Exogenous salicylic acid stimulates physiological and biochemical changes to improve growth, yield and active constituents of fennel essential oil

Fennel (Foeniculum vulgare Mill) is a high-value medicinal and essential oil bearing plant used extensively in pharmaceutical, food and cosmetic industries. A pot experiment was carried out in the natural conditions of net house to resolve whether the foliar application of salicylic acid (SA) might enhance the growth, yield and essential oil production of fennel. Plants were sprayed three times with SA. The first spray was carried out at 40?days after sowing (DAS); the second and third sprays were applied one and 2?weeks later, the plants were sprayed with deionised water (control) and different concentrations of SA (10^?5, 10^?4 and 10^?3?M). The foliar spray of SA at 10^?4?M significantly enhanced the vegetative growth (shoot and root lengths, fresh and dry weights), physiological and biochemical characteristics (chl ??a??, chl ??b??, total chlorophyll and carotenoids contents, nitrate reductase activity, carbonic anhydrase activity, leaf-N, -P and -K contents), yield characteristics (number of umbels and fruits, 1,000-seed weight and seed yield) and essential oil yield of fennel. GLC analysis revealed the significant increase in the components of essential oil, viz. trans-anethole (80.4?C84.7?%), methyl chavicol (2.3?C2.5?%) and fenchone (5.6?C7.9?%). It was concluded that foliar spray of SA at 10^?4?M might be employed for enhancing the plant growth as well as yield and quality of essential oil of fennel.     

Effect of low above-zero temperature on the content of low-molecular antioxidants and activities of antioxidant enzymes in green barley leaves

We studied the effect of low above-zero temperature (2°C) on the content of low-molecular antioxidants (ascorbic acid, glutathione, and carotenoids) and also activities of antioxidant enzymes (ascorbate peroxidase, APO; catalase, CAT; glutathione reductase, GR; and superoxide dismutase, SOD) in green barley (Hordeum vulgare L.) seedlings. Under stress conditions, the content of low-molecular antioxidants, especially that of reduced ascorbate form, increased. Low-temperature stress activated APO, CAT, GR, and SOD. First enzymes responding to the action of stress factor were APO and CAT, i.e., enzymes neutralizing hydrogen peroxide in plant cells, which indicated H₂O₂ active generation at low temperature. Cytoplasmic SOD was more active than its chloroplast isoforms. This indicates that oxidative process initiation under low-temperature stress occurred more active in the cytosol. After termination of stress-factor action, the content of total ascorbate, glutathione, and carotenoids reduced rapidly to the level close to the initial one. During post-stress period, the amount of reduced ascorbate declined as well; however, it remained at the level higher than the initial one. Activities of APO and CAT dropped sharply; activities of GR and SOD reduced gradually. Thus, reduced ascorbate, APO, and CAT play an important role in plant cell defense against above-zero temperatures close to zero; reduced ascorbate, GR, and SOD are especially important during post-stress period.     

Chilling Induced Oxidative Stress in Germinating Wheat Grains as Affected by Water Stress and Calcium

Wheat (Triticum aestivum L.) plants were subjected to mild water stress during grain filling at milk (early, medium, and late) and dough (early, soft, hard) stages. The grains harvested from stressed plants were subjected to low temperature stress of 10 °C for 24 h in presence or absence of 1 mM CaCl₂, and embryos were examined for oxidative injury. The embryos of grains water stressed at milk and soft dough stages showed lowest contents of H₂O₂ and malondialdehyde and highest membrane stability index, ascorbic acid content, and activities of catalase, ascorbate peroxidase, and superoxide dismutase as compared to control embryos or water-stressed at other stages. Presence of Ca²⁺ in the medium reduced H₂O₂ and malondialdehyde content and increased ascorbic acid content, and catalase, ascorbate peroxidase and superoxide dismutase activities.     

