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.     

Oxidative damages induced by short-term exposure to cadmium in bean plants: Protective role of salicylic acid

The role of salicylic acid (SA) in alleviating cadmium (Cd) toxicity was investigated in a hydroponic cultivation system. Short-term exposure of bean (Phaseolus vulgaris) plants to 20 μM Cd inhibited biomass production and intensively increased accumulation of Cd in both roots and leaves. At leaf level, Cd significantly decreased mineral ions, chlorophyll and carotenoids concentrations. Concomitantly, Cd enhanced electrolyte leakage, H₂O₂ content and lipid peroxidation as indicated by malondialdehyde (MDA) accumulation. SA pretreatment decreased the uptake and the transport of Cd, alleviated the Cd-induced inhibition of nutrient absorption and led to a significant increase of chlorophyll and carotenoid content. SA application alleviated the oxidative damages as evidenced by the lowered H₂O₂ and MDA content. SA particularly induced an increase in both CAT and APX activities accompanied by a significant reduction in SOD and POD activities. As important antioxidants, ascorbate and glutathione contents in bean leaves exposed to cadmium were significantly decreased by SA treatment. These results reveal the potentiating effect of salicylic acid in regulating cadmium induced oxidative stress in bean plants.     

Physiological mechanism of hypertolerance of cadmium in Kentucky bluegrass and tall fescue: Chemical forms and tissue distribution

Kentucky bluegrass (Poa pratensis) and tall fescue (Festuca arundinacea) are hypertolerant grasses to soil cadmium contamination. Little information is available on their tolerance mechanism. A sand culture and a hydroponic culture experiment were designed to investigate the Cd chemical form changes and its translocation in different tissues. The results showed that Kentucky bluegrass and tall fescue can tolerate 50–200 mg kg⁻¹ of soil Cd stresses and accumulate as high as 4275 and 2559 mg Cd kg⁻¹ DW, respectively, in their shoots without the loss of shoot biomass. Their Cd hypertolerance was correlated with an increase of the undissolved Cd phosphates in the leaves in both grass species, as determined by sequential solvent extraction procedures. The superior Cd tolerance of tall fescue to Kentucky bluegrass was associated with less Cd translocation into the stele of roots and less Cd transported to leaves. The pectate- and protein-integrated Cd forms may be involved in the symplastic translocation of Cd from cortex into stele, and this may lead the higher Cd concentrations in the stele of roots and then above ground leaves via long-distance transport in Kentucky bluegrass.     

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.     

Salicylic acid-mediated alleviation of cadmium toxicity in hemp plants in relation to cadmium uptake,photosynthesis, and antioxidant enzymes

To assess the role of salicylic acid (SA) in alleviating cadmium (Cd) toxicity in hemp (Cannabis sativa L.) plants, the growth parameters, Cd accumulation, photosynthetic performance and activities of major antioxidant enzymes were investigated in hemp seedlings treated with 500 μM SA, under 0, 25, 50, and 100 mg Cd kg⁻¹ sands (DW) conditions, respectively. Cd exposure resulted in a small reduction in biomass (12.0–26.9% for root, and 8.7–29.4% for shoot, respectively), indicating hemp plants have innate capacity to tolerant Cd stress. This was illustrated by little inhibition in photosynthetic performance, unchanged malondialdehyde content, and enhancement of superoxide dismutase (SOD) and peroxidases (POD) activities in hemp plants. Cd content in root is 25.0–29.5 times’ higher than that in shoot, suggesting the plant can be classified as a Cd excluder. It is concluded that SA pretreatment counteracted the Cd-induced inhibition in plant growth. The beneficial effects of SA in alleviating Cd toxicity can be attributed to the SA-induced reduction of Cd uptake, improvement of photosynthetic capacity, and enhancement of SOD and POD activities.     

