Interaction of nitrate uptake and nitrogen fixation in faba beans

An experiment was carried out to determine the relationship between nitrate uptake and nitrogen fixation of faba beans. Therefore inoculated and uninoculated faba beans were grown in nutrient solution with different nitrate concentrations. Nitrate uptake was measured every two days during the growing period. At the end of the experiment the nitrate uptake kinetics were determined with a short time depletion technique and nitrogen fixation was measured with the acetylene reduction method. A limitation of nitrate uptake due to nitrogen fixation was relatively small. Nitrate concentrations of approximately 1 mol m^–3 and 5 mol m^–3 decreased nitrogen fixation to values of 16% and 1% of the control plants which received no nitrate nitrogen. A reduction of nitrogen fixation was mainly due to a decrease of specific nitrogen fixation per unit nodule weight and to a lesser extent due to a reduction of nodule growth. Only the maximum nitrate influx (I_max) seemed to be influenced by nitrogen fixation. Michaelis-Menten constants (K_m) and minimum NO _inf3 ^– -concentrations (C_min) were not significantly influenced by nitrogen fixation.     

Effect of NH4 +:NO3 - ratios on growth and nitrogen uptake by onions

The modelling of ion uptake by plants requires the measurement of kinetic and growth parameters under specific conditions. The objective of this study was to evaluate the effect of nine NH _inf4 ^sup+ :NO _inf3 ^sup− ratios on onions (Allium cepa L.). Twenty-eight to 84 day-old onion plants were treated with NH _inf4 ^sup+ :NO_f3/sup− ratios ranging from 0 to 100% of each ionic species in one mM solutions in a growth chamber. Maximum N influx (I_max) was assessed using the N depletion method. Except at an early stage, ionic species did not influence significantly I_max, the Michaelis constant (K_m) and the minimum concentration for net uptake (C_min). I_max for ammonium decreased from 101 to 59 pmole cm^-2 s^-1 while I_max for nitrate increased from 26 to 54 pmole cm^-2 s^-1 as the plant matured. On average, K_m and C_min values were 14.29 μM, and 5.06 μM for ammonium, and 11.90 μM and 4.54 μM for nitrate, respectively. In general, the effect of NH₄ ⁺:NO₃ ^- ratios on root weight, shoot weight and total weight depended on plant age. At an early stage, maximum plant growth and N uptake were obtained with ammonium as the sole source of N. At later stages, maximum plant growth and N uptake were obtained as the proportion of nitrate increased in the nutrient solution. The was no apparent nutrient deficiency whatever NH₄ ⁺:NO₃ ^- ratio was applied, although ammonium reduced the uptake of cations and increased the uptake of phosphorus. The research was supported by the Natural Sciences and Engineering Research Council of Canada.     

The effects of different external nitrate concentrations on growth of Phaseolus vulgaris cv. Seafarer at chilling temperatures

The effects of different applied nitrate concentrations (1 to 50 mol m³) on growth of Phaseolus vulgaris cv. Seafarer at temperatures around 15°C was examined. Total plant dry weight and carbon content decreased sharply with increased applied nitrate 1 to 10 mol m^-3 then decreased slightly with further increases in applied N. Total plant reduced -N content increased sharply with increased applied nitrate concentration from 1 to 5 mol m^-3, changed little with increased applied nitrate from 5 to 25 mol m^-3, then increased when applied nitrate was increased from 25 to 50 mol m^-3. Nitrate concentration in all tissues increased sharply with applied nitrate increased from 1 to 10 mol m³ and showed a further increase at 50 mol m³ applied nitrate. Fresh weight to dry weight ratio for all leaves and specific leaf area for all secondary leaves increased sharply with applied nitrate concentration from 1 to 5 mol m^-3 then decreased with applied nitrate 25 to 50 mol m³ Secondary leaf chlorophyll concentration decreased sharply when applied nitrate increased from 1 to 5 mol m^-3 but increased with applied nitrate from 25 to 50 mol m^-3. Initially, the rate of leaf extension was greater at 20 mol m^-3 applied nitrate than 1 mol m^-3 applied nitrate. It is proposed that decreased growth with increased applied nitrate in the range 1 to 10 mol m^-3 is due to increased leaf damage caused by a greater rate of leaf expansion.     

