Physiological and biochemical mechanisms of silicon-induced copper stress tolerance in cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.)

Accumulation of excess copper (Cu) in agricultural soils can decrease growth and quality of crops grown on these soils and a little information is available on the role of silicon (Si) in reducing Cu toxicity in plants. A hydroponic study was conducted to investigate the effects of Si (1.0 mM) on growth and physiology of cotton seedlings grown on different Cu (0, 25, and 50 µM) concentrations. Elevated levels of Cu decreased growth, biomass, photosynthetic pigments, and gas exchange characteristics, and increased the electrolyte leakage (EL), hydrogen peroxide (H₂O₂), and thiobarbituric acid reactive substances (TBARS) contents in leaf, stem, and roots of cotton seedlings. Cu stress alone decreased the activities of key antioxidant enzymes in cotton seedlings. Exogenous application of Si alleviated the toxic effects of Cu on cotton seedlings by improving growth, photosynthetic pigments, and gas exchange characteristics under Cu stress. The Si application decreased Cu concentrations in leaves, stem, and roots as compared with the control plants. Furthermore, Si decreased oxidative stress as evidenced by decreased EL, H₂O₂, and TBARS contents, and increased the antioxidant enzyme activities in cotton seedlings. This study provides evidences of Si-mediated reduction of Cu toxicity in cotton seedlings at physiological and biochemical levels.     

Boron supply alleviates Al-induced inhibition of root elongation and physiological characteristics in rapeseed (Brassica napus L.)

Aluminum (Al) toxicity is one of the major problems affecting crop production. Boron (B) is an essential micronutrient for higher plants. In the present study, we investigated the alleviation of Al-induced inhibition of root growth and physiological characteristics by B in rapeseed. The rapeseeds were grown in different Al concentrations (0 and 300?μM), and for every concentration, two B treatments (2.5 and 25?µM as H₃BO₃) were applied. The results showed that Al toxicity under low B drastically inhibited root growth. The supply of B improved root length, photosynthesis, root activity, total chlorophyll by 60.15%, 104.7%, 102%, and 106.3%, respectively under Al toxicity. This further resulted in improvement of peroxidase, catalase, and ascorbate peroxidase activities while decreasing malondialdehyde, H₂O_2, and Al contents in roots and leaves. It might be supposed that B alleviates Al toxicity by less mobilization of Al in plant parts and through improving antioxidant enzyme activities.     

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.     

Recovery of cowpea seedling roots from exposure to toxic concentrations of trace metals

Rhizotoxic effects of many trace metals are known, but there is little information on recovery after exposure. Roots of 3-d-old cowpea (Vigna unguiculata (L.) Walp. cv. Caloona) seedlings were grown for 4 or 12 h in solutions of 960 μM Ca and 5 μM B at two concentrations (which reduce growth by 50 or 85%) of nine trace metals that rupture the outer layers of roots. Measured concentrations were 34 or 160 μM Al, 0.6 or 1.6 μM Cu, 2.2 or 8.5 μM ?Ga, 2.3 or 12 μM Gd, 0.8 or 1.9 μM Hg, 1.0 or 26 μM In, 2.4 or 7.3 μM La, 1.8 or 3.8 μM Ru, and 1.3 or 8.6 μM Sc. Roots were rinsed, transferred to solutions free of trace metals, and regrowth monitored for up to 48 h. Recovery from exposure to Hg occurred within 4 h, but regrowth was delayed for ≥?12 h with Al, Ga, or Ru. There was poor regrowth after 4 or 12 h exposure to Cu, Gd, In, La, or Sc. Roots recovered after being grown for 12 to 48 h in 170 μM Al, 5.1 μM? Ga, 2.0 μM Hg, or 1.4 μM Ru, but the extent of recovery was reduced with longer exposure time. Microscopy showed marked differences in symptoms on roots recovering from exposure to the various trace metals. Differences in (i) concentrations that are toxic, (ii) ability of roots to recover, (iii) time for recovery to occur, and (iv) symptoms that develop, suggest that each trace metal has a unique combination of rhizotoxic effects.     

