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     

Zinc, Lead and Cadmium Tolerance, Uptake and Accumulation by the Common Reed,Phragmites australis(Cav.) Trin. ex Steudel

Zinc (Zn), lead (Pb) and cadmium (Cd) tolerance in populationsof seedlings ofPhragmites australisraised from seeds collectedfrom a mine site (Plombières, Belgium) contaminated withZn, Pb and Cd and three ‘clean’ sites (Felixstowe,UK; Wisbech, UK; and Mai Po, Hong Kong) were studied under glasshouseconditions. Small differences were found between the metal-contaminatedpopulation and the three ‘clean’ populations whenseedlings were grown in 1.0 µg  ml^-1Zn and 10.0 µgml^-1Pb treatment solutions. In general, however, different populationsof seedlings showed similar growth responses, metal uptake andindices of Zn, Pb and Cd tolerance when cultured in the samemetal-contaminated media for 89 d or in the same metal treatmentsolutions (ZnSO₄:1.0 and 4.0 µg ml^-1Zn; Pb(NO₃)₂: 10.0and 25.0 µg ml^-1Pb; CdSO₄: 0.5 and 1.0 µg ml^-1Cd)for 3 weeks. There was insufficient evidence to support thehypothesis that the metal-contaminated population has evolvedto a Zn-, Pb- or Cd-tolerant ecotype but the results indicatedsome differentiation between the populations with that fromHong Kong being the least productive under the experimentalconditions used. The implications of the findings on selectionof provenances for use in constructed wetlands for wastewatertreatment are discussed. Metal accumulation; heavy metal tolerance; Phragmites australis; population differentiation     

Nitrate Stimulation of Mobilization of Seed Reserves in Temperate Cereals: Importance of Water Uptake

Relationships between nitrate (NO^-₃) supply, uptake and assimilation,water uptake and the rate of mobilization of seed reserves wereexamined for the five main temperate cereals prior to emergencefrom the substrate. For all species, 21 d after sowing (DAS),residual seed dry weight (d.wt) decreased while shoot plus rootd.wt increased (15–30%) with increased applied NO^-₃concentrationfrom 0 to 5–20 mM . Nitrogen (N) uptake and assimilationwere as great with addition of 5 mM ammonium (NH⁺₄) or 5 mMNO^-₃but NH⁺₄did not affect the rate of mobilization of seedreserves. Chloride (Cl^-) was similar to NO^-₃in its effect onmobilization of seed reserves of barley (Hordeum vulgare L.).Increased rate of mobilization of seed reserves with additionalNO^-₃or Cl^-was associated with increases in shoot, root and residualseed anion content, total seedling water and residual seed watercontent (% water) 21 DAS. Addition of NH⁺₄did not affect totalseedling water or residual seed water content. For barley suppliedwith different concentrations of NO^-₃or mannitol, the rate ofmobilization of seed reserves was positively correlated (r >0.95)with total seedling water and residual seed water content. Therate of mobilization of seed reserves of barley was greaterfor high N content seed than for low N content seed. Seed watercontent was greater for high N seed than for low N seed, 2 DAS.Additional NO^-₃did not affect total seedling water or residualseed water content until 10–14 DAS. The effects of seedN and NO^-₃on mobilization of seed reserves were detected 10and 14 DAS, respectively. It is proposed that the increasedrate of mobilization of seed reserves of temperate cereals withadditional NO^-₃is due to increased water uptake by the seedlingwhile the seed N effect is due to increased water uptake bythe seed directly. Avena sativa L.; oat; Hordeum vulgare L.; barley; Secale cereale L.; rye; xTriticosecale Wittm.; triticale; Triticum aestivum L.; wheat; nitrate; seed; germination; seed reserve mobilization     

Zinc (Zn)-phosphorus (P) Interactions in Two Cultivars of Spring Wheat (Triticum aestivum L.) Differing in P Uptake Efficiency