Salicylic acid alleviates mercury toxicity by preventing oxidative stress in roots of Medicago sativa

Salicylic acid (SA) as a signal molecule mediates many biotic and environmental stress-induced physiological responses in plants. In this study, we investigated the role of SA in regulating Hg-induced oxidative stress in the roots of alfalfa (Medicago sativa). Plants pretreated with 0.2 mM SA for 12 h and subsequently exposed to 10 μM Hg²⁺ for 24 h displayed attenuated toxicity to the root. The SA-promoted root growth was correlated with decreased lipid peroxidation in root cells. The ameliorating effect of SA was confirmed by the histochemical staining for the detection of loss of membrane integrity in Hg-treated roots. We show that treatment with 0.2 mM SA increased the activity of NADH oxidase, ascorbate peroxidase (APX) and peroxidase (POD) in the roots exposed Hg. However, a slightly decreased superoxide dismutase (SOD) activity was observed in SA + Hg-treated roots when compared to those of Hg treatment alone. We also measured accumulation of ascorbate (ASC), glutathione (GSH) and proline in the roots of alfalfa and found that roots treated with SA in the presence of Hg accumulated more ASC, GSH and proline than those treated with Hg only. These results suggest that exogenous SA may improve the tolerance of the plant to the Hg toxicity.     

Cadmium tolerance in Thlaspi caerulescens: I. Growth parameters,metal accumulation and phytochelatin synthesis in response to cadmium

A population of the metallophyte, Thlaspi caerulescens, originating from a Cd–Pb–Zn old mining and smelter site at Plombières (Belgium) was studied. T. caerulescens was cultivated hydroponically to investigate Cd uptake and tolerance. Cd was added to Hoagland’s medium at concentration range from 5 to 500 μM. The plants could tolerate 500 μM Cd in the solution showing only minor visible symptoms of toxicity but with a 32% decrease in fresh weight. After 14 days at 500 μM, Cd content in roots and shoots was 707 and 602 mg kg⁻¹ of dry weight (d.w.), respectively. Application of Cd to hydroponically cultivated T. caerulescens induced the accumulation of PCs in plant roots and shoots. Buthionine sulfoximine (BSO) application almost completely reduced (by 98–100%) the accumulation of PCs without simultaneous increase in plants sensitivity to Cd. These results suggest a minor if any role of PCs in tolerance to Cd of the studied population of T. caerulescens in hydroponics. On the other hand, no PC accumulation was detected either in T. caerulescens plants growing in their natural environment at Plombierès or in plants growing in their native soil in a greenhouse. These results suggest that naturally selected tolerance in T. caerulescens population from Plombières is not associated with enhanced PCs synthesis.     

Organic Acids Accumulation and Antioxidant Enzyme Activities in Thlaspi caerulescens under Zn and Cd Stress

Growth, organic acid and phytochelatin accumulation, as well as the activity of several antioxidative enzymes, i.e. superoxide dismutase (SOD), ascorbate peroxidase (APX) guaiacol peroxidase (POX) and catalase (CAT) were investigated under Zn and Cd stress in hydroponically growing plants of Thlaspi caerulescens population from Plombières, Belgium. Tissue Zn and Cd concentration increased (the highest concentration of both was in roots) as the concentration of these metals increased in the nutrient solution. Increasing Zn concentration enhanced plant growth, while with Cd it declined compared to the control. Both metals stimulated malate accumulation in shoots, Zn also caused citrate to increase. Zn did not induce phytochelatin (PC) accumulation. In plants exposed to Cd, PC concentration increased with increasing Cd concentration, but decreased with time of exposure. Under Zn stress SOD activity increased, but APX activity was higher at 500 and 1000 μM Zn and CAT activity only at 500 μM Zn in comparison with the control. CAT activity decreased in Cd- and Zn-stressed plants. The results suggest that relative to other populations, a T. caerulescens population from Plombières, when grown in hydroponics, was characterized by low Zn and Cd uptake and their translocation to shoots and tolerance to both metals. The accumulation of malate and citrate, but not PC accumulation was responsible for Zn tolerance. Cd tolerance seems to be due to neither PC production nor accumulation of organic acids.     

