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Copper (Cu) is an essential mineral nutrient for the proper growth and development of plants; it is involved in myriad morphological, physiological, and biochemical processes. Copper acts as a cofactor in various enzymes and performs essential roles in photosynthesis, respiration and the electron transport chain, and is a structural component of defense genes. Excess Cu, however, imparts negative effects on plant growth and productivity. Many studies have summarized the adverse effects of excess Cu on germination, growth, photosynthesis, and antioxidant response in agricultural crops. Its inhibitory influence on mineral nutrition, chlorophyll biosynthesis, and antioxidant enzyme activity has been verified. The current review focuses on the availability and uptake of Cu by plants. The toxic effects of excess Cu on seed germination, plant growth and development, photosynthesis, and antioxidant response in plants are discussed. Plant tolerance mechanisms against Cu stress, and management of Cu-contaminated soils are presented.

Nanotechnology now plays a revolutionary role in many applications; nanomaterials have experienced significant importance in both basic and applied sciences as well as in bio-nanotechnology. Zinc oxide nanoparticles (ZnO-NPs) have become one of the most important metal oxide NPs in biological applications due to their beneficial impacts. The purpose of this study was to explore the effects of ZnO-NPs in reducing Cd toxicity by studying the growth, photosynthesis reactions, antioxidant system, oxidative stress, and protein content in Lycopersicon esculentum (tomato). ZnO-NPs induced an upregulation of antioxidative enzymes which protect the photosynthetic apparatus in plants. Seeds of tomato were sown to create nursery. At 20 days after sowing (DAS), seedlings were transferred to soil pots. Varied concentrations (0.4, 0.6 or 0.8 mM) of Cd were applied to the soil after 24 and 25 DAS. Zinc (Zn; 50 mg/L) and ZnO-NPs (50 mg/L) treatments were given continuously for 5 days from 31 to 35 DAS and sampling took place at 45 DAS. The results indicate that a Cd-generated oxidative burst in the form of elevated hydrogen peroxide (H2O2) levels resulted in a decline in cell viability through enhanced activity of the antioxidant system and proline content; the data increased on follow-up treatment with ZnO-NPs. Foliar application of ZnO-NPs significantly enhanced plant height, fresh, and dry weight of plant, leaf area, SPAD chlorophyll, photosynthetic attributes, i.e., net photosynthetic rate (PN), transpiration rate (E), internal CO2 concentration (Ci), and stomatal conductance (gs). Application of ZnO-NPs reduced the adverse effects generated by Cd and increased protein content, activities of nitrate reductase and carbonic anhydrase over the control in both stressed and non-stressed plants. Additionally, microscopic studies showed a marked increase in stomatal aperture after ZnO-NPs treatment in the presence or absence of Cd. This was associated with decrease in malondialdehyde and superoxide radical (O2−) levels. The present study suggests that ZnO-NPs can be effectively used to reduce the toxicity of Cd in tomato plants and may also be suitable for testing on other crop species.

Lipoxygenases (LOXs), naturally occurring enzymes, are widely distributed in plants and animals. LOXs can be non-sulfur iron, non-heme iron, or manganese-containing dioxygenase redox enzymes. LOXs catalyze the oxidation of polyunsaturated fatty acids into fatty acid hydroperoxides. Linolenic acid, a precursor in the jasmonic acid (JA) biosynthesis, is converted to 12-oxo-phytodienoic acid through oxygenation with LOX, allene oxide synthase, and allene oxide cyclase. Moreover, JA participates in seed germination, fruit ripening, senescence, and many other physio-biochemical processes. LOXs also play crucial roles in defense responses against biotic stress, i.e., insects, pests, pathogenic attacks, and abiotic stress, such as wounding, UV-rays, extreme temperature, oxidative stress, and drought.

