Salicylic acid pretreatment induces drought tolerance and delays leaf rolling by inducing antioxidant systems in maize genotypes

Salicylic acid (SA) acts as an endogenous signal molecule responsible for inducing abiotic stress tolerance in plants. In this study, the role of SA in improving drought tolerance in two maize cultivars (Zea mays L.) differing in their tolerance to drought was evaluated. The plants were regularly watered per pot and grown until the grain filling stage (R2) under a rainout shelter. At stage R2, parts of the plants were treated with SA, after which drought stress was applied. Leaf samples were harvested on the 10th and 17th days of the drought. Some antioxidant enzyme activity, such as the superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) content, was measured during the drought period. Exogenous SA prevented water loss and delayed leaf rolling in comparison with control leaves in both cultivars. As a consequence of drought stress, lipid peroxidation, measured in terms of malondialdehyde content, was prevented by SA. SA pretreatment induced all antioxidant enzyme activities, and to a greater extent than the control leaves, during drought. SA also caused a reduction in the ascorbate (ASC) and glutathione (GSH) content in two maize cultivars. The H₂O₂ level was higher in SA pretreated plants than the controls in both cultivars. Pretreatment with SA further enhanced the activities of antioxidant enzymes and the concentrations of non-enzymatic antioxidants in the tolerant cultivar compared with the sensitive cultivar. Results suggested that exogenous SA could help reduce the adverse effects of drought stress and might have a key role in providing tolerance to stress by decreasing water loss and inducing the antioxidant system in plants with leaf rolling, an alternative drought protection mechanism.     

Antioxidant system is essential to increase drought tolerance of sugarcane

Drought is one of the main factors affecting the productivity of agricultural crops, and plants respond to such stress by activating various physiological and biochemical mechanisms against dehydration. The present study investigated two varieties of sugarcane (Saccharum spp.) with contrasting responses to drought (RB867515, more tolerant; and RB855536, less tolerant) and subjected them to progressive drought conditions (2, 4, 6 and 8 days) followed by rehydration. Drought caused a decrease in water potential (ψ_w) and osmotic potential (ψ_os) in the leaves, which recovered to normal levels after rehydration only up to the fourth day of drought. Water stress changed the carbon metabolism of leaves by reducing starch and sucrose contents and increasing glucose and fructose contents in both varieties. Water deficit caused a significant reduction in the maximum quantum efficiency of photosystem II (F_v/F_m) and effective quantum yield (Φ_PSII) in both varieties; however, RB867515 recovered faster after rehydration. Under water stress, the more tolerant variety RB867515 exhibited increased activity of the antioxidant enzymes catalase, ascorbate peroxidase and superoxide dismutase compared with the RB855536 variety. The results suggest that RB867515 is more tolerant to drought conditions because of a more efficient antioxidant system, which results in reduced photosynthesis photoinhibition during water stress, thus revealing itself as a potential physiological marker for drought tolerance studies.     

Antioxidant defence system and physiological responses of Iranian crested wheatgrass (<Emphasis Type="Italic">Agropyron cristatum</Emphasis> L.) to drought and salinity stress

Crested wheatgrass (Agropyron cristatum L.) is a cool-season perennial grass, which has demonstrated its potential for use as turfgrass. However, limited information is available on its drought and salinity tolerance. The main purpose of this study was to investigate the changes in the antioxidant defence system and physiological traits of six Iranian crested wheatgrass genotypes under drought and salinity stresses. The experimental design comprised a split plot with water treatments (control well-watered, salinity stress and water stress) as main plots and genotypes as subplots. This study demonstrated the variations in drought and salinity tolerance among crested wheatgrass genotypes. ‘ACSKI’, ‘ACAMI’ and ‘ACDTI’ generally performed better than other genotypes under drought and salinity conditions, mainly by maintaining higher activities of antioxidant enzymes like superoxide dismutase, peroxidase, catalase, ascorbate peroxidase and non-enzyme antioxidants like glutathione, higher proline and total non-structural carbohydrates content. The increased, decreased, and unchanged activities of antioxidant enzymes in the crested wheatgrass genotypes indicates a different forms of metabolism of antioxidant enzymes in response to drought and salinity stress. In general, drought and salinity stress increases the malondialdehyde (MDA) content and hydrogen peroxide (H₂O₂) content; however, ‘ACSKI’, ‘ACAMI’ and ‘ACDTI’ genotypes could tolerate an increase in MDA content and H₂O₂ content; therefore, lower levels of MDA content and H₂O₂ content were observed. The results showed that increasing levels of diamine oxidase and polyamine oxidase have been associated with increasing drought and salinity tolerance. Based on the biochemical and physiological parameters that were evaluated, we concluded that the genotype ‘ACSKI’ was superior in terms of drought and salinity tolerance. This superiority was mainly a result of better enzymatic and non-enzymatic antioxidant defence system and better osmotic adjustment under stress conditions.     

