Applications of ascorbic acid or proline increase resistance to salt stress in barley seedlings

The present study was carried out to examine the effects of seed soaking in 1 mM ascorbic acid (AA) or 1 mM proline on the growth, content of photosynthetic pigments and proline, relative water content, electrolyte leakage, antioxidant enzymes and leaf anatomy of Hordeum vulgare L. Giza 124 seedlings grown in greenhouse under 100 or 200 mM NaCl. The plants exposed to the NaCl stress exhibited a significant reduction in growth, relative water content, leaf photosynthetic pigments, soluble sugars, as well as alterations in leaf anatomy. However, the treatment with AA or proline ameliorated the stress generated by NaCl and improved the above mentioned parameters. NaCl increased electrolyte leakage, proline content, and activities of antioxidant enzymes (SOD, CAT, and POX). The antioxidant enzymes and leaf anatomy exhibited considerable changes in response to AA or proline application in the absence or presence of NaCl.     

Antioxidant defense system,lipid peroxidation,proline-metabolizing enzymes,and biochemical activities in two <Emphasis Type="Italic">Morus alba</Emphasis> genotypes subjected to NaCl stress

Salt stress-induced changes in antioxidant enzymes, lipid peroxidation, proline and glycine betaine contents, and proline-metabolizing enzymes were examined in the leaves of two mulberry cultivars (Local and Sujanpuri). With increasing salinity up to 150 mM NaCl, superoxide dismutase, catalase, ascor-bate peroxidase, guaiacol peroxidase, glutathione reductase, and monodehydroascorbate reductase activities were increased in both cultivars as compared to control, but more pronounced increase was observed in cv. Local. Salt stress enhanced the rate of lipid peroxidation (as indicated by increasing MDA content) in both cultivars. Under NaCl stress, cv. Local showed less change in the MDA content than cv. Sujanpuri. Salt stress resulted in a significant accumulation of free proline in mulberry leaves, and more accumulation was detected in cv. Local than cv. Sujanpuri. The leaves of cv. Local showed 9-fold accumulation of glycine betaine in comparision with cv. Sujanpuri after 20 days at 150 mM NaCl. A decrease in proline oxidase activity and an increase in γ-glutamyl kinase activity were observed with increasing NaClconcentration. The relative water content and electrolyte leakage also decreased after increasing the NaCl concentration, but a decrease was more pronounced in cv. Sujanpuri than in cv. Local. The results indicate that oxidative stress may play an important role in salt-stressed mulberry plants and cv. Local have more efficient antioxidant characteristics, which could provide for a better protection against oxidative stress.     

Influence of exogenous application of glycinebetaine on antioxidative system and growth of salt-stressed soybean seedlings (Glycine max L.)

Glycinebetaine is one of the most competitive compounds which play an important role in salt stress in plants. In this study, the enhanced salt tolerance in soybean (Glycine max L.) by exogenous application of glycinebetaine was evaluated. To improve salt tolerance at the seedling stage, GB was applied in four different concentrations (0, 5, 25 and 50 mM) as a pre-sowing seed treatment. Salinity stress in the form of a final concentration of 150 mM sodium chloride (NaCl) over a 15 day period drastically affected the plants as indicated by increased proline, MDA and Na⁺ content of soybean plants. In contrast, supplementation with 50 mM GB improved growth of soybean plants under NaCl as evidenced by a decrease in proline, MDA and Na⁺ content of soybean plants. Further analysis showed that treatments with GB, resulted in increasing of CAT and SOD activity of soybean seedlings in salt stress. We propose that the role of GB in increasing tolerance to salinity stress in soybean may result from either its antioxidant capacity by direct scavenging of H₂O₂ or its role in activating CAT activity which is mandatory in scavenging H₂O₂.     