NaCl induced cross-acclimation to UV-B radiation in four Barley (<Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.) cultivars

The effect of pre-treatment with 200 mM NaCl on the response of four barley cultivars (Hordeum vulgare L. cv. Bülbül-89, Kalaycı-97, Tarm-92 and Tokak-157/37) to UV-B radiation was investigated. Salt stress as well as UV-B irradiation led to a decrease of the total chlorophyll (chl) content in all cultivars, except in Kalaycı-97. While carotenoids are almost not affected by NaCl treatment, UV-B irradiation caused an increase by 5–20% of carotenoid content of all cultivars. UV-B induced damages of photosynthetic apparatus were estimated by the rate of photosynthetic electron transport measured by chl fluorescence and the rate of oxygen evolution, the latter being more affected. Pre-treatment with NaCl alleviated harmful effect of UV-B irradiation on F _v/F _m and ETR, but not on oxygen evolution. UV-B-induced and UV-B-absorbing compounds with absorption at 300 and 438 nm increased as a result of UV-B treatment. The level of stress marker proline increased considerably as a result of NaCl treatment, while UV-B irradiation resulted in a pronounced increase of the level of H₂O₂. MDA enhanced in the seedlings subjected to salt and UV-B stress. Established cross-acclimation to UV-B as a result of salt treatment could be due to the increased free proline and the level of UV-B absorbing compounds in barley seedlings subjected to NaCl.     

The effects of elevated CO2 (0.5%) on chloroplasts in the tetraploid black locust (Robinia pseudoacacia L.)

Some ploidy plants demonstrate environmental stress tolerance. Tetraploid (4×) black locust (Robinia pseudoacacia L.) exhibits less chlorosis in response to high CO₂ than do the corresponding diploid (2×) plants of this species. We investigated the plant growth, anatomy, photosynthetic ability, chlorophyll (chl) fluorescence, and antioxidase activities in 2× and 4× black locusts cultivated under high CO₂ (0.5%). Elevated CO₂ (0.5%) induced a global decrease in the contents of total chl, chl a, and chl b in 2× leaves, while few changes were found in the chl content of 4× leaves. Analyses of the chl fluorescence intensity, maximum quantum yield of photosystem II (PSII) photochemistry (Fv/Fm), K‐step (V_k), and J‐step (V_J) revealed that 0.5% CO₂ had a negative effect on the photosynthetic capacity and growth of the 2× plants, especially the performance of PSII. In contrast, there was no significant effect of high CO₂ on the growth of the 4× plants. These analyses indicate that the decreased inhibition of the growth of 4× plants by high CO₂ (0.5%) may be attributed to an improved photosynthetic capacity, pigment content, and ultrastructure of the chloroplast compared to 2× plants.     

Impact of foliar herbivory on the development of a root-feeding insect and its parasitoid

The majority of studies exploring interactions between above- and below-ground biota have been focused on the effects of root-associated organisms on foliar herbivorous insects. This study examined the effects of foliar herbivory by Pieris brassicae L. (Lepidoptera: Pieridae) on the performance of the root herbivore Delia radicum L. (Diptera: Anthomyiidae) and its parasitoid Trybliographa rapae (Westwood) (Hymenoptera: Figitidae), mediated through a shared host plant Brassica nigra L. (Brassicaceae). In the presence of foliar herbivory, the survival of D. radicum and T. rapae decreased significantly by more than 50%. In addition, newly emerged adults of both root herbivores and parasitoids were significantly smaller on plants that had been exposed to foliar herbivory than on control plants. To determine what factor(s) may have accounted for the observed results, we examined the effects of foliar herbivory on root quantity and quality. No significant differences in root biomass were found between plants with and without shoot herbivore damage. Moreover, concentrations of nitrogen in root tissues were also unaffected by shoot damage by P. brassicae larvae. However, higher levels of indole glucosinolates were measured in roots of plants exposed to foliar herbivory, suggesting that the development of the root herbivore and its parasitoid may be, at least partly, negatively affected by increased levels of these allelochemicals in root tissues. Our results show that foliar herbivores can affect the development not only of root-feeding insects but also their natural enemies. We argue that such indirect interactions between above- and below-ground biota may play an important role in the structuring and functioning of communities.     

Biochemical and physiological estimation of barley regenerants obtained in selective systems

Callus lines of barley (Hordeum vulgare L.) resistant to aluminum toxicity and osmotic stress from which plants regenerated were obtained in selective systems. Comparative estimation of the intensity of lipid peroxidation, the content of anthocyans and ascorbic acid in leaves, the parameters of CO₂/H₂O gas exchange in leaves of regenerant plants obtained in selective mediums with gradual addition of aluminum ions and polyethylene glycol and in mediums without selective agents was performed in conditions of an artificial climate. The obtained data were compared with biochemical and physiological parameters of barley plants of the initial genotypes grown in the same conditions. Significant changes in the intensity of apparent photosynthesis, transpiration, and efficacy of water use by barley leaves were revealed as a result of callus culturing and especially due to the use of selective systems in vitro.     