Zinc-phosphorus (Zn-P) interactions were investigated in twowheat cultivars (Brookton and Krichauff) differing in P uptakeefficiency. The experiment was carried out in a growth chamber.Rock phosphate or CaHPO₄were used as P sources, and ammoniumnitrate or nitrate only as nitrogen sources. Two Zn levels wereused: 0.22 and 2.2 mg ZnSO₄.5H₂O kg^-1. The results confirmedthat Brookton had a higher P uptake efficiency than Krichauffunder low P conditions, irrespective of nitrogen and Zn supply.Zn supply had little effect on tissue P concentration and Puptake per unit of root weight in either cultivar, irrespectiveof nitrogen supply. An increase in P availability caused a significantreduction in Zn uptake per unit of root weight, and tissue concentrationof Zn in both cultivars. The reduction in tissue Zn concentrationcannot be explained entirely by a dilution effect. Zn uptakeby, and Zn concentrations in, Brookton (with high P uptake efficiency)were significantly lower than those of Krichauff. Zn concentrationsin Brookton were more sensitive to P uptake than those in Krichauff.It is suggested that high P uptake efficiency may depress plantuptake of Zn, and therefore cause a reduction in the concentration(density) of Zn in grains of wheats grown in low P (and possiblylow Zn) soils. Copyright 2001 Annals of Botany Company Phosphorus efficiency, translocation, uptake, zinc-phosphorus interaction, wheat     

Zinc Mobility in Wheat: Uptake and Distribution of Zinc Applied to Leaves or Roots

Little is known about transport of Zn from leaves to other plantorgans. The present study tested a range of Zn forms appliedfoliarly for their suitability to provide adequate Zn nutritionto wheat (Triticum aestivum L.). Transport of⁶⁵Zn applied eitherto leaves or to one side of the root system was also studied.Inorganic (ZnO, ZnSO₄) and chelated sources of Zn (ZnEDTA, glycine-chelatedBiomin Zn) applied foliarly provided sufficient Zn for vigorousgrowth. Zinc concentrations in roots and shoots were in thesufficiency range, except in the -Zn control. Foliar treatmentswith ZnSO₄and chelated Zn forms resulted in shoot Zn concentrationsin 7-week-old plants being about two-fold greater than thosein plants supplied with Zn in the root environment or via foliarspray of ZnO. Adding surfactant to foliar sprays containingchelated forms of Zn did not cause negative growth effects,but surfactant added to ZnO or ZnSO₄foliar sprays decreasedshoot growth. Adding urea to the ZnO foliar spray had no effecton shoot growth. Foliarly-applied⁶⁵Zn was translocated to leavesabove and below the treated leaf as well as to the root tips.Stem girdling confirmed that⁶⁵Zn transport toward lower leavesand roots was via the phloem. Split-root experiments showedintensive accumulation of⁶⁵Zn in the stem and transport to allleaves as well as to the root tips in the non-labelled sideof the root system. Foliar application of Zn in inorganic ororganic form is equally suitable for providing adequate Zn nutritionto wheat. Phloem transport of Zn from leaves to roots was demonstrated.Copyright 2001 Annals of Botany Company Foliar spraying, phloem, surfactant, urea, xylem, wheat, zinc     

The Influence of NO-3 and NH+4 Nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

Respiratory oxygen consumption by roots was 1·4- and1·6-fold larger in NH⁺₄-fed than in NO^-₃-fed wheat (Triticumaestivum L.) and maize (Zea mays L.) plants respectively. Higherroot oxygen consumption in NH⁺₄-fed plants than in NO^-₃-fedplants was associated with higher total nitrogen contents inNH⁺4-fed plants. Root oxygen consumption was, however, not correlatedwith growth rates or shoot:root ratios. Carbon dioxide releasewas 1·4- and 1·2-fold larger in NO⁺3-fed thanin NH⁺₄-fed wheat and maize plants respectively. Differencesin oxygen and carbon dioxide gas exchange rates resulted inthe gas exchange quotients of NH^-₄-fed plants (wheat, 0·5;maize, 0·6) being greatly reduced compared with thoseof NO^-₃-fed plants (wheat, 1·0; maize, 1·1). Measuredrates of HCO^-₃ assimilation by PEPc in roots were considerablylarger in 4 mM NH⁺₄-fed than in 4 NO^-₃ plants (wheat, 2·6-fold;maize, 8·3-fold). These differences were, however, insufficientto account for the observed differences in root carbon dioxideflux and it is probable that HCO^-₃ uptake is also importantin determining carbon dioxide fluxes. Thus reduced root extension in NH⁺₄-fed compared with NO^-₃-fedwheat plants could not be ascribed to differences in carbondioxide losses from roots.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize assimilation, ammonium assimilation, root respiration     

