Hydrogen peroxide pre-treatment induces salt-stress acclimation in maize plants

The effect of exogenously applied H2O2 on salt stress acclimation was studied with regard to plant growth, lipid peroxidation, and activity of antioxidative enzymes in leaves and roots of a salt-sensitive maize genotype. Pre-treatment by addition of 1 microM H2O2 to the hydroponic solution for 2 days induced an increase in salt tolerance during subsequent exposure to salt stress. This was evidenced by plant growth, lipid peroxidation and antioxidative enzymes measurements. In both leaves and roots the variations in lipid peroxidation and antioxidative enzymes (superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, glutathione reductase, and catalase) activities of both acclimated and unacclimated plants, suggest that differences in the antioxidative enzyme activities may, at least in part, explain the increased tolerance of acclimated plants to salt stress, and that H2O2 metabolism is involved as signal in the processes of maize salt acclimation.     

小麦幼苗中过氧化物歧化酶活性与幼苗脱水忍耐力相关性的研究

本文着重探讨当前国际上提出的一种植物抗旱机制的新理论——即生物自由基与植物保护酶系统间相互作用的理论。在水分逆境情况下,植物细胞内自由基产生和清除的平衡遭到破坏,过剩的自由基会伤害细胞膜系统。过氢化物歧化酶是氧自由基的清除剂,因此它的活性的高低应与植物的抗旱能力密切相关。以小麦幼苗为材料,采用光化学方法测定了过氧化物歧化酶活性,同时测定幼苗的脱水耐受力,发现在不同组织和器官中,在不同发育阶段的幼苗中,二者均存在有正相关性。此外,还测定了幼苗脱水前后过氧化物歧化酶活性的变化,发现酶活性在脱水处理时升高。我们推测小麦幼苗经受干旱的过程中,细胞内可能包含生物自由基的有害作用。     

Effects of fulvic acid on the photosynthetic and physiological characteristics of Paeonia ostii under drought stress

ABSTRACTPaeonia ost iihas become an economically important oil crop in recent years, but its growth is seriously affected by drought stress in dry areas. In this study, the alleviating effect of fulvic acid (FA) on potted P. ostii under natural drought stress was investigated. The natural drought stress adopted in this experiment was mainly characterized by the low soil water content, and the roots of plants cannot absorb enough water to compensate for the consumption of transpiration, which affects the normal physiological activities and causes damage. The results showed that FA treatment significantly increased the leaf water content and antioxidant enzyme activities and decreased reactive oxygen species (ROS) accumulation, the proline (Pro) content, and the relative electrical conductivity (REC). Moreover, FA treatment improved photosynthetic parameters and chlorophyll (Chl) fluorescence parameters, maintained the integrity of chloroplasts and mesophyll cells, and increased the expression level of drought-tolerant genes. These results indicated that FA treatment could induce antioxidant enzymes to eliminate ROS, reduce membrane lipid peroxidation and decrease damage to photosynthesis in P. ostii under drought stress, which would provide a measure for alleviating the damage of P. ostii caused by drought stress.KEYWORDS: P. ostii, fulvic acid, drought stress, antioxidant enzymes, photosynthesis     

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.     

Seed biopriming with drought tolerant isolates of Trichoderma harzianum promote growth and drought tolerance in Triticum aestivum