Biofertilizers and/or some micronutrients role on wheat plants grown on newly reclaimed soil

Pot experiment was conducted to study the effect of biofertilizers (inoculation with different bacterial isolates), foliar spraying with some micronutrients (Mn, Zn, Fe and Mn+Zn+Fe) and their interaction on growth, physiological parameters and nutrients content of wheat plants grown on reclaimed soil. Pot experiment was conducted in the greenhouse of National Research center, The experimental design was split plot with four replicates. Four biofertilizer treatments (un‐inoculated, Bacillus polymyxa, Azotobacter chroococcum or Azosprillium barasilense) were used and randomly distributed in the main pots. The foliar treatments with micronutrients were randomly distributed in the sub plots. The growth parameters (plant height, leaf area, roots, shoots and whole plant dry weights and number of tillers & leaves per plant); some physiological parameters (soluble sugar %, protein %, polysaccharide %, chl. A+b μg cm^?1 leaf per plant, carotenoids μg g^?1, IAA mg kg^?1 and psll mol DCPIP reduced per mg chl. per h) and nutrient contents (N, P, K, Mg, Mn, Zn and Cu) of wheat plants were significantly increased by inoculating wheat grains with different bacteria as compared with un‐inoculated plants (control). The highest values of all the mentioned parameters were obtained by using Azospirillum brasilense followed by Azotobacter chroococcum and Bacillus polymyxa in decreasing order. Foliar spraying treatments significantly increased the growth parameters, physiological parameters as well as nutrients content of wheat plants as compared with control. Highest values were obtained by using (Mn+Fe+Zn) treatment followed by Zn, Fe and Mn in decreasing order. Micronutrients in wheat plants differed as the foliar treatments were differed, so application of any micronutrient individually significantly increased its content and enhanced the content of other micronutrients in wheat. Interaction between the used biofertilizers and foliar spraying with micronutrients significantly affected all the studied parameters of wheat plants, the highest were obtained by inoculating wheat grains with Azospirillum brasilense and spraying the plants with (Mn+Fe+Zn) treatment, while the lowest values were attained by un‐inoculated grains (control) and spraying the wheat plants with tap water (control). Effective microorganisms in combination with micronutrients could be recommended to farmers to lead higher wheat yield.     

Exogenous salicylic acid alleviates NaCl toxicity and increases antioxidative enzyme activity in <Emphasis Type="Italic">Lycopersicon esculentum</Emphasis>

Effects of exogenous salicylic acid (SA) on plant growth, contents of Na, K, Ca and Mg, activities of superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), glutathione reductase (GR) and catalase (CAT), and contents of ascorbate and glutathione were investigated in tomato (Lycopersicon esculentum L.) plants treated with 100 mM NaCl. NaCl treatment significantly increased H₂O₂ content and lipid peroxidation indicated by accumulation of thiobarbituric acid reactive substances (TBARS). A foliar spray of 1 mM SA significantly decreased lipid peroxidation caused by NaCl and improved the plant growth. This alleviation of NaCl toxicity by SA was related to decreases in Na contents, increases in K and Mg contents in shoots and roots, and increases in the activities of SOD, CAT, GPX and DHAR and the contents of ascorbate and glutathione.     

Die Hemmung der Carotinoidsynthese im Stickstoffmangel

Summary The ability to synthesize carotenoids during normal or strongly reduced nitrogen supply was tested in a higher plant (Hordeum vulgare L.) and in a green alga (Scenedesmus obliquus). Though carotenoids are nitrogenfree, their synthesis is strongly inhibited during nitrogen deficiency. The formation of secondary carotenoids, which apparently proceeds at the expense of the primary carotenoid components, occurs in Scenedesmus but not in Hordeum plants. It is concluded that the dimerisation of geranly-geranyl-pyrophosphate is the limiting step in the carotenoid synthesis during nitrogen deficiency. The synthesis of prenyl chains is less affected by nitrogen deficiency.     