Ammonium release by nitrogen sufficient diatoms in response to rapid increases in irradiance

It has been hypothesized that nitrogen-replete diatoms, butnot flagellates, may release NO₂^–, NH₄⁺ or dissolved organicnitrogen (DON) following rapid increases in irradiance (andconsequently an increase in cellular electron energy), as mightbe expected to occur in a vertically well mixed estuarine system.Just as the increase in irradiance leads to an increase in cellularenergy, so too would a decrease in temperature, due to the temperaturedependency of biosynthetic enzymes. This hypothesis was testedby comparing the response of nitrogen-replete diatoms (Skeletomenacostatum, Thalassiosira weissflogii and Chaetoceros sp.) andflagellates (Dunaliella tertiolecta, Pavlova lutheri and Prorocentrumminimum) to rapid increases in irradiance and decreases in temperature.Short-term (<3 h) changes in extracellular NO₂^– andNH₄⁺ concentrations were measured in cultures following theseexperimental shifts, as well as in cultures retained at thegrowth irradiance. Net rates of NO₂^– and NH₄⁺ releasewere calculated from the time course of extracellular nitrogenconcentrations. As a fraction of NO₃^– uptake, NO₂^–release rates under the increased irradiance increased marginallyrelative to NO₂^– release rates under the growth irradiance.Release rates of NH₄⁺ under the increased irradiance increasednearly fivefold over release rates at the growth irradiance,and accounted for 84% of the NO₃^– uptake rate. In directcontrast to the diatom species, the flagellate species releasedNO₂^– under the higher experimental irradiance at ratesone half those of the release rates under the growth irradiance,and continued to take up NH₄⁺ under both irradiance conditions.Within the experimental boundaries, these findings have importantphysiological and ecological implications. The magnitude ofthe observed nitrogen release represents a significant physiologicalsink for electrons and, in fact, calculations suggest that upto 62% of the total electrons harvested could be consumed. Froman ecological perspective, these findings add to the body ofliterature which suggests that a significant fraction of thenitrogen that is taken up is ultimately released in dissolvedform. More importantly, these data suggest that DON is not theonly compound that phytoplankton may release in the aquaticenvironment.     

A carrier-mediated mechanism for pyridoxine uptake by human intestinal epithelial Caco-2 cells: regulation by a PKA-mediated pathway

Vitamin B₆ is essential for cellular functions and growth due to its involvement in important metabolic reactions. Humans and other mammals cannot synthesize vitamin B₆ and thus must obtain this micronutrient from exogenous sources via intestinal absorption. The intestine, therefore, plays a central role in maintaining and regulating normal vitamin B₆ homeostasis. Due to the water-soluble nature of vitamin B₆ and the demonstration that transport of other water-soluble vitamins in intestinal epithelial cells involves specialized carrier-mediated mechanisms, we hypothesized that transport of vitamin B₆ in these cells is also carrier mediated in nature. To test this hypothesis, we examined pyridoxine transport in a model system for human enterocytes, the human-derived intestinal epithelial Caco-2 cells. The results showed pyridoxine uptake to be 1) linear with time for up to 10 min of incubation and to occur with minimal metabolic alteration in the transported substrate, 2) temperature and energy dependent but Na⁺ independent, 3) pH dependent with higher uptake at acidic compared with alkaline pHs, 4) saturable as a function of concentration (at buffer pH 5.5 but not 7.4) with an apparent Michaelis-Menten constant (K_m) of 11.99 ± 1.41 µM and a maximal velocity (V_max) of 67.63 ± 3.87 pmol · mg protein^-1 · 3 min^-1, 5) inhibited by pyridoxine structural analogs (at buffer pH 5.5 but not 7.4) but not by unrelated compounds, and 6) inhibited in a competitive manner by amiloride with an apparent inhibitor constant (K_i) of 0.39 mM. We also examined the possible regulation of pyridoxine uptake by specific intracellular regulatory pathways. The results showed that whereas modulators of PKC, Ca⁺²/calmodulin (CaM), and nitric oxide (NO)-mediated pathways had no effect on pyridoxine uptake, modulators of PKA-mediated pathway were found to cause significant reduction in pyridoxine uptake. This reduction was mediated via a significant inhibition in the V_max, but not the apparent K_m, of the pyridoxine uptake process. These results demonstrate, for the first time, the involvement of a specialized carrier-mediated mechanism for pyridoxine uptake by intestinal epithelial cells. This system is pH dependent and amiloride sensitive and appears to be under the regulation of an intracellular PKA-mediated pathway. vitamin B₆; intestinal transport; transport regulation; Caco-2 cell     