Cell Wall Accumulation of Cu Ions and Modulation of Lignifying Enzymes in Primary Leaves of Bean Seedlings Exposed to Excess Copper

Copper is both a nutrient and an environmental toxin that is taken up by plants. In order to determine the subcellular localization of copper and to assess the resulting metabolic changes, we exposed 14-day-old bean seedlings to nutrient solutions containing varying concentrations of Cu²⁺ ions for 3 days. Biochemical analyses revealed that the cell wall was the major site of Cu²⁺ accumulation in the leaves of treated plants. Excess copper modified the activity of lignifying peroxidases in both soluble and ionic cell wall-bound fraction. The activity of ionic GPX (guaiacol peroxidase, EC 1.11.1.7) was increased by 50 and 75 μM CuSO_4. The activities of both ionic CAPX (coniferyl alcohol peroxidase, EC 1.11.1.4) and NADH oxidase were increased by both copper concentrations tested. While soluble CAPX activity decreased in leaves treated by all copper concentrations tested, the activity of soluble NADH oxidase remained unchanged at 50 μM and was enhanced at 75 μM. Treatment with CuSO₄ also increased the abundance of total phenol compounds and induced stimulation in the activity of PAL (phenylalanine ammonia lyase, EC. 4.3.1.5). Using histochemistry in combination with fluorescence microscopy we show that bean leaves from copper-exposed plants displayed biochemical and structural modifications reinforcing the cell walls of their xylem tissues. On the other hand, the perivascular fiber sclerenchyma appeared to be less developed in treated leaves.     

Cadmium-induced oxidative damage in mustard [Brassica juncea (L.) Czern. & Coss.] plants can be alleviated by salicylic acid

The present study was carried out to examine the effects of salicylic acid (SA) on growth, activities of antioxidant enzymes and some physio-biochemical attributes in mustard [Brassica juncea (L.) Czern. & Coss.] plants exposed to cadmium (Cd) stress. Increasing concentrations of Cd led to decreased growth, shoot biomass, relative water content (RWC) and rate of photosynthesis (A). SA allayed the adverse effects of Cd on growth, RWC, and A, but the inhibitory effect of Cd on stomatal conductance (g_s) and transpiration rate (E) was further promoted due to SA treatment. Cadmium-induced oxidative stress increased proline, lipid peroxidation and electrolyte leakage, but on exposure to SA, these parameters showed a marked decrease. Lower concentrations of Cd caused enhanced Cd transport into the plant. Cadmium suppressed the uptake of macro- and micro-nutrients, but exogenous application of SA restored the capability of plants to accumulate essential elements. SA mitigated the Cd-induced inhibition in the growth of mustard plants. Cadmium-induced increase in the activities of some key antioxidant enzymes, superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), and glutathione reductase (GR) was reduced by the exogenous application of SA. This reflects that SA might have acted as one of the potential antioxidants in mustard plants under Cd stress.     

Effects of Exogenous Salicylic Acid and Nitric Oxide on Physiological Characteristics of Perennial Ryegrass Under Cadmium Stress

