement period despite having a smaller proportion ofits dry weight in photosynthetic tissue (laminae) than grassover much of it. The latter was compensated for, initially bya higher specific leaf area than grass, and later by a higherphotosynthetic rate per unit leaf weight. The results are discussedin relation to observed declines in the clover content of swardsafter the addition of nitrogen fertilizer in the field. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, nitrogen, photosynthesis, carbon balance     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 2. A Comparison of Monocultures and Mixed Swards

White clover (Trifolium repens L.) and Perennial ryegrass (Loliumperenne L.) plants were grown, in Perlite, in simulated swardsas either monocultures or mixtures of equal plant numbers. Theywere supplied with a nutrient solution either high (220 µgg^–1) or low (40 µg g^–1) in ¹⁵N-labelled nitrateand grown to ceiling yield at either high (20°C day/15°Cnight) or low (10°C day/8°C night) temperature. Temperature had little effect on the maximum rates of grosscanopy photosynthesis which were similar in High-N grass andHigh-N and Low-N clover monocultures. However these maxima werereached more slowly in clover than grass, and more slowly atlow rather than high temperature. Nitrogen supply increasedphotosynthesis in grass but not in clover. Clover had higherN contents than grass in all four treatments, although in anygiven treatment its N content was lower, and contribution ofN₂-fixation relative to nitrate uptake higher, in mixture thanin monoculture. Conversely, grass had higher N contents in mixturethan monoculture, because more nitrate was available per plantand not because of transfer of biologically fixed N from clover. Under Low-N, clover outyielded grass in mixture, particularlyat high temperature. The grass plants in the Low-N mixtureshad higher N contents and higher SLA, LAR and shoot: root ratiosthan those in monoculture. It is proposed that competition forlight is the cause of the low relative yield and negative aggressivityof grass in these swards. Under High-N, grass outyielded cloverin monoculture and mixture, at both temperatures but particularlyat low temperature when grass had a high aggressivity. Nitrogenand yield component analyses shed no light on clover's apparentlylow competitive ability and evidence is drawn from the previouspaper to demonstrate that grass grew faster than clover onlyas spaced individuals during non-com petitive growth. The relativemerits of measures of competitive ability based on final harvestdata and physiological data taken over a growth period are discussed. Trifolium repens L., white clover, Lolium perenne, perennial ryegrass, competition, temperature, nitrogen     

Copper Nutrition of Sugarbeet

Sugarbeet (Beta vulgaris L.) plants were grown in refined sandat graded levels of copper ranging from acute deficiency (0.000325µg Cu cm^–3) to excess (65 µg Cu cm^–3).Visible effects of copper deficiency appeared up to 0-00065µg Cu cm^–3and depression in growth up to 00065µCucm^–3. Copper deficiency decreased the concentrations ofDNA and RNA and the activities of polyphenol oxidase, cytochrome-coxidase, catalase and aldolase; and it increased the activitiesof peroxidase, ribonuclease and acid phosphatase in leaves.The maximum sucrose concentration in roots was obtained at 0-65µCucm^–3 Twenty four h after infiltration of a solution of 65µCucm^–3into copper deficient leaves, the activities of cytochrome-coxidase and peroxidase had increased even in the presence ofcycloheximide but that of polyphenol oxidase increased onlyin the absence of this inhibitor. Key words: Beta vulgaris, Cu deficiency: Enzymes     

Phosphate Uptake and Transport by Roots and Stolons of Intact White Clover (Trifolium repens L.)