Green house study was aimed to investigate the effect of seed biopriming with drought tolerant isolates of Trichoderma harzianum, viz. Th 56, 69, 75, 82 and 89 on growth of wheat under drought stress and to explore the mechanism underlying plant water stress resilience in response to Trichoderma inoculation. Measurements of relative water content, osmotic potential, osmotic adjustment, leaf gas exchange, chlorophyll fluorescence and membrane stability index were performed. In addition, analysis of the phenolics, proline, lipid peroxidation and measurements of phenylalanine ammonia‐lyase activity were carried out. Seed biopriming enhanced drought tolerance of wheat as drought induced changes like stomatal conductance, net photosynthesis and chlorophyll fluorescence were delayed. Drought stress from 4 to 13 days of withholding water induced an increase in the concentration of stress induced metabolites in leaves, while Trichoderma colonisation caused decrease in proline, malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), and an increase in total phenolics. A common factor that negatively affects plants under drought stress conditions is accumulation of toxic reactive oxygen species (ROS), and we tested the hypothesis that seed biopriming reduced damages resulting from accumulation of ROS in stressed plants. The enhanced redox state of colonised plants could be explained by higher l ‐phenylalanine ammonia‐lyase (PAL) activity in leaves after 13 days of drought stress in Trichoderma treated plants. Similar activity was induced in untreated plants in response to drought stress but to a lower extent in comparison to treated plants. Our results support the hypothesis that seed biopriming in wheat with drought tolerant T. harzianum strains increased root vigour besides performing the process of osmoregulation. It ameliorates drought stress by inducing physiological protection in plants against oxidative damage, due to enhanced capacity to scavenge ROS and increased level of PAL, a mechanism that is expected to augment tolerance to abiotic stresses.     

The Positive Correlations Between the Activity of Superoxide Dismutase and Dehydration Tolerance in Wheat Seedlings

Damage to crops by drought is still a serious problem in large areas of the world. Considerable research has been undertaken to discover the mechanisms of drought injury and drought resistance of plants. However, the critical features of drought injury have not yet been identified. In the past ten years a free radical hypothesis has been suggested to account for subcellular damage caused by severe environments. Superoxide (oxygen radical) is normally produced in hydrated tissues. It is controlled by free radical scavenging reactions. One such scavenger is the enzyme superoxide dismutase (SOD). Under water stress, production of excess free radicals may occur in dehydrated plant tissues and this probably damages the membranes by causing peroxidation of the lipid components. So far few studies have been done to determine if drought injury is correlated with the free radical mechanism. In the present study, the SOD activities in wheat seedlings under water stress have been investigated by measuring the photoreduction of nitro blue tetrazolium using a spectrometric method. Meanwhile, the viabilities of wheat seedlings during drying were followed by tetrazolium test. These results provided information on the relationship between SOD activity and the dehydration tolerance of the plant. Results indicated that SOD activity changed with the time after germination. The activity of SOD of 24 h seedlings was 1.9 times higher than those of 72 h seedlings based on fresh weight. SOD activity in shoot was also higher than in root. These results were consistent with the results obtained from rating of the viabilities of seedlings during drying. The 24h seedlings were more tolerant of dehydration than 72 h seedlings and root were more sensitive of drought than shoot. In addition, shoot and root tips showed the higher SOD activities than non-tip region and they also showed a higher survival ability upon dehydration. In dehydration and subsequent rehydration, SOD activity, different from many other enzymes in plants, increased rather than declined during drying. After rehydration SOD activity returned to nearly the original level. Therefore, the positive correlations were found to exist between SOD activity and dehydration tolerance. It is reasonable to suggest that SOD enzyme may play a protective role against damage caused by free radicals which may be produced excessively during dehydration in wheat seedling.     

The influence of the mycorrhiza Glomus etunicatum on drought acclimation in safflower and wheat

A study was done to determine the effects of vesicular‐arbuscular mycorrhizal (VAM) colonization on drought acclimation of host plants. Safflower ( Carthamus tinctorius L. cv. S555) and wheat ( Triticum aestivum L. cv. Anza) were grown under environmentally controlled conditions with or without the VAM fungus, Glomus etunicatum Becker and Gerd., and were either acclimated (by pre‐exposing plants to a 10–11 day drought period) or unacclimated to drought. Plants from all treatments were then exposed to drought for 9 days, and plant water status and root water uptake were measured. To minimize interactions between drought and P uptake, growth periods were adjusted so that acclimated and unacclimated plants were similar in size when measurements were made. When wheat was acclimated to drought, osmotic adjustment occurred (leaf solute potentials of leaf tissue rehydrated to full turgor were approximately 0.5 MPa lower in acclimated than unacclimated plants); in safflower, osmotic adjustment was minimal when plants were acclimated. Consequently, acclimated wheat plants were able to tolerate drought better than unacclimated plants, and maintained higher leaf water potentials and relative water contents as soil water was depleted. For both safflower and wheat, acclimated plants had higher water use efficiency, and therefore produced more biomass when water availability was limited, than unacclimated plants. However, mycorrhizal colonization did not affect osmotic adjustment, plant water status, water use efficiency or water uptake in either plant species, and therefore had no effect on drought acclimation or resistance.     

