Antioxidant defense system and cadmium uptake in barley genotypes differing in cadmium tolerance

Using two barley (Hordeum vulgare) cultivars (cvs. Tokak and Hamidiye) nutrient solution experiments were conducted in order to study the genotypic variation in tolerance to Cd toxicity based on (i) development of leaf symptoms, (ii) decreases in dry matter production, (iii) Cd concentration and (iv) changes in antioxidative defense system in leaves (i.e., superoxide dismutase, ascorbate peroxidase, glutathione reductase, catalase, ascorbic acid and non-protein SH-groups). Plants were grown in nutrient solution under controlled environmental conditions, and subjected to increasing concentrations of Cd (0, 15, 30, 60 and 120 micromol/L Cd) for different time periods. Of the barley cultivars Hamidiye was particularly sensitive to Cd as judged by the severity and earlier development of Cd toxicity symptoms on leaves. Within 48 h of Cd application Hamidiye rapidly developed severe leaf Cd toxicity symptoms whereas in Tokak the leaf symptoms appeared only slightly. Hamidiye also tended to show more decrease in growth caused by Cd supply when compared to Tokak. The differences in sensitivity to Cd between Tokak and Hamidiye were not related to Cd concentrations in roots and shoots or Cd accumulation per plant. With the exception of catalase, activities of the enzymes involved in detoxification of reactive oxygen species (ROS) were markedly enhanced in Hamidiye by increasing Cd supply. By contrast, in Tokak there was either only a slight increase or no change in the activities of the antioxidative enzymes. Similarly, levels of ascorbic acid and especially non-protein SH-groups were increased in Hamidiye by Cd supply, but not affected in Tokak. The results indicate the existence of a large genotypic variation between barley cultivars for Cd tolerance. The differential Cd tolerance found in the barley cultivars was not related to uptake or accumulation of Cd in plants, indicating importance of internal mechanisms in expression of differential Cd tolerance in barley. As a response to increasing Cd supply particular increases in antioxidative mechanisms in the Cd-sensitive barley cultivar Hamidiye suggest that the high Cd sensitivity of Hamidiye is related to enhanced production and oxidative damage of ROS.     

Influence of waterlogging on some anti-oxidative enzymatic activities of two barley genotypes differing in anoxia tolerance

The experiment was conducted to investigate the formation of oxidative stress and the development of anti-oxidative enzymes in two barley genotypes differing in anoxia tolerance. Waterlogging led to significant reduction in root and shoot weight, green leaf area and tillers per plant, but tolerant Xiumai 3 was much less reduced than sensitive Gerdner. Malondialdehyde (MDA) content, an indicator of membrane lipid peroxidation, significantly increased in Gerdner when the plants were subjected to waterlogging, but remained little changed in Xiumai 3. Superoxide dismutase (SOD) activity was increased with waterlogging treatment and the sensitive cultivar had higher activity than the tolerant one during the experimental duration. At early stage of waterlogging treatment, both peroxidase (POD) and catalase (CAT) activities significantly increased in Xuimai 3, while obviously decreased in Gerdner. Moreover, both cultivars showed substantial increase in both POD and CAT with the progress of waterlogging exposure. Glutathione reductase (GR) activity was increased in both tolerant- and sensitive cultivars under waterlogging. It may be assumed from the current results that SOD activity appears to be not a constraining factor limiting the scavenging of ROS, and it is the change of POD and CAT activity under waterlogging that determine the status of oxidative stress. The difference between genotypes in waterlogging tolerance could be distinguished from the changed patterns of these enzymatic activities.     