Exogenous application of calcium chloride in wheat genotypes alleviates negative effect of drought stress by modulating antioxidant machinery and enhanced osmolyte accumulation

Plants use various mechanisms to cope with drought constraints at morphological, physiological, and biochemical level by means of different adaptive mechanisms. All organisms use a network of signal transduction pathways to control their metabolism and to adapt to the environment. Among these pathways, calcium (Ca²⁺) ions play an important role as a universal second messenger. Calcium has unique properties and universal ability to transmit diverse signals that trigger primary physiological actions in the cell in response to hormones, pathogens, and stress factors. Calcium plays a fundamental role in regulating the polar growth of cells and tissues and participates in plant adaptation to various stress factors. This study was conducted to examine the role of Ca²⁺ in ameliorating the adverse effect of drought stress responses in two contrasting wheat genotypes, HD 2733 (drought sensitive) and HD 2987 (drought tolerant), differing in their drought tolerance. The plants were treated with mannitol or Hoagland solution and then supplemented with CaCl₂ (10 mM). Measurements of seed germination, shoot growth, and chlorophyll content showed that calcium treatment increased all these factors in tolerant genotype (HD 2987) under induced stress condition. Drought stress reduced relative water content, osmolyte, and soluble sugar accumulation in both the genotypes, but CaCl₂ supplementation increased all the components under stress condition in HD 2987 as compared to HD 2733. The oxidative damage caused by induced stress was lower in HD 2987 compared to HD 2733 genotypes as assessed by their higher photosynthetic capacity and lower electrolyte leakage, malondialdehyde (MDA) content as well as H₂O₂ accumulation. Less accumulation of superoxide and H₂O₂ was also observed in HD 2987 genotype after CaCl₂ supplementation combined with mannitol treatment. In addition, the enhanced accumulation of calcium in the HD 2987 genotype is correlated with the higher activities of antioxidant enzymes than HD 2733 genotype under similar stress conditions. Our findings provide evidence of the protective role of exogenous calcium in conferring better tolerance against mannitol-induced drought stress in wheat genotypes, which could be useful as genetic stock to develop wheat tolerant varieties in breeding programs.     

Natural variation of drought response in Brachypodium distachyon

Brachypodium distachyon (Brachypodium) is a temperate wild grass species and is a powerful model system for studying grain, energy, forage and turf grasses. Exploring the natural variation in the drought response of Brachypodium provides an important basis for dissecting the genetic network of drought tolerance. Two experiments were conducted in a greenhouse to assess the drought tolerance of 57 natural populations of Brachypodium. Principle component analysis revealed that reductions in chlorophyll fluorescence (Fv/Fm) and leaf water content (LWC) under drought stress explained most of the phenotypic variation, which was used to classify the tolerant and susceptible accessions. Four groups of accessions differing in drought tolerance were identified, with 3 tolerant, 16 moderately tolerant, 32 susceptible and 6 most susceptible accessions. The tolerant group had little leaf wilting and fewer reductions in Fv/Fm and LWC, while the most susceptible groups showed severe leaf wilting and more reductions in Fv/Fm and LWC. Drought stress increased total water soluble sugar (WSS) concentration, but no differences in the increased WSS were found among different groups of accessions. The large phenotypic variation of Brachypodium in response to drought stress can be used to identify genes and alleles important for the complex trait of drought tolerance.     