Jasmonates modulate the promotion effects induced by SNP on root development of wheat under osmotic stress through lipoxygenase activation

We investigated promotion effects of exogenous sodium nitroprusside (SNP) on wheat seedling (Triticum aestivum L.) lateral root (LR) and root hair development, and the relationship between endogenous jasmonate (JA) production and activity changes of lipoxygenase (LOX) isoenzymes under osmotic stress generated by 15 % PEG-6000. Our results showed that 25 or 50 μM SNP could significantly increase LR length and number whether or not the seedlings were under PEG stress. When 50 μM cPTIO, 50 μM SHAM or 50 μM NDGA was supplemented, the promotion effects of SNP were blocked. SNP could also induce the production of endogenous JAs in roots, and 25 μM SNP induced the maximum JA content. The effect of SNP on JA production could also be blocked by adding cPTIO, SHAM or NDGA. Furthermore, the activity of lipoxygenase (LOX) in roots was affected by SNP; the maximal activity of LOX also occurred in the roots treated by 25 μM SNP under PEG stress, or 50 μM SNP without PEG stress. LOX isoenzymes in roots were detected by electrophoresis; the results showed that 25 μM SNP could noticeably increase the activities of LOXII and LOXIII under PEG stress. Our results suggest that, under osmotic stress generated by PEG, the promotion effects of exogenous SNP on wheat LR and root hair development could be mediated by endogenous JAs through LOX activation.     

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.     

5-Aminolevulinic Acid Ameliorates the Growth, Photosynthetic Gas Exchange Capacity, and Ultrastructural Changes Under Cadmium Stress in Brassica napus L.

Heavy-metal toxicity in soil is one of the major constraints for oilseed rape (Brassica napus L.) production. One of the best ways to overcome this constraint is the use of growth regulators to induce plant tolerance. Response to cadmium (Cd) toxicity in combination with a growth regulator, 5-aminolevulinic acid (ALA), was investigated in oilseed rape grown hydroponically in greenhouse conditions under three levels of Cd (0, 100, and 500 μM) and three levels of foliar application of ALA (0, 12.5, and 25 mg l^?1). Cd decreased plant growth and the chlorophyll concentration in leaves. Foliar application of ALA improved plant growth and increased the chlorophyll concentration in the leaves of Cd-stressed plants. Significant reductions in photosynthetic parameters were observed by the addition of Cd alone. Application of ALA improved the net photosynthetic and gas exchange capacity of plants under Cd stress. ALA also reduced the Cd content in shoots and roots, which was elevated by high concentrations of Cd. The microscopic studies of leaf mesophyll cells under different Cd and ALA concentrations showed that foliar application of ALA significantly ameliorated the Cd effect and improved the structure of leaf mesophyll cells. However, the higher Cd concentration (500 μM) could totally damage leaf structure, and at this level the nucleus and intercellular spaces were not established as well; the cell membrane and cell wall were fused to each other. Chloroplasts were totally damaged and contained starch grains. However, foliar application of ALA improved cell structure under Cd stress and the visible cell structure had a nucleus, cell wall, and cell membrane. These results suggest that under 15-day Cd-induced stress, application of ALA helped improve plant growth, chlorophyll content, photosynthetic gas exchange capacity, and ultrastructural changes in leaf mesophyll cells of the rape plant.     

Significant contribution of boron stored in seeds to initial growth of rice seedlings

Rice plants are relatively insensitive to low boron environment. We examined the role of boron stored in seeds in growth of rice seedlings in regard to its insensitivity to low boron condition. In experiments using seeds without the removal of the endosperm, the effect of low boron treatments (0.18 and 0.03 μM B) was not evident on growth of seedlings until 5 weeks from germination. Analysis of boron distribution among different organs in rice grain demonstrated that the endosperm contained 92% of total boron in brown rice and husks contained twice as much boron as the brown rice. To examine the role of boron stored in the endosperm, endosperm were removed 5 days after de-husking and imbibition and the plants were subjected to boron treatments. The growth of the plants without endosperm was much reduced after 10 days under the 0.03 μM boron compared with the plants grown with the normal level (18 μM B). With the normal level of boron supply, the growth did not differ between the plants with and without endosperm, suggesting that growth difference between seedlings with and without endosperm under low B condition is not due to lack of starch or proteins, but due to lack of boron stored in seeds. Tracer experiments confirmed that a substantial amount of B was supplied from the seed to the seedling and that B limitation increased the B supply from seeds to seedlings, especially to roots. Our results established that boron stored in seeds, principally in the endosperm, contributes significantly to the growth of rice seedlings when external boron supply is limited.     

