Oxidative stress induced by long-term plum pox virus infection in peach (Prunus persica)

In this study, the effect of long-term plum pox virus (PPV) infection on the response of certain antioxidant enzymes at the subcellular level was studied in peach plants ( Prunus persica (L.) Batch) (cv. GF305), which are characterized by great susceptibility to the virus. In infected plants, a decrease in the efficiency of excitation energy capture by PSII ( F _v'/ F _m') was observed, which was accompanied by a decrease in non-photochemical quenching (NPQ). p -Hydroxy-mercury benzoic acid (pHMB)-insensitive ascorbate peroxidase (APX) activity (class III peroxidase) was detected in both chloroplast and soluble fractions. In soluble fractions from inoculated peaches, a significant increase in pHMB-sensitive APX activity and a significant decrease in superoxide dismutase (SOD) activity were observed. These changes were correlated with the observations in isolated chloroplasts, where an increase in both pHMB-sensitive and pHMB-insensitive APX activities was observed, whereas significant decreases in SOD, monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR) activities were produced. According to these results, as a consequence of PPV infection, an oxidative stress, indicated by an increase in lipid peroxidation and protein oxidation, was produced in peach leaves, which was monitored by the diaminobenzidine (DAB) peroxidase-coupled H₂O₂ probe. PPV infection produced an alteration in chloroplast ultrastructure, giving rise to dilated thylakoid membranes. PPV-infected peach leaves showed a decreased amount of starch in chloroplasts from palisade parenchyma, as well as an increase in the number and size of plastoglobuli, in relation to control plants. The results suggest that long-term PPV infection produces an oxidative stress, and that an antioxidative metabolism imbalance may be related to the progress of PPV infection and symptoms in peach plants.     

Response of antioxidative enzymes to plum pox virus in two apricot cultivars

Recent evidence has indicated that activated oxygen species (AOS) may function as molecular signals in the induction of defence genes. In the present work, the response of antioxidative enzymes to the plum pox virus (PPV) was examined in two apricot (Prunus armeniaca L.) cultivars, which behaved differently against PPV infection. In the inoculated resistant cultivar (Goldrich), a decrease in catalase (CAT) as well as an increase in total superoxide dismutase (SOD) and dehydroascorbate reductase (DHAR) activities were observed. Ascorbate peroxidase (APX), glutathione reductase (GR) and monodehydroascorbate reductase (MDHAR) did not change significantly in relation to non-inoculated (control) plants. In the susceptible cultivar (Real Fino), inoculation with PPV brought about a decrease in CAT, SOD and GR, whereas a rise in APX, MDHAR and DHAR activities was found in comparison to non-inoculated (control) plants. Apricot leaves contain only CuZn-SOD isozymes, which responded differently to PPV depending on the cultivar. Goldrich leaves contained 6 SODs and both SOD 1 and SOD 2 increased in the inoculated plants. In leaves from Real Fino, 5 SODs were detected and only SOD 5 was increased in inoculated plants. The different behaviour of SODs (H2O2-generating enzymes) and APX (an H2O2-remover enzyme) in both cultivars suggests an important role for H2O2 in the response to PPV of the resistant cultivar, in which no change in APX activity was observed. This result also points to further studies in order to determine if an alternative H2O2-scavenging mechanism takes place in the resistant apricot cultivar exposed to PPV. On the other hand, the ability of the inoculated resistant cultivar to induce SOD 1 and SOD 2 as well as the important increase of DHAR seems to suggest a relationship between these activities and resistance to PPV. This is the first report about the effect of PPV infection on the antioxidative enzymes of apricot plants. It opens the way for the further studies, which are necessary for a better understanding of the role of antioxidative processes in viral infection by PPV in apricot plants.     