Physiological performance of two contrasting rice varieties under water stress

Two rice varieties PR-115 and Super-7 were imposed to water stress and different physiological traits were monitored to evaluate the performance of these varieties under drought. Under water stress condition although the relative water content, osmotic potential, chlorophyll content, photosynthesis rate, carbon discrimination and biomass decreased in both the varieties however, the reduction was more pronounced in Super-7 variety. Oryzanol a trans-ester of ferulic acid functions as antioxidant and it increased along with total phenolic and anthocyanin content in both the varieties under drought stress. However, gallic acid, 4 hydroxy benzoic acid, syringic acid and chlorogenic acid showed differential pattern in both of the varieties under water limiting conditions. Under drought, grain yield was penalized by 17 and 54% in PR-115 and Super-7 varieties, respectively in comparison to watered plants. Super-7 variety showed pronounced electrolyte leakage and MDA enhancement under water stress condition. High non photochemical quenching and reduction in Y(NO) and Y(II) indicated balanced energy management in tolerant PR-115 variety. The studies showed that PR-115 is a drought tolerant variety while Super-7 is drought sensitive in nature.     

ABA application to maize hybrids contrasting for drought tolerance: changes in water parameters and in antioxidant enzyme activity

The objective of this study was to evaluate the effects of abscisic acid (ABA) related to the increase of water-stress tolerance in two drought contrasting maize hybrids: DKB 390 (tolerant) and BRS 1030 (sensitive). The characterization of water status (pre-dawn leaf water potential, Ψ_pd; midday leaf water potential, Ψ_md and stem water potential, Ψ_st) and antioxidant enzyme activity was conducted on greenhouse grown plants. The ABA, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) contents were also analyzed. Water deficit was imposed for 10 days at the flowering stage and a dosage of 100 μM ABA was applied to plant canopy. Measurements were taken during 10 days after the water recovery. With 5 days of stress, the tolerant hybrid showed lower MDA content, decrease in the water status, and higher activity of the enzymes superoxide dismutase, catalase, ascorbate peroxidase, as well as guaiacol, glutathione reductase, dehydroascorbate reductase, polyphenol oxidase, and l-phenylalanine ammonia-lyase, as compared to the sensitive hybrid. With 10 days of stress, DKB 390 had a decrease in the activity of enzymes whereas BRS 1030 showed a higher activity. In addition, the latter showed greater amounts of H₂O₂ and MDA. ABA application led to a higher tolerance only in DKB 390, due to the increase of water status and the enzymatic activity, mainly the catalase.     

Physiological response of wheat varieties to elevated atmospheric CO2 and low water supply levels

In the phytotron experiment, the effect of elevated atmospheric CO₂ (EC, 750 μmol mol^?1) on the drought tolerance was studied in two winter varieties (Mv Mambo, tolerant; Mv Regiment, moderately tolerant) and in one spring variety of wheat (Lona, sensitive to drought). Changes in net photosynthetic rate (P _N), stomatal conductance, transpiration, wateruse efficiency, effective quantum yield of photosystem II, and activities of glutathione reductase (GR), glutathione-Stransferase (GST), guaiacol peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) were monitored during water withdrawal. Drought caused a faster decline of P _N at EC, leading to the lower assimilation rates under severe drought compared with ambient CO₂ (NC). In the sensitive variety, P _N remained high for a longer period at EC. The growth at EC resulted in a more relaxed activation level of the antioxidant enzyme system in all three varieties, with very low activities of GR, GST, APX, and POD. The similar, low values were due to decreases in the varieties which had higher ambient values. A parallel increase of CAT was, however, recorded in two varieties. As the decline in P _N was faster at EC under drought but there was no change in the rate of electron transport compared to NC values, a higher level of oxidative stress was induced. This triggered a more pronounced, general response in the antioxidant enzyme system at EC, leading to very high activities of APX, CAT, and GST in all three varieties. The results indicated that EC had generally favourable effects on the development and stress tolerance of plants, although bigger foliage made the plants more prone to the water loss. The relaxation of the defence mechanisms increased potentially the risk of damage due to the higher level of oxidative stress at EC under severe drought compared with NC.     