Induction of Light Dependent Pyruvate Transport into Chloroplasts of Mesembryanthemum crystallinum by Salt Stress

Mode of photosynthesis in Mesembryanthemum crystallinum changesfrom C₃ to Crassulacean acid metabolism (CAM) when the plantswere stressed with high salinity. [¹⁴C]Pyruvate uptake for 30s into intact chloroplasts isolated from leaves of the CAM modeof M. crystallinum was enhanced more than 5-fold in the lightcompared with that in the dark. The stromal concentration ofpyruvate in the light reached to more than 2.5 times of themedium. In contrast, little or no pyruvate uptake occurred inchloroplasts from C₃ leaves in either light or dark condition.The initial uptake rate (10 s incubation at 4°C) into theCAM chloroplasts in the light was about 3-fold higher than therate in the dark. K_m and V_max of the initial uptake in the lightwere 0.54 mM and 8.5 µmol (mg Chl)^–1 h^–1 respectively.These suggest that pyruvate was actively incorporated into theCAM chloroplasts against its concentration gradient across theenvelope in the light. When hydroponically grown M. crystallinumwere stressed by 350 mM NaCl, the capacity of chloroplasts forpyruvate uptake was induced in 6 d corresponding to the inductionof the activities of PEP-carboxylase and NAD(P)⁺-malic enzymesin response to salt stress. (Received October 12, 1995; Accepted January 19, 1996)     

Salinity Reduces Water Use and Nitrate-N-use Efficiency of Citrus

Five-month-old Cleopatra mandarin (Citrus reticulata Blanco)(CM) and Volkamer lemon (Citrus volkameriana Ten. and Pasq.)(VL) seedlings were grown in a glasshouse in 2·3-1 potsof Candler fine sand. Plants were irrigated with either non-saline(EC_e = 0·23 dS m^-1) or saline (6·13 dS m^-1) waterusing 3:1 NaCl:CaCl₂ solution over a 4-week period. A singleapplication of K¹⁵NO₃ (19·64 atom % excess ¹⁵N) at 212mg N1^-1, was substituted for a normal weekly fertilization after3 weeks and plants were harvested 7 d later. The transpirationrate, uptake of nitrogen, growth and nitrogen-use efficiency(NUE) on a dry weight basis (mg d. wt mg^-1 N) of both specieswas reduced by salinity. Based on growth, water-use and chloride(Cl) accumulation in leaves, VL was more salt-sensitive thanCM, but ¹⁵N uptake was equally reduced by salinity in both species.Salinity reduced ¹⁵N uptake relatively more than shoot growthover the 7-d period, such that the ¹⁵NUE (mg d. wt µg^-115N) of new shoot growth of both species increased. There wasno evidence of Cl antagonism of nitrate (NO₃) uptake but totalplant ¹⁵NO₃ uptake was positively correlated with whole planttranspiration in both species. Thus, it appears that reductionsin NO₃ uptake are more strongly related to reduced water usethan to Cl antagonism from salt stress.Copyright 1993, 1999Academic Press Sodium, chloride, salinity, calcium, nitrate, ¹⁵NO₃ uptake, nitrogen allocation, nitrogen-use efficiency, water use, Citrus reticulata, Citrus volkameriana     

A comparison of the impacts of various nitrogen sources on acid-base balance in C3 Triticum aestivum L. and C4 Zea mays L. plants