Antioxidant response of wheat roots to drought acclimation

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. The antioxidant defense in terms of redox metabolites and enzymes in root cells and mitochondria was examined in relation to membrane damage. Acclimated seedlings exhibited higher relative water content and were able to limit the accumulation of H₂O₂ and membrane damage during subsequent severe water stress conditions. This was due to systematic up-regulation of superoxide dismutase, ascorbate peroxidase (APX), catalase, peroxidases, and ascorbate–glutathione cycle components at both the whole cell level as well as in mitochondria. In contrast, direct exposure of severe water stress to non-acclimated seedlings caused greater water loss, excessive accumulation of H₂O₂ followed by elevated lipid peroxidation due to the poor antioxidant enzyme response particularly of APX, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and ascorbate–glutathione redox balance. Mitochondrial antioxidant defense was found to be better than the cellular defense in non-acclimated roots. Termination of stress followed by rewatering leads to a rapid enhancement in all the antioxidant defense components in non-acclimated roots, which suggested that the excess levels of H₂O₂ during severe water stress conditions might have inhibited or down-regulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced tolerance toward oxidative stress in the root tissue of wheat seedlings due to both reactive oxygen species restriction and well-coordinated induction of antioxidant defense.     

Hydrogen peroxide pretreatment of roots enhanced oxidative stress response of tomato under cold stress

In the view of physiological role of H₂O₂, we investigated whether exogenous H₂O₂ application would affect short-term cold response of tomato and induce acclimation. Pretreatments were performed by immersing roots into 1 mM H₂O₂ solution for 1 h when transferring seedlings from seedling substrate to soil (acclimated group). Cold stress (3 °C for 16 h) caused significant reduction in relative water content (RWC) of control and non-acclimated (distilled water treated) groups when compared with unstressed plants. H₂O₂ promoted maintenance of relatively higher RWC under stress. Anthocyanin level in leaves of acclimated plants under cold stress was significantly higher than that of unstressed control and non-acclimated plants. Malondialdehyde (MDA) levels demonstrated low temperature induced oxidative damage to control and non-acclimated plants. MDA remained around unstressed conditions in acclimated plants, which demonstrate that H₂O₂ acclimation protected tissues against cold induced lipid peroxidation. H₂O₂ acclimation caused proline accumulation in roots under cold stress. Ascorbate peroxidase (APX) activity in roots of cold stressed and unstressed H₂O₂ acclimated plants increased when compared with control and non-acclimated plants, with highest increase in roots of acclimated plants under cold stress. CAT levels in roots of acclimated plants also increased, whereas levels remained unchanged in unstressed plants. Endogenous H₂O₂ levels significantly increased in roots of control and non-acclimated plants under cold stress. On the other hand, H₂O₂ content in roots of acclimated plants was significantly lower than control and non-acclimated plants under cold stress. The results presented here demonstrated that H₂O₂ significantly enhanced oxidative stress response by elevating the antioxidant status of tomato.     