Modelling the uptake of nitrate by a growing plant with an adjustable root nitrate uptake capacity

A new model is presented to predict the plant uptake of nitrate supplied by diffusion and mass flow to its roots. Plant growth, root-shoot ratio and the plant's nitrate uptake capacity are all set dependent on the plant's N nutrition state. By thoroughly integrating processes occurring in both plant and soil, the model enables to control the relative importance of both under a wide range of different nutritional scenarios.Soil parameters D₀ diffusion coefficient in water (m² day^-1) - D_e diffusion coefficient in soil (m² day^-1) - C nitrate concentration in soil (mol m^-3) - f tortuosity (-) - volumetric moisture content (-) - R radial distance from root axis (m) Plant parameters b₁, b₂ parameters of biomass partitioning Equation (10) - IR interroot distance (m) - K_mU Michaelis-Menten constant of the uptake system (mol m^-3) - K_mNRA Michaelis-Menten constant of nitrogen reduction system (mol g^-1) - k₁, k₂, k₃ parameters of growth model Equation (9) - L_v Root length density (m m^-3) - NO_{3 set} ^- Set point of the cytoplasmatic nitrate pool (mol g^-1 dw) - NO_{3 c} ^- cytoplasmatic nitrate concentration (mol g^-1 dw) - NO_{3 v} ^- vacuolar nitrate concentration (mol g^-1 dw) - NRA_max maximum nitrate reductase activity (mol g^-1 dw day^-1) - N_re reduced nitrogen content (mol) - N_remax maximum reduced N concentration in the plant (mol g^-1 dw) - P partitioning coefficient of nitrate between cyplasm and vacuole - R(1) root radius (m) - RGR relative growth rate (day^-1) - U uptake rate (mol day^-1 m^-2) - U_max maximum uptake rate (Eq. 6) (day^-1 m^-2) - Vo water flux at root surface (m day^-1) - W_r root dry weight (g) - W_sh shoot dry weight (g) - X model parameter: number of root compartments - Y model parameter: number of nodes     

The effect of ammonium ions on membrane potential and anion flux in roots of barley and tomato

The effects of ammonium (0–5 mol m^?3) on root hair membrane potential and on the influx of nitrate and phosphate were investigated in roots of intact barley and tomato plants. In both species, addition of ammonium to the medium bathing the roots caused an almost immediate depolarization of the membrane potential; the depolarization was greater at higher concentrations of ammonium. Influx of ¹³NC₃^? and ³²Pi was inhibited over the same time scale and concentration range. In tomato roots, there was little further depolarization of the membrane potential or inhibition of anion influx at ammonium concentrations above 0.4 mol m^?3. In barley roots, the inhibition of nitrate influx and the depolarization of the membrane potential did not saturate below 5 mol m^?3 ammonium.     

Seasonal variations in nitrate content,total nitrogen,and nitrate reductase activities of macrophytes from a chalk stream in Upper Bavaria

Summary 11 macrophytic species from a groundwater influenced chalk stream in Upper Bavaria were investigated during a period of one year in order to determine differences in the endogenous nitrate content, in total nitrogen content and in nitrate reductase activity (NRA). Nitrate concentrations of different plants taken from the same site of the river varied by a factor of approximately 10³. A maximum of 1,958 mol NO ₃ ^- g^-1 dry w. could be measured in the petioles of Nasturtium officinale, which accounts for 12% of plant dry w. Very high values were also found in Callitriche obtusangula and Veronica angallis-aquatica. In comparison to the ambient water, mean accumulation rates of up to 131 could be found. In Fontinalis antipyretica, the plant poorest in nitrate, the ratio was only 1.24:1. Elodea canadensis belonged to a group of plants having very low nitrate concentrations. Since NRA was very low too, it is assumed that nitrogen nutrition of this species depends rather on ammonia than on nitrate. With a few exceptions nitrate content of different plant organs varied markedly. In general they were lowest in leaves and highest in shoot axes. Appreciable amounts of nitrate were also found in the roots of plants. No correlation could be found between endogenous nitrate content and NRA. In contrast to endogenous nitrate content and NRA, total nitrogen concentrations of the plants did not differ significantly.     