The apoplastic antioxidant system in Prunus: response to long-term plum pox virus infection

This work describes, for the first time, the changes taking place in the antioxidative system of the leaf apoplast in response to plum pox virus (PPV) in different Prunus species showing different susceptibilities to PPV. The presence of p-hydroxymercuribenzoic acid (pHMB)-sensitive ascorbate peroxidase (APX) (class I APX) and pHMB-insensitive APX (class III APX), superoxide dismutase (SOD), peroxidase (POX), NADH-POX, and polyphenoloxidase (PPO) was described in the apoplast from both peach and apricot leaves. PPV infection produced different changes in the antioxidant system of the leaf apoplast from the Prunus species, depending on their susceptibility to the virus. In leaves of the very susceptible peach cultivar GF305, PPV brought about an increase in class I APX, POX, NADH-POX, and PPO activities. In the susceptible apricot cultivar Real Fino, PPV infection produced a decrease in apoplastic POX and SOD activities, whereas a strong increase in PPO was observed. However, in the resistant apricot cultivar Stark Early Orange, a rise in class I APX as well as a strong increase in POX and SOD activities was noticed in the apoplastic compartment. Long-term PPV infection produced an oxidative stress in the apoplastic space from apricot and peach plants, as observed by the increase in H2O2 contents in this compartment. However, this increase was much higher in the PPV-susceptible plants than in the resistant apricot cultivar. Only in the PPV-susceptible apricot and peach plants was the increase in apoplastic H2O2 levels accompanied by an increase in electrolyte leakage. No changes in the electrolyte leakage were observed in the PPV-inoculated resistant apricot leaves, although a 42% increase in the apoplastic H2O2 levels was produced. Two-dimensional electrophoresis analyses revealed that the majority of the polypeptides in the apoplastic fluid had isoelectric points in the range of pI 4-6. The identification of proteins using MALDI-TOF (matrix-assisted laser desorption/ionization-time of flight) and peptide mass fingerprinting analyses showed the induction of a thaumatin-like protein as well as the decrease of mandelonitrile lyase in peach apoplast due to PPV infection. However, most of the selected polypeptides showed no homology with known proteins. This fact emphasizes that, at least in Prunus, most of the functions of the apoplastic space remain unknown. It is concluded that long-term PPV infection produced an oxidative stress in the leaf apoplast, contributing to the deleterious effects produced by PPV infection in leaves of inoculated, susceptible Prunus plants.     

Antioxidative defense system in an upland rice cultivar subjected to increasing intensity of water stress followed by recovery

Rice ( Oryza sativa L.) cv. Tulsi is recommended for Eastern India, for upland ecological cultivation systems where a crop experiences natural cycles of water deficit and water sufficiency, depending upon the monsoon rains. In this experiment, this cultivar was subjected to three cycles of water stress of increasing stress intensity. Each stress cycle was terminated by rewatering the plants for a 48-h period. The level of stress was measured by quantification of H₂O₂. The response of antioxidant metabolites such as ascorbate and glutathione, and enzymes such as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2) and guaiacol peroxidase (POX, EC 1.11.1.7) was analysed in terms of activity and isozyme pattern for each cycle of stress and recovery. The differential response of the antioxidant enzymes with increasing stress intensity followed by recovery, highlight the different role of each in the drought acclimation process of upland rice. SOD and POX activity in stressed plants was higher than the controls in all the three cycles. The second level of stress saw an increase in all the enzymes with APX and GR showing its maximum activity and there was a better management of H₂O₂ levels. There was an induction of a new CAT isoform in stressed plants in the third cycle of water stress. The co-ordinated defense helped the plants to recover in terms of growth on rewatering after stress cycles.     

Antioxidant enzymes as biochemical markers for sharka resistance in apricot

The activity of antioxidant enzymes in different apricot (Prunus armeniaca L.) cultivars, resistant or susceptible to Plum pox virus (PPV), was analyzed during the years 2002 and 2003. Resistant cultivars showed higher activities of catalase (CAT), ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) than susceptible cultivars. Only CuZn-SOD isozymes were detected in the apricot cultivars. However, no correlation was observed between this isozyme pattern and the resistance to PPV. On the other hand, PPV-resistant apricot cultivars could have a greater capability for elimination of H₂O₂ and recycling of ascorbate-glutathione cycle, and they have at least two of these enzymatic activities (CAT, APX and DHAR) over the average. In contrast, this response was not observed in the susceptible cultivars. All these data suggest that the activities of CAT, APX and DHAR could be used as biochemical markers of sharka resistance in apricot.     