Root exudation and Fe uptake and transport in wheat genotypes differing in tolerance to Zn deficiency

Tolerance to Zn deficiency in wheat germplasm may be inversely related to uptake and transport of Fe to shoots. The present study examined eight bread (Triticum aestivum) and two durum (T. turgidum L. conv. durum) wheat genotypes for their capacity to take up and transport Fe when grown under either Fe or Zn deficiency. Bread wheat genotypes Aroona, Excalibur and Stilleto showed tolerance to Zn and Fe deficiency, while durum wheat genotypes are clearly less tolerant to either deficiency. Roots of bread wheats tolerant to Zn deficiency exuded more phytosiderophores than sensitive bread and durum genotypes. Greater amounts of phytosideophores were exuded by roots grown under Fe than Zn deficiency. A relatively poor relationship existed between phytosiderophore exudation or the Fe uptake rate and relative shoot growth under Fe deficiency. At advanced stages of Zn deficiency, genotypes tolerant to Zn deficiency (Aroona and Stilleto) had a greater rate of Fe uptake than other genotypes. Zinc deficiency depressed the rate of Fe transport to shoots in all genotypes in early stages, while advanced Zn deficiency had the opposite effect. Compared with Zn-sufficient plants, 17-day-old Zn-deficient plants of genotypes tolerant to Zn deficiency had a lower rate of Fe transport to shoots, while genotypes sensitive to Zn deficiency (Durati, Yallaroi) had the Fe transport rate increased by Zn deficiency. A proportion of total amount of Fe taken up that was transported to shoots increased with duration of either Fe or Zn deficiency. It is concluded that greater tolerance to Zn deficiency among wheat genotypes is associated with the increased exudation of phytosiderophores, an increased Fe uptake rate and decreased transport of Fe to shoots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hydrogen sulfide alleviates aluminum toxicity in barley seedlings

Aims

Aluminum (Al) toxicity is one of the major factors that limit plant growth. Low concentration of hydrogen sulfide (H₂S) has been proven to function in physiological responses to various stresses. The objective of this study is to investigate the possible role of H₂S in Al toxicity in barley (Hordeum vulgare L) seedlings.

Methods

Barley seedlings pre-treated with sodium hydrosulfide (NaHS), a H₂S donor, and subsequently exposed to Al treatment were studied for their effects on root elongation, Al accumulation in seedlings, Al-induced citrate secretion and oxidative stress, and plasma membrane (PM) H⁺-ATPase expression.

Results

Our results showed that H₂S had significant rescue effects on Al-induced inhibition of root elongation which was correlated well with the decrease of Al accumulation in seedlings. Meanwhile, Al-induced citrate secretion was also significantly enhanced by NaHS pretreatment. Al-induced oxidative stress as indicated by lipid peroxidation and reactive oxygen species burst was alleviated by H₂S through the activation of the antioxidant system. Moreover, Al-induced reduction in PM H⁺-ATPase expression was reversed by exogenous NaHS.

Conclusions

Altogether, our results suggest H₂S plays an ameliorative role in protecting plants against Al toxicity by inducing the activities of antioxidant enzymes, increasing citrate secretion and citrate transporter gene expression, and enhancing the expression of PM H⁺-ATPase.     

The influence of salinity on cell ultrastructures and photosynthetic apparatus of barley genotypes differing in salt stress tolerance

A hydroponic experiment was conducted to elucidate the difference in growth and cell ultrastructure between Tibetan wild and cultivated barley genotypes under moderate (150 mM NaCl) and high (300 mM NaCl) salt stress. The growth of three barley genotypes was reduced significantly under salt stress, but the wild barley XZ16 (tolerant) was less affected relative to cultivated barley Yerong (moderate tolerant) and Gairdner (sensitive). Meanwhile, XZ16 had lower Na⁺ and higher K⁺ concentrations in leaves than other two genotypes. In terms of photosynthetic and chlorophyll fluorescence parameters, salt stress reduced maximal photochemical efficiency (F _v/F _m), net photosynthetic rate (Pn), stomatal conductance (Gs), and intracellular CO₂ concentration (Ci). XZ16 showed relatively smaller reduction in comparison with the two cultivated barley genotypes. The observation of transmission electron microscopy found that fundamental cell ultrastructure changes happened in both leaves and roots of all barley genotypes under salt NaCl stress, with chloroplasts being most changed. Moreover, obvious difference could be detected among the three genotypes in the damage of cell ultrastructure under salt stress, with XZ16 and Gairdner being least and most affected, respectively. It may be concluded that high salt tolerance in XZ16 is attributed to less Na⁺ accumulation and K⁺ reduction in leaves, more slight damage in cell ultrastructure, which in turn caused less influence on chloroplast function and photosynthesis.     