Nitrate and reduced-N concentrations in the xylem sap of Stellaria media, Xanthium strumarium and six legume species

Abstract At an applied nitrate concentration of 1 mol m^?3, the proportion of xylem sap nitrogen as nitrate was < 15% for Cajanus cajan, Lupinus albus and Trifolium repens, 33% for Pisum sativum and within the range 57–62% for Glycine max, Phaseolus vulgar is, Stellaria media and Xanthium strumarium. At an applied nitrate concentration of 10 mol m～³ the value had increased to 66% for T. repens while at 20 mol m^?3 nitrate values had increased to 46, 51 and 49% for C. cajan, L. albus and Pisum sativum, respectively, and 89% and 85% for 5. media and X. strumarium, respectively. Glycine max and Phaseolus vulgaris differed from the other species in that the proportion of their xylem sap nitrogen as nitrate remained constant (～ 60%) as applied nitrate concentration increased from 1 to 20 mol m^?3. The proportion of total plant nitrate reductase activity in the shoot of C. cajan, S. media and X. strumarium increased as applied nitrate concentration increased from 1 to 20 mol m^?3. Values at the lower and upper concentrations were, respectively, 26 and 72% for C. cajan. 48 and 80% for X. strumarium and 68 and 87% for S. media. The partitioning of nitrate assimilation between root and shoot in these species is discussed.     

Kinetics of nitrate uptake by different species from nutrient-rich and nutrient-poor habitats as affected by the nutrient supply

The species Urtica dioica L., Plantago major ssp. major L., Plantago lanceolata L., Hypochaeris radicata L. ssp. radicata and Hypochaeris radicata ssp. ericetorum Van Soest were grown under high and low nutrient conditions (1/4 Hoagland and 2% of 1/4 Hoagland further called the 100% and 2% treatment, containing 3.75 mM NO^-₃ and 0.075 mM NO^-₃, respectively). After a certain period half of the plants were transferred from low to high or high to low nutrients, yielding the 100%/2% and the 2%/100% treatments. The kinetics of nitrate uptake in the range of system I of the five species grown under the different nutrient conditions were measured during a three week experimental period. The nitrate uptake of all the species showed the characteristic features of Michaelis-Menten kinetics. Under low nutrient conditions the apparent V_max of U. dioica expressed per g dry root was lower than under high nutrient conditions. For H. radicata ssp. radicata and for H. radicata ssp. ericetorum the reverse was found. The V_max values of P. major ssp. major were almost the same for the two treatments. The apparent V_max in young plants of P. lanceolata was higher in the 100% treatment than in 2%; whereas the reverse was found in mature plants. The results are explained in relation to the relative growth rate, the shoot to root ratio and the natural environment of the species. The apparent K_m values were not influenced by the different treatments. Differences in K_m between the species, if any, were very small. It is suggested that the V_max is a more important parameter for the distribution of plant species in the field than the K_m. The rate of nitrogen accumulation was calculated from growth data and the contents of nitrate and reduced nitrogen. It is concluded that the V_max of system I for nitrate uptake in most cases was sufficient to explain the observed growth rates.     

Nitrate and ammonium absorption by plants growing at a sufficient or insufficient level of phosphorus in nutrient solutions

Summary Absorption of nitrate and ammonium was studied in water culture experiments with 4 to 6 weeks old plants of barley (Hordeum vulgare L.), buckwheat (Fagopyrum esculentum L. Moench) and rape (Brassica napus L.). The plants were grown in a complete nutrient solution with nitrate (5.7±0.2 mM) or nitrate (5.6±0.2 mM) + ammonium (0.04±0.02 mM). The pH of the nutrient solution was kept at 5.0 using a pH-stat. It was found that phosphorus deficiency reduced the rate of nitrate uptake by 58±3% when nitrate was the sole N source and by 83±1% when both nitrate and ammonium were present. The reduction occurred even before growth was significantly impeded by P deficiency. The inhibition of the uptake of ammonium was less,i.e. ammonium constituted 10±1% of the total N uptake in the P sufficient plants and 30±5% in the P deficient plants. The reduction of nitrate absorption greatly decreased the difference between the uptake of anions and cations. It is suggested that P deficiency reduced the assimilation of NO ₃ ^– into the proteins, which might cause a negative feedback on NO ₃ ^– influx and/or stimulate NO ₃ ^– efflux.     