Effects of NaCl on growth, ion accumulation, protein, proline contents and antioxidant enzymes activity in callus cultures of Jatropha curcas

Jatropha curcas is an oil bearing species with multiple uses and considerable economic potential as a biofuel crop. The effect of NaCl stress on growth, ion accumulation, contents of protein, proline, and antioxidant enzymes activity in callus cultures of J. curcas was investigated. Exposure of callus to NaCl decreased growth in a concentration dependent manner. NaCl treated callus accumulated Na and declined in K, Ca and Mg contents. Na/K ratio increased steadily as a function of external NaCl treatment. NaCl induced significant differences in quality and quantity of proteins, whereas, proline accumulation remained more or less constant with treatment. NaCl stress enhanced the activity of superoxide dismutase (SOD; E.C. 1.15.1.1) and peroxidase (POX; E.C. 1.11.1.7). Further in the isoenzyme studies, four SOD isoenzymes (SOD 1, 2, 3, and 4) and two POX isoenzymes (POX 1 and 2) were detected with the treatment. NaCl strongly induced activity of SOD 4 isoenzyme in 40, 60, 80 mM and POX 2 isoenzyme in 40 and 80 mM NaCl concentrations. Increase in antioxidant enzymes activity could be a response to cellular damage induced by NaCl. This increase could not stop the deleterious effects of NaCl, but it reduced stress severity and thus allowed cell growth to occur.     

Effect of short-term exposure to NaCl on photochemical activity and antioxidant enzymes in <Emphasis Type="Italic">Bruguiera parviflora</Emphasis>, a non-secretor mangrove

Effect of short-term (6 days) exposure to high salinity (500 mM NaCl) was studied in Bruguiera parviflora, a tree mangrove. NaCl treatment decreased photochemical activity, but had no effect on growth. Thylakoid protein profile and spectral characteristic were not changed. There was no significant effect on chlorophylls and carotenoids content, total proteins and total free amino acids. However, there was an increase in free proline. The activity of antioxidant enzymes like catalase, ascorbate peroxidase was enhanced, but no significant change in guaiacol peroxidase was observed. Salinity did not cause any alteration in malondialdehyde formation indicating intactness of membrane integrity upon high salinity. We conclude that the effect of high NaCl stress is not revealed in morphology of the plants, but in the metabolic changes as increase in proline and antioxidant enzyme activity. These effects are the adaptive mechanisms that operates under high salt stress in this mangrove; however, the decrease in photochemical activity may be due to onset of senescence which helps plant in remobilization of photosynthate to new leaves after adaptation.     

The effect of NaCl on antioxidant enzyme activities in potato seedlings

The effect of NaCl on the growth and activity of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) were investigated in the seedlings of four potato cultivars (Agria, Kennebec; relatively salt tolerant, Diamant and Ajax; relatively salt sensitive). The shoot fresh mass of Agria and Kennebec did not changed at 50 mM NaCl, whereas in Diamant and Ajax it decreased to 50 % of that in the controls. In Agria and Kennebec, SOD activity increased at 50 mM NaCl, but no significant changes observed in Diamant and Ajax. At higher NaCl concentration, SOD activity reduced in all cultivars. CAT and POD activities increased in all cultivars under salt stress. Unlike the other cultivars, in Ajax seedlings, APX activity increased in response to NaCl stress. We also observed new POD and SOD isoenzyme activities and changes in isoenzyme compositions under salt stress. These results suggest that salt-tolerant potato cultivars may have a better protection against reactive oxygen species (ROS) by increasing the activity of antioxidant enzymes (especially SOD) under salt stress.     

Effect of salinity and waterlogging on growth,anatomical and antioxidative responses in <Emphasis Type="Italic">Mentha aquatica</Emphasis> L.

