Chromium effect on ROS generation and detoxification in pea (Pisum sativum) leaf chloroplasts

Pea plants were exposed to 0, 20, 50, and 100 µM chromium [Cr(VI)] to investigate oxidative stress in isolated chloroplasts. Leaf area and biomass accumulation were significantly reduced at higher Cr supply. Generation of superoxide, hydrogen peroxide, and ·OH radical generation was enhanced in the chloroplasts isolated from Cr-exposed pea plants. Cr(VI) significantly reduced F _v/F _m ratio of chlorophyll (Chl) fluorescence, Chl content, and whole chain electron transport rate. Superoxide dismutase (SOD) activity increased at lower Cr supply while it decreased at higher Cr supply. Ascorbate peroxidase (APX) was found to be most sensitive to Cr stress. Monodehydroascorbate reductase activity remained higher at 20 and 50 µM Cr but decreased at 100 µM Cr. Increased activities of dehydroascorbate reductase (DHAR) and glutathione reductase (GR) in the isolated chloroplasts were observed during the initial 3 days of Cr exposure of pea plants. Activities of DHAR and GR were increased up to day 3 only. Ascorbate and glutathione (GSH) pools showed similar decrease that was more evident in the GSH pool as the duration of Cr treatment increased. Observed changes in reactive oxygen species concentration, photosynthetic characteristics, and antioxidant system indicate that chloroplasts in Cr-exposed pea plants are an important target of oxidative stress.     

Glaucocalyxin A and B Regulate Growth and Induce Oxidative Stress in Lettuce (Lactuca sativa L.) Roots

Glaucocalyxin (Gla) A–C are major ent-kaurane diterpenoids isolated from Isodon japonicus var. glaucocalyx (Maxim.) H. W. Li. This study investigated the possible interference of these diterpenoids with root growth and its mechanism of action in lettuce (Lactuca sativa L.) seedlings. Results indicated the dual stimulatory and inhibitory effects of Gla A and B on root growth and their phytotoxic effects on root hair development. The promotion of root growth by lower levels of Gla A and B (20–40 μM) resulted from enhanced cell length and increased mitotic activity. However, higher concentrations (80–200 μM) of Gla A and B had inhibitory effects. In addition, Gla A and B inhibited root hair development of lettuce seedlings in a dose-dependent manner at concentrations between 20 and 200 μM. Exposure of lettuce roots to Gla A and B at 200 μM increased levels of malondialdehyde and the generation of O ₂ ^·? , indicating lipid peroxidation and induction of oxidative stress. Activities of the antioxidant enzymes superoxide dismutase, catalase, and peroxidase were significantly elevated. Reactive oxygen species (ROS) scavengers dihydroxybenzene disulfonic acid (Tiron) and dimethylthiourea at 100 μM could efficiently alleviate the phytotoxicity induced by Gla A and B at 200 μM. These results demonstrated that the deleterious effect of Gla A and B at higher concentrations (80–200 μM) on roots may occur through the imposition of oxidative stress on cell growth and cell division. Due to the lack of an α,β-unsaturated ketone in α-methylenecyclopentanone moiety, Gla C could not induce ROS generation and exhibited no effect on the roots, even at the highest concentration (200 μM). Therefore, the α-methylenecyclopentanone moiety in the ent-kaurene diterpenoids was presented as an essential possible active center for the phytotoxicity.     

Aluminum inhibits root growth and induces hydrogen peroxide accumulation in Plantago algarbiensis and P. almogravensis seedlings

We have evaluated the impact of aluminum (Al) on germination, relative root growth, Al accumulation in roots tips, H₂O₂ levels, plasma membrane integrity, pigment levels, protein content, and the activities of superoxide dismutase (SOD) and catalase (CAT) in seedlings of the endangered Portuguese species Plantago algarbiensis and Plantago almogravensis. We found that up to 400 μM Al had no impact on the germination percentage in either species but inhibited root growth in a concentration-dependent manner (more severely in P. algarbiensis). Al accumulation in the root tips of both species was concentration dependent up to 200 μM but declined thereafter despite the absence of membrane damage. We observed a concentration-dependent induction of SOD activity but no change in CAT activity resulting in the accumulation of H₂O₂ (a known growth inhibitor), although its impact in P. almogravensis may be partially ameliorated by the accumulation of carotenoid pigments. Our data suggest an association between Al uptake, H₂O₂ production, and the inhibition of root growth during early seedling development in P. algarbiensis and P. almogravensis, although the latter is more tolerant towards higher concentrations of the metal.     