Persistence of photosynthetic components and photochemical efficiency in ears of water-stressed wheat (Triticum aestivum)

Ear photosynthesis may be an important source of C for grain growth in water-stressed plants of cereals. The main objectives of this work were to determine the stability of the photosynthetic apparatus and the photochemical efficiency of ears in plants subjected to post-anthesis drought. Plants of wheat ( Triticum aestivum L. cv. Granero INTA) were grown in pots under a rain shelter and subjected to water stress (soil water potential around −0.6 to −0.8 MPa) starting 4  days after anthesis. Post-anthesis drought substantially accelerated the loss of chlorophyll, Rubisco and the light-harvesting complex of photosystem II (LHCII) in the flag leaf, but the degradation of these photosynthetic components was much less affected by water deficit in awns and ear bracts. Quantum yield of PSII (Φ_PSII) decreased in leaves of water-stressed plants. In contrast, ear bracts had a higher Φ_PSII than leaves, and Φ_PSII of ear bracts did not decrease at all in response to drought. Removing the grains immediately before fluorescence measurements (less than 30 min) slightly reduced Φ_PSII, indicating that CO₂ supplied by grain respiration may contribute to the high photochemical efficiency of ears in droughted plants. However, other factors may be involved in maintaining high Φ_PSII, since even in the absence of grains Φ_PSII remained much higher in ear bracts than in the flag leaf. The relative stability of ear photosynthetic components and their relatively high photochemical efficiency may help to maintain ear photosynthesis during the grain filling period in droughted plants.     

Partitioning of excitation energy in two wheat cultivars with different grain protein contents grown under three nitrogen applications in the field

Influences of different nitrogen applications on photosynthesis and utilization of excitation energy were explored by comparing two field-grown wheat ( Triticum aestivum L.) cultivars with high or low grain protein content [High protein cultivar '8901' (HC) and low protein cultivar '1391' (LC)]. High nitrogen application significantly decreased both CO₂ assimilation and photorespiration in both cultivars during the early stages after anthesis. However, the actual photosystem II (PSII) efficiency ( Ф _PSII) was not significantly different between high, moderate and low nitrogen applications in the HC. As a result, the ratio of Ф _PSII to the quantum yield of carbon metabolism ( Φ _CO2) measured under non-photorespiratory conditions in the HC was higher under high nitrogen application than under low or medium nitrogen application. The grain protein content of the HC was also increased by high nitrogen application. In contrast, high nitrogen application decreased the actual PSII efficiency in the flag leaves of the LC in the early stages after anthesis and different nitrogen applications did not significantly alter the Ф _PSII/ Φ _CO2 ratio or grain protein content in the LC. No significant difference was detected in the activity of superoxide dismutase or ascrobate peroxidase between different nitrogen treatments in either cultivar throughout the entire experimental period. These results indicate that more excitation energy is partitioned to nitrogen metabolism in the flag leaves of the HC under high nitrogen application whereas the partitioning of excitation energy in the LC was not affected by nitrogen application.     

Drought acclimation confers oxidative stress tolerance by inducing co-ordinated antioxidant defense at cellular and subcellular level in leaves of wheat seedlings