Morpho-physiological responses of sugar beet (Beta vulgaris L.) genotypes to drought stress

The identification of morpho-physiological traits related to drought tolerance and high yield potential is a challenge when selecting sugar beet genotypes with greater tolerance to water stress. In this paper, root morphological parameters, antioxidant systems, leaf relative water content (RWC) and H⁺-ATPase activity as key morpho-physiological traits involved in drought tolerance/susceptibility of sugar beet were studied. Genotypes showing a different drought tolerance index (DTI) but a similar yield potential, under moderate (?0.6 Mpa) and severe (?1.2 MPa) water stress, were selected and their morpho-physiological traits were investigated. The results showed a wide genetic variation in morpho-physiological parameters which demonstrated the different adaptive strategies under moderate and severe drought conditions in sugar beet. In particular, an efficient antioxidant system and redox signalling made some sugar beet genotypes more tolerant to drought stress. The alternative strategy of other genotypes was the reduction of root tissue density, which produced a less dense root system improving the axial hydraulic conductivity. These results could be considered as interesting challenge for a better understanding of the drought tolerance mechanisms in sugar beet.     

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.     

Adaptation to high temperature mitigates the impact of water deficit during combined heat and drought stress in C3 sunflower and C4 maize varieties with contrasting drought tolerance

Heat and drought stress frequently occur together, however, their impact on plant growth and photosynthesis (P_N) is unclear. The frequency, duration and severity of heat and drought stress events are predicted to increase in the future, having severe implications for agricultural productivity and food security. To assess the impact on plant gas exchange, physiology and morphology we grew drought tolerant and sensitive varieties of C3 sunflower (Helianthus annuus) and C4 maize (Zea mays) under conditions of elevated temperature for 4 weeks prior to the imposition of water deficit. The negative impact of temperature on P_N was most apparent in sunflower. The drought tolerant sunflower retained ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RubisCO) activity under heat stress to a greater extent than its drought sensitive counterpart. Maize exhibited no varietal difference in response to increased temperature. In contrast to previous studies, where a sudden rise in temperature induced an increase in stomatal conductance (G_s), we observed no change or a reduction in G_s with elevated temperature, which alongside lower leaf area mitigated the impact of drought at the higher temperature. The drought tolerant sunflower and maize varieties exhibited greater investment in root‐systems, allowing greater uptake of the available soil water. Elevated temperatures associated with heat‐waves will have profound negative impacts on crop growth in both sunflower and maize, but the deleterious effect on P_N was less apparent in the drought tolerant sunflower and both maize varieties. As C4 plants generally exhibit water use efficiency (WUE) and resistance to heat stress, selection on the basis of tolerance to heat and drought stress would be more beneficial to the yields of C3 crops cultivated in drought prone semi‐arid regions.     

Responses of antioxidant enzymes to cold and high light are not correlated to freezing tolerance in natural accessions of Arabidopsis thaliana

Low temperatures and high light cause imbalances in primary and secondary reactions of photosynthesis, and thus can result in oxidative stress. Plants employ a range of low‐molecular weight antioxidants and antioxidant enzymes to prevent oxidative damage, and antioxidant defence is considered an important component of stress tolerance. To figure out whether oxidative stress and antioxidant defence are key factors defining the different cold acclimation capacities of natural accessions of the model plant Arabidopsis thaliana, we investigated hydrogen peroxide (H₂O₂) production, antioxidant enzyme activity and lipid peroxidation during a time course of cold treatment and exposure to high light in four differentially cold‐tolerant natural accessions of Arabidopsis (C24, Nd, Rsch, Te) that span the European distribution range of the species. All accessions except Rsch (from Russia) had elevated H₂O₂ in the cold, indicating that production of reactive oxygen species is part of the cold response in Arabidopsis. Glutathione reductase activity increased in all but Rsch, while ascorbate peroxidase and superoxide dismutase were unchanged and catalase decreased in all but Rsch. Under high light, the Scandinavian accession Te had elevated levels of H₂O₂. Te appeared most sensitive to oxidative stress, having higher malondialdehyde (MDA) levels in the cold and under high light, while only high light caused elevated MDA in the other accessions. Although the most freezing‐tolerant, Te had the highest sensitivity to oxidative stress. No correlation was found between freezing tolerance and activity of antioxidant enzymes in the four accessions investigated, arguing against a key role for antioxidant defence in the differential cold acclimation capacities of Arabidopsis accessions.     