Copper oxide nanoparticle toxicity in mung bean (Vigna radiata L.) seedlings: physiological and molecular level responses of in vitro grown plants

In this study, the toxic effect of copper oxide nanoparticles (CuONPs) at the physiological and molecular level was investigated in mung bean (Vigna radiata L.) plants. The seedlings were grown in half strength Murashige and Skoog medium supplemented with different concentrations of CuONPs (0, 20, 50, 100, 200 and 500 mg l^?1) for 21 days under controlled growth conditions. Exposure to 200 and 500 mg l^?1 of CuONPs significantly reduced shoot length and biomass. Significant reduction in root length and biomass was observed upon exposure to all concentrations of CuONPs. Retardation of primary and lateral root growth was observed upon exposure to different concentrations of CuONPs. At 100, 200 and 500 mg l^?1 of CuONPs exposure, the total chlorophyll contents reduced significantly. Exposure to different concentrations of CuONPs has not resulted in any significant change in carotenoid contents. The proline content significantly increased upon exposure to 100, 200 and 500 mg l^?1 of CuONPs. Significant increase in hydrogen peroxide content and lipid peroxidation was observed in roots upon exposure to 20, 50, 100, 200 and 500 mg l^?1 of CuONPs. Histochemical staining with nitroblue tetrazolium and treatment with 3′-(p-hydroxyphenyl) fluorescein indicated a concentration-dependent increase in reactive oxygen species generation in roots. Exposure to CuONPs has resulted in excess lignification of roots cells as revealed by phloroglucionol-HCl staining. Gene expression analysis using real-time polymerase chain reaction showed modulations in the expression of CuZn superoxide dismutase, catalase and ascorbate peroxidase genes in roots of CuONPs exposed plants.     

Chromium (VI)-induced hormesis and genotoxicity are mediated through oxidative stress in root cells of Allium cepa L.

Chromium (VI) genotoxicity was evaluated in Allium bioassay by using different treatment protocols. Treatment of bulbs of Allium cepa L. with Cr(VI) at a range of concentrations for 5 days (120 h) exhibited low dose (12.5 μM) stimulation and high dose (25–200 μM) inhibition of root growth apparently indicating hormesis. Inhibition of root growth was correlated with the dose-dependent increase in generation of reactive oxygen species (ROS), cell death, lipid peroxidation, repression of antioxidative enzymes (catalase, superoxide dismutase, ascorbate peroxidase), induction of DNA damage, chromosome aberrations or micronuclei in root cells. The above effects were, however, reversed when the duration of Cr(VI) treatment was limited to 3–24 h followed by recovery in tap water for 4 days that resulted in the dose-dependent stimulation of root growth, mitosis and increased activity of the antioxidative enzymes that obliterated oxidative stress and genotoxicity. The above Cr(VI)-induced stimulation of root growth was effectively countered by pre- or post-treatments of dimethylthiourea, a ROS-scavenger. These findings underscored that Cr(VI), depending on the magnitude of the dose (concentration × time), could either be stimulatory or inhibitory for root growth that underlined the crucial role of ROS having obvious implications in agriculture, post harvest technology and human health.     

Quantitative real-time expression profiling of aquaporins-isoforms and growth response of Brassica juncea under arsenite stress

The present study analyzed the expression level of aquaporins of plasma membrane intrinsic protein (PIP) class in response to arsenite (AsIII) exposure of 100 μM from 0.5 h to 8 days in Brassica juncea. The expression levels of most of the PIPs were down-regulated during the course of AsIII exposure. This led to decrease in total water content of plants, which in turn hampered seedling growth. The level of reactive oxygen species (superoxide radicals and hydrogen peroxide), lipid peroxidation and root oxidizability increased significantly upon exposure to AsIII as compared to that of control leading to an increase in cell death. The study proposes that the down-regulation of PIPs happened presumably to regulate AsIII levels, which, however, occurred at the cost of reduced growth, disturbed water balance and induced oxidative stress.     

