Interaction between a copper-tolerant and a copper-sensitive population of Silene cucubalus

Interactions between copper-tolerant and copper-sensitive plants of Silene cucubalus (L.) Wib. were absent when grown in mixed culture in a nutrient solution with a normal Cu²⁺ concentration (0.5 μ M ). When grown in mixed culture in a nutrient solution with 40.5 μ M CuSO₄, however, the biomass production of the sensitive plants was less affected than when grown in monoculture. At 40.5 μ M Cu²⁺, in the presence of tolerant plants, the concentration of copper in both roots and shoots of sensitive plants was significantly diminished in comparison to a monoculture without tolerant plants. At the same time the copper concentration in the roots of the tolerant plants was higher in the presence of sensitive plants. The possibility of external detoxification of the copper by tolerant plants as a mechanism of heavy metal resistance is discussed.     

Bioaccumulation du Cd et du Zn chez les plants de tomates (Lycopersicon esculentum L.)

This work aims at evaluating the accumulation of cadmium (Cd) and zinc (Zn) (trace elements) in the organs of young tomato plants (Lycopersicon esculentum L. var. Rio Grande) and their effects on the rate of chlorophyll and enzyme activities involved in the antioxidant system: catalase (CAT), glutathion-S-transferase (GST) and peroxysase ascorbate (APX). Plants previously grown on a basic nutrient solution were undergoing treatment for 7 days, either by increasing concentrations of CdCl₂ or ZnSO₄ (0, 50, 100, 250, 500 μM) or by the combined concentrations of Cd and Zn (100/50, 100/100, 100/250, 100/500 μM). The results concerning the determination of metals in the various compartments of tomato plants as a function of increasing concentrations of Cd or Zn, suggest a greater accumulation of Cd and Zn in the roots compared to leaves. The combined treatment (Cd/Zn) interferes with the absorption of the two elements according to their concentrations in the culture medium. The presence of Zn at low concentrations (50 μM of Zn/100 μM Cd) has little influence on the accumulation of Cd in the roots and leaves, while the absorption of these two elements in the leaves increases and decreases in roots when their concentrations are equivalent (100/100 μM) compared to treatment alone. When the concentration of Zn is higher than that of Cd (500 μM of Zn/100 μM Cd) absorption of the latter is inhibited in the roots while increasing their translocation to the leaves. Meanwhile, the dosage of chlorophylls shows that they tend to decrease in a dose-dependent for both treatments (Cd or Cd/Zn), however, treatment with low concentrations of Zn (50 and 100 μM) stimulates chlorophyll synthesis. However, treatment with different concentrations of Cd seems to induce the activity of the enzymes studied (CAT, APX, GST). It is the same for treatment with different concentrations of Zn and this particularly for the highest concentrations. Finally, the combined treatment (Zn/Cd) also appears to cause enzyme inductions: CAT, APX and GST.     

The influence of potassium humate on 137Cs accumulation by barley growing on soil with different fertility

The changes studied in 137Cs uptake by plants and its distribution between vegetative and generative organs of barley cultivated with the application of potassium humate. A relationship has been found between 137Cs accumulation size in barley at various ontogenesis stages and way of potassium humate application (treatment of seeds or plants), as well as availability of mineral nutrients in the soil. Changes in K+ and NH4+ concentrations in soil solution are shown to be of prevailing importance in regulating 137Cs uptake by plants compared with potassium humate effects.     

Brassinosteroids and iron plaque affect arsenic and cadmium uptake by rice seedlings grown in hydroponic solution