Enhanced sensitivity to oxidative stress in transgenic tobacco plants with decreased glutathione reductase activity leads to a decrease in ascorbate pool and ascorbate redox state

To investigate the possible mechanisms of glutathione reductase (GR) in protecting against oxidative stress, we obtained transgenic tobacco (Nicotiana tabacum) plants with 30–70% decreased GR activity by using a gene encoding tobacco chloroplastic GR for the RNAi construct. We investigated the responses of wild type and transgenic plants to oxidative stress induced by application of methyl viologen in vivo. Analyses of CO₂ assimilation, maximal efficiency of photosystem II photochemistry, leaf bleaching, and oxidative damage to lipids demonstrated that transgenic plants exhibited enhanced sensitivity to oxidative stress. Under oxidative stress, there was a greater decrease in reduced to oxidized glutathione ratio but a greater increase in reduced glutathione in transgenic plants than in wild type plants. In addition, transgenic plants showed a greater decrease in reduced ascorbate and reduced to oxidized ascorbate ratio than wild type plants. However, there were neither differences in the levels of NADP and NADPH and in the total foliar activities of monodehydroascorbate reductase and dehydroascorbate reductase between wild type and transgenic plant. MV treatment induced an increase in the activities of GR, ascorbate peroxidase, superoxide dismutase, and catalase. Furthermore, accumulation of H₂O₂ in chloroplasts was observed in transgenic plants but not in wild type plants. Our results suggest that capacity for regeneration of glutathione by GR plays an important role in protecting against oxidative stress by maintaining ascorbate pool and ascorbate redox state.     

Effect of prolonged root hypoxia on the antioxidant content of tomato fruit

Tomato fruit quality depends on its metabolite content, which in turn is determined by numerous metabolic changes occurring during fruit development and ripening. The aim of this work was to investigate whether flooding affects the nutritional quality of tomato fruit, focusing on compounds essential to human health: carotenoids and ascorbate. To this end, tomato plants (Solanum lycopersicum L. cv Micro-Tom) were submitted to prolonged root hypoxia (1–2% O₂) at first flower anthesis. Fruits were harvested at five stages of the ripening process and analysed for their carotenoid and ascorbate contents. Our results showed that the ripening of fruits that developed on hypoxia treated plants was not inhibited. However, root hypoxia significantly limits carotenoid and ascorbate accumulation in pericarp during fruit ripening, the strongest effects being observed at late stages of ripening. Limitation of both carotenoids and ascorbate accumulation seems to be primarily mediated by the reduced level of expression of genes of the corresponding metabolic pathway.     

Repellent and toxic properties of plant oils and extracts on Cydalima perspectalis Walker (Lepidoptera: Crambidae)

Buxus plants in Europe become frequently infested by the Box tree moth, Cydalima perspectalis Walker (Lepidoptera: Crambidae) but applicable biological control methods are limited. To make a first step in preventing egg deposition on Buxus plants, seven essential oils (Carum carvi, Juniperus communis, Lavendula angustifolia, Melaleuca alternifolia, Pinus sylvestris, Rosmarinus officinalis and Thymus vulgaris), one seed oil (Sambucus nigra) and six plant extracts (Equisetum arvense, Juniperus communis, Quassia amara, Salvia officinalis, Tanacetum vulgare and Thymus vulgaris) were investigated on their potential to influence oviposition preference in the greenhouse. The four essential oils exhibiting the highest repellency values (RV = 0.61–0.94) were further assessed in laboratory leaf disc bioassays on larvae. Despite the strong repellent effect of S. nigra on female oviposition, no impact could be detected on the larvae. These were significantly susceptible to exposed leaf discs treated with T. vulgaris and M. alternifolia essential oils including feeding deterrence, survival and larval weight. T. vulgaris was the most effective oil considering both effects. The effective lethal concentration (LC₅₀) of T. vulgaris was calculated to be 1% at 72 h. Our results suggest the possibility of using essential oils to protect Buxus plants in order to reduce the overall risk for defoliation by larval feeding.     