This work aimed to study the impacts of acquisition and assimilationof various nitrogen sources, i.e. NO₃^–, NH₄⁺ or NH₄NO₃,in combination with gaseous NH₃ on plant growth and acid-basebalance in higher plants. Plants of C₃ Triticum aestivum L.and C₄ Zea mays L. grown with shoots in ambient air in hydroponicculture solutions with 2 mol m^–3 of nitrogen source asNO₃^–, NH₄⁺ or NH₄NO₃ for 21 d and 18 d, respectively,had their shoots exposed either to 320 µg m^–3 NH₃or to ambient air for 7 d. Variations in plant growth (leaves,stubble and roots), and OH^– and H⁺ extrusions as wellas the relative increases in nitrogen, carbon and carboxylatewere determined. These data were computed as H⁺/N, H⁺/C, (C-A)/N,and (C-A)/C to analyse influences of different nitrogen sourceson acid-base balance in C₃ Triticum aestivum and C₄ Zea maysplants. Root growth in dry weight gain was significantly reduced bytreatment with 320 µg m^–3 NH₃ in Triticum aestivumand Zea mays growing with different N-forms, whereas leaf growthwas not significantly affected by NH₃. In comparison with C₃Triticum aestivum, non-fumigated C₄ Zea mays had low ratiosof OH^–/N in NO₃^–3-grown plants and of H⁺/N in NH₄⁺- and NH ₄NO₃-grown plants. Utilization of NH₃ from the atmospherereduced both the OH^–N ratios in NO₃^– -grown plantsand the H⁺/N ratio in NH₄⁺ - and NH₄NO₃ -grown plants of bothspecies. Furthermore, Zea mays had higher ratios of (C-A)/Nin NH₄⁺ - and NH₄NO₃-grown plants than Triticum aestivum. Thismeans that C₄ Zea mays had synthesized more organic anion perunit increase in organic N than C₃ Triticum aestivum plants.Within both species, different nitrogen sources altered theratios of (C-A)/N in the order: NH₄NO₃>NH₄⁺>NO₃^–.Fumigation with NH₃ increased organic acid synthesis in NO₃^–- and NH₄⁺ - grown plants of Triticum aestivum, whereas it decreasedorganic acid synthesis in Zea mays plants under the same conditions.Furthermore, these differences in acid-base regulation betweenC₃ Triticum aestivum and C₄ Zea mays plants growing with differentnitrogen sources are discussed. Key words: Acid-base balance, ammonia, ammonium, nitrate, ammonium nitrate, C₃ Triticum aestivum L., C₄ Zea mays L.     

Effect of Sudden Salt Stress on Ion Fluxes in Intact Wheat Suspension Cells

Although salinity is one of the major problems limiting agriculturalproduction around the world, the underlying mechanisms of highNaCl perception and tolerance are still poorly understood. Theeffects of different bathing solutions and fusicoccin (FC),a known activator of plasma membrane ATPase, on plasma membranepotential (E_m) and net fluxes of Na⁺, K⁺and H⁺were studied inwheat suspension cells (Triticum aestivum) in response to differentNaCl treatments. E_mof cells in Murashige and Skoog (MS) mediumwas less negative than in cells exposed to a medium containing10 mM KCl + 0.1 m M CaCl₂(KSM) and to a basic salt medium (BSM),containing 1 m M KCl and 0.1 m M CaCl₂. Multiphasic Na⁺accumulationin cells was observed, peaking at 13 min after addition of 120m M NaCl to MS medium. This time scale was in good agreementwith net Na⁺flux changes measured non-invasively by moving ion-selectivemicroelectrodes (the MIFE system). When 120 m M NaCl was addedto all media studied, a quick rise of Na⁺influx was reversedwithin the first 20 min. In both 120 and 20 m M NaCl treatmentsin MS medium, net Na⁺efflux was observed, indicating that activeNa⁺transporters function in the plant cell response to saltstress. Lower external K⁺concentrations (KSM and BSM) and FCpre-treatment caused shifts in Na⁺fluxes towards net influxat 120 m M NaCl stress. Copyright 2000 Annals of Botany Company Sodium, potassium, proton, membrane potential, fusicoccin, salt stress, wheat, Triticum aestivum     

Preconditioning of cortical neurons by oxygen-glucose deprivation: tolerance induction through abbreviated neurotoxic signaling