High tolerance of aluminum in the grass species Cynodon aethiopicus

Concentrations of aluminum (Al) were determined in leaves of native terrestrial plants, macrophytes and fruit parts (watermelon and tomato) using inductively coupled plasma mass spectrometry. Al concentrations in water and soil were determined by inductively coupled plasma optical emission spectrometry. Potamogeton thunbergii (macrophyte) and Cynodon aethiopicus (terrestrial grass) had the highest leaf Al concentrations (2 and 1 g kg^?1 dw, respectively). Transfer factors (mg kg^?1 dw plants/mg kg^?1 dw soil) based on total Al concentrations in soil varied from 2 × 10^?3 to 0.05 and from 1.9 to 78 based on mobile Al concentrations determined after sequential extraction. Bioconcentration factors (mg kg^?1 dw plants/mg L^?1 water) varied from 19 to 9.5 × 10³ L kg^?1 dw. Plants can accumulate high concentrations of Al when growing in neutral pH soils and slightly alkaline lakes in the Ethiopian Rift Valley. Controlled experiments showed that C. aethiopicus can accumulate high levels of Al both in root and shoot. Compared to Arabidopsis thaliana, C. aethiopicus was more tolerant to Al exposure as ≥400 μM AlCl₃ was needed to inhibit root growth compared to 200 μM in A. thaliana. After exposing C. aethiopicus and A. thaliana in 800 μM AlCl_3, alkaline comet assay indicates significant DNA (deoxyribonucleic acid) damage in A. thaliana while C. aethiopicus was unaffected. No significant induction of reactive oxygen species (ROS), in terms of leaf H₂O₂ levels, could be observed in C. aethiopicus. C. aethiopicus has mechanisms to suppress both Al-induced ROS and DNA damage, thereby increasing tolerance of the species to high Al concentrations.     

Genotypic difference in response of peroxidase and superoxide dismutase isozymes and activities to salt stress in barley

Difference in isozymes and activities of peroxidase (POD) and superoxide dismutase (SOD) in two barley (Hordeum vulgare L.) genotypes differing in salt tolerance (Gebeina, tolerant; Quzhou, sensitive) was investigated using a hydroponic experiment. The activities of both enzymes were significantly increased when the plants of the two barley genotypes were exposed to salt stress, with salt-tolerant genotype being generally higher than the sensitive one. The variation in the POD and SOD isozymes was dependent on barley genotype, salt level and exposure time. When the plants were exposed to salt stress for 10 days, two new POD isozymes were found, R _m0.26 (R _m, relative mobility of enzyme to dye) in Gebeina and R _m0.45 in Quzhou. Both isozymes disappeared after 20 days of salt stress, but R _m0.26 appeared again 30 days after the stress. Two new SOD isozymes of R _m0.19 and R _m0.46 were found in Gebeina when exposed to NaCl for 10 days, but only R _m0.46 in Quzhou. As the time of salt stress extended, more new SOD isozymes were detected, R _m0.35 in both genotypes in all different salt treatments and R _m0.48 in Gebeina under 200 mM NaCl stress. At 30 days after the stress, all the new SOD isozymes disappeared except for R _m0.48 in Gebeina under 200 mM NaCl stress. The results suggest that the increased POD and SOD activities could be partly due to the formation of some new isozymes and the tolerant variety had better ability to form new isozymes to overcome salt stress.     

Light level does not alter ethylene sensitivity in radish or pea

Ethylene accumulation occurs in many plant growth environments. In some instances, low photosynthetic photon flux (PPF) is also a stress factor. Ethylene helps regulate the shade-avoidance mechanism and synthesis rates can be altered by light. We thus hypothesized that ethylene sensitivity in whole plants may be altered in low light. Radish (Raphanus sativus) and pea (Pisum sativum) plants were selected as models due to their rapid growth, use in previous studies and difference in growth habit. We first characterized radish and pea sensitivity to ethylene. Radish vegetation was less sensitive to ethylene than pea vegetation. Pea reproductive yield was highly sensitive. Plants grown under low light levels are typically etiolated and less robust than plants grown under higher light. In a second series of studies we examined the interaction of ethylene (50 ppb pea, 200 ppb radish) with PPFs from 50 to 400 μmol m^?2 s^?1. There was no statistically significant interaction between ethylene sensitivity and PPF, indicating that high PPF does not mitigate the detrimental effects of chronic low-level ethylene exposure. This also suggests there is no crosstalk between the shade avoidance pathway and the primary ethylene signaling pathway.     