Brassinosteroids (Brs) have drawn wide attention due to their protective role against toxicity of heavy metals in plants. To better understand the role of Br in arsenic (As) and cadmium (Cd) uptake by rice plants, a hydroponic experiment was conducted to investigate the combined effect of 24-epibrassinolide (Br24) or 28-homobrassinolide (Br28) and iron plaque (IP) on As and Cd uptake and accumulation in rice seedlings. Six-week-old seedlings were sprayed with 0.2 or 0.02 μM Br24 or Br28 and grown in nutrient solution for 3 d, and then 20 or 60 mg Fe²⁺ dm^-3 (Fe20 and Fe60) was used to induce root IP formation for 3 d. These seedlings with or without Br and with or without IP were exposed to solution containing 0.5 mg dm^-3 As^III or Cd for 9 d. The results showed that rice growth decreased when Br24 were applied, but it increased when combination of Br24 and IP was applied. Fe concentrations in dithionite-citratebicarbonate (DCB) extracts were increased after 0.2 or 0.02 μM Br24 application in the absence of IP, but decreased by Br24 in the presence of IP. In the absence of IP, As and Cd content in leaves was significantly reduced by 0.02 μM Br24 and 0.2 μM Br28, respectively. The As content in leaves was also reduced by the combination of 0.02 and 0.2 μM Br28 and IP, and the Cd content in leaves was reduced by the combined effect of 0.2 μM Br24 and IP. These results indicate that Br24 and Br28 could impede As and Cd accumulation, and the interactions between Br and IP may have a potential in restricting the transport of As and Cd into rice shoots.     

Carboxylate metabolism in sugar beet plants grown with excess Zn

The effects of Zn excess on carboxylate metabolism were investigated in sugar beet (Beta vulgaris L.) plants grown hydroponically in a growth chamber. Root extracts of plants grown with 50 or 100 μM Zn in the nutrient solution showed increases in several enzymatic activities related to organic acid metabolism, including citrate synthase and phosphoenolpyruvate carboxylase, when compared to activities in control root extracts. Root citric and malic acid concentrations increased in plants grown with 100 μM Zn, but not in plants grown with 50 μM Zn. In the xylem sap, plants grown with 50 and 100 μM Zn showed increases in the concentrations of citrate and malate compared to the controls. Leaves of plants grown with 50 or 100 μM Zn showed increases in the concentrations of citric and malic acid and in the activities of citrate synthase and fumarase. Leaf isocitrate dehydrogenase increased only in plants grown with 50 μM Zn when compared to the controls. In plants grown with 300 μM Zn, the only enzyme showing activity increases in root extracts was citrate synthase, whereas the activities of other enzymes decreased compared to the controls, and root citrate concentrations increased. In the 300 μM Zn-grown plants, the xylem concentrations of citric and malic acids were higher than those of controls, whereas in leaf extracts the activity of fumarase increased markedly, and the leaf citric acid concentration was higher than in the controls. Based on our data, a metabolic model of the carboxylate metabolism in sugar beet plants grown under Zn excess is proposed.     

Proline protects <Emphasis Type="Italic">Atropa belladonna</Emphasis> plants against nickel salt toxicity

Atropa belladonna L. plants were grown in water culture for 8 weeks before the nutrient medium was supplemented with NiCl₂ to final concentrations of 0 (control treatment), 50, 100, 150, 200, 250, and 300 μM. After 4 days of plant growing in the presence of nickel chloride, the content of water, proline, Ni, Fe, free polyamines, as well as lipid peroxidation rates were measured. The addition of 100–150 μM Ni to the medium significantly reduced the fresh weight increments and water content in comparison with these parameters for untreated plants; 200 μM Ni caused serious, although nonlethal damage to the plants, whereas 250 and 300 μM Ni proved to be lethal. In the aboveground organs, the major part of Ni was accumulated in the apical leaves. When the plants were treated with 200 μM Ni, the Ni content in apical leaves was 220 μg/g dry wt, while Ni content in roots reached 1500 μg/g dry wt. The treatment of plants with proline in the presence of 200 μM Ni inhibited Ni accumulation in tissues. The proline-treated plants exhibited elevated iron content in leaves and especially in roots and were characterized by comparatively low rates of lipid peroxidation and by sustained leaf water status. When 200 μM Ni was applied, the content of free putrescine decreased, while the contents of spermine and spermidine in leaves increased appreciably with respect to the control values. The toxic effect of nickel was accompanied not only by an enhanced accumulation of high- molecular-weight polyamines but also by their oxidative degradation, which was evident from the 14-fold increase in the content of 1,3-diaminopropane. The protective effect of exogenous proline in the presence of high nickel concentrations was manifested in lowered lipid peroxidation rates, alleviation of iron deficiency, and in retarded oxidative degradation of polyamines.     