Changes in apoplastic antioxidants induced by powdery mildew attack in oat genotypes with race non-specific resistance

Three oat (Avena sativa L.) lines which show differential responses to attack by the biotrophic fungal pathogen Blumeria graminis DC f. sp. avenae Marchal, which causes powdery mildew, were studied: Maldwyn shows the strongest resistance in adult plants; Selma shows greater susceptibility; while a Selma × Maldwyn hybrid, OM1387, has a similar degree of resistance to Maldwyn. Host responses to pathogen attack were complete 48 h after inoculation but largely accomplished within the first 24 h, the point when material was taken for enzyme and metabolic assays. In Maldwyn and OM1387 about 80% of attacked cells showed localized autofluorescent host-cell responses but this fell to less than 20% in Selma. A cytoplasmic marker enzyme, glucose 6-phosphate dehydrogenase, was used to determine contamination of the apoplastic extracts by cellular components. After correction for cytoplasmic contamination, up to 4% of the total foliar activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase activities were detected in the apoplast. The apoplast contained about 2% of the total foliar glutathione pool and dehydroascorbate, but not ascorbate, at values amounting to 10% of the total foliar ascorbate plus dehydroascorbate pool. Twenty-four hours after inoculation the foliar or apoplastic ascorbate pools were similar in inoculated and control leaves. Foliar catalase activity increased in both susceptible and resistant responses. Resistance correlated with increased total foliar glutathione, an increase in the ratio of reduced to oxidized glutathione and with decreased total activities of foliar ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase. Received: 17 April 1998 / Accepted: 28 August 1998     

Leucanthemum Vulgare Lam. Germination,Growth and Mycorrhizal Symbiosis Under Crude Oil Contamination

Oil contamination of soil limits plants’ access to water and nutrients. Leucanthemum vulgare colonized by mycorrhizae could provide an effective tool in remedying oil contamination. Seeds of L. vulgare were planted in pots containing soil mixed with petroleum at 0, 2.5, 5, 7.5, and 10% w/w and propagules of mycorrhizal fungi. Plants were grown under ambient conditions for 16 weeks. Seed germination data were collected weekly for three weeks. Mycorrhizal percentage, spore counts, length and weight of roots and shoots were determined after harvesting. Results showed significant differences in seed germination rates between oil-treated, mycorrhizal and non-mycorrhizal plants. The overall germination rate was greater at 7.5% w/w crude oil contamination (ρ = 0.05) in mycorrhizal and non-mycorrhizal pots with significant differences between their respective Root:Shoot ratios (both length and weight). Results of this research showed L. vulgare could be germinated and grown in crude oil contaminated soils and could be used to augment plant establishment as part of phytoremediation practices.     

Morphactin effects on soybean leaf anatomy and chlorophyll content

The effect of chlorflurenol (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate) (CF) on chlorophyll (chl) content was studied in intact plants and floating leaf disks. For intact soybean (Glycine max (L.) Merrill) plants grown in the growth chamber, 2.5 g/ml CF applied 10 to 20 d after planting retarded chl decline in senescing tissues such as cotyledons and unifoliate leaves and increased chl content in recently expanded tissues such as trifoliate leaves. CF did not retard chl decline in the dark unless regulator application was followed by a period of 24 h in the light prior to darkness. In floating leaf disk tests, CF retarded chl decline in dock (Rumex obtusifolius L.) and radish (Raphanus sativus L.) at concentrations of 10^–4 M, but was ineffective at lower concentrations. Chl decline was significantly hastened by CF in tobacco (Nicotiana tabacum L.) and soybean, but was unchanged in barley (Hordeum vulgare L.).CF treatment increased tissue weight (g fresh wt/cotyledon; g dry wt/ cm² for unifoliate and trifoliate leaves), decreased moisture content, and increased leaf thickness, palisade layer thickness, and palisade and spongy mesophyll cell counts. We conclude that plants treated with morphactins show greater green coloration predominantly because of growth effects, and only in small part because of prevention of chl decline in senescing tissues.This paper reports results of research only. Mention of a pesticide in this paper does not constitute a recommendation by USDA nor does it imply registration under FIFRA. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products or vendors that may also be suitable.     

Net photosynthetic rate,ascorbate peroxidase and glutathione reductase activities of <Emphasis Type="Italic">Erythrina orientalis</Emphasis> in polluted and non-polluted areas

We investigated net photosynthetic rate and antioxidative enzyme activities in Erythrina orientalis grown in three different sites: Makati and Quezon (cities with high levels of air pollution, HP) and La Mesa (a non-polluted area, NP). Photosynthetic activity of E. orientalis was significantly reduced in the HP cities. In contrast, activities of the antioxidative enzymes ascorbate peroxidase and glutathione reductase were significantly higher in HP cities than in the NP area.     