Differential responses of antioxidant enzymes to aluminum toxicity in two rice (Oryza sativa L.) cultivars with marked presence and elevated activity of Fe SOD and enhanced activities of Mn SOD and catalase in aluminum tolerant cultivar

Seedlings of two Indica rice (Oryza sativa L.) cvs. HUR-105 and Vandana, differing in Al-tolerance were used to identify the key mechanisms involved in their differential behaviour towards Al toxicity. Cv. HUR-105 appeared to be Al sensitive by showing significant reduction (p ≤ 0.01) in root/shoot length, fresh weight, dry weight and water content in presence of 421 μM Al³⁺ in growth medium whereas cv. Vandana appeared to be fairly Al³⁺ tolerant. A conspicuous and significant reduction in dry weight of root and shoot was observed in Al sensitive cv. HUR-105 with 178 μM Al³⁺ treatment for 3 days. Al was readily taken up by the roots and transported to shoots in both the rice cultivars. Localization of absorbed Al was always greater in roots than in shoots. Our results of the production of reactive oxygen species (ROS) H₂O₂ and O₂ ^.? and activities of major antioxidant enzymes such as total superoxide dismutase (SOD), Cu/Zn SOD, Mn SOD, Fe SOD, catalase (CAT) and guaiacol peroxidase revealed Al induced higher oxidative stress, greater production of ROS and lesser capacity to scavenge ROS in cv. HUR-105 than Vandana. With Al treatment, higher oxidative stress was noted in shoots than in roots. Greatly enhanced activities of SOD (especially Fe and Mn SOD) and CAT in Al treated seedlings of cv. Vandana suggest the role of these enzymes in Al tolerance. Furthermore, a marked presence of Fe SOD in roots and shoots of the seedlings of Al tolerant cv. Vandana and its significant (p ≤ 0.01) increase in activity due to Al-treatment, appears to be the unique feature of this cultivar and indicates a vital role of Fe SOD in Al-tolerance in rice.     

Effect of Coronatine on Antioxidant Enzyme Response of Chickpea Roots to Combination of PEG-Induced Osmotic Stress and Heat Stress

Abiotic stresses, such as high temperature and drought, are major limiting factors of crop production and growth. Coronatine (COR), a structural and functional analog of jasmonates, is suggested to have a role in abiotic stress tolerance. The aim of our study was to examine whether pretreatment with COR enhances the tolerance of chickpea (Cicer arietinum L. cv ICC 4958) roots to PEG-induced osmotic stress, heat stress, and their combination. Therefore, seedlings raised hydroponically in a growth chamber for 15 days were pretreated with or without COR at 0.01 μM for 24 h and then exposed to 6 % PEG 6000-induced osmotic stress or heat (starting at 35 °C and then gradually increased 1 °C every 15 min and kept at 44 °C for 1 h) stress for 3 days. After different treatment periods, the changes in relative growth rate (RGR); malondialdehyde (MDA), proline (Pro), and hydrogen peroxide (H₂O₂) contents; and the activities of antioxidant enzymes/isoenzymes in roots of chickpea seedlings with or without 0.01 μM COR application were studied. RGR in roots was increased by COR application. Under all stress conditions, H₂O₂, MDA, and Pro levels increased sharply, but pretreatment with COR significantly reduced them. Moreover, COR increased the activities of H₂O₂ scavenger enzymes such as catalase (CAT) under heat stress, ascorbate peroxidase (POX) under PEG stress, and CAT and POX under combined stresses. Therefore, COR might alleviate adverse effects of PEG stress and heat stress and combined stresses on roots of chickpea by reduction of H₂O₂ production, enhancing or keeping the existent activity of antioxidant enzymes, thereby preventing membrane peroxidation.     

Boron-aluminum interactions affect organic acid metabolism more in leaves than in roots of <Emphasis Type="Italic">Citrus grandis</Emphasis> seedlings

Sour pummelo (Citrus grandis) seedlings were irrigated with nutrient solution containing four boron concentrations (i.e., 2.5, 10, 25 and 50 μM H₃BO₃) and two aluminum concentrations [i.e., 0 (-Al) and 1.2 mM AlCl₃ · 6 H₂O (+Al)]. It was found that B did not affect, but Al increased, the Al content in the roots. The Al and citrate contents in the -Al leaves either did not change or slightly increased with increasing B concentration. On the other hand, the Al and citrate contents in the +Al leaves rapidly decreased as B concentration increased from 2.5 to 50 μM, then decreased at the highest B concentration. The Al and citrate contents were higher in the +Al than in the -Al leaves, except for at 25 μM B when they were similar. The leaf malate content did not change in response to B or Al, except for an increase in the +Al leaves and a decrease in the -Al leaves at 2.5 μM B. Similarly, the root malate and citrate contents did not change in response to B with or without Al, except for a decrease in the malate and citrate contents in the +Al roots at 50 μM B and an increase in the citrate content in the -Al roots at 50 μM B. The activities of acid-metabolizing enzymes were less affected by B-Al interactions in the roots than in the leaves.     