The influence of humic acids derived from earthworm-processed organic wastes on plant growth

Some effects of humic acids, formed during the breakdown of organic wastes by earthworms (vermicomposting), on plant growth were evaluated. In the first experiment, humic acids were extracted from pig manure vermicompost using the classic alkali/acid fractionation procedure and mixed with a soilless container medium (Metro-Mix 360), to provide a range of 0, 50, 100, 150, 200, 250, 500, 1,000, 2,000, and 4,000 mg of humate per kg of dry weight of container medium, and tomato seedlings were grown in the mixtures. In the second experiment, humates extracted from pig manure and food wastes vermicomposts were mixed with vermiculite to provide a range of 0, 50, 125, 250, 500, 1,000, and 4,000 mg of humate per kg of dry weight of the container medium, and cucumber seedlings were grown in the mixtures. Both tomato and cucumber seedlings were watered daily with a solution containing all nutrients required to ensure that any differences in growth responses were not nutrient-mediated. The incorporation of both types of vermicompost-derived humic acids, into either type of soilless plant growth media, increased the growth of tomato and cucumber plants significantly, in terms of plant heights, leaf areas, shoot and root dry weights. Plant growth increased with increasing concentrations of humic acids incorporated into the medium up to a certain proportion, but this differed according to the plant species, the source of the vermicompost, and the nature of the container medium. Plant growth tended to be increased by treatments of the plants with 50-500 mg/kg humic acids, but often decreased significantly when the concentrations of humic acids derived in the container medium exceeded 500-1,000 mg/kg. These growth responses were most probably due to hormone-like activity of humic acids from the vermicomposts or could have been due to plant growth hormones adsorbed onto the humates.     

Interactive effects of salicylic acid and nitric oxide on soybean plants under NaCl salinity

The effects of salicylic acid (SA), sodium nitroprusside (SNP), a nitric oxide donor, and their combination (SA+SNP) on some physiological parameters of 23-day-old soybean seedlings grown under saline and nonsaline conditions were studied. The changes in the leaf area, shoot fresh and dry weights, contents of chlorophylls and carotenoids, amounts of MDA and hydrogen peroxide showed that the addition of 100 μM SA and/or 100 μM SNP markedly declined the oxidative damage to soybean plants induced by 50 and 100 μM NaCl. Our results proved that combined action of SA and nitric oxide donor significantly activated catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX), which contributed to the decay of H₂O₂ in soybean leaves under NaCl toxicity. The protective action of (SA+SNP) against saltinduced oxidative damage was often more efficient than effects of SA and SNP alone. We also observed that the accumulation of proline was apparently accelerated by these substances under salt stress. As well, it was observed that the interaction between SA and nitric oxide had synergistic effects in decreasing of the damages induced by NaCl salinity.     

Biological effects of high copper and zinc concentrations and their interaction in rapeseed plants

In experiments with rapeseed (Brassica napus L., cv. Westar) plants, it was confirmed that copper was considerably more toxic than zinc. The toxic effects of 50 and 150 μM CuSO₄ were comparable to those of 1000 and 2500 μM ZnSO₄. The analysis of the effects of these concentrations of copper and zinc on photosynthetic pigment contents and on the rate of lipid peroxidation did not reveal any reasons for different toxicities of these heavy metals (HM). Among biological effects studied, significant differences were found in the organ distribution of these metals in plants grown on both the standard medium and the medium with high concentrations of copper or zinc. Copper retained in the roots in relatively small amounts and was poorly transported over the aboveground part of the plants. It stayed mainly in the lower leaves, and its distribution changed only a little during the recovery of plants following the HM treatment. In contrast, zinc proved to be highly mobile, it was concentrated in the upper leaves and actively transported when the plants were transferred to a medium with the optimal HM concentrations. High copper concentrations slowed strongly zinc uptake by the roots but practically did not change its movement over the plant. In contrast, high zinc concentrations facilitated copper uptake by the roots but reduced its transfer to the aboveground organs. The data presented here allow us to hypothesize that biological peculiarities of organ and cellular distribution of copper and zinc in plants and interaction of these HM play an important role in the toxic effects of high concentrations of these HM and the mechanisms of adaptation to them at industrial environmental pollution used by rapeseed plants.     