Plant and fertiliser effects on rhizodegradation of crude oil in two soils with different nutrient status

Soils and sediments polluted with crude oil are of major environmental concern on various contaminated sites. Outdoors pot experiments were conducted to test the phytodegradation potential of common reed (Phragmites australis) and poplar (Populus nigra × maximowiczii) in fertilised and non-fertilised control treatments. Two topsoils (E, G) of different texture were mixed with crude oil. Soil analysis included hydrocarbon (HC) measurements, detection of labile phosphorus and mineralised nitrogen as well as dehydrogenase activity. Increased HC degradation by native soil biota was clearly related to higher P availability in soil G and to fertilisation in soil E. Except of the non-fertilised common reed treatment, plants did not enhance crude oil degradation. We found even inhibited degradation of high molecular weight HC in the presence of plants together with declining labile phosphorous concentrations due to planting on soil E. Native soil biota were able to use the whole range of crude oil compounds (C₁₀ to C₆₀) as a carbon source in the presence of sufficient nutrient concentrations in soil. This study is the first to show that reduced HC degradation in the higher molecular weight crude oil fraction (C₂₀ to C₄₀) is likely to be a consequence of decreased phosphorus availability for microorganisms in the plant rhizosphere.     

Seasonal variations of gas exchange, photosynthetic pigments, and antioxidants in Turkey oak (Quercus cerris L.) and Hungarian oak (Quercus frainetto Ten.) of different age

Seasonal changes in physiological and biochemical parameters were studied in 35-, 55- and 140-year-old trees of Turkey oak (Quercus cerris L.) and Hungarian oak (Q. frainetto Ten.), growing in natural stands in Eastern Balkan Mountains (Bulgaria). During the seasonal drought period (August), assimilation activity, transpiration rate, stomatal conductance and water potential had a seasonal minimum in all the studied tree ages and species. The foliar concentrations of glutathione, ascorbate, α-tocopherol, as well as photosynthetic pigments in oak leaves were significantly affected by season. With the increasing age of the studied trees, we observed a decrease of the physiological activity and an increase of the antioxidants’ accumulation. Both the species were drought tolerant and anisohydric, where Q. frainetto exhibited higher rates of gas exchange than Q. cerris. Moreover, they differed in the extent of increase in the foliar antioxidants and carotenoids.     

Zinc‐induced oxidative stress in Verbascum thapsus is caused by an accumulation of reactive oxygen species and quinhydrone in the cell wall

Oxidative stress is one aspect of metal toxicity. Zinc, although unable to perform univalent oxido‐reduction reactions, can induce the oxidative damage of cellular components and alter antioxidative systems. Verbascum thapsus L. plants that were grown hydroponically were exposed to 1 and 5 mM Zn²⁺. Reactive oxygen species (ROS) accumulation was demonstrated by the fluorescent probe H₂DCFDA and EPR measurements. The extent of zinc‐induced oxidative damage was assessed by measuring the level of protein carbonylation. Activities and isoform profiles of some antioxidant enzymes and the changes in ascorbate and total phenolic contents of leaves and roots were determined. Stunted growth because of zinc accumulation, preferentially in the roots, was accompanied by H₂O₂ production in the leaf and root apoplasts. Increased EPR signals of the endogenous oxidant quinhydrone, ^?CH₃ and ^?OH, were found in the cell walls of zinc‐treated plants. The activities of the antioxidative enzymes ascorbate peroxidase (APX) (EC 1.11.1.11), soluble superoxide dismutase (SOD) (EC 1.15.1.1), peroxidase (POD), (EC 1.11.1.7) and monodehydroascorbate reductase (EC 1.6.5.4) were increased; those of glutathione reductase (EC 1.6.4.2), dehydroascorbate reductase (EC 1.8.5.1) and ascorbate oxidase (AAO) (EC 1.10.3.3) were decreased with zinc treatment. Zinc induced a cell‐wall‐bound SOD isoform in both organs. Leaves accumulated more ascorbate and phenolics in comparison to roots. We propose a mechanism for zinc‐promoted oxidative stress in V. thapsus L. through the generation of charge transfer complexes and quinhydrone because of phenoxyl radical stabilisation by Zn²⁺ in the cell wall. Our results suggest that the SOD and APX responses are mediated by ROS accumulation in the apoplast. The importance of the POD/Phe/AA (ascorbic acid) scavenging system in the apoplast is also discussed.