The alleviation of zinc toxicity by silicon is related to zinc transport and antioxidative reactions in rice

The objective of this study is to elucidate the roles of silicon (Si) in enhancing tolerance to excess zinc (Zn) in two contrasting rice (Oryza sativa L.) cultivars: i.e. cv. TY-167 (Zn-resistant) and cv. FYY-326 (Zn-sensitive). Root morphology, antioxidant defense reactions and lipid peroxidation, and histochemical staining were examined in rice plants grown in the nutrient solutions with normal (0.15 μM) and high (2 mM) Zn supply, without or with 1.5 mM Si. Significant inhibitory effects of high Zn treatment on plant growth were observed. Total root length (TRL), total root surface area (TRSA) and total root tip amount (TRTA) of both cultivars were decreased significantly in plants treated with high Zn, whereas these root parameters were significantly increased when Zn-stressed plants were supplied with 1.5 mM Si. Supply of Si also significantly decreased Zn concentration in shoots of both cultivars, indicating lower root-to-shoot translocation of Zn. Moreover, superoxide dismutase (SOD), catalase (CAT), and asorbate peroxidase (APX) activities were increased, whereas malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) concentrations were decreased in Si-supplied plants of both Zn-sensitive and Zn-resistant rice cultivars exposed to Zn stress. These alleviative effects of Si, further confirmed by the histochemical staining methods, were more prominent in the Zn-resistant cultivar than in the Zn-sensitive one. Taken together, all these results suggest that Si-mediated alleviation of Zn toxicity is mainly attributed to Si-mediated antioxidant defense capacity and membrane integrity. The possible role of Si in reduction of root-to-shoot translocation of Zn can also be considered.     

Effects of Arsenic on Growth, Photosynthetic Activity, and Accumulation in Two New Hyperaccumulating Populations of Isatis cappadocica Desv.

The effect of arsenic on leaf photosynthetic rate, growth responses, and accumulation capability of Isatis cappadocica Desv., a Brassica collected from Iranian arsenic-contaminated mine spoils and control populations, was investigated. Both populations of I. cappadocica were considerably more tolerant than the reference Brassica species (Descurainia sophia). The 1,000 μM arsenate exposure inhibited root growth completely in D. sophia, but only by 50 and 40 % in the nonmine and mine populations of I. cappadocica, respectively. Furthermore, the chlorophyll contents of both populations of I. cappadocica were not statistically different, especially when plants were exposed to 5–800 μM As. The chlorophyll a fluorescence kinetics (F _v/F _m) and electron transfer rate values of treated I. cappadocica populations remained unaffected, indicating normal photosynthetic efficiency and strength of plants in the presence of arsenic. After 28 days of exposure to 1,300 μM As, shoot arsenic concentrations of mine and nonmine populations reached 310 and 345 mg kg^?1, respectively, with the arsenic transfer factor and bioaccumulation greater than 1.0. According to these results, it was shown that I. cappadocica had strong tolerance to and the capability to hyperaccumulate arsenic; therefore, it is a potential As hyperaccumulator.     