Effects of nitric oxide and Fe supply on recovery of Fe deficiency induced chlorosis in peanut plants

The effects of nitric oxide (NO) and/or iron (Fe) supplied to Fe deficient plants have been investigated in peanut (Arachis hypogaea L.) grown in Hoagland nutrient solution with or without Fe. Two weeks after Fe deprivation, recovery was induced by addition of 250 μM sodium nitroprusside (SNP, a NO donor) and/or 50 μM Fe (Fe-EDTA) to the Fe deprived (-Fe) nutrient solution. Activities of antioxidant enzymes, leaf chlorophyll (Chl), and active Fe content decreased, whereas activities of H⁺-ATPase, ferric-chelate reductase (FCR), nitrate reductase, and nitric oxide synthase and NO production increased in Fe deficient plants, consequently an Fe chlorosis symptom appeared obviously. In contrast, these symptoms disappeared gradually after two weeks with NO and/or Fe supply, which caused an increases in leaf Chl and active Fe content, especially following by co-treatment with NO and Fe to values found in Fe sufficient plants. Increased activities of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase) and decreased accumulation of reactive oxygen species (H₂O₂, O ₂ ^?? ) and malondialdehyde enhanced the ability of resistance to oxidative stress. Supplied NO alone had the obvious effect on increased NO production and on activity of H⁺-ATPase and FCR, whereas root length and root/shoot ratio were most effectively increased by Fe supplied alone. Co-treatment with NO and Fe did the best effects on recovery peanut chlorosis symptoms by significantly increased Chl and available Fe content and adjusted distribution of Fe and other mineral elements (Ca, Mg, and Zn) in both leaves and roots.     

Salinity attenuates nickel-accumulating capacity of <Emphasis Type="Italic">Atropa belladonna</Emphasis> L. plants

Comparative analysis of growth and composition of Atropa belladonna L. plants was performed after separate and combined additions of NaCl and NiCl₂ to the nutrient medium. Plants were grown in water culture on modified Johnson solution for 8 weeks until the formation of the fifth leaf pair. Thereafter, NiCl₂ was introduced at final concentrations of 100 and 150 μM into the medium either separately or in combination with 100 mM NaCl. After completing the 7-day treatment with Ni ions, the plants' weight and the content of water and photosynthetic pigments were determined. The content of Ni, free polyamines (putrescine, spermidine, spermine), and atropine was determined in plant roots and leaves, whereas the content of Fe, proline, and malondialdehyde (MDA) was examined in leaves only. The distribution of Ni in various tissues was inspected using the dimethylglyoxime method. The presence of NiCl₂ in growth media diminished the increments in fresh weight of shoots and roots; lowered the content of water, pigments, and iron in leaves; and initiated chlorosis. The leaves of Ni-treated plants accumulated larger amounts of atropine, putrescine, proline, and MDA with respect to the control levels of these compounds. In contrast to the action of Ni alone, the combined application of NaCl and NiCl₂ was followed by the increased content of water and pigments in leaves. The presence of NaCl in the medium restricted the entry of Ni into roots and diminished the levels of MDA and proline in leaves. After growing the plants in the presence of 100 and 150 μM NiCl₂, nickel was located in the root outer cortex and the rhizoderm. In plants treated with 150 μM NiCl₂, nickel was also observed in tissues of the central cylinder, mostly in the pericycle, phloem, and xylem. In plants grown in the presence of 150 μM NiCl₂ and 100 mM NaCl, the decreased accumulation of nickel was noted in the tissues of the central cylinder in the root hair zone. Thus, the combined action of Ni and moderate salinity reduced nickel accumulation in roots and aboveground organs of A. belladonna plants. The reduced Ni content in plants mitigated the toxic effect of Ni present in the medium. This was manifested in stabilization of leaf water status, an increase in the content of photosynthetic pigments, and alleviation of oxidative stress, which was assessed from the content of low-molecular organic compounds exhibiting stress-protective and antioxidant action (proline, MDA, free polyamines, and atropine).     