Salinity and waterlogging are two stresses which in nature often occur simultaneously. In this work, effects of combined waterlogging and salinity stresses are studied on the anatomical alteration, changes of enzymatic antioxidant system and lipid peroxidation in Mentha aquatica L. plants. Seedlings were cultured in half-strength Hoagland medium 50 days after sowing, and were treated under combination of three waterlogging levels (well drained, moderately drained and waterlogging) and NaCl (0, 50, 100, 150 mM) for 30 days. Moderately drained and waterlogging conditions induced differently aerenchyma formation in roots of M. aquatica salt-treated and untreated plants. Moreover, stele diameter and endodermis layer were also affected by salt stress and waterlogging. Salt stress significantly decreased growth, relative water content (RWC), protein level, catalase (CAT) and polyphenol oxidase (PPO) activities, and increased proline content, MDA content, H₂O₂ level and activities of superoxide dismutase (SOD), peroxidase (POX), and ascorbate peroxidase (APX). Waterlogging in salt-untreated plants increased significantly growth parameters, RWC, protein content, antioxidant enzyme activity, and decreased proline content, H₂O₂ and MDA levels. In salt-treated plant, waterlogging caused strong induction of antioxidant enzymes activities especially at severe stress condition. These results suggest M. aquatica is a waterlogging tolerant plant due to significant increase of antioxidant activity, membrane stability and growth under water stress. High antioxidant capacity under waterlogging can be a protective strategy against oxidative damage, and help to salt stress alleviation.     

Growth stage-based modulation in antioxidant defense system and proline accumulation in two hexaploid wheat (Triticum aestivum L.) cultivars differing in salinity tolerance

Changes in the antioxidant defense system and proline accumulation were examined at different growth stages (vegetative, boot and reproductive) in plants of two hexaploid spring wheat cultivars (S-24, salt tolerant; MH-97, salt sensitive), grown in hydroponics and salinity-affected with 0, 50, 100 and 150 mM of NaCl. Salt stress provoked a marked decline in plant dry mass and ascorbic acid contents, and increased proline, total soluble proteins and H₂O₂ contents in both wheat cultivars at all growth stages. However, higher accumulation of proline and H₂O₂ contents was recorded at the vegetative and boot stages, respectively, in both wheat cultivars. Salt stress caused a consistent rise in the activities of some key antioxidant enzymes (CAT, SOD, POD, and APX) at all growth stages only in the salt tolerant cultivar S-24, whereas such pattern of enhanced activities of enzymatic antioxidants in cv. MH-97 was found only at the vegetative stage under saline regimes. Maximum activities of various enzymatic antioxidants were observed at the vegetative stage in both wheat cultivars under varying external salt treatments. The results showed that high salinity tolerance of cv. S-24, as manifested by lower decrease in its dry matter under salt stress, was associated with higher activities of antioxidant enzymes, increased accumulation of proline, and lower levels of H₂O₂, as compared with cv. MH-97 at all growth stages under saline regimes.     

Calcium chloride effects on salinity-induced oxidative stress, proline metabolism and indole alkaloid accumulation in Catharanthus roseus

Catharanthus roseus (L.) G. Don. plants were grown with NaCl and CaCl2 in order to study the effect of CaCl2 on NaCl-induced oxidative stress in terms of lipid peroxidation (TBARS content), H2O2 content, osmolyte concentration, proline (PRO)-metabolizing enzymes, antioxidant enzyme activities, and indole alkaloid accumulation. The plants were treated with solutions of 80 mM NaCl, 80 mM NaCl with 5 mM CaCl2 and 5 mM CaCl2 alone. Groundwater was used for irrigation of control plants. Plants were uprooted randomly on 90 days after sowing (DAS). NaCl-stressed plants showed increased TBARS, H2O2, glycine betaine (GB) and PRO contents, decreased proline oxidase (PROX) activity, and increased gamma-glutamyl kinase (gamma-GK) activity when compared to control. Addition of CaCl2 to NaCl-stressed plants lowered the PRO concentration by increasing the level of PROX and decreasing the gamma-GK activities. Calcium ions increased the GB contents. CaCl2 appears to confer greater osmoprotection by the additive role with NaCl in GB accumulation. The antioxidant enzymes superoxide dismutase (SOD), peroxidase (POX) and catalase (CAT) were increased under salinity and further enhanced due to CaCl2 treatment. The NaCl-with-CaCl2-treated C. roseus plants showed an increase in total indole alkaloid content in shoots and roots when compared to NaCl-treated and untreated plants.     

Role of Salicylic Acid in Promoting Salt Stress Tolerance and Enhanced Artemisinin Production in Artemisia annua L.