Nitric oxide (as sodium nitroprusside) supplementation ameliorates Cd toxicity in hydroponically grown wheat roots

Cadmium (Cd) is a non-redox toxic heavy metal present in the environment and induces oxidative stress in plants. We investigated whether exogenous nitric oxide (NO) supplementation as sodium nitroprusside (SNP) has any ameliorating action against Cd-induced oxidative damage in plant roots and thus protective role against Cd toxicity. Cd treatment (50 or 250 μM) alone or in combination with 200 μM SNP was given to hydroponically grown wheat roots for a short time period of 24 h and then these were shifted to distilled water to observe changes in levels of oxidative markers (lipid peroxidation, H₂O₂ content and electrolyte leakage). Supplementation of Cd with SNP significantly reduced the Cd-induced lipid peroxidation, H₂O₂ content and electrolyte leakage in wheat roots. It indicated a reactive oxygen species (ROS) scavenging activity of NO. However, even upon removal of Cd-treatment solution, the levels of oxidative markers increased during 24 h recovery stage and later at 48 h these decreased. Cd treatment resulted in an upregulation of activities of antioxidant enzymes—superoxide dismutase (SOD, 1.15.1.1), guaiacol peroxidase (GPX, 1.11.1.7), catalase (CAT, 1.11.1.6), and glutathione reductase (GR, 1.6.4.2). SNP supply resulted in a reduction in Cd-induced increased activities of scavenging enzymes. The protective role of exogenous NO in decreasing Cd-induced oxidative damage was also evident from the histochemical localization of lipid peroxidation, plasma membrane integrity and superoxides. The study concludes that an exogenous supply of NO protects wheat roots from Cd-induced toxicity.     

干旱-复水处理对杠柳幼苗光合作用及活性氧代谢的影响

采用人工控制土壤水分试验,以80%田间持水量处理为对照,研究了3次干旱-复水处理对2年生杠柳幼苗叶片光合作用,根、茎、叶膜脂过氧化和抗氧化系统的影响.结果表明: 在干旱条件下, 杠柳叶片相对含水量和光合速率(P_n)显著降低,光合色素含量升高. 干旱复水后,叶片相对含水量完全恢复,反映了杠柳较强的旱后修复能力;叶绿素含量和P_n均明显高于对照,表现出补偿效应,适度干旱诱导了杠柳的抗旱适应性.干旱胁迫使P_n下降,以气孔限制为主,但在中午发生了非气孔限制;幼叶、新茎和细根中的超氧阴离子产生速率升高,丙二醛含量则降低,说明这些幼嫩组织未受到氧化伤害.不同器官中SOD、CAT和POD活性及变化趋势不同,以细根的3种保护酶的反应最为灵敏,说明细根是杠柳适应干旱环境的重要器官.器官间的相互合作与协调使杠柳能有效地适应干湿交替的干旱环境.     

Generation of Hydroxyl Radicals and Its Relation to Cellular Oxidative Damage in Plants Subjected to Water Stress

The hydroxyl radical ('OH) is one of the roost reactive mdieales known to chemistry and is believed to be a major active free radicle responsible for modifications of macmmolecules and cellular damage. Two lines of evidence strongly indicate that 'OH radicals are generated in a Fenton-type Haber-Weiss reactions in plants subjected to water stress. Firstly, water stress causes an increase in the concentration of catalytic metals, which are critical for Fenton-like reactions to proceed in vivo. Furthermore, subrmillimolar concentrations of H2O2 and ascorbic acid(or O2－ ) in the drought-stressed plants are large enough to support the Fentontype Haber-Weiss reactions. Secondly, there is oxidation of proteins and lipids in the drought-stressed plants; a process that requires a catalytic metal and that, at least for protein oxidation, is mediated by the 'OH radicals. Protein oxidation is thought to involve binding of metal ions to the proteins and subsequent site-specific attack by the 'OH radicals arising from the roetal-catalysed decomposition of H2O2. It has been proposed that protein oxidation may be a better index than lipid peroxidation because the latter fields many different products and these only appear after a lag period. The validity of malondialdehyde (MDA), an early product of lipid peroxidation, as an index of lipid peroxidation has been argued by the non-specific method of its measurement. The 'OH radicals are not the only necessary initiator for lipid peroxidation and lipid peroxidation is not usually involved in plants exposed to water stress.     