The extent to which phosphate can be absorbed directly fromthe outer medium by stolon internodes and contribute to thetotal accumulation of phosphate by intact plants of white clover(Trifolium repens L. cv. Blanca) was assessed in hydroponicexperiments in a controlled environment room. The uptake ofphosphate by intact roots or stolons was measured by sealinga segment (6-0 mm long) across a flow-cell in which ³²P-labellednutrient solution was circulated for 24 h, the rest of the rootsystem receiving unlabelled nutrient solution. The rate of uptakeof phosphate (µmol g^–1 d^–1 dry wt. basis)by roots was more than 300 times that by intact stolons. Pretreatmentof stolons by gentle abrasion to remove cuticle, so as to simulatethe condition of stolons in the field, increased the uptakeof phosphate 7-fold compared with that of intact stolons. However,the potential of stolons to contribute to the P status of whitedover in the field was calculated to be small (5%). When an incision was made through the hypodermal layer of stolons,the rate of phosphate uptake greatly increased, attaining 71%of that by root segments. This increase, which was greater athigher phosphate concentrations, indicates that the suberi.zedhypodermis constitutes a major barrier to the influx of phosphatein the stolon. After withholding phosphate for different time intervals, thesubsequent rate of phosphate uptake by roots was increased 2-3-foldafter 2 d phosphate deprivation and 3-4-fold after 6 d or 13d phosphate deprivation. A higher proportion of absorbed phosphatewas transported to shoots in phosphate-deprived plants. After1 d of uptake following restoration of the phosphate supply,the concentrations of labelled phosphate in shoots were greaterthan in control plants, although the concentrations of labelin roots was less. However, the rate of uptake of phosphateby stolons, following deprivation, was not significantly increased.These results suggest that the mechanism regulating the enhancedrate of phosphate loading into the xylem, initiated by a periodof phosphate deprivation, is specific to roots and is not inducedin stolons. The results are discussed in relation to the growth and acquisitionof phosphate by white clover in the field. Key words: Nutrient deficiency, phosphate, stolons, transport (ions), Trifolium repens     

Uptake of zinc from chelate-buffered nutrient solutions by wheat genotypes differing in zinc efficiency

Zinc-efficient Triticum aestivum (cv. Warigal) and Zn-inefficientTriticum turgidum conv. durum (cv. Durati) were grown in chelate-buffered,complete nutrient solutions providing either deficient or sufficientZn supply. When transferred to fresh chelatebuffered nutrientsolutions containing a wide range of Zn supplies (0–1.28µmol m^–3 Zn²⁺ activity) for 24–48 h, bothgenotypes increased net Zn uptake linearly with an increasein solution Zn²⁺ activities. Zincefficient Warigal accumulatedZn at a greater rate than Zn-inefficient Durati. The greaterrate of net Zn uptake was observed by plants of both genotypeswhen pretreated at deficient Zn supply. Net loss of Zn to thesolution was higher in plants pretreated with sufficient Znand was inversely related to Zn²⁺ activity in the external solution.When continuously supplied with 40 nmol m^–3 Zn²⁺, netZn uptake by Zn-efficient Warigal was significantly greaterthan that of Zn-inefficient Durati, but the difference diminishedwith plant age. Shoot concentrations of Fe, Mn and Cu were higherwhen plants were grown at deficient than at sufficient Zn supply.The Zn-efficient genotype transported less Zn and Fe to shootsand had higher Fe concentrations in roots than the Zn-inefficientgenotype, supporting the hypothesis that Zn efficiency may beconnected with inefficient transport of Fe from roots to shootsand thus initiation of the Fe-deficiency response resultingin increased release of Zn- and Fe-binding phytosiderophores.It is concluded that differential Zn efficiency of wheat genotypesis at least partly due to a greater ability of efficient genotypesto accumulate Zn. Key words: Chelate-buffering, genotypes, micronutrients, Triticum spp., uptake, zinc efficiency     

The Uptake of Nitrate by Lolium perenne from Flowing Nutrient Solution: I. EFFECT OF CONCENTRATION

Experiments with simulated swards of perennial ryegrass (Loliumperenne L.) show the relationship between concentration in flowing nutrient solution, nitrate uptake,plant growth, and the chemical composition of roots and shoots.Rates of uptake exceeding 1 g N m^–2 d^–1 were maintainedat concentrations in solution down to 0•02 mg N l^–1. Short-term studies confirmed that at such lowconcentrations the plants were able to maintain rates of uptakeof about 85% of maximum. Between 0•2 and 200 mg N l^–1the concentration of in solution had little effect on rate of uptake or plant growth. With at 1000 and 2000 mg N l^–1 there was a marked reductionin weight of the shoots and, more particularly, in the lengthand tensile strength of the roots. There were several significanttrends in mineral composition of the plants (notably in S, Ca,Mg) which were apparently correlated with increasing concentrationof in solution.     