The ecotoxicological and interactive effects of chromium and aluminum on growth,oxidative damage and antioxidant enzymes on two barley genotypes differing in Al tolerance

The effects of single or combined stress of aluminum (Al) and chromium (Cr) on plant growth, root dehydrogenase, oxidative stress and antioxidative enzymes were studied using two barley genotypes differing in Al tolerance in a hydroponic experiment. Al or Cr stress decreased plant growth, lowered root dehydrogenase activity and caused oxidative damage, as characterized by increased MDA and H₂O₂ contents. Under Al or Cr stress, the activities of antioxidative enzymes, including superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT), were dramatically increased in plant tissues. Gebeina, an Al-tolerant genotype, had less oxidative damage than Shang 70-119, an Al-sensitive genotype. The extent of oxidative damage induced by Cr varied with the pH of the culture solution, with lower pH values (4.0) being more severe than higher pH values (6.5). The combination of Cr and Al caused a further decrease in plant growth, a decrease in root dehydrogenase activity and an increase in MDA and H₂O₂ contents as well as the activities of antioxidative enzymes. There was also a marked difference between the two barley genotypes in the extent of increased antioxidative enzyme activity under the Cr and Al stresses.     

Differential accumulation of proteins in leaves and roots associated with heat tolerance in two Kentucky bluegrass genotypes differing in heat tolerance

To understand the mechanisms of heat stress responses in perennial grasses, differential proteins in leaves and roots of two genotypes of Kentucky bluegrass (Poa pratensis), including heat-tolerant ‘Midnight’ and heat-sensitive ‘Brilliant’, were analyzed with two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS). Plants were exposed to heat stress for 28 days in growth chambers. Under 7–28 days of heat stress, leaf photochemical efficiency declined significantly while electrolyte leakage increased in leaves and roots, and to a lesser extent for heat-tolerant ‘Midnight’ than for heat-sensitive ‘Brilliant’. Compared with leaves, cell membrane damage due to heat stress was more severe in roots. The 2-DE and MS analysis identified 37 heat-responsive proteins in leaves, 28 heat-responsive proteins in roots; 14 proteins in leaves and 9 proteins in roots exhibited differential expression between the two genotypes. The results indicate that proteins involved in metabolism and energy in leaves and those in antioxidant defense in roots are associated with heat tolerance in Kentucky bluegrass. The differential accumulation of these proteins might be the reason for different heat tolerance in two Kentucky bluegrass genotypes in aerial and underground parts.     

Interaction of NaCl and Cd stress on compartmentation pattern of cations,antioxidant enzymes and proteins in leaves of two wheat genotypes differing in salt tolerance

Physiological mechanisms of salinity–Cd interactions were investigated in inter- and intracellular leaf compartments of salt-tolerant wheat × Lophopyrum elongatum (Host) A. Löve (syn. Agropyron elongatum) amphiploid and its salt-sensitive wheat parent (Triticum aestivum L. cv Chinese Spring). In comparison with the intracellular fluid, only very low Na⁺ concentrations (up to about 4 mM) were found in the intercellular leaf compartment of wheat after a 75 mM supply of NaCl. NaCl salinity led to a higher Cd concentration in leaves of the salt-sensitive genotype. Cd in the intercellular leaf compartment was not detectable. Higher K⁺ concentrations in the intercellular leaf compartment of the salt-sensitive genotype suggest a higher plasma membrane permeability caused by NaCl + Cd stress. Ascorbate peroxidase (APX) activity was increased in leaves of the salt-sensitive genotype under the combined NaCl and Cd stress. The highest non-specific peroxidase activities were detected under the combined stresses. It is suggested that NaCl and Cd stress in combination enhance the production of oxygen radicals and H₂O₂, especially in leaves of the salt-sensitive genotype. As a consequence, disturbed membrane function may cause elevated Cd concentrations in the intracellular leaf compartment under salinity. Cd did not change protein concentration and pattern in leaves. The protein content in inter-and intracellular leaf compartments of both genotypes was increased under salinity. A different protein pattern was obtained in inter- and intracellular leaf compartments. Thus, several physiological interactions between NaCl stress and Cd were found in the two wheat genotypes.     