The mycorrhizal status of an emergent aquatic, Lythrum salicaria L., at different levels of phosphorus availability

 The relationship between nutrient availability and mycorrhizal status has been well studied for terrestrial plant species, but has been examined rarely in aquatic and emergent aquatic species. The purpose of this study was to determine the effect of phosphorus availability on the arbuscular mycorrhizal (AM) status of an emergent aquatic, Lythrum salicaria L. L. salicaria was grown in hydroponic sand culture at five phosphorus concentrations (0, 100, 1000, 10 000, and 47 500 μg PO₄/l nutrient solution) for 49 days with or without mycorrhizal inoculum obtained from wetland soil. Inoculated plants at the lowest three phosphorus concentrations were colonized by AM, whereas there was no colonization of plants grown at the highest two phosphorus concentrations. Colonization by AM fungi occurred in conjunction with symptoms of phosphorus deficiency in L. salicaria under experimental conditions: plants at the lowest three phosphorus concentrations had lower biomass and higher root: shoot weight ratios than plants at the highest two concentrations. However, total biomass and internal phosphorus concentration did not differ between inoculated and control plants. Further studies are needed under conditions more closely mimicking natural dynamics. Accepted: 7 August 1999     

INFLUENCE OF ALUMINUM IN BIOLOGIC EFFECTS OF ELF MAGNETIC FIELD STIMULATION

We studied the influence of a weak, extremely low-frequency magnetic field (MF) with a frequency of 50 Hz and a peak amplitude of 103 μT and aluminum solution (in the form of AlCl₃) at different concentrations (0, 40, 70, 100, 130, 160, 400, 800, 2000, and 5000 μM) on the growth of spruce seedlings (Picea abies). The results showed that stimulatory and statistically significant MF effects on the growth of seedlings were observed only with a 100-μM aluminum solution. Slight stimulative effects were also observed within the range of concentrations between 40 and 160 μM Al³⁺ (all the stimulated groups taken together). Germination and fresh weight were not significantly influenced. At these concentrations the aluminum solution alone (without MF) or the MF alone (without Al³⁺) did not influence the growth parameters. These results suggest the importance of synergistic action of the MF with environmental factors as well as the existence of “physiologic windows” in addition to the frequency and power ones.     

Manganese requirement for optimum photosynthesis and growth in NAD-malic enzyme C-4 species

Despite the evidence for a critical role of Mn in malate decarboxylation and CO₂ release for carbon fixation reactions in C-4 plants, there is a lack of information on their Mn requirement. The objective of this study was to establish Mn levels needed for optimum growth and photosynthesis of four agriculturally important C-4 species, NAD-ME C-4 pearl millet and purple amaranth, and NADP-ME C-4 corn and sorghum, as compared to two C-3 species, wheat and squash. Plants were grown hydroponically in a complete nutrient solution with Mn concentrations ranging from 0 to 100 μM. We report that under these conditions, C-3 and NADP-ME C-4 plants reached their maximum biomass production with 2–5 μM Mn, the concentration commonly used in plant nutrient media. In contrast, Mn concentrations supporting maximum performance of NAD-ME C-4 plants were up to 20-fold higher and ranged between 50 and 100 μM. Although leaf tissue Mn concentrations increased in parallel with Mn nutrition in all plants, the higher leaf Mn had no effect on NADP-ME C-4 or C-3 plants, but it caused a large, up to 100%, increase in net photosynthetic rate in NAD-ME C-4 species. The highest photosynthetic rates across the spectrum of photon flux density were recorded for C-3 and NADP-ME C-4 plants receiving 2–5 μM Mn, and for NAD-ME C-4 species millet and amaranth supplied with 50 or 100 μM Mn, respectively. Squash (C-3) plants were the most sensitive to Mn and their photosynthetic rate was severely depressed with more than 10 μM Mn. The increase in photosynthetic rates of NAD-ME C-4 species occurred without an increase in stomatal conductance, eliminating CO₂ uptake as the main cause. We propose that the higher photosynthetic rates in NAD-ME C-4 species supplied with higher Mn were a result of increased activation of the Mn-dependent NAD-ME in bundle sheath cells, producing greater CO₂ supply for Calvin cycle reactions. This is, to our knowledge, the first report on a significantly higher Mn requirement for optimum photosynthesis and biomass production of NAD-ME C-4 species.     