The reported study investigated the interaction between salicylic acid (SA) and citric acid (CA) in cadmium (Cd)-stressed Brassica juncea plants. Seedling received Cd (0.6 mM) stress through soil at 5-day stage of growth. SA (0.01 mM) and CA (0.6 mM) treatments were applied at 25 days after sowing. Growth, photosynthesis, oxidative burst, and antioxidant systems were examined at 30-day stage of growth. Growth and photosynthetic parameters reduced significantly in the presence of Cd, and elevated levels of H2O2 were indicative of oxidative burst which resulted in decline of cell viability. Foliar spray of SA and CA alone or in combination mitigated the toxic effects generated by Cd and enhanced plant growth parameters. The inhibitory effects of Cd toxicity on width of stomatal pore resulted in reduced internal CO2 concentration and carbonic anhydrase activity which consequently limited the photosynthetic rate. SA and CA alleviated the inhibitory effect of Cd on photosynthesis by stimulating the stomatal activity and pore size. The Cd-generated oxidative burst was reduced via enhanced antioxidant activity (catalase, peroxidase, and superoxide dismutase) upon follow-up treatment with SA and CA alone or in combination. A combined dose of SA and CA countered Cd-induced damage by reducing levels of reactive oxygen species and strengthening plant antioxidant defense systems, which resulted in membrane stabilization and recovery from stress. Combined dose of SA and CA proved more effective than their individual application towards Cd stress which suggests an effective synergism between the two acids.

In the last few decades, use of copper oxide nanoparticles (CuO NPs) has been increased significantly that eventually included as a growth stimulator. This makes it essential to examine their impact on several plants. In the study detailed here, we investigated the effects of CuO NPs on the growth, physiological efficiency, biochemical assays, and antioxidant system in the mustard plant. Varying concentrations (0, 2, 4, 8, and 16 mg/L) of CuO NPs were applied at 25 days after sowing (DAS), and sampling took place at 30 and 45 DAS. The results indicate that CuO NPs-treated plants registered an increase in the growth and biomass over their respective control. Among different concentrations of CuO NPs (0, 2, 4, 8, and 16 mg/L), 8 mg/L proved to be the optimum foliar spray treatment and increase the chlorophyll content, net photosynthetic rate, leaf proline content, and antioxidant enzymes activity. We concluded that CuO NPs interact with meristematic cells triggering biochemical pathways conductive to an enhancement of the growth attributes. Further studies are needed to investigate the mechanisms of CuO NPs in mustard.

The salt stress limits the production of mustard throughout the world and it is one of the major abiotic stresses. Crop productivity is declining due to the limited area of fertile land. In order to investigate the effects NO donor on salt tolerance and the recovery of Brassica juncea. (L) cv. RGN-48, sodium nitroprusside (SNP) was applied at 1, 10 or 100 µM concentrations as foliar spray for five days consecutively. SNP triggered a significant increase in the main antioxidative enzymes including catalase (CAT), peroxidase (POX) and superoxide dismutase (SOD) along with the increase in the enzymes involved in nitrogen metabolism (nitrate reductase), photosynthesis and respiration (carbonic anhydrase, rubisco, fumarase, hexokinase and succinate dehydrogenase). On the other hand, decrease in programmed cell death (PCD) and the contents of hydrogen peroxide (H2O2), superoxide anion (O2.−) and malondialdehyde (MDA) was observed in NaCl-stressed plants subjected to the different concentrations of SNP. Consequently, the spray of SNP restored several photosynthetic attributes i.e. SPAD chlorophyll, chlorophyll fluorescence and gas exchange parameters in NaCl-stressed plants. These results suggested that exogenous application of SNP is useful in ameliorating the toxicity generated by NaCl in mustard plants.

Nitric oxide (NO) acts as a gaseous diffusible plant growth regulator. It plays an important role in growth and development of plants. Therefore, in present study mustard plants were sprayed with different concentrations of sodium nitroprusside (0, 10–4 M, 10–5 M and 10–6 M), a donor of NO, at 25 days after sowing to assess different physiological parameters. The results indicate that foliar spray of SNP upregulate chlorophyll content, chlorophyll fluorescence along with gaseous exchange parameters which further boost overall photosynthetic efficiency. A gradual increase in carbon metabolism (total reducing sugars, total carbohydrate content, glucose, fructose, sucrose and starch content) was also observed in SNP-treated plants as compared to control. Nutrient status (carbon, nitrogen, phosphorus, sulfur, potassium and magnesium) of leaves also shows a significant increase. The activity of various enzymes associated with nitrogen metabolism, CO2/HCO3− homeostasis, glycolysis, Calvin cycle and Krebs cycle (nitrate reductase, carbonic anhydrase, hexokinase, rubisco, fumarase and succinate dehydrogenase) were also increased in the presence of SNP. It was also reported that O2−, H2O2 and MDA were decreased in SNP-treated samples. SNP application also upregulate antioxidative defense system by increasing the activity of antioxidant enzymes, i.e., CAT, POX and SOD. Thus, it can be concluded from the present observation that SNP proved beneficial and alter most of the parameters which ultimately improve photosynthetic efficiency in mustard plants.