Transient exposure of rat cortical cultures to nonlethal oxygen-glucose deprivation (OGD preconditioning) induces tolerance to otherwise lethal oxygen-glucose deprivation (OGD) or N-methyl-D-aspartate 24 h later. This study evaluates the role of cytosolic and mitochondrial Ca²⁺-dependent cellular signaling. Mechanistic findings are placed in context with other models of ischemic preconditioning or known neurotoxic pathways within cortical neurons. Tolerance to otherwise lethal OGD is suppressed by performing OGD preconditioning in the presence of the broad-scope catalytic antioxidants Mn(III)tetra(4-carboxyphenyl)porphyrin (MnTBAP) or Zn(II)tetra(4-carboxyphenyl)porphyrin [Zn(II)TBAP], but not by a less active analog, Mn(III)tetra(4-sulfonatophenyl)porphyrin, or a potent superoxide scavenger, Mn(III)tetra(N-ethyl-2-pyridyl)porphyrin chloride. Inhibitors of adenosine A₁ receptors, nitric oxide synthase, mitogen-activated protein kinase, and poly(ADP-ribose) polymerase fail to suppress OGD preconditioning despite possible links with reactive oxygen species in other models of ischemic preconditioning. Preconditioning is suppressed by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which has been ascribed elsewhere to inhibition of superoxide transport to the cytosol through mitochondrial anion channels. However, although it induces mitochondrial Ca²⁺ uptake, neuronal preconditioning is largely insensitive to mitochondrial uncoupling with carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone or 2,4-dinitrophenol. Un-couplers will prevent production of mitochondrial reactive oxygen species, implying nonmitochondrial targets by MnTBAP, Zn(II)TBAP, and DIDS. Emphasizing the importance of an increase in cytosolic Ca²⁺ during preconditioning, a Ca²⁺/calmodulin-dependent protein kinase II inhibitor, KN-62, suppresses development of subsequent tolerance. Summarizing, only those cellular transduction pathways that have the potential to be neurotoxic may be activated by preconditioning in cortical neurons. Finally, a marked decrease in extracellular glutamate is observed during otherwise lethal OGD in preconditioned cultures, suggesting that this end effector may represent a point of convergence across different preconditioning models. N-methyl-D-aspartate; Ca²⁺; antioxidants; mitochondria     

Concurrent Rates of N2 Fixation, Nitrate and Ammonium Uptake by White Clover in Response to Growth at Different Root Temperatures

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown for 71 d in flowing nutrient solutions containingN as 10 mmol m_–3 NH₄NO₃, under artificial illumination,with shoots at 20/15°C day/night temperatures and root temperaturereduced decrementally from 20 to 5°C. Root temperatureswere then changed to 3, 7, 9, 11, 13, 17 or 25°C, and theacquisition of N by N₂ fixation, NH₄+ and NO₃^– uptakewas measured over 14 d. Shoot specific growth rates (d. wt)doubled with increasing temperature between 7 and 17°C,whilst root specific growth rates showed little response; shoot:root ratios increased with root temperature, and over time at11°C. Net uptake of total N per plant (N₂ fixation + NH₄++ NO₃^–) over 14 d increased three-fold between 3 and 17°C.The proportion contributed by N₂ fixation decreased with increasingtemperature from 51% at 5°C to 18% at 25°C. Uptake ofNH₄+ as a proportion of NH₄+ + NO₃^– uptake over 14 d variedlittle (55–62%) with root temperature between 3 and 25°C,although it increased with time at most temperatures. Mean ratesof total N uptake per unit shoot f. wt over 14 d changed littlebetween 9 and 25°C, but decreased progressively with temperaturebelow 9°C, due to the decline in the rates of NH₄+ and NO₃^–uptake, even though N₂ fixation increased. The results suggestthat N₂ fixation in the presence of sustained low concentrationsof NH₄+ and NO₄^– is less sensitive to low root temperaturethan are either NH₄+ or NO₃^– uptake systems. White clover, Trifolium repens L. cv. Huia, root temperature, nitrogen fixation, ammonium, nitrate     