Role of Exogenous Nitric Oxide in Alleviating Iron Deficiency Stress of Peanut Seedlings (<Emphasis Type="Italic">Arachis hypogaea</Emphasis> L.)

A greenhouse hydroponic experiment was performed to evaluate how peanut seedlings (Arachis hypogaea L.) responded to iron (Fe) deficiency stress in the presence of sodium nitroprusside (SNP), a nitric oxide (NO) donor. The results showed that Fe deficiency inhibited peanut plant growth, decreased chlorophyll and active Fe concentrations, and dramatically disturbed ion balance. The addition of 50, 100, 250, and 500 µM SNP, significantly promoted the absorption of Fe in the cell wall, cell organelles, and soluble fractions, increased the concentrations of active Fe and chlorophyll in peanut plants, and alleviated the excess absorption of manganese (Mn) and copper (Cu) induced by Fe deficiency. In addition, SNP also significantly increased the activities of superoxide dismutase, peroxidase, and catalase, which is beneficial to inhibit the accumulation of malondialdehyde and reactive oxygen species. Addition of 250 µM SNP had the most significant alleviating effect against Fe-deficiency stress, and after 15 days of treatment, the plants with the 250 µM SNP treatment achieved comparable NO levels with those grown under optimal nutrition conditions. However, the effects of SNP were reversed by addition of hemoglobin (Hb, a NO scavenger). These results suggest that NO released from SNP decomposition was responsible for the effect of SNP-induced alleviation on Fe deficiency.     

The effect of ammonium and nitrate nitrogen sources on copper uptake and amino acid status of cereals

Summary The effects of different nitrogen sources (NH₄, NO₃, and NH₄ NO₃) on the uptake of copper by wheat and barley growing in solution culture were compared in three experiments. Both the copper concentration and weight gain of shoots and roots were found to decrease in the order NO₃>NH₄ NO₃>NH₄ irrespective of the solution copper concentration. Ammonium nitrogen was also found to decrease the copper concentration of wheat grown on a copper deficient soil compared with a nitrate source of nitrogen. Increasing concentrations of ammonium ions in solution culture caused ammonium toxicity and reduced both plant copper concentrations and vegetative yield. Biochemical investigations using paper chromatography revealed that the amino acid asparagine was the major detoxification product of ammonia in wheat. Copper deficient plants were found to have elevated levels of amino acids compared with controls, irrespective of the nitrogen source.     

Catechol oestrogens stimulate and direct prostaglandin synthesis

We have tested the action of a catechol oestrogen -2,3,17β- trihydroxy oestra-1,3,5 (10)-triene (2-OH oestradiol) in stimulating prostaglandin (PG) production by an homogenate of rat uterus. Marked and dose dependent stimulation was observed in PGF_2α and PGE₂ production using 20–250 μM concentrations of catechol oestrogen; a concentration of 250 μM 2-OH oestradiol resulted in a 23 fold increase in PGF_2α production with a 50% reduction in the synthesis of 6-keto PGF_1α. Tryptophan, catechol and glutathione were without effect on PGF_2α and PGE₂ production whereas adrenalin stimulated the production of all PGs, although the increase was less than that seen with 2-OH oestradiol. Oestradiol had a slight stimulatory action on PGF_2α production which reached a maximum at around 40 μM but had a more marked stimulation of 6-keto PGF_1α formation. Stimulation of prostaglandin production by oestradiol and 2-OH oestradiol showed no variation at different stages of the rat oestrous cycle. The use of 5 to 100 mg of tissue/ml gave similar product distribution although the effect of catechol oestrogen both in terms of stimulation of E and F formation (expressed per mg of tissue) and in its action on product distribution was more marked at lower concentrations of tissue.     