Influence of phosphate status on phosphate uptake kinetics of maize (Zea mays) and soybean (Glycine max)

To obtain plants of different P status, maize and soybean seedlings were grown for several weeks in flowing nutrient solution culture with P concentrations ranging from 0.03–100 µmol P L^-1 kept constant within treatments. P uptake kinetics of the roots were then determined with intact plants in short-term experiments by monitoring P depletion of a 3.5 L volume of nutrient solution in contact with the roots. Results show maximum influx, I_max, 5-fold higher in plants which had been raised in solution of low compared with high P concentration. Because P concentrations in the plants were increased with increase in external P concentration, I_max was negatively related to % P in shoots. Michaelis constants, K_m, were also increased with increased pretreatment P concentration, only slightly with soybean, but by a factor of 3 with maize. The minimum P concentration, C_min, where net influx equals zero, was found between 0.06 and 0.3 µmol L^-1 with a tendency to increase with pretreatment P concentration. Filtration of solutions at the end of the depletion experiment showed that part of the external P was associated with solid particles.It was concluded that plants markedly adapt P uptake kinetics to their P status, essentially by the increase of I_max, when internal P concentration decreases. Changes of K_m and C_min were of minor importance.     

Nitrate transport processes in Fagus- Laccaria-Mycorrhizae

The contribution of influx and efflux of NO₃ ^- on NO₃ ^- net uptake has been studied in excised mycorrhizae of 18–20 week old beech (Fagus sylvatica L.) trees. Net uptake rates of NO₃ ^- followed uniphasic Michaelis-Menten kinetics in the concentration range between 10 μM and 1.0 mM external NO₃ ^-, with an apparent K_m of 88±7 μM, and a V_max of 110±7 nmol g^-1 root f.wt. h^-1. The relative xylem loading of N, i.e. the portion of NO₃ ^- taken up that was loaded into the xylem vessels as NO₃ ^- plus reduced N, was constant over the concentration range tested (4.6–7.7%). NO₃ ^- influx proceeded linearly with increasing external NO₃ ^- supply. When the assumed regulators of net NO₃ ^- uptake, i.e. NH₄ ⁺ or L-glutamate, were applied together with NO₃ ^-, net uptake rates of NO₃ ^- decreased. This inhibitory effect was caused by a reduction of NO₃ ^- influx rather than an enhanced efflux. The comparison of the present data with a recent study with non-mycorrhizal beech roots (Kreuzwieser et al., 1997; J. Exp. Bot. 48, 1431–1438) revealed that mycorrhization leads to reduced rates of NO₃ ^- net uptake. This effect is caused by reduced influx plus enhanced efflux of NO₃ ^- as compared with non-mycorrhizal beech roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Multiphasic uptake of phosphate by corn roots

Abstract The concentration dependence of phosphate uptake was studied using root sections of corn (Zea mays L. cv. Ganga 5). Detailed and wide-range (57 concentrations in the range 1 μmol m^?3-75 mol m^?3), precise (average SEM < 2.5%, n= 6) and reproducible (similar patterns in three independent experiments and for 5, 10, 15, 20, 25 and 30°C) data revealed six (or seven) concentration-dependent phases separated by ‘jumps’ or sharp breaks. These transitions were independent of temperature and occurred over relatively narrow concentration ranges (0.0001–0.0004, 0.08–0.31, 1.0–3.5, (7.5–10), 18–20 and 57–59 mol m^?3). The intermediate phases obeyed Michaelis-Menten kinetics, whereas sigmoidal kinetics were observed at lower concentrations. Uptake within each of the two highest phases increased more rapidly with increasing external phosphate concentration than predicted from Michaelis-Menten kinetics but also saturated more rapidly. The latter finding is not consistent with free diffusion across the plasmalemma at high external phosphate concentrations. Kinetic models yielding continuous isotherms, e.g. the sum of one or two Michaelis-Menten terms and a diffusion term, cannot account for the data.     