N2 Fixation and Nitrate Uptake by White Clover Swards in Response to Root Temperature in Flowing Solution Culture

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown as simulated swards for 71 d in flowing nutrientsolutions with roots at 11 C and shoots at 20/15 C, day/night,under natural illumination. Root temperatures were then changedto 3, 5, 7, 11, 13, 17 or 25 C and the total N₂, fixation over21 d was measured in the absence of a supply mineral N. Alltreatments were subsequently supplied with 10 mmol m^–2NO₂ ^– in the flowing solutions for 14 d, and the relativeuptake of N by N₂, fixation and NO₃ ^– uptake was compared.Net uptake of K⁺ was measured on a daily basis. Root temperature had little effect on root d. wt over the 35-dexperimental period, but shoot d. wt increased by a factor of3.5 between 3 and 25 C, with the sharpest increase occurringat 7–11 C. Shoot: root d. wt ratios increased from 25to 68 with increasing temperature at 7–25 C. N₂-fixationper plant (in the absence of NO₂ ^–) increased with roottemperature at 3–13C, but showed little change above13 C. The ratios of N₂ fixation: NO₂ ^– uptake over 14d (mol N: mol N) were 0.47–0.77 at 3–7 C, 092–154at 11–17 C, and 046 at 25 C, reflecting the dominanceof NO₃ ^– uptake over N₂ fixation at extremes of high andlow root temperature. The total uptake of N varied only slightlyat 11–25 –C (095–110 mmol N plant^–1),the decline in N₂ fixation as root temperature increased above11 C was compensated for by the increase in NO^– ₃ uptake.The % N in shoot dry matter declined with decreasing root temperature,from 32% at 13 C to 15% at 3 C. In contrast, concentrationsof N expressed on a shoot water content basis showed a modestdecrease with increasing temperature, from 345 mol m^–3at 3 C to 290 mol m^–3 at 25 C. Trifolium repens L, white clover, root temperature, N₂ fixation, potassium uptake, nitrate uptake, flowing solution culture     

Hypoxia Induces Membrane Depolarization and Potassium Loss from Wheat Roots but does not Increase their Permeability to Sorbitol

This paper deals with the responses of roots of wheat {Triticumaestivum L.) to hypoxia with special emphasis on the effectsof severe O2 deficiency on membrane integrity, loss of K+ fromthe root and root membrane potentials. Seminal and crown roots of 26-d-old plants exposed to severehypoxia (0.003 mol O₂ m^–3) for 3 h or 10 d prior to excisionand subsequently exposed to hypoxic solutions, had slightlylower rates of sorbitol influx and a slightly smaller apparentfree space than roots in aerated solutions. These results indicatethat neither a few hours nor a 10-d exposure to hypoxia hadadverse effects on the membrane integrity of the bulk of thecells in the roots. However, both 6-d-old seedlings and 26-d-oldplants lost K⁺ from the roots following their transfer fromaerated to hypoxic nutrient solutions. In the 26-d-old plants,which were of high nutritional status, there was a net K⁺ effluxfrom the roots to the external solution. In contrast, with the6-d-old seedlings, which were of low nutritional status, thedecrease in the K⁺ content of the roots was smaller than thenet K⁺ uptake to the shoots. Exposure of excised roots to 0.008 mol O₂ m^–3caused arapid and reversible membrane depolarization from –120to ––80 mV. These data and the magnitude of thenet effluxes strongly suggest that K⁺ losses during the earlystages of hypoxia are due to membrane depolarization ratherthan to increases in the permeability of membranes to K ⁺. Key words: Hypoxia, membrane integrity, membrane potentials, seminal and crown roots