【作者】 曾凡荣；

【导师】 张国平；

【作者基本信息】 浙江大学 ， 作物学， 2010， 博士

【摘要】 土壤铬污染已对农业生产和人类健康造成了严重的威胁。环境中的铬主要以Cr3+和Cr6+两种氧化形态存在。虽然Cr3+是人类和动物必需的营养元素,但Cr3+和Cr6+对植物都具有严重的毒害作用,且Cr6+的毒性远强于Cr3+。与镉、铅、铝等重金属相比,铬的电子化学结构更为复杂,在自然条件下Cr3+与Cr6+极易通过氧化还原反应发生相互转化,这些特性使铬毒害和耐性研究相对迟缓。迄今,铬在植物生长发育和生理代谢上的毒害作用虽已有一定的了解,但有关铬毒害和耐性的机理尚缺乏深入研究。本研究以铬耐性与积累差异显著的水稻基因型(秀水113和单K5)为材料,从铬的环境化学、铬的毒害效应、水稻对铬的吸收和积累、铬在水稻组织的亚细胞分布以及水稻铬胁迫响应的蛋白质组学等方面系统地研究了水稻铬耐性的生理和分子机理,并探索了缓解铬毒害的化学调控途径。主要研究结果如下：1.土壤pH值和有机质含量对土壤重金属有效态以及水稻籽粒中重金属含量的影响植物对重金属的吸收一方面与植物的遗传和生理特性有关,另一方面主要受土壤中重金属迁移性和有效性的影响,而它们则由土壤的各种理化特性决定。本试验通过测定Cr、Cu、Fe、Mn、Pb和Zn等重金属在不同土壤中的有效态含量和水稻植株与籽粒中的浓度,研究了土壤pH值和有机质含量对稻田重金属有效性和水稻吸收重金属的影响。结果显示,土壤中有效态重金属含量与土壤pH值呈显著负相关,而与有机质含量呈显著正相关。多元回归分析表明,土壤pH和有机质含量对Cu、Pb和Zn的有效性变化均有显著影响,而Cr、Fe的有效性主要受有机质含量的影响,Mn的有效性受土壤pH的影响较大。结果还显示,土壤中有效态重金属含量高的试验点水稻植株重金属含量相对较高,说明土壤重金属的有效性影响着水稻对重金属的吸收和积累。相关分析表明,水稻植株和籽粒中重金属的含量与土壤pH值呈负相关,而与土壤有机质含量呈正相关。多元回归分析结果揭示,土壤pH值是影响水稻植株中重金属含量的重要因素。由此推测,大田生产上通过相关农艺措施调节土壤的酸碱度,可有效降低稻米的重金属含量。2.水稻籽粒中铬、镉、铅含量的基因型和环境差异重金属的吸收和积累在不同作物种类之间以及同一作物不同基因型之间存在着显著差异,选育或培育具有重金属低积累特性的作物品种是降低重金属污染地区作物产品重金属含量的有效途径。在低积累重金属作物品种的筛选上,有效地评估和鉴定食用器官的重金属含量是关键技术。本试验以138份水稻品种(品系)为材料,分析了它们种植在不同污染程度土壤中的植株和籽粒重金属含量。结果显示,水稻籽粒Cr、Cd和Pb含量随土壤污染程度的加重而增加,同时不同基因型之间存在着显著的差异。在三种污染程度下,138份水稻材料的籽粒Cr浓度最大值／最小值比均达20倍以上,基因型间的变异系数超过55%。水稻基因型与环境(污染程度)对籽粒中Cr、Cd以及Pb含量有显著的互作效应。本研究筛选到一批籽粒Cr、Cd、Pb含量在三种污染程度下始终保持高水平或低水平的水稻品种(品系),如HG-5、单K5和湖优-1的籽粒在三种污染程度下均具有高Cr含量；而秀水113、秀水09和明珠1号的籽粒则具有低Cr含量；秀水11、嘉02-5和明珠1号为籽粒高镉积累材料,而春江026、春江11和湖97-98为籽粒低镉积累材料；单K5、单K8、嘉单繁18为籽粒高铅积累材料,而嘉02-5、嘉C1和单K15为籽粒低铅积累材料。3.铬胁迫对水稻抗氧化系统和养分吸收积累的影响本试验以两个前期筛选出的籽粒铬积累特性不同的水稻品种(秀水113和单K5)为材料,分析了铬胁迫对水稻抗氧化系统和养分吸收积累的影响。低水平铬处理(10μM Cr)对MDA含量影响不大,但增加SOD和POD的活性。在高铬水平(100μM)下,水稻叶片和根系的MDA含量显著增加,SOD和POD活性则显著降低,说明高水平铬处理对水稻幼苗造成了严重的氧化胁迫。另外,高浓度铬处理显著降低水稻根、茎、叶、籽粒中营养元素的含量,说明铬胁迫干扰植物养分的吸收和分配。