In the present investigation, the role of salicylic acid (SA) in inducing salinity tolerance was studied in Artemisia annua L., which is a major source of the antimalarial drug artemisinin. SA, when applied at 1.00 mM, provided considerable protection against salt stress imposed by adding 50, 100, or 200 mM NaCl to soil. Salt stress negatively affected plant growth as assessed by length and dry weight of shoots and roots. Salinity also reduced the values of photosynthetic attributes and total chlorophyll content and inhibited the activities of nitrate reductase and carbonic anhydrase. Furthermore, salt stress significantly increased electrolyte leakage and proline content. Salt stress also induced oxidative stress as indicated by the elevated levels of lipid peroxidation compared to the control. A foliar spray of SA at 1.00 mM promoted the growth of plants, independent of salinity level. The activity of antioxidant enzymes, namely, catalase, peroxidase, and superoxide dismutase, was upregulated by salt stress and was further enhanced by SA treatment. Artemisinin content increased at 50 and 100 mM NaCl but decreased at 200 mM NaCl. The application of SA further enhanced artemisinin content when applied with 50 and 100 mM NaCl by 18.3 and 52.4%, respectively. These results indicate that moderate saline conditions can be exploited to obtain higher artemisinin content in A. annua plants, whereas the application of SA can be used to protect plant growth and induce its antioxidant defense system under salt stress.     

Effect of nitric oxide signaling in bacterial-treated soybean plant under salt stress

To understand protective roles of nitric oxide against salt stress, the effects of exogenous sodium nitroprusside on activities of lipoxygenase, peroxidase, phenylalanine ammonialyase, catalase, superoxide dismutase enzymes, proline accumulation, and distribution of sodium in soybean plants under salt were determined. Application of sodium nitroprusside + bacterium enhanced plant growth-promotion characteristics, activities of different enzymes, and proline accumulation in the presence of sodium nitroprusside under salt stress. Treatment with NaCl at 200 mM and sodium nitroprusside (0.1 mM) reduced Na⁺ levels but increased K⁺ levels in leaves in comparison with the NaCl-treated plants. Correspondingly, the plants treated with exogenous sodium nitroprusside and NaCl maintained a lower ratio of [Na⁺]/[K⁺] in NaCl-stressed plants.     

Salt stress responses of a halophytic grass <Emphasis Type="Italic">Aeluropus lagopoides</Emphasis> and subsequent recovery

To investigate the salt tolerance mechanisms, Aeluropus lagopoides as a halophytic plant was used. Plants were treated with 0, 150, 450, 600, and 750 mM NaCl and harvested at 0, 4, 8, and 10 days after treatment and 1 day and 1 week after recovery. Optimal growth, measured as fresh and dry weights, occurred at 150 mM NaCl, but it was suppressed by 450, 600, and 750 mM NaCl. Recovery significantly increased fresh and dry weights only in 750 mM NaCl-treated plants. Water content was decreased after NaCl treatment and increased after recovery. Na⁺ and proline contents and activity of superoxide dismutase (SOD) were increased after NaCl treatment and decreased after recovery in all treated plants. In contrast, K⁺ content and ascorbate peroxidase activity decreased after NaCl treatment and increased after recovery in all treated plants. Catalase (CAT) was activated only in 750 mM NaCl-treated plants. Total content of soluble protein was slightly changed after NaCl treatment. It was concluded that proline accumulation for osmotic adjustment, SOD activation for O₂^·− scavenging, and CAT activation at the higher level of salt stress to detoxify produced H₂O₂ were main A. lagopoides strategies under salt stress. A. lagopoides salt tolerance was not based on the restriction of Na⁺ uptake.     