Halopriming of seeds imparts tolerance to NaCl and PEG induced stress in Vigna radiata (L.) Wilczek varieties

The investigation was carried out to study the effect of halopriming on NaCl and polyethylene glycol-6000 (PEG-6000) induced stress tolerance potential of three Vigna radiata (L.) Wilczek varieties, with varied abiotic stress tolerance potential. Halopriming is a seed priming technique in which the seeds were soaked in various salt solutions (in this study NaCl was used). The results of the study indicated that the application of stresses (both NaCl and PEG) induced retardation of growth attributes (measured in terms of shoot length, fresh weight, dry weight) and decrease in physiological attributes like total chlorophyll content, metabolites, photosynthetic and mitochondrial activity of the seedlings in all three V. radiata (L.) varieties. However, halopriming of the seeds could reduce the extent of decrease in these biological attributes. NaCl and PEG stress also caused increase in MDA content (a product of membrane lipid peroxidation) in all the varieties studied and this increase was significantly minimized under halopriming. From the present investigation it was evident that among the green gram varieties studied, Pusa Vishal, a NaCl tolerant variety showed enhanced tolerance to NaCl and PEG induced stress, when the seeds were subjected to halopriming followed by Pusa Ratna (stress sensitive variety). Pusa 9531 (drought tolerant variety) also showed positive halopriming effects but it was less significant when compared to other two varieties. It could be concluded that halopriming improved the drought and salinity stress tolerance potential of all varieties and it was significantly higher in the Pusa Vishal as compared to Pusa 9531 and Pusa Ratna.     

H_2S对干旱胁迫下白三叶幼苗抗氧化防御能力的影响

以‘拉丁诺’白三叶(Trifolium repens cv.‘Ladino’)为试验材料,研究外源H2S处理对PEG6 000(聚乙二醇)模拟干旱胁迫下白三叶叶片相对含水量(RWC)、膜脂过氧化、活性氧成分、抗氧化酶、抗坏血酸-谷胱甘肽循环代谢和非酶抗氧化物质的影响,以揭示H_2S调控白三叶抗旱性的生理机制。结果显示:(1)0.2 mmol/L的外源NaHS(H_2S供体)能显著提高干旱胁迫下白三叶的叶片相对含水量,维持显著较低的电解质渗透率(EL)和丙二醛(MDA)含量。(2)与直接干旱胁迫相比,干旱胁迫下外源添加NaHS处理的白三叶叶片内超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)活性显著增强,抗坏血酸-谷胱甘肽循环代谢中关键酶抗坏血酸过氧化物酶(APX)、脱氢抗坏血酸还原酶(DHAR)、单脱水抗坏血酸还原酶(MDHAR)和谷胱甘肽还原酶(GR)活性及其抗氧化中间产物抗坏血酸(AsA)、谷胱甘肽(GSH)含量也显著提高。(3)叶片类黄酮、总酚和原花青素的含量在一定的胁迫时间范围内亦显著增加,并伴随着活性氧成分O_2~(-·)产生速率和H_2O_2水平降低。研究认为,外源H2S能通过促进干旱胁迫下白三叶体内的多重抗氧化防御能力来提高其幼苗的抗旱性。     

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.     

Antioxidative response in different sorghum species under short-term salinity stress

Seedlings of sorghum varieties (M35-1, a drought tolerant species; SPV-839, a drought sensitive one) differing in their drought tolerance were subjected to 150 mM NaCl stress for a short duration of time (up to 72 h). Both the varieties failed to exhibit efficient ion exclusion mechanism like that of salt tolerant species, but in turn resulted in higher accumulation of Na⁺ and Cl⁻ ions over a period of 72 h salt stress. In addition, accumulation of calcium, potassium and proline in seedlings of sorghum varieties was moderate to short-term NaCl stress. The modulation of antioxidant components significantly diverged between the two varieties during seed germination, further the efficiency of antioxidant scavenging system is maintained during short-term NaCl treatments. In comparison to tolerant variety, the sensitive variety depicted higher SOD activity under control and salinity treatments but specific activity of catalase was significantly reduced. In contrast, drought tolerant variety exhibited efficient hydrogen peroxide scavenging mechanisms with higher catalase and GST activities under control and salt stress conditions, but not in the sensitive one. In conclusion, our comparative studies indicate that drought tolerant and susceptible varieties of sorghum induce efficient differential oxidative components of enzymatic machinery for scavenging ROS thereby alleviating the oxidative stress generated by salt stress during seedling growth.     