Physiology and Growth of Wheat Across a Subambient Carbon Dioxide Gradient

Two cultivars of wheat (Triticum aestivum L.), 'Yaqui 54' and'Seri M82', were grown along a gradient of daytime carbon dioxideconcentrations ([CO₂]) from near 350-200 µmol CO₂ mol^-1air in a 38 m long controlled environment chamber. Carbon dioxidefluxes and evapotranspiration were measured for stands (plantsand soil) in five consecutive 7·6-m lengths of the chamberto determined potential effects of the glacial/interglacialincrease in atmospheric [CO₂] on C₃ plants. Growth rates andleaf areas of individual plants and net assimilation per unitleaf area and daily (24-h) net CO₂ accumulation of wheat standsrose with increasing [CO₂]. Daytime net assimilation (P_D, mmolCO₂ m^-2 soil surface area) and water use efficiency of wheatstands increased and the daily total of photosynthetic photonflux density required by stands for positive CO₂ accumulation(light compensation point) declined at higher [CO₂]. Nighttimerespiration (R_N, mmol CO₂ m^-2 soil surface) of wheat, measuredat 369-397 µmol mol^-1 CO₂, apparently was not alteredby growth at different daytime [CO₂], but R_N /P_D of stands declinedlinearly as daytime [CO₂] and P_D increased. The responses ofwheat to [CO₂], if representative of other C₃ species, suggestthat the 75-100% increase in [CO₂] since glaciation and the30% increase since 1800 reduced the minimum light and waterrequirements for growth and increased the productivity of C₃plants.Copyright 1993, 1999 Academic Press Atmospheric carbon dioxide, carbon accumulation, evapotranspiration, light compensation point, net assimilation, respiration, Triticum aestivum, water use efficiency, wheat     

Modulation of hepatocellular swelling-activated K+ currents by phosphoinositide pathway-dependent protein kinase C

K⁺ channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K⁺ current (I_KVol). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K⁺ flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to I_KVol activation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP₂) increased I_KVol, whereas conditions that decrease cellular PIP₂ decreased I_KVol. GTP and AlF₄^– stimulated I_KVol development, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased I_KVol and inhibited the stimulatory response to PIP₂ or GTP. Protein kinase C (PKC) is involved, because K⁺ current was enhanced by 1-oleoyl-2-acetyl-sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both I_KVol and the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated I_KVol activation involves the cytoskeleton. Under isotonic conditions, a slowly developing K⁺ current similar to I_KVol was activated by PIP₂, lipid phosphatase inhibitors to counter PIP₂ depletion, a PLC-coupled ₁-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate I_KVol through a PIP₂/PKC-dependent pathway. The results indicate that PIP₂ indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation. KCNQ1; patch clamp; phosphatidylinositol 4,5-bisphosphate; regulatory volume decrease     

Nitrogen utilization by plant species from acid heathland soils: II. Growth and shoot/root partitioning of NO3- assimilation at constant low pH and varying NO3-/NH4+ ratio

The growth of four heathland species, two grasses (D. flexuosa,M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix),was tested in solution culture at pH 4.0 with 2 mol m^–3N, varying the N0₃^–/NH₄⁺ ratio up to 40% nitrate. In addition,measurements of NRA, plant chemical composition, and biomassallocation were carried out on a complete N0₃^–/NH₄⁺ replacementseries up to 100% nitrate. With the exception of M. caerulea, the partial replacement ofNH₄⁺ by NO₃^– tended to enhance the plant's growth ratewhen compared to NH₄⁺ only. In contrast to the other species,D. flexuosa showed a very flexible response in biomass allocation:a gradual increase in the root weight ratio (RWR) with NO₃^–increasing from 0 to 100%. In the presence of NH₄⁺, grassesreduced nitrate in the shoot only; roots did not become involvedin the reduction of nitrate until zero ambient NH₄⁺. The dwarfshrubs, being species that assimilate N exclusively in theirroots, displayed an enhanced root NRA in the presence of nitrate;in contrast to the steady increase with increasing NO₃^–in Calluna roots, enzyme activity in Erica roots followed arather irregular pattern. Free nitrate accumulated in the tissuesof grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO₃^– [(proportion of nitratein N uptake)/(proportion of nitrate in N supply)] was lowestin M. caerulea and highest in D. flexuosa. Whereas M. caeruleaand the dwarf shrubs always absorbed ammonium highly preferentially(relative uptake ratio for NO₃^– <0.20), D. flexuosashowed a strong preference for NO₃^– at low external nitrate(the relative uptake ratio for N0₃^– reaching a value of2.0 at 10% NO₃^–). The ecological significance of thisprominent high preference for NO₃^– at low NO₃^–/NH₄⁺ratio by D. flexuosa and its consequences for soil acidificationare briefly discussed. Key words: Ammonium, heathland lants, N0₃^–/NH₄⁺ ratio, nitrate, nitrate reductase activity, soil acidification, specific absorption rate     

Effects of Calmodulin Antagonists on the Mitochondrial and Microsomal Ca2$ Uptake in Apple Fruit