The present study evaluated the influence of melatonin (MEL) on copper toxicity in terms of morphophysiological, microscopic, histochemical, and stress resilience responses in Brassica juncea . Different levels of Cu (0, 30, and 60 mg kg –1 ) were given in air-dried soil, and 25 days after sowing (DAS), plants were sprayed with 30, 40, or 50 μM of MEL. The results demonstrated that under Cu stress, a significant amount of Cu accumulated in plant tissues, particularly in roots than in upper ground tissues, thereby suppressing the overall growth as evidenced by decrease in tolerance index and photosynthesis and increase in oxidative stress biomarkers (reactive oxygen species, malondialdehyde, and electrolyte leakage content) and cell death. Interestingly, the follow-up treatment of MEL, mainly 40 μM, efficiently improved the physio-biochemical and growth parameters, sugar accumulation, and metabolism. The potential of MEL in modulating Cu stress is attributed to its involvement in enriching the level of nutrient and improving chloroplast and stomatal organization besides lowering oxidative stress via enhanced levels of antioxidants. MEL improved the Cu reclamation potential in plants by enhancing Cu uptake and its translocation to aerial tissues. Principal component analysis showed that most of the morphophysiological and growth attributes were positively linked with MEL and negatively related to Cu levels, whereas all the stress-enhancing attributes showed a strong relationship with excessive Cu levels in soils. The present study suggested that MEL has the potential to improve growth and photosynthesis resulting in improved stress resilience under Cu stress along with increased remediation capability of mustard for remediation of Cu-contaminated soils.

It is known that auxin and nitric oxide have the ability to ameliorate various abiotic stresses in plants individually. However, their interactive effects on the stress generated by salt in plants are not reported yet. Therefore, the present study was conducted to explore the effectiveness of sodium nitroprusside (SNP), a donor of nitric oxide and IAA (Indole acetic acid) individually and in combination on the photosynthetic efficiency, antioxidative enzyme activities, and plant biomass under salt stress. Twenty-five-day-old plants were exposed to SNP and IAA individually and in combination for 5 days continuously, and it was found that both SNP and IAA showed almost similar response at 30 and 45 days of old plants and increased studied parameters over their respective controls. However, the interactive effect of both SNP and IAA excelled over the individual response of SNP and IAA. On the other hand, the presence of salt in soil significantly decreased the parameters studied. Furthermore, stress generated by NaCl was overcome by the exogenous application of SNP, IAA alone as well as in combination at both the stages of samplings. However, the combination of IAA and SNP was more effective at 45 DAS where the value of plant biomass and photosynthetic efficiency was more in comparison to control plants. The level of antioxidant enzymes and proline was further enhanced by SNP and IAA and maximum values were reported in the plants received both SNP and IAA under NaCl stress. This enhanced antioxidant system and proline accumulation could ameliorate the stress induced by NaCl in plants that was reflected by the improved plant biomass, and photosynthetic efficiency in comparison to control plants.

In plants, glucose (Glc) acts as a crucial signaling molecule in mediating metabolism, growth, stress tolerance mechanism, etc. However, little is known about Glc supplementation in salinity tolerance. This experiment was designed to study the ameliorative effect of Glc in mustard under salt stress. The seeds were soaked in three concentrations of NaCl (0, 50, or 100 mM) for 8 h and then treated with four concentrations of Glc (0, 2, 4, or 8%) as foliar spray for 5 days at 25-day stage. The plants were harvested at three growth stages (30, 45, and 60) for examining morpho-physiological and proteomic studies. Glc application as foliar spray increases growth, photosynthesis, and antioxidative enzyme activities in NaCl-treated plants. Glc applied in plants also showed reduction in superoxide anion, hydrogen peroxide, and malondialdehyde content under salt stress. Amongst all doses of Glc, spray of 4% Glc proved best in alleviating the harmful effects of salinity.