Chromium ions inactivate electron transport and enhance superoxide generation in vivo in pea (Pisum sativum L. cv. Azad) root mitochondria

The effect in vivo of hexavalent chromium (Cr⁶⁺) on the respiratory electron transport activity and production of superoxide (O₂^–) radicals, was studied in submitochondrial particles (SMPs) prepared from mitochondria isolated from roots of 15‐day‐old pea (Pisum sativum L. cv. Azad) plants exposed to environmentally relevant (20 µm ) and acute (200 µm ) concentrations of chromium for 7 d. A concentration ‐dependent inactivation of electron transport activity from both NADH to O₂ (NADH oxidase) and succinate to O₂ (succinate oxidase) was observed. The electron transport activity was more sensitive to Cr⁶⁺ with NADH as the substrate than with succinate as the substrate. Although NADH dehydrogenase and succinate dehydrogenase were less affected, NADH: cytochrome c oxidoreductase and succinate: cytochrome c oxidoreductase activities were prominently affected by Cr⁶⁺. Cytochrome oxidase was the most susceptible complex of mitochondrial membranes to Cr⁶⁺, exhibiting maximal inactivation of activity both at 20 and 200 µm chromium concentrations. Cr⁶⁺ increased the generation of O₂^– radicals. This effect was more evident at 200 than at 20 µm . A significant increase in lipid peroxidation of mitochondrial membranes at 200 µm Cr⁶⁺ was the physiological impact of the metal‐induced enhanced generation of O₂^– radicals. An increase in superoxide dismutase (SOD) activity at 20 µm Cr⁶⁺ towards enhanced production of O₂^– radicals appeared to be a defence response in pea root mitochondria that, however, could not be sustained at 200 µm Cr⁶⁺. The results obtained concerning inactivation of mitochondrial electron transport and subsequent enhancement in the generation of O₂^– radicals suggest that root mitochondria are an important target of Cr⁶⁺‐induced oxidative stress in pea.     

N-acetyl-cysteine alleviates Cd toxicity and reduces Cd uptake in the two barley genotypes differing in Cd tolerance

A hydroponic experiment was carried out to study the physiological mechanisms of N-acetyl cysteine (NAC) in mitigating cadmium (Cd) toxicity in two barley (Hordeum vulgare L.) genotypes, Dong 17 (Cd-sensitive) and Weisuobuzhi (Cd-tolerant). Addition of 200 μM NAC to a culture medium containing 5 μM Cd (Cd + NAC) markedly alleviated Cd-induced growth inhibition and toxicity, maintained root cell viability, and dramatically depressed O ₂ ^·? and ·OH, and malondialdehyde accumulation, significantly reduced Cd concentration in leaves and roots, especially in the sensitive genotype Dong 17. External NAC counteracted Cd-induced alterations of certain antioxidant enzymes, e.g., brought root superoxide dismutase and glutathione reductase, leaf/root peroxidase and glutathione peroxidase activities of the both genotypes down towards the control level, but elevated Cd-stress-depressed leaf catalase in Dong 17 and root ascorbate peroxidase activities in both genotypes. NAC counteracted Cd-induced alterations in amino acids and microelement contents. Furthermore, NAC significantly reduced Cd-induced damage to leaf/root ultrastructure, e.g. the shape of chloroplasts in plants treated with Cd + NAC was relatively normal with well-structured thylakoid membranes and parallel pattern of lamellae but less osmiophilic plastoglobuli compared with Cd alone treatment; nuclei of root cells were better formed and chromatin distributed more uniformly in both genotypes. These results suggested that under Cd stress, NAC may protects barley seedlings against Cd-induced damage by directly and indirectly scavenging reactive oxygen species and by maintaining stability and integrity of the subcellular structure.     

Morphological changes in the apex of pea roots during and after recovery from aluminium treatment

We investigated how the pea (Pisum sativum cv. Harunoka) root, upon return to an Al-free condition, recovers from injury caused by exposure to Al. The growing region of the root during and after treatment with Al was examined by marking the root at intervals with India ink. Al-induced cell death was detected by staining with Evans blue. Root growth in 40 μM Al solution relative to that in Al-free solution (RRG) was approximately 45% from 6 h to12 h after the start of the treatment. However, values of RRG from 12 h to 24 h in Al-free solution for recovery or in the same Al solution were about 75% and 35%, respectively, indicating recovery from Al-induced growth inhibition. Images of the root characterized by zonal staining with Evans blue were observed in the sub-apical region (more than 1 mm from the tip) in Al-stressed roots. However, the interval of the stained zone was widened in the root after recovery from Al-induced growth inhibition, though it was narrower and more densely stained with time in the Al-stressed roots. During the recovery, the root apex may resume elongation in a specified region without Al-induced death or injury in cells detected by Evans blue.     