Proline antioxidant role in the common ice plant subjected to salinity and paraquat treatment inducing oxidative stress

Leaves of 4-week-old (juvenile) and 9-week-old (adult) plants of the halophyte Mesembryanthemum crystallinum L. (the common ice plant), cultured under controlled conditions in the phytotron, were treated with paraquat (0.1 μM), which produces superoxide radical, and (or) paraquat combined with introduction of NaCl (100 mM) or proline (5 mM) into nutrient medium. After a 20-h dark period (23°C), plants were transferred into light (4 h at 54.1 W/m² of photosynthetically active radiation) for stimulation of O°₂⁻ formation in plastids. Activities of antioxidant enzymes, the contents of MDA, H₂O₂, chlorophyll, and free proline were measured in leaves. Plant responses in two age groups, which differed in the type of photosynthesis (juvenile plants had C₃ type of photosynthesis, whereas adult plants were at the transition stage to Crassulacean Acid Metabolism (CAM) photosynthesis), differed in the levels of constitutive proline and proline, induced by NaCl and paraquat, as well as in activities of superoxide dismutase (SOD) and catalase. Changes in SOD activity and proline accumulation in response to paraquat treatment combined with NaCl revealed opposite dependence to accumulation of proline: the more proline accumulated in leaves, the lower activity of the enzyme. In response to paraquat treatment, the content of chlorophylls a and b most drastically declined in juvenile plants. Negative effect of salinity on the content of chlorophylls was lower than that of paraquat and was almost the same in plants of both age groups. Protective effect of exogenous proline was most profound in the case of paraquat treatment. Exogenous proline decreased the rate of lipid peroxidation, the content of superoxide radical and, consequently, SOD activity (almost fivefold), and increased the content of chlorophylls (a and b) in leaves of adult plants. The obtained data suggest that stress-induced accumulation of proline in the common ice plant has both osmoprotectory and antioxidant functions.     

Differential responses to salinity of two Atriplex halimus populations in relation to organic solutes and antioxidant systems involving thiol reductases

Atriplex halimus L. is a xero-halophyte species widespread in the Mediterranean basin. The tolerance to water stress and high salinity of two Atriplex populations from semi-arid (Djelfa) and arid saline (Laghouat) Algerian regions has been investigated in relation with organic solutes and antioxidant systems. Whereas no noticeable difference was observed between the two populations under water stress resulting from withholding watering or PEG treatment, Laghouat plants display significantly higher fresh and dry weights than Djelfa plants when exposed to high salinity. At 300mM NaCl, Laghouat plants exhibit higher concentrations in Na(+), proline and quaternary ammonium compounds, and a higher catalase activity than Djelfa plants. We then analysed the involvement of recently characterized plastidial thiol reductases, peroxiredoxins (Prxs) and methionine sulphoxide reductases (MSRs), key enzymes scavenging organic peroxides and repairing oxidized proteins, respectively. Upon salt treatment (300mM NaCl), we observed higher amounts of PrxQ and over-oxidized 2-Cys Prx in Laghouat than in Djelfa. An increased abundance of plastidial MSRA and a higher total MSR activity were also noticed in Laghouat plants treated with 300mM NaCl compared to Djelfa ones. We propose that mechanisms based on organic solutes and antioxidant enzymes like catalases, peroxiredoxins and MSRs party underlie the better tolerance of the Laghouat population to high salt.     

Mechanisms of salt stress tolerance development in barley plants under the influence of 5-aminolevulinic acid

Growing barley (Hordeum vulgare L.) plants for 7 days on NaCl solutions (20–200 mM) decreased chlorophyll (Chl) a and b content with respect to that in untreated control plants. The content of free proline and the plant ability to synthesize 5-aminolevulinic acid (ALA) started to increase in parallel at salt concentrations of 20–50 mM. The maximum amount of ALA accumulated in plants grown at 100 mM NaCl was twofold higher than in control plants grown on fresh water. In this case the proline content increased 2.8-fold. On further increase in salt concentration, the rate of ALA accumulation decreased, approaching control values at 150 mM NaCl; even lower rates were observed at 200 mM NaCl. The reduced ability to synthesize ALA was accompanied by an increase in proline content. The albino tissue of plants treated at the seed stage with the antibiotic streptomycin lost its ability to synthesize ALA needed for Chl formation. The proline content in the albino tissue was tenfold higher than in control green plants and was 30-fold higher when the plants were grown on solutions with 100 mM NaCl. No effect of NaCl on ALA-dehydratase activity was noted. As NaCl concentration was raised, there occurred the decrease in magnesium chelatase activity, accumulation of reactive oxygen species (ROS), the increase in ascorbate peroxidase activity, and a slight decrease in lipid peroxidation level. Growing plants in the presence of 150 mM NaCl and 10 or 60 mg/l exogenous ALA led to the increase in proline content (by a factor of 1.8 and 4.2, respectively) and to the decrease in ROS content, in comparison with plants grown on salt solutions without ALA. Furthermore, in the presence of exogenous ALA, the parameters of seedling growth became similar to those of NaCl-untreated plants. The role of ALA in plants as an antistress agent is considered. ALA is supposed to confer tolerance to salt stress by taking part in Chl and heme biosynthesis and also through functioning as a plant growth regulator. A hypothesis is put forward that the impairment of ALA-synthesizing ability may redirect metabolic conversions of glutamic acid from Chl and heme synthesis to the proline synthesis pathway, which would stimulate proline biosynthesis and improve salt tolerance.     