Simultaneous hydrolysis of tri- and tetrasaccharides by industrial mixtures of glucoamylase and α-amylase: kinetics and thermodynamics

The present paper deals with the study of the kinetics and thermodynamics of batch enzymic hydrolysis of multisubstrate media in the presence of more than one glucanase. A kinetic model, similar to the one proposed for competitive inhibition, is presented to describe the competition between two substrates, tri- and tetrasaccharides, for the same enzyme, glucoamylase. A literature research on the most recurrent values of the Michaelis-Menten constant also shows a linear relationship between this parameter and the molecular weight of the sugar substrate for a given glucanase.List of Symbols a dimensionless empirical parameter in Eq. (1) - b dimensionless empirical parameter in Eq. (1) - B dimensionless empirical parameter in Eq. (6) - E kg/m³ enzyme concentration - E _a kcal/mol activation energy - kc kg/m³ competition constant - k _M kg/m³ Michaelis-Menten constant - MW kg·10^–3 molecular weight - r mol/(min·kg) specific hydrolysis rate of glucoamylase - R cal/(°K·mol) ideal gas constant - S kg/m³ substrate concentration - T °K absolute temperature - v kg/(m³·h) hydrolysis rate of glucoamylase Indices 1 values referring to trisaccharides - 2 values referring to tetrasaccharides - 0 starting values - max maximum values     

Are phloem amino acids involved in the shoot to root control of NO3- uptake in Ricinus communis plants?

The putative role of phloem amino acids as negative feedback signals for root NO3^- uptake was investigated in Ricinus communis L. The NO3^--grown plants were subjected to N-deficiency due either to complete N-deprivation, or to localized N-deprivation on one side of a split-root system. In comparison with controls, complete N-deprivation resulted in a transient increase in ¹⁵NO3^- influx, and in profound changes in downward phloem transport of amino acids. Total amino acid concentration in the phloem sap decreased by 40%, but responses markedly differed between the individual amino acids. Concentrations of Gln and Ser were rapidly lowered by 50%, while those of Val, Phe, Leu, and Ile displayed a marked increase. Localized N-deprivation on one side of the split root system also resulted in the up-regulation of ¹⁵NO3^- influx in the roots still supplied with NO3^-. However, the amino acid composition of the phloem sap directed to these roots was not modified by the treatment, and remained similar to that in N-sufficient control plants. Only amino acid transport to the N-deprived roots was affected as observed in response to complete N-deprivation. The results from split-root plants indicate that the response of root NO3^- influx to N-deficiency is controlled by shoot-borne regulatory signals, and provide a case study where these signals are not related to a qualitative change or a significant decrease in downward phloem transport of amino acids.     

EFFECTS OF NITRATE CONCENTRATION ON THE GROWTH AND PHYSIOLOGY OF LAMINARIA SACCHARINA (PHAEOPHYTA) IN CULTURE1,2

Laminaria saccharina Lamour. sporophytes were grown in enriched and synthetic media through a range of nitrate concentrations, There was an approximately linear relationship between growth and nutrient concentration up to 10 μ substrate concentration. The half-saturation constant (K_{2) was ca. 1.4 μ NO3-. The internal levels of NO3- increased at substrate concentrations above 10 μM b>3- and reached levels several thousand times higher than the surrounding medium. Thus there is evidence for luxury consumption of NOsb>3}^-. The chlorophyll content and photosynthetic capacities of plants also increased with increasing external NO₃^- The ecological implications of this work are considered.     