Effects of aluminum on superoxide dismutase and peroxidase activities,and lipid peroxidation in the roots and calluses of soybeans differing in aluminum tolerance

The seedlings of two soybean genotypes, Al-tolerant PI 416937 (PI) and Al-sensitive Young, were cultured in the solution containing 0, 25 or 50 μM Al (AlCl₃·6H₂O) for 24, 36 or 48 h in the hydroponics, and the calluses induced from two genotypes were cultured in medium containing 0, 10, 50 or 100 μM Al for 5, 10 or 15 days, respectively. The effects of Al on growth of seedling roots and calluses, antioxidant enzyme activities of superoxide dismutase (SOD) and peroxidase (POD) and lipid peroxidation were investigated. Under Al stress, PI was more tolerant to Al toxicity than Young at both intact plant and tissue levels and lower concentrations of Al significantly stimulated the root and callus growth of PI. Al application enhanced the activities of SOD and POD and lipid peroxidation in both roots and calluses of two genotypes. Although the differences of SOD activities between two genotypes in response to Al toxicity depended on Al concentration and durations of treatment, SOD activities in the roots of PI were higher than those in the roots of corresponding Young in the presence of Al for 36 or 48 h. Meanwhile, the POD activities in PI roots increased as the Al levels and durations of treatment increased, significantly higher than those in the corresponding Young roots. Moreover, Al-treated PI had significantly lower lipid peroxidation than Young at both root and callus levels. These results suggest that the enhanced antioxidant-related enzyme activities and reduced lipid peroxidation in PI might be one of Al-tolerant mechanisms.     

Boron alleviates aluminum toxicity in pea (Pisum sativum)

One important target of boron (B) deficiency and aluminum (Al) toxicity is cell wall. Thus we studied the hypothesis that B is capable of alleviating Al toxicity in pea (Pisum sativum). Short-term and prolonged Al exposure to pea roots at different B levels was carried out on uniform seedlings pre-cultured at a low B level. When seedlings with a low B level were supplied with or without B for 1 and 2 days before 24 h Al exposure, roots were longer while root diameter was thinner after B addition especially for 2 days even with exposure to Al; root elongation was inhibited while root diameter was enlarged by Al exposure. Callose induction by Al toxicity was higher with B added, but this was reversed after the removal of the cotyledons. Hematoxylin staining was lighter in the root tips given B, and Al content in the root tips and cell walls dropped after exposure to B. This indicates that B alleviated Al toxicity in the root tips during short-term Al exposure by decreasing Al binding in root cell walls. An increase in chlorophyll and biomass and reduced chlorosis were found at the higher level of B during prolonged Al treatment, which was coincided with the decreased Al contents, indicating that B alleviated Al toxicity to shoots. B supplementation alleviates some of the consequences of Al toxicity by limiting some Al binding in cell walls, resulting in less injury to the roots as well as less injury to the shoots.     

Does the degree of pectin esterification influence aluminium sorption by the root apoplast?

This study investigates the influence of the degree of pectin esterification (DE) on the sorption of aluminium (Al) by plant roots. Ca-pectates, with varying degrees of esterification, are major constituents of the soil–root interface and of the root apoplast. Ca-pectate networks (Ca–PG and Ca–Al–PG) were formed at three DEs (0%, 26%, 65%) with custom-made cells and used as a model system for the root cell wall. Sorption of Al was conducted for 24 h at a range of oxalic acid concentrations (0–500 μM) at pH 4.50 to examine two different metal resistance mechanisms of plants. In fact, plants release organic acids either to desorb or to complex metals to prevent their sorption by plant roots.Thermal analysis showed that Al sorption did not seem to affect the stability of the pectate gels and the presence of hydrophobic groups (–CH₃) at DE?>?0% seemed to even increase the stability of the gels decreasing thermal decomposition. Results suggest two potential Al tolerance mechanisms: (a) high oxalic acid concentrations (500 μM) were able to desorb almost 100% and 72% at DE 65 and 0%, respectively; (b) high oxalic acid concentrations (500 μM) and thus molar ratios of 5:1 (oxalate/Al) reduced Al sorption by 98% and 86% at DE 65 and 0%, respectively. In conclusion, both mechanisms indicate that high degrees of esterification as 65% are much more efficient in excluding Al from the apoplast and might therefore contribute to Al resistance in plants.     