Abscisic acid in developing grains of wheat and barley genotypes differing in grain weight

Two genetically related wheat lines growing in cabinets were given different temperatures during grain filling, and abscisic acid (ABA) was measured in whole grains by gas chromatography with an electron-capture detector. Three genetically related barley lines grown in the field were assayed for ABA content in endosperm and embryo fractions separately by radiommunoassay.Maximum grain growth rate and final weight per grain of the two wheat lines differed by 50–60% at low temperature and 30–40% at high temperature. During grain development two peaks in ABA level were observed at low temperature but only one at high temperature. At times when differences in grain growth rate between genotypes and between temperature treatments were large, the corresponding differences in ABA concentration were small. In barley, one line (Iabo 14) had 30% heavier grains than the other two (Onice and Opale). Endosperm ABA concentrations showed no clear differences between genotypes until grain filling was nearly complete. Embryo ABA levels were up to 10-times greater than those in the endosperm, with Opale having significantly less ABA in the embryo than the other two cultivars.Our experiments did not provide evidence for a causal relationship between ABA levels during grain filling and grain growth rate or final weight.Abbreviations ABA Abscisic acid - DAA days after anthesis - DW dry weight - FW fresh weight     

Effects of boron toxicity on root and leaf anatomy in two Citrus species differing in boron tolerance

Key message

Typical toxic symptom only occurred in B-toxic C. grandis leaves. B-toxicity induced PCD of C. grandis leaf phloem tissue. The lower leaf free B might contribute to the higher B-tolerance of C. sinensis.

Abstract

Seedlings of ‘Xuegan’ (Citrus sinensis) and ‘Sour pummelo’ (Citrus grandis) differing in boron (B)-tolerance were irrigated with nutrient solution containing 10 (control) or 400 (B-toxic) μM H₃BO₃ for 15 weeks. Thereafter, the effects of B-toxicity on leaf photosynthesis, chlorophyll, plant B absorption and distribution, root and leaf anatomy were investigated to elucidate the possible B-tolerant mechanisms of Citrus plants. Typical toxic symptom only occurred in B-toxic C. grandis leaves. Similarly, B-toxicity only affected C. grandis photosynthesis and chlorophyll. Although total B concentration in B-toxic roots and leaves was similar between the two species, leaves from B-toxic C. grandis plant middle had higher free B and lower bound B as compared with those from C. sinensis. Effects of B-toxicity on leaf structure were mainly limited to the mesophyll cells and the phloem of leaf veins. Although irregular cell wall thickening was observed in leaf cortex cells and phloem tissue of B-toxic C. grandis and C. sinensis leaves, exocytosis only occurred in the companion cells and the parenchyma cells of B-toxic C. sinensis leaf phloem. Also, B-toxicity induced cell death of phloem tissue through autophagy in C. grandis leaf veins. B-toxicity caused death of root epidermal cells of the two Citrus species. B-toxicity restrained degradation of middle lamella, but did not alter ultrastructure of Golgi apparatus and mitochondria in root elongating zone cells. In conclusion, C. sinensis was more tolerant to B-toxicity than C. grandis. The lower leaf free B and higher bound B might contribute to the higher B-tolerance of C. sinensis.     

Differential capacity for high-affinity manganese uptake contributes to differences between barley genotypes in tolerance to low manganese availability