Transmembrane potential measurements on plant cells using the voltage-sensitive dye ANNINE-6

The charging of the plasma membrane is a necessary condition for the generation of an electric-field-induced permeability increase of the plasmalemma, which is usually explained by the creation and the growth of aqueous pores. For cells suspended in physiological buffers, the time domain of membrane charging is in the submicrosecond range. Systematic measurements using Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) protoplasts stained with the fast voltage-sensitive fluorescence dye ANNINE-6 have been performed using a pulsed laser fluorescence microscopy setup with a time resolution of 5 ns. A clear saturation of the membrane voltage could be measured, caused by a strong membrane permeability increase, commonly explained by enhanced pore formation, which prevents further membrane charging by external electric field exposure. The field strength dependence of the protoplast’s transmembrane potential V _M shows strong asymmetric saturation characteristics due to the high resting potential of the plants plasmalemma. At the pole of the hyperpolarized hemisphere of the cell, saturation starts at an external field strength of 0.3 kV/cm, resulting in a measured transmembrane voltage shift of ?V _M?=??150 mV, while on the cathodic (depolarized) cell pole, the threshold for enhanced pore formation is reached at a field strength of approximately 1.0 kV/cm and ?V _M?=?450 mV, respectively. From this asymmetry of the measured maximum membrane voltage shifts, the resting potential of BY-2 protoplasts at the given experimental conditions can be determined to V _R?=??150 mV. Consequently, a strong membrane permeability increase occurs when the membrane voltage diverges |V _M|?=?300 mV from the resting potential of the protoplast. The largest membrane voltage change at a given external electric field occurs at the cell poles. The azimuthal dependence of the transmembrane potential, measured in angular intervals of 10° along the circumference of the cell, shows a flattening and a slight decrease at higher fields at the pole region due to enhanced pore formation. Additionally, at the hyperpolarized cell pole, a polarization reversal could be observed at an external field range around 1.0 kV/cm. This behavior might be attributed to a fast charge transfer through the membrane at the hyperpolarized pole, e.g., by voltage-gated channels.     

Modulation of osmotic adjustment and antioxidant status in salt-stressed leaves of Thermopsis turcica

Thermopsis turcica is distributed naturally in saline soils. Interestingly, how T. turcica can live in harsh salt conditions is unknown. To study its defense responses under salinity, T. turcica was grown in a medium containing 100 and 200 mM NaCl for 7 and 14 days. Physiological parameters, ion contents, reactive oxygen species accumulation, activities of antioxidant enzymes/isozymes, NADPH oxidase enzyme/isozyme, lipid peroxidation (TBARS) and osmolyte contents were investigated. Stress caused a rapid decline in relative growth rate, relative water content and chlorophyll fluorescence (F _v/F _m) under both NaCl treatments. These traits were more suppressed at 200 mM NaCl. The decline in osmotic potential (Ψ _Π) with salinity increased the gradient for water flux into the cell and assisted in turgor maintenance. The increased membrane permeability under stress caused the entrance of excess Na⁺ and K⁺ into the cell. Stress decreased superoxide dismutase, catalase and peroxidase after 14 days of growth in 200 mM NaCl, whereas glutathione reductase (GR) increased throughout the experiment. While ascorbate peroxidase (APX) increased by 44 % at 7 days, it decreased after 14 days exposure to 200 mM NaCl. 200 mM NaCl caused the highest increase in TBARS at 14 days, indicating a decrease in OH^· scavenging activity. Increasing concentrations of salinity caused an increase in glycine betaine (GB) and choline (Cho), though an increase in proline was only observed at 200 mM NaCl for 14 days. Briefly, H₂O₂ was more efficiently eliminated in 100 mM-treated plants by the ascorbate–glutathione cycle in which APX acts a strong catalyst together with GR. Also, Cho and GB help to maintain osmotic adjustment and cytoplasmic function.     