Wheat ( Triticum aestivum L.) seedlings of a drought-resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods (drought acclimation). The antioxidant defense in terms of redox metabolites and enzymes in leaf cells, chloroplasts, and mitochondria was examined in relation to ROS-induced membrane damage. Drought-acclimated seedlings modulated growth by maintaining favorable turgor potential and RWC and were able to limit H₂O₂ accumulation and membrane damage as compared with non-acclimated plants during severe water stress conditions. This was due to systematic upregulation of H₂O₂-metabolizing enzymes especially ascorbate peroxidase (APX, EC 1.11.1.11) and by maintaining ascorbate–glutathione redox pool in acclimated plants. By contrast, failure in the induction of APX and ascorbate–glutathione cycle enzymes makes the chloroplast susceptible to oxidative stress in non-acclimated plants. Non-acclimated plants protected the leaf mitochondria from oxidative stress by upregulating superoxide dismutase (SOD, EC 1.15.1.1), APX, and glutathione reductase (GR, EC 1.6.4.2) activities. Rewatering led to rapid enhancement in all the antioxidant defense components in non-acclimated plants, which suggested that the excess levels of H₂O₂ during severe water stress conditions might have inhibited or downregulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced oxidative stress tolerance by well-co-ordinated induction of antioxidant defense both at the chloroplast and at the mitochondrial level.     

The oxidative stress caused by salinity in two barley cultivars is mitigated by elevated CO2

Changes in antioxidant metabolism because of the effect of salinity stress (0, 80, 160 or 240 m M NaCl) on protective enzyme activities under ambient (350 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂ concentrations were investigated in two barley cultivars ( Hordeum vulgare L., cvs Alpha and Iranis). Electrolyte leakage, peroxidation, antioxidant enzyme activities [superoxide dismutase (SOD), EC 1.15.1.1; ascorbate peroxidase (APX), EC 1.11.1.11; catalase (CAT), EC 1.11.1.6; dehydroascorbate reductase (DHAR), EC 1.8.5.1; monodehydroascorbate reductase (MDHAR), EC 1.6.5.4; glutathione reductase (GR), EC 1.6.4.2] and their isoenzymatic profiles were determined. Under salinity and ambient CO₂, upregulation of antioxidant enzymes such as SOD, APX, CAT, DHAR and GR occurred. However, this upregulation was not enough to counteract all ROS formation as both ion leakage and lipid peroxidation came into play. The higher constitutive SOD and CAT activities together with a higher contribution of Cu,Zn-SOD 1 detected in Iranis might possibly contribute and make this cultivar more salt-tolerant than Alpha. Elevated CO₂ alone had no effect on the constitutive levels of antioxidant enzymes in Iranis, whereas in Alpha it induced an increase in SOD, CAT and MDHAR together with a decrease of DHAR and GR. Under combined conditions of elevated CO₂ and salinity the oxidative damage recorded was lower, above all in Alpha, together with a lower upregulation of the antioxidant system. So it can be concluded that elevated CO₂ mitigates the oxidative stress caused by salinity, involving lower ROS generation and a better maintenance of redox homeostasis as a consequence of higher assimilation rates and lower photorespiration, being the response dependent on the cultivar analysed.     

Antioxidant status of the potato tuber and Ca2+ deficiency as a physiological stress

The antioxidant status of potato ( Solanum tuberosum L.) tubers of two genotypes, cv. Désirée and clone 10337de40 was investigated in relation to susceptibility to internal rust spot (IRS), a Ca²⁺-related physiological disorder. Concentrations of total calcium within the perimedulla tissue of tubers, grown with a restricted (1 m M CaCl₂) Ca²⁺ supply, were similar in cv. Désirée (IRS resistant) and clone 10337de40 (IRS susceptible). A range of antioxidants was assayed in order to assess antioxidant status in both genotypes under the two Ca²⁺ treatments. Although no appreciable differences were detected between low Ca²⁺ and control treatments, certain antioxidants were present at significantly higher levels in the IRS resistant genotype, cv. Désirée. These included dehydroascorbate reductase (EC 1.8.5.1) activity (more than 100% higher), total glutathione content (ca 40% higher), glutathione reductase (EC 1.6.4.2) activity (almost 50% higher), peroxidase (EC 1.11.1.7) activity (ca 60% higher) and superoxide dismutase (EC 1.15.1.1) activity (almost 80% higher). There was no difference in ascorbate content, ascorbate free radical reductase activity (EC 1.6.5.4), α-tocopherol levels and catalase activity (EC 1.11.1.6) between the two genotypes. The possible relationship between resistance to IRS and a superior antioxidant status, found in cv. Désirée, is discussed.     