Growth and physiological alterations in Niger cultivars under drought stress

Aim of the present study was to determine differential responses in growth and physiology of tolerant (cv. IGPN 2004) and sensitive (cv. GA 10) cultivars of Niger (Guizotia abyssinica Cass.) using in vitro grown calli under water deficit conditions. The calli were subjected to drought stress using PEG-8000 (–0.16,–0.45,–0.87,–1.42 bar) for 15 d and relative growth rate (RGR), percent tissue water content (% TWC), osmolytes (proline–Pro, glycine betaine—GB, total soluble sugars—TSS) accumulation, malondialehyde (MDA) content as well as antioxidant enzyme activities such as superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) were analysed. Our findings showed that RGR and percent TWC was decreased significantly with the intensity of drought stress in both cultivars, but the RGR reduction was least (five folds) in cv. IGPN 2004 than in cv. GA 10 (6.2 folds). In osmolyte accumulation such as Pro and GB, cv. IGPN 2004 was found superior (5.5 and ten folds higher, respectively) to tolerate drought stress than GA 10; however, no change was observed in TSS accumulation. Further, it was noted that cv. IGPN 2004 caused least oxidative damage to the membranes. It also exhibited better SOD, CAT and APX activities and had higher α-tocopherol content. The least reduction in growth and MDA content and higher osmolytes and antioxidant activities in cv. IGPN 2004 revealed more drought stress tolerance at cellular level. It was suggested that increased drought tolerance of cv. IGPN 2004 was coupled with its better maintenance of RGR, percent TWC, reduced lipid peroxidation, more accumulation of osmolytes and higher antioxidant enzymes.     

Physiological responses of somaclonal variants of triploid bermudagrass (<Emphasis Type="Italic">Cynodon transvaalensis</Emphasis> × <Emphasis Type="Italic">Cynodon dactylon</Emphasis>) to drought stress

Eight somaclonal variants with enhanced drought tolerance were isolated from regenerated plants of triploid bermudagrass (Cynodon dactylon × Cynodon transvaalensis cv., TifEagle). Three of them (10-17, 89-02, 117-08) with strong drought tolerance were selected for investigations of physiological responses to drought stress. Compared to the parent control, TifEagle, the somaclonal variants had higher relative water contents and relative growth, and lower ion leakages in the greenhouse tests, while no difference in evapotranspirational water losses and soil water contents was observed between the variants and TifEagle. The variants also had less leaf firing in the field tests under drought stress. Superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities decreased gradually in responses to drought stress in all plants and exhibited negative correlations with ion leakage, indicating that the declined activities of these antioxidant enzymes were associated with drought injury in the triploid bermudagrass. However, CAT activities were significantly higher in all three variants than in TifEagle during drought stress. Two variants, 10-17 and 89-02, also had significantly higher APX activities than TifEagle before and during the first 4 days of drought treatments. These two lines also showed higher SOD activities after prolonged drought stress. Proline, total soluble sugars and sucrose were accumulated under drought stress in all plants and exhibited positive correlations with ion leakage. More proline and sugars were accumulated in TifEagle than in the variants. The results indicated that higher activities of the antioxidant enzymes in the variants during drought stress are associated with their increased drought tolerance.     

Comparative study on calli from two reed ecotypes under heat stress

The different physiological responses to heat stress in calli from two ecotypes of common reed (Phragmites communis Trin.) plants (dune reed (DR) and swamp reed (SR)) were studied. The relative water content, the relative growth rate, cell viability, membrane permeability (MP), H₂O₂ content, MDA content, proline level, and the activities of enzymes, such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), and lipoxygenase (LOX) were assayed. Results showed that under heat stress, DR callus could maintain the higher relative growth rate and cell viability than SR callus, while H₂O₂ content, MDA content, and MP in SR callus increased more than in DR callus. The activities of antioxidant enzymes, such as SOD, CAT, POD, APX, and GR in two calli were enhanced by high temperature. However, antioxidant enzymes in DR callus showed the higher thermal stability than those in SR callus. LOX activity increased more in SR callus than in DR callus under heat stress. High temperature markedly elevated proline content in DR callus whereas had no effect on that in SR callus. Taken together, DR callus is more thermotolerant than SR callus, which might be due to the higher activity of antioxidant enzymes and proline level compared with SR callus under heat stress.