Change in antioxidant responses against oxidative damage in black chickpea following cold acclimation

Cold stress is an important factor affecting chickpea (Cicer arietinum L.) plants in winter and early spring. We evaluated the effects of cold stress by measuring lipid peroxidation, membrane permeability, and some enzyme activities involved in the ROS-scavenging system under acclimation and non-acclimation conditions in black chickpea Kaka, a popular genotype planted, and accession 4322, as a landrace genotype. Under non-acclimation conditions, the genotype 4322 prevented the H₂O₂ accumulation more efficiently, which led to a decrease in lipid peroxidation and membrane permeability compared to Kaka. Studying the activities of antioxidant enzymes showed that catalase was more effective enzyme in cell protection against H₂O₂ in 4322 plants. Such response in acclimated plants was more pronounced than in control and nonacclimated plants. In this study, the increase in guaiacol peroxidase and ascorbate peroxidase activities did not preserve cell membranes from oxidative damage in Kaka plants. It was observed that short-term acclimation can induce greater cold tolerance upon the increase of oxidative stress in chickpea plants. This was due to low levels of MDA and electrolyte leakage index, indicating the lower lipid peroxidation and higher membrane stability under the cold stress compared to non-acclimated plants.     

Antioxidant responses to water deficit by drought‐tolerant and ‐sensitive sugarcane varieties

Water deficit is the major yield‐limiting factor of crop plants. The exposure of plants to this abiotic stress can result in oxidative damage due to the overproduction of reactive oxygen species. The aim of this work was to study the antioxidant‐stress response of drought‐tolerant (SP83‐2847 and SP83‐5073) and drought‐sensitive (SP90‐3414 and SP90‐1638) sugarcane varieties to water‐deficit stress, which was imposed by withholding irrigation for 3, 10 and 20 days. The drought‐sensitive varieties exhibited the lowest leaf relative water content and highest lipid peroxidation, hydrogen peroxide (H₂O₂) and proline contents during the progression of the drought‐stress condition. The antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPOX) and glutathione reductase (GR) activities changed according to variety and stress intensity. SP83‐2847 exhibited higher CAT and APX activities than the other varieties in the early stage of drought, while the activities of GPOX and GR were the highest in the other varieties at the end of the drought‐stress period. A Cu/Zn SOD isoenzyme was absent at the end of drought period from the SP90‐3414‐sensitive variety. The results indicate that lipid peroxidation and early accumulation of proline may be good biochemical markers of drought sensitivity in sugarcane.     

Free radical and freezing injury to cell membranes of winter wheat

The symptoms of injury in microsomal membranes isolated from crowns of seedlings of Triticum aestivum , L. cultivar Fredrick after a lethal freeze-thaw stress included an increased lipid phase transition temperature, loss of lipid phosphate (lipid-P), and increased free fatty acid levels. However, minimal changes in fatty acid saturation were observed, suggesting minimal amounts of lipid peroxidation. All of these injury symptoms, including the lack of lipid peroxidation, were simulated in vitro by treatment of isolated membranes with oxygen free radicals, generated from either xanthine oxidase (EC 1.1.3.22) or paraquat (l,r-dimethyl-4,4'-bipyridinium dichloride). Further evidence indicating a relationship between free radicals and freezing injury comes from the observation that both protoplasts and microsomal membranes isolated from wheat seedlings, that had been acclimated to induce freezing tolerance, also had increased tolerance of oxygen free radicals, and contained higher lipid-soluble antioxidant levels, than those from non-acclimated seedlings. Lipid-soluble antioxidants accumulated in the crown tissue of the wheat seedling during the acclimation period. Freezing stress accelerated the formation of oxygen free radicals. Membranes isolated from crowns after a freeze–thaw stress tended to produce higher levels of superoxide as shown by the reduction of Tiron (1,2-dihydroxy-l,3-benzenedisulfonic acid). In protoplasts, increased superoxide production coincided with lethal freezing injury. These results are discussed in terms of the possible involvement of oxygen free radicals in mediating aspects of freezing injury to cell membranes.     