Enhancement of nickel and lead accumulation and their toxic growth-inhibitory effects on amaranth seedlings in the presence of calcium

The effects of Ni(NO₃)₂ and Pb(NO₃)₂ on Amaranthus sp. L. root growth and the effect of calcium on heavy metal (HM) accumulation in the growing root zone and root growth inhibition were studied. The seeds were germinated in the Pb(NO₃)₂ solutions at concentrations of 50, 100, 200, 500 and 700 μM or Ni(NO₃)₂ solutions at concentrations of 10, 50, 70, 100, and 500 μM in the presence of 100 μM Ca(NO₃)₂ or without it. HM toxicity was assesses in 7 days after seed sowing by the root length. Distribution of HM over the tissues of the growing root part was examined histochemically. Ni was more toxic to root growth than Pb. In the presence of Ca, Ni and Pb accumulation in the amaranth root growing part increased markedly, and this enhanced their growth-inhibitory of action. A comparison of results obtained in this work and available from the literature permitted a conclusion that the routes of HM penetration into the root differ in different plant species, and this determines ambiguity of protective Ca action.     

Calcium and Salt Toleration by Bean Plants

The role of calcium in the salt relations of the bean plant, Phaseolus vulgaris, was examined. Brittle wax bush bean plants were cultured in nutrient solutions containing 50 mM NaCl. In the absence of added calcium the plants showed a general breakdown of the roots. A low concentration of calcium in the nutrient solution (0.1 mM) prevented this. Without added calcium the plants absorbed and translocated sodium at such a rate that high concentrations of it built up in the leaves within two days. With increasing concentrations of calcium in the nutrient solution the leaves contained progressively less sodium, and at 3 mM CaSO₄ the concentrations of sodium in the leaves was equal to that of the control plants grown without addition of salt. Even after both roots and stems had reached a high concentration of sodium, the leaves of plants grown in the presence of adequate concentrations of calcium contained little sodium.     

Cadmium toxicity in tomato (Lycopersicon esculentum) plants grown in hydroponics

The effects of Cd have been investigated in tomato (Lycopersicon esculentum) plants grown in a controlled environment in hydroponics, using Cd concentrations of 10 and 100 μM. Cadmium treatment led to major effects in shoots and roots of tomato. Plant growth was reduced in both Cd treatments, leaves showed chlorosis symptoms when grown at 10 μM Cd and necrotic spots when grown at 100 μM Cd, and root browning was observed in both treatments. An increase in the activity of phosphoenolpyruvate carboxylase, involved in anaplerotic fixation of CO₂ into organic acids, was measured in root extracts of Cd-exposed plants. Also, significant increases in the activities of several enzymes from the Krebs cycle were measured in root extracts of tomato plants grown with Cd. In leaf extracts, significant increases in citrate synthase, isocitrate dehydrogenase and malate dehydrogenase activities were also found at 100 μM Cd, whereas fumarase activity decreased. These data suggest that at low Cd supply (10 μM) tomato plants accumulate Cd in roots and this mechanism may be associated to an increased activity in the PEPC–MDH–CS metabolic pathway involved in citric acid synthesis in roots. Also, at low Cd supply some symptoms associated with a moderate Fe deficiency could be observed, whereas at high Cd supply (100 μM) effects on growth overrule any nutrient interaction caused by excess Cd. Cadmium excess also caused alterations on photosynthetic rates, photosynthetic pigment concentrations and chlorophyll fluorescence, as well as in nutrient homeostasis.