Proteome analysis of roots of wheat seedlings under aluminum stress

The root apex is considered the first sites of aluminum (Al) toxicity and the reduction in root biomass leads to poor uptake of water and nutrients. Aluminum is considered the most limiting factor for plant productivity in acidic soils. Aluminum is a light metal that makes up 7 % of the earth’s scab dissolving ionic forms. The inhibition of root growth is recognized as the primary effect of Al toxicity. Seeds of wheat cv. Keumkang were germinated on petridish for 5 days and then transferred hydroponic apparatus which was treated without or with 100 and 150 μM AlCl₃ for 5 days. The length of roots, shoots and fresh weight of wheat seedlings were decreased under aluminum stress. The concentration of K⁺, Mg²⁺ and Ca²⁺ were decreased, whereas Al³⁺ and P₂O₅ ^? concentration was increased under aluminum stress. Using confocal microscopy, the fluorescence intensity of aluminum increased with morin staining. A proteome analysis was performed to identify proteins, which are responsible to aluminum stress in wheat roots. Proteins were extracted from roots and separated by 2-DE. A total of 47 protein spots were changed under Al stress. Nineteen proteins were significantly increased such as sadenosylmethionine, oxalate oxidase, malate dehydrogenase, cysteine synthase, ascorbate peroxidase and/or, 28 protein spots were significantly decreased such as heat shock protein 70, O-methytransferase 4, enolase, and amylogenin. Our results highlight the importance and identification of stress and defense responsive proteins with morphological and physiological state under Al stress.     

Effects of Hydrogen Sulfide on Growth,Antioxidative Capacity,and Ultrastructural Changes in Oilseed Rape Seedlings Under Aluminum Toxicity

The present study evaluates the beneficial effects of the hydrogen sulfide (H₂S) donor, sodium hydrosulfide (0 and 0.3 mM), on the growth of oilseed rape (Brassica napus L. cv. ZS 758) seedlings under aluminum (Al) stress (0, 0.1, and 0.3 mM). Results showed that Al stress decreased the seedling growth by reducing the shoot and root length, biomass, and antioxidant enzymes, which could be illustrated by increased levels of malondialdehyde (MDA), production of hydrogen peroxide (H₂O₂), and accumulation of Al in the shoots. Pretreatment with H₂S reduced MDA and H₂O₂ levels in the leaves and roots of B. napus seedlings. Moreover, activities of antioxidant enzymes (APX, CAT, APX, SOD, POD, and GR) were elevated significantly with the application of H₂S under Al stress. The microscopic examination confirmed that higher levels of Al completely impaired leaf mesophyll and root tip cells. Chloroplasts were spongy shaped with dissolved thylakoid membranes and more starch grains. Root tip cells showed visible symptoms under Al toxicity such as deposition of Al in vacuoles and disruption of whole cell organelles. Under pretreatment with exogenous H₂S, cell structures were improved and presented a clean mesophyll cell and chloroplast possessing well-developed thylakoid membranes as well as fewer starch grains. A number of modifications could be observed in root tip cells, that is, mature mitochondria, long endoplasmic reticulum as well as golgi bodies, under the combined application of H₂S and Al. On the basis of our results, we can conclude that H₂S has a promotive effect which could improve plant survival under Al stress.     

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.     

Aluminum resistance in wheat involves maintenance of leaf Ca<Superscript>2+</Superscript> and Mg<Superscript>2+</Superscript> content,decreased lipid peroxidation and Al accumulation,and low photosystem II excitation pressure