Photosynthetic CO<Subscript>2</Subscript> uptake in seedlings of two tropical tree species exposed to oscillating elevated concentrations of CO<Subscript>2</Subscript>

Do short-term fluctuations in CO₂ concentrations at elevated CO₂ levels affect net CO₂ uptake rates of plants? When exposed to 600 μl CO₂ l^?1, net CO₂ uptake rates in shoots or leaves of seedlings of two tropical C₃ tree species, teak (Tectona grandis L. f.) and barrigon [Pseudobombax septenatum (Jacq.) Dug.], increased by 28 and 52% respectively. In the presence of oscillations with half-cycles of 20 s, amplitude of ca. 170 μl CO₂ l^?1 and mean of 600 μl CO₂ l^?1, the stimulation in net CO₂ uptake by the two species was reduced to 19 and 36%, respectively, i.e. the CO₂ stimulation in photosynthesis associated with a change in exposure from 370 to 600 μl CO₂ l^?1 was reduced by a third in both species. Similar reductions in CO₂-stimulated net CO₂ uptake were observed in T. grandis exposed to 40-s oscillations. Rates of CO₂ efflux in the dark by whole shoots of T. grandis decreased by 4.8% upon exposure of plants grown at 370 μl CO₂ l^?1 to 600 μl CO₂ l^?1. The potential implications of the observations on CO₂ oscillations and dark respiration are discussed in the context of free-air CO₂ enrichment (FACE) systems in which short-term fluctuations of CO₂ concentration are a common feature.     

Nickel-induced Inhibition of Wheat Root Growth is Related to H2O2 Production, but not to Lipid Peroxidation

Effects of exogenous nickel (Ni: 10 and 200 μM) on growth, mitotic activity, Ni accumulation, H₂O₂ content and lipid peroxidation as well as the activities of various antioxidative enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GSH-Px) were investigated in wheat roots. A considerable Ni accumulation in the roots occurred at both the concentrations. Although Ni at 10 μM did not have any significant effect on root growth, it strongly inhibited the root growth at 200 μM. Mitotic activity in the root tips was not significantly affected by exposure of the seedlings to 10 μM Ni; however, it was almost completely inhibited at 200 μM treatment. Ni stress did not result in any significant changes in CAT and APX activities as well as lipid peroxidation. However, H₂O₂ concentration increased up to 82% over the control in the roots of seedlings exposed to 200 μM Ni. There was a significant decline in both SOD (50%) and GSH-Px (20–30%) activities in the roots when the seedlings were treated with 200 μM Ni. The results indicated that a strong inhibition of wheat root growth caused by Ni stress was not due to enhanced lipid peroxidation, but might be related to the accumulation of H₂O₂ in root tissue.     

Antioxidant system activation by mercury in Pfaffia glomerata plantlets

Oxidative stress caused by mercury (Hg) was investigated in Pfaffia glomerata plantlets grown in nutrient solution using sand as substrate. Thirty-day-old acclimated plants were treated for 9 days with four Hg levels (0, 1, 25 and 50 μM) in the substrate. Parameters such as growth, tissue Hg concentration, toxicity indicators (δ-aminolevulinic acid dehidratase, δ-ALA-D, activity), oxidative damage markers (TBARS, lipid peroxidation, and H₂O₂ concentration) and enzymatic (superoxide dismutase, SOD, catalase, CAT, and ascorbate peroxidase, APX) and non-enzymatic (non-protein thiols, NPSH, ascorbic acid, AsA, and proline concentration) antioxidants were investigated. Tissue Hg concentration increased with Hg levels. Root and shoot fresh weight and δ-ALA-D activity were significantly decreased at 50 μM Hg, and chlorophyll and carotenoid concentration were not affected. Shoot H₂O₂ concentration increased curvilinearly with Hg levels, whereas lipid peroxidation increased at 25 and 50 μM Hg, respectively, in roots and shoots. SOD activity showed a straight correlation with H₂O₂ concentration, whereas CAT activity increased only in shoots at 1 and 50 μM Hg. Shoot APX activity was either decreased at 1 μM Hg or increased at 50 μM Hg. Conversely, root APX activity was only increased at 1 μM Hg. In general, AsA, NPSH and proline concentrations increased upon addition of Hg, with the exception of proline in roots, which decreased. These changes in enzymatic and non-enzymatic antioxidants had a significant protective effect on P. glomerata plantlets under mild Hg-stressed conditions.