The effects of Cd, in combination with salicylic acid (SA) and sodium nitroprusside (SNP), on ryegrass seedlings were studied. Exposure of plants to 0.1 mM CdCl₂ for 2 weeks resulted in toxicity symptoms such as chlorosis and necrotic spots on leaves. The addition of 0.2 mM SA or 0.1 mM SNP slightly alleviated the toxic effects of Cd. After application of both SA and SNP, these symptoms significantly decreased. Treatment with Cd resulted in a decrease of dry weight of roots and shoots, chlorophyll content, net photosynthetic rate (P _n), transpiration rate (T _r), and the uptake and translocation of mineral elements. In Cd-treated plants, levels of lipoxygenase activity and malondialdehyde, hydrogen peroxide (H₂O₂), and proline contents significantly increased, whereas the activities of antioxidant enzymes, such as superoxide dismutase, guaiacol peroxidase, catalase, and ascorbate peroxidase, decreased in both roots and shoots. The results indicated that Cd caused physiological stresses in ryegrass plants. The Cd-stressed plants exposed to SA or SNP, especially to SA + SNP, exhibited improved growth compared with Cd-stressed plants. Application of SA or SNP, especially the combination SA + SNP, considerably reduced root-to-shoot translocation of Cd and increased the activities of antioxidant enzymes in both roots and shoots of Cd-stressed plants. The interaction of SA and SNP increased chlorophyll content, P _n and T _r in leaves, and the uptake and translocation of mineral elements, and decreased lipid peroxidation and H₂O₂ and proline accumulation in roots and shoots. These results suggest that SA or SNP, and, in particular, their combination counteracted the negative effects of Cd on ryegrass plants.     

Effects of cadmium on meristem activity and nucleus ploidy in roots of Pisum sativum L. cv. Frisson seedlings

The effects of cadmium (Cd) administration on primary root growth, mitotic activity of apical meristems, mitotic aberrations and percentage of nucleus ploidy classes of differentiated roots were examined in Pisum sativum L. cv. Frisson. Cadmium caused a reduction of root length related to concentration, with an almost complete block of growth in plants treated with 250 μM Cd, from 24 h of treatment. Root lengthening is generally related to apical meristem activity, however, in the examined pea plants, mitotic activity was suppressed by 2.5 and 25 μM Cd treatment, while the highest Cd concentration, 250 μM, caused the occurrence of mitotic figures consisting almost exclusively of prophases. The lack of relation between root lengthening and mitotic activity was explained by the meristematic activity in the first period of treatment and by a different cell elongation. Lower (0.25, 0.5 and 1 μM), non-blocking Cd concentrations induced a number of mitotic aberrations, mainly consisting of sticky metaphases and anaphase bridges, whose frequency increased with Cd concentration. Besides, Cd induced variations of the percentages of nucleus populations in the differentiated roots, increasing the percentage of 4C nuclei and decreasing that of 2C. The mechanisms involved in the nuclear response to Cd, and the possible relations between Cd alteration of meristem cell activity and nuclear ploidy of differentiated cells are discussed.     

外源水杨酸改善Cd胁迫下路易斯安娜鸢尾的生长和养分吸收

采用Hoagland溶液培养,研究了外源水杨酸(SA)预处理对镉(Cd)胁迫下路易斯安娜鸢尾生物量、含水量、光合作用、根系活力、Cd积累、矿质养分吸收和抗氧化酶活性的影响.结果表明： Cd胁迫下,SA预处理的植株根叶干质量、根系活力和净光合速率提高.与单独Cd处理相比,SA预处理后的胁迫植株Cd含量无明显变化,而叶片N、P、S含量显著提高,K含量显著下降,根部抗氧化酶活性显著提高.表明SA预处理可明显缓解路易斯安娜鸢尾的Cd伤害,其原因不在于减少Cd吸收量而是提高了对Cd的抗性,矿质元素的吸收积累调节和根部抗氧化酶活性的提高是其毒害缓解的重要原因.     

Exogenous treatment with salicylic acid attenuates cadmium toxicity in pea seedlings