The phytotoxic aluminum species (Al³⁺) is considered as the primary factor limiting crop productivity in over 40 % of world’s arable land that is acidic. We evaluated the responses of two wheat cultivars (Triticum aestivum L.) with differential Al resistance, cv. Yecora E (Al-resistant) and cv. Dio (Al-sensitive), exposed to 0, 37, 74 and 148 μM Al for 14 days in hydroponic culture at pH 4.5. With increasing Al concentration, leaf Ca²⁺ and Mg²⁺ content decreased, as well as the effective quantum yield of photosystem II (PSII) photochemistry (Φ _PSII ), while a gradual increase in leaf membrane lipid peroxidation, Al accumulation, photoinhibition (estimated as F _v /F _m ), and PSII excitation pressure (1 ? q _p ) occurred. However, the Al-resistant cultivar with lower Al accumulation, retained larger concentrations of Ca²⁺ and Mg²⁺ in the leaves and kept a larger fraction of the PSII reaction centres (RCs) in an open configuration, i.e. a higher ratio of oxidized to reduced quinone A (Q_A), than plants of the Al-sensitive cultivar. Four times higher Al concentration in the nutrient solution was required for Al-resistant plants (148 μM Al) than for Al-sensitive (37 μM Al), in order to establish the same closed RCs. Yet, the decline in photosynthetic efficiency in the cultivar Dio was not only due to closure of PSII RCs but also to a decrease in the quantum yield of the open RCs. We suggest that Al³⁺ toxicity may be mediated by nutrient deficiency and oxidative stress, and that Al-resistance of the wheat cultivar Yecora E, may be due at least partially, from the decreased Al accumulation that resulted to decreased reactive oxygen species (ROS) formation. However, under equal internal Al accumulation (exposure Al concentration: Dio 74 μM, Yecora E 148 μM) that resulted to the same oxidative stress, the reduced PSII excitation pressure and the better PSII functioning of the Al-resistant cultivar was probably due to the larger concentrations of Ca²⁺ and Mg²⁺ in the leaves. We propose that the different sensitivities of wheat cultivars to Al³⁺ toxicity can be correlated to differences in the redox state of Q_A. Thus, chlorophyll fluorescence measurements can be a promising tool for rapid screening of Al resistance in wheat cultivars.     

<Emphasis Type="Italic">In vitro</Emphasis> evaluation of excess copper affecting seedlings and their biochemical characteristics in <Emphasis Type="Italic">Carthamus tinctorius</Emphasis> L. (variety PBNS-12)

The present study was focused to recognize the changes in the Safflower (Carthamus tinctorius L. variety PBNS-12), when exposed to different concentration of copper (25, 50 and 100 μM) along with control (0.5 μM) for 10 and 20 days. This experiment used Hoagland’s nutrient solution to meet the external nutrient conditions, which includes micro and macronutrients equivalent to soil solution with copper sulphate (CuSO₄. 5H₂O) as a metal stress. The plant samples were harvested after 10 and 20 days. The effect of increased concentrations of copper was indicated by the reduction in overall growth with reduced fresh and dry weight. Copper stress caused significant increase in the non- enzymatic antioxidants (polyphenols and flavonoids) in leaves of treated safflower seedlings as compared to the control. Also, enhanced accumulation of proline was observed in the safflower leaves. In response to excess copper concentration, the level of MDA content was found to be increased. The results showed that the copper has time and dose-dependent effects on safflower seedlings.     

Improvement of element uptake and antioxidative defense in Brassica napus under lead stress by application of hydrogen sulfide

Heavy metal pollution is one of the major constraints in oilseed rape (Brassica napus L.) production. In this study, protective role of hydrogen sulfide (H₂S) on plant growth under lead (Pb) stress was studied in B. napus. Plants were grown hydroponically in greenhouse conditions under three levels (0, 100, and 400 μM) of Pb and three levels (0, 100 and 200 μM) of H₂S donor sodium hydrosulfide. Outcomes demonstrated that Pb stress significantly reduced the plant biomass, leaf chlorophyll contents, nutrients uptake in the leaves and roots of B. napus plants. Exogenous application of H₂S significantly improved the plant biomass, chlorophyll contents and concentration of macro- and micronutrients in the leaves and roots of B. napus plants under Pb-toxicity conditions. The data indicated that application of Pb alone significantly increased the reactive oxygen species (ROS) as well as malondialdehyde (MDA) in the leaves and roots of plants. Meanwhile, application of H₂S decreased the production of MDA and ROS in the leaves and roots by increasing antioxidant activities under Pb stress. Moreover, this study also revealed that plants treated with H₂S at different concentrations enhanced the contents of total glutathione and glutathione reduced/glutathione oxidized ratio in leaves and roots under different levels of Pb. The results depicted that H₂S improved the plant biomass, uptake of nutrients in the leaves and roots of B. napus plants and enhanced the performance of antioxidant defense system due to its ameliorative potential under Pb stress conditions.