Permeability behaviour of lipid vesicles prepared from plant plasma membranes--impact of compositional changes

Exposure of oat seedlings to repeated moderate water deficit stress causes a drought acclimation of the seedlings. This acclimation is associated with changes in the lipid composition of the plasma membrane of root cells. Here, plasma membranes from root cells of acclimated and control plants were isolated using the two-phase partitioning method. Membrane vesicles were prepared of total lipids extracted from the plasma membranes. In a series of tests the vesicle permeability for glucose and for protons were analysed and compared with the permeability of model vesicles. Further, the importance of critical components for the permeability properties was analysed by modifying the lipid composition of the vesicles from acclimated and from control plants. The purpose was to add specific lipids to vesicles from acclimated plants to mimic the composition of the vesicles from control plants and vice versa. The plasma membrane lipid vesicles from acclimated plants had a significantly increased permeability for glucose and decreased permeability for protons as compared to control vesicles. The results point to the importance of the ratio phosphatidylcholine (PC)/phosphatidylethanolamine (PE), the levels of cerebrosides and free sterols and the possible interaction of these components for the plasma membrane as a permeability barrier.     

Influence of arbuscular mycorrhiza on lipid peroxidation and antioxidant enzyme activity of maize plants under temperature stress

The influence of the arbuscular mycorrhizal (AM) fungus, Glomus etunicatum, on characteristics of growth, membrane lipid peroxidation, osmotic adjustment, and activity of antioxidant enzymes in leaves and roots of maize (Zea mays L.) plants was studied in pot culture under temperature stress. The maize plants were placed in a sand and soil mixture under normal temperature for 6 weeks and then exposed to five different temperature treatments (5oC, 15oC, 25oC, 35oC, and 40oC) for 1 week. AM symbiosis decreased membrane relative permeability and malondialdehyde content in leaves and roots. The contents of soluble sugar content and proline in roots were higher, but leaf proline content was lower in mycorrhizal than nonmycorrhizal plants. AM colonization increased the activities of superoxide dismutase, catalase, and peroxidase in leaves and roots. The results indicate that the AM fungus is capable of alleviating the damage caused by temperature stress on maize plants by reducing membrane lipid peroxidation and membrane permeability and increasing the accumulation of osmotic adjustment compounds and antioxidant enzyme activity. Consequently, arbuscular mycorrhiza formation highly enhanced the extreme temperature tolerance of maize plant, which increased host biomass and promoted plant growth.     

Exogenous salicylic acid alleviates effects of long term drought stress and delays leaf rolling by inducing antioxidant system

Salicylic acid (SA) is one of the important signal molecules modulating plant responses to environmental stress. In this study, the effects of exogenous SA on leaf rolling, one of drought avoidance mechanisms, and antioxidant system were investigated in Ctenanthe setosa during long term drought stress. The plants were subjected to 38-day drought period and they were treated with or without SA (10⁻⁶ M) on the 25th, 27th and 29th days of the period. Leaf samples were harvested on the 30th, 34th and 38th days. Some antioxidant enzyme activities (superoxide dismutase, catalase, ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, glutathione reductase), reactive oxygen species (hydrogen peroxide and superoxide) and lipid peroxidation were determined during the drought period. Treatment with SA prevented water loss and delayed leaf rolling in comparison with control leaves. Exogenous SA induced all antioxidant enzyme activities more than control leaves during the drought. Ascorbate and glutathione, α-tocopherol, carotenoid and endogenous SA level were induced by the SA treatment. Levels of reactive oxygen species were higher in SA treated plants than control ones on the 34th day. Their levels on the 38th day, however, fastly decreased in SA treated plants. SA treatment prevented lipid peroxidation while the peroxidation increased in control plants. The results showed that exogenous SA can alleviate the damaging effect of long term drought stress by decreasing water loss and inducing the antioxidant system in the plant having leaf rolling, alternative protection mechanism to drought.     