Increased susceptibility to photoinhibition in pre-existing needles experiencing low temperature at spring budbreak in Sakhalin spruce (Picea glehnii) seedlings

The seasonal changes in photosynthetic properties in 1-year-old needles of Sakhalin spruce ( Picea glehnii ) were measured using the chlorophyll fluorescence technique at various temperatures (5, 10, 20, 25 and 30°C). In the course of seasonal change, a temporary decrease in the quantum yield of PSII electron transport (Φ_PSII) was observed just before budbreak. A decline in photochemical quenching ( q _P) was observed at the same time as that of Φ_PSII but only at the two lowest temperatures (5 and 10°C). Photochemical efficiency of open PSII ( F _v'/ F _m') also declined just before budbreak at 25 and 30°C. An increase in thermal energy dissipation as indicated by a decrease in F _v'/ F _m' before budbreak was not significant at lower temperatures (5 and 10°C) in spite of the declines in q _P. This implies that thermal energy dissipation necessitated by the decline in Φ_PSII might not be sufficiently strong to prevent a decline in q _P at lower temperatures. On the other hand, at higher temperatures no decline was observed in q _P because Φ_PSII decreased to a relatively small extent, therefore thermal energy dissipation is sufficient in coping with the excessive energy accumulation in PSII. Seedlings of Sakhalin spruce exposed to ambient air temperature below 10°C before budbreak exhibited photoinhibition indicated by a decrease in the maximal photochemical efficiency of PSII ( F _v/ F _m) after an overnight dark adaptation. The present study suggests that 1-year-old shoots of Sakhalin spruce have an increased susceptibility to photoinhibition at low temperature just before budbreak.     

Effects of Fusaric Acid on Reactive Oxygen Species and Antioxidants in Tomato Cell Cultures

Generation of O₂^– and H₂O₂ as well as the activities of superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, dehydroascorbate reductase and ascorbate content were studied in tomato cell cultures in response to fusaric acid – a nonspecific toxin of phytopathogenic Fusarium species. Toxin treatment resulted in decreased cell viability which was preceded by culture medium alkalinization up to 0.65 pH unit and enhanced extracellular O₂^– production. The H₂O₂ level was not significantly affected. In toxin-treated cultures, a transient, significant increase occurred in intracellular superoxide dismutase, catalase, guaiacol peroxidase and ascorbate peroxidase activities. Fusaric acid-induced ascorbate turnover modulation led to up to a twofold increase in dehydroascorbic acid accumulation, and a decrease in the associated ascorbate redox ratio. It was concomitant with a significant decrease in dehydroascorbate reductase activity. These results support previous observations that the pro- and anti-oxidant systems are involved in response to fusaric acid treatment although differential response of H₂O₂ and its metabolism-related enzymes between the whole leaf and cell culture assays was found.     

Oxidative stress induction by Prunus necrotic ringspot virus infection in apricot seeds

Prunus necrotic ringspot rvirus (PNRSV) was able to invade the immature apricot seed including the embryo. The amount of virus was very high inside the embryo compared with that present in the cotyledons. PNRSV infection produced an oxidative stress in apricot seeds as indicated by the increase in lipid peroxidation, measured as thiobarbituric acid-reactive substances. This lipid peroxidation increase was parallelled with an imbalance in the seed antioxidant enzymes. A significant decrease in the ascorbate–GSH cycle enzymes as well as in peroxidase (POX) activity took place in infected seeds, suggesting a low capability to eliminate H₂O₂. No changes in superoxide dismutase (SOD) or catalase activity were observed. A significant decrease in polyphenoloxidase (PPO) activity was also observed. Native PAGE revealed the presence of three different SOD activity bands in apricot seeds: a Mn-containing SOD and two CuZn-containing SODs. Only an isozyme with catalase, glutathione reductase (GR) or PPO activity was detected in both healthy and infected apricot seeds. Regarding POX staining, three bands with POX activity were detected in native gels in both healthy and infected seeds. The gel results emphasise that the drop detected in POX, GR and PPO activities in PNRSV-infected apricot seeds by kinetic analyses was also evident from the results obtained by native PAGE. The oxidative stress and the imbalance in the antioxidant systems from PNRSV-infected apricot seeds resemble the hypersensitive response observed in some virus–host interactions. This defence mechanism would inactivate PNRSV during seed formation and/or the storage period or even during seed germination. Those results can explain the decrease in seed germination and the low transmission of PNRSV by seeds in apricot trees.     