Antioxidant Enzyme Responses to NaCl Stress in Cassia angustifolia

Seeds of Cassia angustifolia Vahl. were subjected to 0, 20, 50, 100 mM NaCl for 7 d in order to study the effect of salt stress on growth parameters, endogenous Na⁺ and Cl^– concentrations, antioxidant system, lipid peroxidation, hydrogen peroxide, and proline contents. Salinity affected all of the considered parameters and caused a great reduction in plant biomass. The root and shoot length, fresh and dry mass and germination percentage were inhibited by NaCl treatments. These changes were associated with an increase in the Na⁺ and Cl^– contents in the seedlings and increased activities of superoxide dismutase, catalase, peroxidase, and polyphenol oxidase. The increased enzyme activity coincided with decreased ascorbate content and enhanced H₂O₂ and proline content.     

Influence of ascorbic acid and 24-epibrassinolide on physiological characteristics of pot marigold under water-stress condition*

Water deficit is considered as a major limiting environmental factor for plant growth and yield. To ameliorate the adverse effects of water restriction, an experiment was conducted in the research field of Mohaghegh Ardabili University in two successive years (2014 and 2015). Foliar spraying of different concentration of epibrassinolide (EBL) (0, 10^?8, and 10^?7?M) and ascorbic acid (AsA) (0 and 10?mM) was carried out and water-stress trials included 50 and 100?mm evaporation from class A pan. Water stress significantly enhanced essential oil content, but reduced capitula yield and relative water content (RWC) of leaves. Water-stress damage ameliorated by foliar application of 10?mM AsA with 10^?7?M EBL and the essential oil yield and antioxidant enzymes activity improved significantly. Enhancing of malondialdehyde (MDA) content and electrolyte leakage indicates that water-deficit stress caused oxidative damage to the membrane by enhancing hydrogen peroxide (H₂O₂) level. Combined-application of regulators significantly declined the amounts of H₂O_2, MDA, and electrolyte leakage under water stress. Antioxidant enzymes activity and also proline and protein content were enhanced by drought stress as well as regulators. Also, the application of EBL and AsA induced tolerance to water deficit and reduced the reactive oxygen species by increasing antioxidant enzymes activity and osmotic adjustment.     

Kinetin improves photosynthetic and antioxidant responses of <Emphasis Type="Italic">Nigella sativa</Emphasis> to counteract salt stress

The present study involves analysis of growth, photosynthesis, oxidant (H₂O₂) accumulation, and antioxidant enzyme activities in Nigella sativa L. as affected by foliar kinetin (KIN) application during salt stress. The test plants were treated with 75 or 150 mM NaCl since germination and sprayed with either water or 10 μM KIN in 25 days after emergence. Salt stress, especially at the higher NaCl concentration, was found to induce a substantial decrease in leaf relative water content and subsequently in leaf area and stomatal conductance; chlorophyll content and δ-aminolevulinic acid dehydratase (ALA-D) activity were also affected, resulting in the lower net photosynthetic rate and dry matter production. Moreover, H₂O₂ content increased in the salt-treated plants, concomitant with an increase in superoxide dismutase and peroxidase activities; however, the activity of catalase declined. Meanwhile KIN was found to reduce appreciably the adverse effects of salinity, besides favorably modulating antioxidant enzyme activities and alleviating oxidative stress in the test plants, to result in a higher yield as compared to the untreated stressed plants. Overall, the results indicate an optimization of antioxidant defense mechanisms and physiological processes by KIN and a significant role of exogenous phytohormones in conferring salt tolerance.     