In apple fruit, active ATP-dependent microsomal Ca^2$ uptakeand respiration-dependent mitochondrial Ca^2$ uptake were observed. The mitochondrial Ca^2$ uptake was depressed by the calmodulinantagonists chlorpromazine hydrochloride (CPZ) and N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamidehydrochloride (W-7). The Ca^2$-ATPase from apple mitochondriawas also inhibited by CPZ or W-7. The apparent K_m value forCa^2$ in mitochondrial Ca^2$ uptake (K_m=0.35 mM) was similar tothat of mitochondrial Ca^2$-ATPase (K_m=0.32 mM). The inhibitoryeffect of W-7 on the activity of the mitochondrial Ca^2$ uptakewas closely correlated with the inhibition by W-7 of mitochondrialCa^2$-ATPase (r=0.996). These findings indicate that the mitochondrialuptake of Ca^2$ in apple fruit depends on the calmodulin-mediatedactivation of Ca^2$-ATPase. The microsomal Ca^2$ uptake was depressed by CPZ, suggestingthat the microsomal Ca^2$ uptake may also be modulated by calmodulin. ¹ Contribution No. C-72, Fruit Tree Research Station. (Received June 7, 1982; Accepted October 19, 1982)     

The Specificity of the Carrier-Mediated Uptake of ABA by Root Segments of Phaseolus coccineus L.

Earlier work has established that the saturable component ofuptake of RS-[2¹⁴C]ABA by bean (Phaseolus coccineus L. cv. Prizewinner)root segments can be attributed to the action of a carrier.We now show that the carrier-mediated uptake is unaffected byRS-2-trans-ABA and lunularic acid and the unnatural R-ABA alsoappears to be ineffective. The specificity for S-ABA requiresthe halving of the K_m value for ABA determined previously (2.6mmol m_-3 for RS-; 1.3 mmol m_-3 for S-ABA). The RS-1', 4'-cis-dioland RS-1'-deoxy ABA reduce the uptake of RS-[2-₁₄C]ABA aboutas strongly as does unlabelled ABA, the K¹ for 1'-deoxy ABAwas similar to the K_m for ABA. The K₁ for RS-1', 4'-trans-diolwas 15.7 mmol m_-3. Consideration of the stereochemistry of thesecompounds suggests that the face of the ring of ABA away fromthe 1'-hydroxyl group interacts with the carrier site. Labelled material diffused out of undamaged root surfaces whichhad absorbed RS-[³H]ABA through an apical cut, suggesting thatABA is present in the apoplast. A simplified hypothesis is presented that can account for polartransport of ABA based on a gradient of a carrier in a tissuebut where the carrier is distributed uniformly on the apicaland basal ends of each cell. Key words: Uptake carrier, Abscisic acid, 1', 4'-Diol, Lunularic acid, Phaseolus coccineus, Polar-transport, Deoxyabscisic acid     

Diel variability in nitrogenous nutrient uptake by phytoplankton in the Chesapeake Bay plume

Diel patterns in the uptake of nitrogenous nutrients were observedin the coastal plume of the Chesapeake Bay system, but the specificpatterns varied with season. During the winter months, ratesof NH₄⁺ and urea uptake were significantly higher at night thanduring the day, and rates of NO₃^– uptake were higher duringthe day. During the summer, rates of NH₄⁺ and urea uptake weresignificantly higher at night only during half the studies conducted;during the remaining studies, there was either no significantdifference or rates of uptake of NH₄⁺ were higher during theday. Rates of NO₃^– uptake during the summer months werealso higher during the day than at night. Seasonal differenceswere also apparent in the time of day at which maximum observeduptake rates of each nitrogen nutrient occurred. During thewinter-spring months, maximum observed rates of NO₃^– uptakeoccurred between first light and noon, whereas during the summermonths, maximum observed uptake rates of NO₃^– occurredboth morning and afternoon, and consistently 9–16 h afterthe maximum observed peak in the uptake of reduced nitrogen.We interpret these findings in terms of seasonal shifts in nitrogennutritional status of the assemblages, as well as species-specificdifferences in the effect of a given stimulus (e.g. a nitrogenpulse at the mouth of the Bay) to entrain an uptake response,and we suggest that the extent of this variability must be understoodbefore generalizations about the use of f-ratios as characteristicsof specific populations or water masses can be drawn.