The present study investigated the possible mediatory role of salicylic acid (SA) in protecting plants from cadmium (Cd) toxicity. The exposure of pea plants to increasing Cd concentrations (0.5, 1.0, 2.0 and 5.0 μM) during early stages of their establishment, caused a gradual decrease in shoot and root fresh weight accumulation, the rate of CO₂ fixation and the activity of ribulose-1,5-bisphosphate carboxylase (RuBPC, E.C. 4.1.1.39), the effect being most expressed at higher Cd concentrations. In vivo the excess of Cd-induced alterations in the redox cycling of oxygen-evolving centers and the assimilatory capacity of the pea leaves as revealed by changes in thermoluminescence emission after flash illumination. The levels of some important parameters associated with oxidative stress, namely lipid peroxidation, electrolyte leakage and proline production were increased. Seed pretreatment with SA alleviated the negative effect of Cd on growth, photosynthesis, carboxylation reactions, thermoluminescence characteristics and chlorophyll content, and led to decrease in oxidative injuries caused by Cd. The data suggest that the beneficial effect of SA during an earlier growth period could be related to avoidance of cumulative damage upon exposure to cadmium thus reducing the negative consequences of oxidative stress caused by heavy metal toxicity. In addition, the observed high endogenous levels of SA after treatment with Cd suggests that SA may act directly as an antioxidant to scavenge the reactive oxygen species and/or indirectly modulate redox balance through activation of antioxidant responses.Taken together these evidences could explain at some extend the protective role of SA on photochemical activity of chloroplast membranes and photosynthetic carboxylation reactions in Cd-stressed pea plants.     

Cadmium-induced oxidative damage and antioxidative defense mechanisms in <Emphasis Type="Italic">Vigna mungo</Emphasis> L.

Cadmium (Cd)-induced oxidative stress and antioxidant defense mechanisms were analyzed in roots and leaves of Vigna mungo L. Seeds were germinated in perlite-vermiculite and irrigated with Hoagland nutrient solution. At day 6, seedlings were exposed to 40 μM Cd under semi-hydroponic conditions for a period of 12 days. Growth anomalies and abnormal chromatin condensation were observed in Cd-treated plants, in comparison with control ones. Cd accumulation was observed in roots of treated plants. The analyses of antioxidative defense and oxidative parameters in roots, stems and leaves showed different tissue-specific responses. Superoxide dismutase (SOD) and guaiacol peroxidase (GPx) activities and the level of lipid peroxidation (MDA content) decreased in roots. However, they increased in leaves. Catalase activity and chlorophyll content, on the other hand, decreased over exposure to Cd stress. Total glutathione, non-protein thiols, reduced glutathione (GSH) and phytochelatins increased significantly, while oxidized glutathione (GSSG) decreased, as compared with control plants. The present data suggest that the presence of Cd in soil and water can cause oxidative damage that may be detrimental for optimum production of nutritional mung.     

Effects of root and foliar applications of exogenous NO on alleviating cadmium toxicity in lettuce seedlings

The effects of sodium nitroprusside (SNP, a donor of NO) on cadmium (Cd) toxicity in lettuce seedlings were studied. SNP was added into hydroponic systems or sprayed directly on the leaves of plants grown with and without Cd. Excess supply of Cd (100 μM) caused growth inhibition, dramatically increased Cd accumulation in both leaves and roots, and inhibited the absorption of Ca, Mg, Fe and Cu. Excess Cd also decreased activities of superoxide dismutase peroxidase and catalase in leaves and roots, and increased the accumulation of superoxide anion (O ₂ ^·? ), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA). Root or foliar applications of exogenous NO alleviated Cd-induced growth suppression, especially root application of 250 μM SNP and foliar addition of 500 μM SNP. Addition of SNP promoted the chlorophyll synthesis suggesting that the photosynthesis was up-regulated. Exogenous NO increased Cd-decreased activities of antioxidant enzymes and markedly diminished Cd-induced reactive oxygen species (ROS) and MDA accumulation. Moreover, the absorption of Ca, Mg, Fe and Cu was increased, indicating that exogenous NO stimulated H⁺-ATPase activity to promote sequestration or uptake of ions. In addition, exogenous NO also inhibited Cd transfer from roots to shoots, which may indicate that Cd retention in roots induced by NO plays a significant role in Cd tolerance in lettuce seedlings. These data suggest that under Cd stress, exogenous NO improves photosynthesis by increasing chlorophyll synthesis, protects lettuce seedlings against oxidative damage by scavenging ROS, helps to maintain the uptake of nutrient elements, and inhibits Cd transferred to shoots effectively.     