Effect of nitric oxide and putrescine on antioxidative responses under NaCl stress in chickpea plants

Chickpea plants were subjected to salt stress for 48 h with 100 mM NaCl, after 50 days of growth. Other batches of plants were simultaneously treated with 0.2 mM sodium nitroprusside (NO donor) or 0.5 mM putrescine (polyamine) to examine their antioxidant effects. Sodium chloride stress adversely affected the relative water content (RWC), electrolyte leakage and lipid peroxidation in leaves. Sodium nitroprusside and putrescine could completely ameliorate the toxic effects of salt stress on electrolyte leakage and lipid peroxidation and partially on RWC. No significant decline in chlorophyll content under salt stress as well as with other treatments was observed. Sodium chloride stress activated the antioxidant defense system by increasing the activities of peroxidase (POX), catalase (CAT) superoxide dismutase (SOD) and ascorbate peroxidase (APX). However no significant effect was observed on glutathione reductase (GR) and dehydro ascorbate reductase (DHAR) activities. Both putrescine and NO had a positive effect on antioxidant enzymes under salt stress. Putrescine was more effective in scavenging superoxide radical as it increased the SOD activity under salt stress whereas nitric oxide was effective in hydrolyzing H₂O₂ by increasing the activities of CAT, POX and APX under salt stress.     

The modulation of various physiochemical changes in <Emphasis Type="Italic">Bruguiera cylindrica</Emphasis> (L.) Blume affected by high concentrations of NaCl

Bruguiera cylindrica is a major mangrove species in the tropical mangrove ecosystems and it grows in a wide range of salinities without any special features for the excretion of excess salt. Therefore, the adaptation of this mangrove to salinity could be at the physiological and biochemical level. The 3-month-old healthy plantlets of B. cylindrica, raised from propagules were treated with 0 mM, 400 mM, 500 mM and 600 mM NaCl for 20 days under hydroponic culture conditions provided with full strength Hoagland medium. The modulation of various physiochemical changes in B. cylindrica, such as chlorophyll a fluorescence, total chlorophyll content, dry weight, fresh weight and water content, Na⁺ accumulation, oxidation and antioxidation (enzymatic and non-enzymatic) features were studied. Total chlorophyll content showed very minute decrease at 500 mM and 600 mM NaCl treatment for 20 days and the water content percentage was decreased both in leaf and root tissues with increasing concentration. A significant increase of Na⁺ content of plants from 84.505 mM/plant dry weight in the absence of NaCl to 543.38 mM/plant dry weight in plants treated with 600 mM NaCl was recorded. The malondialdehyde and the metabolites content associated with stress tolerance (amino acid, total phenols and proline) showed an increasing pattern with increasing NaCl concentration as compared to the control in both leaf and root tissues but the increase recorded in plantlets subjected to 500 mM was much less, indicating the tolerance potential of this species towards 500 mM NaCl. The significant decrease of sugar content was found only in 600 mM NaCl on 20 days of treatment, showing that the process of sugar synthesis was negatively affected but the same process remains less affected at 500 mM NaCl. A slight reduction in ascorbate and glutathione content and very less increase in carotenoid content were observed at 500 mM and 600 mM NaCl stress. Antioxidant enzymes (APX, GPX, SOD and CAT) showed an enhanced activity in all the treatments and the increased activity was more significant in 600 mM treated plants. The result establishes that B. cylindrica tolerates high NaCl concentration, to the extent of 500 mM NaCl without any major inhibition on photosynthesis and metabolite accumulation. Understanding the modulation of various physiological and biochemical changes of B. cylindrica at high levels of NaCl will help us to know the physiochemical basis of tolerance strategy of this species towards high NaCl.