Nitric oxide alleviates oxidative damage induced by high temperature stress in wheat

Effect of sodium nitroprusside (SNP), a donor of nitric oxide (NO) was examined in two wheat (Triticum aestivum L.) cultivars, C 306 (heat tolerant) and PBW 550 (comparatively heat susceptible) to study the extent of oxidative injury and activities of antioxidant enzyme in relation to high temperature (HT) stress. HT stress resulted in a marked decrease in membrane thermostability (MTS) and 2, 3, 5-triphenyl tetrazolium chloride (TTC) cell viability whereas content of lipid peroxide increased in both the cultivars. The tolerant cultivar C 306 registered less damage to cellular membranes compared to PBW 550 under HT stress. Activities of antioxidant enzymes viz, superoxide dismutase, catalase, ascorbate peroxidase, guaicol peroxidase and glutathione reductase increased with HT in both the cultivars. Following treatment with SNP, activities of all antioxidant enzymes further increased in correspondence with an increase in MTS and TTC. Apparently, lipid peroxide content was reduced by SNP more in shoots of heat tolerant cultivar C 306 indicating better protection over roots under HT stress. The up-regulation of the antioxidant system by NO possibly contributed to better tolerance against HT induced oxidative damage in wheat.     

Photosynthesis is improved by exogenous glycinebetaine in salt-stressed maize plants

The effects of exogenous application of glycinebetaine (GB) (10 m M ) on growth, leaf water content, water use efficiency, photosynthetic gas exchange, and photosystem II photochemistry were investigated in maize plants subjected to salt stress (50 and 100 m M NaCl). Salt stress resulted in the decrease in growth and leaf relative water content as well as net photosynthesis and the apparent quantum yield of photosynthesis. Stomatal conductance, evaporation rate, and water use efficiency were decreased in salt-stressed plants. Salt stress also caused a decrease in the actual efficiency of PSII ( Φ _PSII), the efficiency of excitation energy capture by open PSII reaction centers ( F _v'/ F _m'), and the coefficients of photochemical quenching ( q _P) but caused an increase in non-photochemical quenching (NPQ). Salt stress showed no effects on the maximal efficiency of PSII photochemistry ( F _v/ F _m). On the other hand, in salt-stressed plants, GB application improved growth, leaf water content, net photosynthesis, and the apparent quantum yield of photosynthesis. GB application also increased stomatal conductance, leaf evaporation rate, and water use efficiency. In addition, GB application increased Φ _PSII, F _v'/ F _m', and q _P but decreased NPQ. However, GB application showed no effects on F _v/ F _m. These results suggest that photosynthesis was improved by GB application in salt-stressed plants and such an improvement was associated with an improvement in stomatal conductance and the actual PSII efficiency.     

Production and detoxification of H2O2 in lettuce plants exposed to selenium

Selenium is considered an essential element for animals. Despite that it has not been demonstrated to be essential for higher plants, it has been attributed with a protective role against reactive oxygen species in plants subjected to stress. In this study, lettuce plants ( Lactuca sativa cv. Philipus) received different application rates (5, 10, 20, 40, 60, 80 and 120 μM) of selenite or selenate, with the aim of testing the effect of Se on the production and detoxification of H₂O₂ in non-stressed plants. The results indicate that the form selenate is less toxic than selenite; that is, the plants tolerated and responded positively to this element, and even increasing in growth up to a rate of 40 μM for the form selenate. On the contrary, the application of selenite triggered a higher foliar concentration of H₂O₂ and a higher induction of lipid peroxidation [malondialdehyde content and lipoxygenase activity] in comparison to that observed after the selenate application. Also, the plants treated with selenate induced higher increases in enzymes that detoxify H₂O₂, especially ascorbate peroxidase and glutathione (GSH) peroxidase, as well as an increase in the foliar concentration of antioxidant compounds such as ascorbate and GSH. These data indicate that an application of selenate at low rates can be used to prevent the induction in plants of the antioxidant system, thereby improving stress resistance.     