There is considerable variability among barley (Hordeum vulgare) genotypes in their ability to grow in soils containing a low level of plant available manganese (Mn). The physiological basis for the tolerance to low Mn availability is unknown. In this work, Mn2+ influx and compartmentation in roots of the Mn-efficient genotype Vanessa and the Mn-inefficient genotype Antonia were investigated. Two separate Mn transport systems, mediating high-affinity Mn2+ influx at concentrations up to 130 nm and low-affinity Mn2+ influx at higher concentrations, were identified in both genotypes. The two genotypes differed only in high-affinity kinetics with the Mn-efficient genotype Vanessa having almost 4 times higher V(max) than the inefficient Antonia, but similar K(m) values. Online inductively coupled plasma-mass spectrometry measurements verified that the observed differences in high-affinity influx resulted in a higher Mn net uptake of Vanessa compared to Antonia. Further evidence for the importance of the differences in high-affinity uptake kinetics for Mn acquisition was obtained in a hydroponic system with mixed cultivation of the two genotypes at a continuously low Mn concentration (10-50 nm) similar to that occurring in soil solution. Under these conditions, Vanessa had a competitive advantage and contained 55% to 75% more Mn in the shoots than did Antonia. Subcellular compartmentation analysis of roots based on 54Mn2+ efflux established that up to 93% and 83% of all Mn was present in the vacuole in Vanessa and Antonia, respectively. It is concluded that differential capacity for high-affinity Mn influx contributes to differences between barley genotypes in Mn efficiency.     

Effects of arbuscular mycorrhizal fungi and drought stress on growth and nutrient uptake of two wheat genotypes differing in drought resistance

 The effects of an arbuscular mycorrhizal (AM) fungus and drought stress on the growth, phosphorus, and micronutrient uptake of two wheat genotypes exhibiting differences in drought resistance were investigated. Plants were grown on a low P (4 mg kg^–1 soil) silty clay (Typic Xerochrept) soil-sand mix. Mycorrhizal infection was higher under well-watered than under dry soil conditions and the drought-resistant genotype CR057 had a higher mycorrhizal colonization than the drought-sensitive genotype CR006. Total and root dry matter yields and total root length were higher in mycorrhizal than in nonmycorrhizal plants of both genotypes. CR057 had higher total dry matter but not root dry matter than CR006 plants. The enhancement in total dry matter due to AM inoculation was 42 and 39% under well-watered and 35 and 45% under water-stressed for CR057 and and CR006, respectively. For both genotypes, the contents of P, Zn, Cu, Mn, and Fe were higher in mycorrhizal than in nonmycorrhizal plants and higher under well-watered than under dry soil conditions. The enhancement of P, Zn, Cu, Mn, and Fe uptake due to AM inoculation was more pronounced in CR006 than in CR057, particularly under water-stressed conditions. Thus CR006 benefitted from AM infection more than the CR057 under dry soil conditions, despite the fact that CR057 roots were highly infected. It appears that CR006 is more dependent on AM symbiosis than CR057. Accepted: 12 February 1997     

Growth response to subsurface soil acidity of wheat genotypes differing in aluminium tolerance

Subsurface soil acidity coupled with high levels of toxic Al is a major limiting factor in wheat production in many areas of the world. This study examined the effect of subsurface soil acidity on the growth and yield of two near-isogenic wheat genotypes differing in Al tolerance at a single genetic locus in reconstructed soil columns. In one experiment, plants were grown in columns with limed topsoil and limed or acidic subsurface soils, and received water only to the subsurface soil at a late part of the growth period. While shoot dry weight, ear number and grain yield of Al-tolerant genotype (ET8) were not affected by subsurface soil acidity, liming subsurface soil increased shoot weight and grain yield of Al-sensitive genotype (ES8) by 60% and ear number by 32%. Similarly, root length density of ET8 was the same in the limed and acidic subsurface soils, while the root length density of ES8 in the acidic subsurface soil was only half of that in the limed subsurface soil. In another experiment, plants were grown with limed topsoil and acidic subsurface soil under two watering regimes. Both genotypes supplied with water throughout the soil column produced almost twice the dry weight of those receiving water only in the subsurface soil. The tolerant genotype ET8 had shoot biomass and grain yield one-third higher than ES8 when supplied with water throughout the whole column, and had yield 11% higher when receiving water in the subsurface soil only. The tolerant genotype ET8 produced more than five times the root length in the acidic subsurface soil compared to ES8. Irrespective of watering regime, the amount of water added to maintain field capacity of the soil was up to 2-fold higher under ET8 than under ES8. The results suggest that the genotypic variation in growth and yield of wheat grown with subsurface soil acidity results from the difference in root proliferation in the subsurface soil and hence in utilizing nutrient and water reserves in the subsurface soil layer.