Regulation of nitrogen uptake on the whole plant level

The largest part of nitrogen requirements of crops is mostly covered by nitrate. The uptake of this ion is thermodynamically uphill and thus dependent on metabolism. This article considers regulation of N uptake in higher plants putting emphasis on NO₃ ^- and the whole plant level.In field conditions the transport rate depends on the concentration at the root surface in Michaelis-Menten-Kinetics. Maximum net influx of NO₃ ^- (I_max) was often reported at concentrations of 100 M NO₃ ^- and even lower. There are indications that for unrestricted growth the NO₃ ^- concentration at root surface has to be in the order of magnitude allowing I_max if plants are not able to compensate for lower NO₃ ^- concentrations by increasing root surface per unit of shoot.I_max is not a constant but depends for a given variety on N status of plants, the availability of NO₃ ^- and plant age. The decrease of I_max with increasing plant age is closely related to relative growth rate as long as the relationship between N demand and new growth is linear and the root:shoot ratio keeps constant. It seems that I_max is a meaningful physiological characteristic of NO₃ ^- uptake reflecting absolute N demand. There is evidence that shoot demand is linked to NO₃ ^- uptake of the root through an amino acid transport pool cycling in the plant via phloem and xylem.The N demand of a crop depends on increase of dry mass and might not be linear if the critical level of nitrogen in plant dry matter changes during crop development or if retranslocation of nitrogen from older leaves to meristematic tissue occurs. Radiation and temperature drive plant growth and thus N demand of crops. These relationships can be described by mathematical models.     

Time-course of Nitrate Uptake and Nitrate Reductase Activity in Nitrogen-depleted Dwarf Bean

The rate of nitrate uptake by N-depleted French dwarf bean (Phaseolus vulgaris L. cv. Witte Krombek) increased steadily during the first 6 h after addition of NO₃ -After this initial phase the rale remained constant for many hours. Detached root systems showed the same time-course of uptake as roots of intact plants. In vivo nitrate reductase activity (NRA) was assayed with or without exogenous NO₃^- in the incubation medium and the result ing activities were denoted potential and actual level, respectively. In roots the difference between actual and potential NRA disappeared within 15 min after addition of nitrate, and NRA increased for about 15 h. Both potential and actual NRA were initially very low. In leaves, however, potential NRA was initially very high and was not affected by ambient nitrate (0.1–5 mol m^-3) for about 10 h. Actual and potential leaf NRA became equal after the same period of time. In the course of nitrate nutrition, the two nitrate reductase activities in leaves were differentially inhibited by cycloheximide (3.6 mmol m^-3) and tungstate (1 mol m^-3). We suggest that initial potential NRA reflects the activity of pre-existing enzyme, whereas actual NRA depends on enzyme assembly during NO₃^- supply. Apparent induction of nitrate uptake and most (85%) of the actual in vivo NRA occurred in the root system during the first 6 h of nitrate utilization by dwarf bean.     

The characterization of inhibition of net nitrate uptake by salt in salt-tolerant barley (Hordeum vulgare L. cv. California Mariout)

Barley seedlings (Hordeum vulgare L. cv. California Mariout) grown hydroponically for 14-19 d without addition of NaCl were used for describing the effects of salt application on net nitrate uptake and for the calculation of kinetic parameters. The addition of NaCl, KCl, CaCl2, and Na2SO4 to the uptake solution in the experiments led to similar inhibition of nitrate uptake, only at low and very high salt concentrations were ion-specific effects found. The same decrease in nitrate uptake can also be achieved by sorbitol or betaine at corresponding osmolalities. Thus it was concluded that the inhibition of uptake was caused mainly by the osmotic effects of salts. Differences in the mechanisms of inhibition were detected between the two systems of nitrate uptake (high affinity system: HATS, and low affinity system: LATS). The HATS was inhibited non-competitively by NaCl, an apparent Ki of 60 mol m^-3 was calculated using a Dixon-plot. Fitting an equation assuming a non-competitively inhibited HATS by computer program to the raw data resulted in an apparent Ki of about 37 mol m^-3. In contrast, the LATS was affected in a complex way: up to 60 mol m^-3 NaCl the affinity was increased, which led to a stimulation of nitrate uptake at low nitrate concentrations (<2 mol m^-3). An inhibition of the LATS became obvious at concentrations above 3 mol m^-3 nitrate (for all applied salt concentrations) or with 100 mol m^-3 NaCl (throughout the whole nitrate range). Related plots of the data pointed to a competitive effect.Key words: Hordeum vulgare L., net nitrate uptake, high affinity transport system (HATS), low affinity transport system (LATS), salt, inhibition, apparent kinetic parameters.      