Potato Responds to Salt Stress by Increased Activity of Antioxidant Enzymes

To understand the response of potato to salt stress, antioxidant enzyme activities and ion content were analyzed for a sensitive and a tolerant cultivar. Nodal cuttings of the tolerant cultivar, Kennebec, and the sensitive cultivar, Concord, were exposed to media without or with 30, 60, 90 or 120 mmol/L NaCl for 4 weeks. On exposure to NaCl, the length and fresh and dry weight of both shoots and roots of Concord showed greater decrease than those of Kennebec. The decrease in shoot growth was more severe than that of the root for both cultivars. The K+ content of shoots and roots of both cultivars was reduced in a dose-dependent manner by exposure to NaCl; the Na+ content Increased. Activities of ascorbate peroxidase, catalase and glutathione reductase were increased in NaCl-exposed shoots of Kennebec; the corresponding activities inNaCl-exposed shoots of Concord were decreased. Roots of both cultivars showed similar changes in the activities of these enzymes on exposure to NaCl. These studies established that enzyme activities In Concord shoots are inversely related to the NaCl concentration, whereas those in Kennebec do not show a dose dependency, which is also the case for the roots of both cultivars. Our findings suggest that an Increase in activity of antioxidant enzymes, such as ascorbate peroxidase,cetalase and glutathione reductase, can contribute to salt tolerance in Kennebec, a salt resistant cultivar of potato.     

Variation in heat stress-induced antioxidant enzyme activities among three mulberry cultivars

The effects of high temperature on antioxidant enzymes were investigatedin three mulberry (Morus alba L.) cultivars (cv. K-2, MR-2and BC2-59). High temperature was imposed by maintaining the plants at 40°Cfor 120, 240 and 360 min in an environmentalplant growth chamber.The activities of superoxide disumutase (SOD), catalase (CAT), guaiacolperoxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR)wereassayed in the leaf extracts of control and high temperature-treated plants.Antioxidant enzyme activities were high in all the mulberry cultivars inresponse to high temperature treatment. However, cv. BC2-59 showedsignificantlyhigher activities of all the five antioxidant enzymes in response to hightemperature compared to those from the leaves of K-2, and MR-2 mulberrycultivars. The present study suggested that the cv. BC2-59 has an efficientantioxidant system among the three cultivars, which could prevent the oxidativedamage in the leaves caused by high temperature stress.     

Photosynthesis, carbohydrate levels and chlorophyll fluorescence-estimated intercellular CO2 in water-stressed Casuarina equisetifolia Forst. & Forst.

The effect of long-term water stress on photosynthetic carbon metabolism in Casuarina equisetifolia Forst. & Forst. was analysed by measuring CO₂ assimilation, stomatal conductance, the quantum yield of photosystem II ( Φ _PSII), enzyme activities, and the levels of photosynthetic intermediates and carbohydrates. CO₂ assimilation decreased under water stress while the intercellular CO₂ concentration ( C _i) as estimated by gas exchange measurements remained high. However, the estimates of C _i from measurements of Φ _PSII suggest that the decrease in photosynthesis can be explained in terms of stomatal closure. Water stress decreased total stromal fructose-1,6-bisphosphatase activity and did not alter the activities and activation states of ribulose bisphosphate carboxylase oxygenase and NADP-dependent malate dehydrogenase (NADP-MDH). The concentration of photosynthetic metabolites, glucose, fructose and sucrose decreased, whereas starch concentrations increased under drought conditions.