Pre-anthesis high-temperature acclimation alleviates damage to the flag leaf caused by post-anthesis heat stress in wheat

The objective of this study was to investigate the effect of pre-anthesis high-temperature acclimation on leaf physiology of winter wheat in response to post-anthesis heat stress. The results showed that both pre- and post-anthesis heat stresses significantly depressed flag leaf photosynthesis and enhanced cell membrane peroxidation, as exemplified by increased O₂⁻ production rate and reduction in activities of antioxiditave enzymes. However, under post-anthesis heat stress, plants with pre-anthesis high-temperature acclimation (HH) showed much higher photosynthetic rates than those without pre-anthesis high-temperature acclimation (CH). Leaves of HH plants exhibited a higher Chl a/b ratio and lower chlorophyll/carotenoid ratio and superoxide anion radical release rate compared with those of the CH plants. In addition, antioxidant enzyme activities in HH plants were significantly higher than in CH. Coincidently, expressions of photosythesis-responsive gene encoding Rubisco activase B (RcaB) and antioxidant enzyme-related genes encoding mitochondrial manganese superoxide dismutase (Mn-SOD), chloroplastic Cu/Zn superoxide dismutase (Cu/Zn-SOD), catalase (CAT) and cytosolic glutathione reductase (GR) were all up-regulated under HH, whereas a gene encoding a major chlorophyll a/b-binding protein (Cab) was up-regulated by post-anthesis heat stress at 10 DAA, but was down-regulated at 13 DAA. The changes in the expression levels of the HH plants were more pronounced than those for the CH. Collectively, the results indicated that pre-anthesis high-temperature acclimation could effectively alleviate the photosynthetic and oxidative damage caused by post-anthesis heat stress in wheat flag leaves, which was partially attributable to modifications in the expression of the photosythesis-responsive and antioxidant enzymes-related genes.     

Silicon Improves the Tolerance to Water-Deficit Stress Induced by Polyethylene Glycol in Wheat (Triticum aestivum L.) Seedlings

Drought stress usually causes a serious yield reduction in wheat production. Silicon (Si) has been reported to be able to alleviate drought stress damage; however, the mechanism is still poorly understood. In this article, the effects of Si (as sodium silicate) on some parameters related to oxidative damage, proline, soluble sugar, and inorganic ions in the leaves of wheat under 20% (w/v) polyethylene glycol (PEG-6000) simulative drought stress are investigated. PEG stress depressed the growth of shoot and root and decreased leaf water potential and chlorophyll concentration. Addition of 1.0 mM Si could partially improve the growth of shoot (but not root) and increase the leaf chlorophyll concentrations of stressed plants. Inclusion of Si in culture solution also maintained leaf water potential of stressed plants at the same level as that of the control plants. PEG stress induced significant accumulation of leaf hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) as well as an increase in electrolyte leakage, which were all decreased by added silicon. These results suggest that stress-induced membrane lipid peroxidation could be partly alleviated by added silicon. Moreover, the results were also supported by the observation that PEG stress-induced decrease in glutathione concentration in the leaves was reversed by added silicon. The proline concentration in the leaves was markedly increased under PEG stress, whereas added silicon partially reversed this. PEG stress decreased the leaf soluble sugar concentration. There were significant negative regressions between proline concentration and both shoot dry weight and leaf chlorophyll concentrations, whereas there were positive regressions between the proline concentration and both H₂O₂ and MDA concentrations in the leaves, supporting the view that proline accumulation is a symptom of stress damage rather than stress tolerance. Addition of Si obviously increased Si accumulation in the shoot. Analyses of Na, Mg, K, and Ca showed no accumulation of these ions in the shoot (on the basis of per tissue dry weight) under water stress, and added Si even decreased their concentrations. These results suggest that under short-term PEG-induced water stress conditions (1 week), antioxidant defense, rather than osmotic adjustment, contributed to the improved wheat growth by Si.