The influence of elevated rhizosphere dissolved inorganic carbon concentrations on respiratory O2 and CO2 flux in tomato roots

Respiratory CO2 and O2 flux were measured in hydroponically grown Lycopersicon esculentum (L.) Mill. cv. F144 plants at either low (O mol mol^-1) or elevated CO2 concentrations (>2000 mol mol^-1) supplied to the roots. In NO3^- fed plants the consumption of O2 and the engagement of the alternative pathway were increased by elevated dissolved inorganic carbon (DIC = CO2 + HCO3^-) concentrations. This was ascribed to the influence of organic acids on the TCA cycle and electron transport pathways. Inhibition of O2 consumption by elevated DIC in NH4^--fed plants may be due to the reduction requirements of anaplerotic carbon entering the TCA cycle or the removal of carbon from the TCA cycle for amino acid synthesis. In both NO3^- and NH4⁺-fed plants elevated DIC inhibited CO2 release due to high rates of DIC incorporation by phosphoenolpyruvate carboxylase (PEPc) activity. Transient net CO2 consumption due to the inhibition of respiration by salicylhydroxamic acid and KCN, together with high respiratory quotients after the addition of inhibitors of carbonic anhydrase (CA) activity, were also ascribed to high rates of DIC incorporation at elevated DIC concentrations. Ethoxyzolamide, an inhibitor of CA activity, inhibited both DI¹⁴C incorporation into organic products and NO3^- uptake by 81% and 40%, respectively. This, together with a 32% increase in DI¹⁴C accumulation and inhibition of NO3^- uptake upon inhibition of anion transport by diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS) may indicate the exchange of HCO3^- for NO3^- across the root plasmalemma. It was concluded that dark incorporation of HCO3^- by PEPc increased at elevated rhizosphere DIC concentrations and that the products of DIC incorporation may stimulate respiratory electron transport. Additional reducing energy and carbon skeletons from the tricarboxylic acid (TCA) cycle would therefore be available for respiration and the reduction and incorporation of NO3^- into amino acids.Key words: Tomato, PEPc, respiration, carbon dioxide nitrate.      

Mechanisms of arsenic hyperaccumulation in<Emphasis Type="Italic"> Pteris</Emphasis> species: root As influx and translocation

Several species of fern from the Pteris genus are able to accumulate extremely high concentrations of arsenic (As) in the fronds. We have conducted short-term unidirectional As influx and translocation experiments with ⁷³As-radiolabeled arsenate, and found that the concentration-dependent influx of arsenate into roots was significantly larger in two of these As-hyperaccumulating species, Pteris vittata (L.) and Pteris cretica cv. Mayii (L.), than in Nephrolepis exaltata (L.), a non-accumulating fern. The arsenate influx could be described by Michaelis-Menten kinetics and the kinetic parameter K _m was found to be lower in the Pteris species, indicating higher affinity of the transport protein for arsenate. Quantitative analysis of kinetic parameters showed that phosphate inhibited arsenate influx in a directly competitive manner, consistent with the hypothesis that arsenate enters plant roots on a phosphate-transport protein. The significantly augmented translocation of arsenic to the shoots that was seen in these As hyperaccumulator species is proposed to be due to a combination of the increased root influx and also decreased sequestration of As in the roots, as a larger fraction of As could be extracted from roots of the Pteris species than from roots of N. exaltata. This leaves a larger pool of mobile As available for translocation to the shoot, probably predominantly as arsenite.Abbreviations As ^V Arsenate - As ^III Arsenite - K _m Michaelis-Menten constant - P _i Phosphate - V _max Maximum rate of an enzyme-catalyzed reaction