Treatment with salicylic acid decreases the effect of cadmium on photosynthesis in maize plants

The present study investigated the possible mediatory role of salicylic acid (SA) in protecting photosynthesis from cadmium (Cd) toxicity. Seeds of maize (Zea mays L., hybrid Norma) were sterilized and divided into two groups. Half of the seeds were presoaked in 500muM SA solution for only 6h, after which both groups were allowed to germinate for 3d and were then grown for 14d in Hoagland solution at 22/18 degrees C in a 16/8-h light/dark period and 120mumolm(-2)s(-1) PAR. All seedlings (without H(2)O and SA controls) were transferred to Cd-containing solutions (10, 15, and 25muM) and grown for 14d. The rate of CO(2) fixation and the activity of ribulose 1,5-bisphosphate carboxylase (RuBPC, EC 4.1.1.39) and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were measured. Changes in the levels of several important parameters associated with oxidative stress, namely H(2)O(2) and proline production, lipid peroxidation, electrolyte leakage, and the activities of antioxidative enzymes (superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), catalase (CAT, EC 1.11.1.6), and guaiacol peroxidase (POD, EC 1.11.1.7)) were measured. Exposure of the plants to Cd caused a gradual decrease in the shoot and root dry weight accumulation, with the effect being most pronounced at 25muM Cd. Seed pretreatment with SA alleviated the negative effect of Cd on plant growth parameters. The same tendency was observed for the chlorophyll level. The rate of CO(2) fixation was lower in Cd-treated plants, and the inhibition was partially overcome in SA-pretreated plants. A drop in the activities of RuBPC and PEPC was observed for Cd-treated plants. Pretreatment with SA alleviated the inhibitory effect of Cd on enzyme activity. Proline production and the rates of lipid peroxidation and electrolyte leakage increased in Cd-treated plants, whereas the values of these parameters were much lower in SA-pretreated plants. Treatment of plants with Cd decreased APX activity, but more than doubled SOD activity. Pretreatment with SA caused an increase in both APX and SOD activity, but caused a strong reduction in CAT activity. The data suggest that SA may protect cells against oxidative damage and photosynthesis against Cd toxicity.     

Effects of cadmium on activity of nitrate reductase and on other enzymes of the nitrate assimilation pathway in bean

We examined the influence of cadmium (Cd) exposure on nitrate assimilation in bean (Phaseolus vulgaris L. cv Morgane). Bean plants were submitted to either a short- (24 h) or long-term (7 d) supply of Cd in the nutrient solution. Addition of Cd decreases very significantly both the water and nitrate uptake of the treated plants when compared to untreated plants. Cadmium also induces a decrease in nitrate reductase (NR, EC 1.6.6.1) activation state after 24 h of exposure whereas, after 7 d, NR activation state was similar to that of the control bean plants. On the other hand, the level of NR protein was decreased by about 80 % after 7 d of Cd exposure and by only 15 % 1 d after Cd addition. We then investigated the in vitro effect of Cd on NR catalytic activities and inactivation by phosphorylation. The molybdenum cofactor-binding domain of NR seemed to be the most affected by Cd which did not interfere with the in vitro inactivation process of NR by MgATP. Glutamate synthase and NR activities were more inhibited by supply of Cd in the long-term experiment than the activities of nitrite reductase and glutamine synthetase. Conversely, an increase in glutamate dehydrogenase activity was observed in parallel with an increase in ammonium concentration. It thus appears that Cd treatment induces both rapid and long-term changes in the activity of the enzymes involved in nitrate assimilation, partly in response to reduced water and nutrient uptake. Moreover we have also shown that Cd itself can have a direct effect on the activities of these enzymes.