水稻单株养分积累量在10μM Cr处理下最高,在100μM Cr处理下最低,其原因与铬胁迫造成水稻植株生长受阻和根细胞损伤有关。铬对养分积累的影响因基因型而异,铬胁迫下单K5中N、P、K、Ca、Zn的单株积累量均显著高于秀水113。4.铬胁迫对水稻根际pH值变化和有机酸分泌的影响本试验通过测定水稻培养液的pH值和检测水稻根系分泌的有机酸含量,研究了不同铬处理水平下根际pH值和有机酸分泌的变化。结果表明,水稻根际的pH值随铬处理浓度的增加和处理时间的延长而显著升高。当营养液中铬处理浓度增加到100μM时,根际pH值明显高于初始调节的pH值(5.10),说明100μM Cr处理严重影响水稻的根系活力以及阴阳离子的吸收平衡。随着铬胁迫水平的升高,根系分泌的有机酸含量显著增加。在测定的六种有机酸中,草酸和苹果酸含量明显高于柠檬酸、乙酸、乳酸和琥珀酸,可以认为前两种是铬胁迫下水稻分泌的主要有机酸。相关分析显示,铬在水稻体内的积累量与根际pH值、草酸、苹果酸、柠檬酸含量呈显著正相关,说明根际pH值升高以及草酸、苹果酸和柠檬酸分泌量增加导致水稻植株中铬积累量增多。另外,根际pH值也与草酸、苹果酸和柠檬酸分泌量呈显著正相关,表明这三种有机酸的分泌对根际pH值的变化可能起有重要作用。5.水稻铬吸收的动力学研究无论Cr3+还是Cr6+对各种生物都具有显著的毒害作用,而且Cr6+的毒性远高于Cr3+。植物对铬的吸收是铬植物体内积累和对植物造成毒害的前提,但是,迄今为止植物对铬的吸收动态及模式尚缺乏研究。本试验以水稻为材料,研究了溶液培养条件下水稻根对Cr3+和Cr6+吸收的动力学特征。结果显示,在Cr3+0-250μM和Cr6+0-500μM范围内,水稻根系对Cr3+和Cr6+的吸收速率均随着溶液中铬浓度的提高而增加,并逐渐在高铬浓度处理下趋于饱和。拟合分析表明,Cr3+和Cr6+的吸收均可用Michaelis-Menten动力学方程描述,Cr3+具有较低的Km值和较高的Vmax值,水稻根系对Cr3+的亲和力和吸收能力均大于Cr6+。在0.5-48 h处理时间内,水稻根系中铬浓度随着时间的推移而增加。但是, Cr3+含量在t=12h后趋于饱和,而Cr6+含量仍持续增加。水稻根系对Cr3+和Cr6+的响应速率表现不同,100μM Cr3+处理0.5h后,水稻根中铬含量可达400 mg g-1 DW,而100μM Cr6+处理0.5h后,水稻根中铬含量不到20 mg g-1 DW,说明Cr3+比Cr6+更易被水稻根系吸收,且其达到吸收饱和较快。另外,Cr吸收的动力学特征显示两供试基因型存在着差异,单K5对Cr3+和Cr6+的Km值和Vmax值都明显高于秀水113,说明单K5对Cr的亲和力虽不如秀水113,但其铬吸收潜力较大。营养液中添加正钒酸盐和2,4-DNP(代谢抑制剂),Cr6+的吸收显著降低,而Cr3+的吸收影响较小,说明Cr6+的吸收需要能量参与,而Cr3+的吸收需能较少由此可见Cr6+与Cr3+分别具有不同的吸收途径或方式。6.铬在水稻组织和亚细胞水平上的分布和化学形态特征分析重金属在植株体内过量积累是植物受重金属毒害的重要原因,而植物组织中重金属的亚细胞水平分布与重金属毒害关系密切。本试验以铬积累特性不同的水稻品种为材料,采用同步辐射微束X射线荧光光谱分析μ-SRXRF)、差速离心以及逐步化学提取法研究了铬在水稻组织和亚细胞水平上的分布特点和化合形态特征,试图阐明铬在水稻植株体内的吸收、积累的机理以及水稻的耐性机制。μ-SRXRF结果显示,水稻根系中的Cr绝大部分累积在表皮细胞中；水稻茎和叶中的铬则主要分布于表皮和维管束组织,其他组织中的铬含量非常低,说明表皮层细胞对铬的阻隔作用可能是植物体内铬迁移受到限制的重要原因,而水稻根系吸收的Cr则主要通过木质部导管向地上部组织运输。在亚细胞水平上,不管在何种Cr处理浓度下,水稻根系中的Cr绝大部分都分布在细胞壁上；而在水稻茎秆和叶片细胞中,随着Cr处理水平的升高,Cr在可溶性组分中的分配比例显著增加。以上结果表明,细胞壁和液泡是水稻细胞内Cr的主要分布部位,而细胞中具有重要代谢功能的细胞器(如：叶绿体、线粒体等)Cr分布相对较少。另外,逐步化学试剂提取法对水稻植株体内铬的化学形态研究表明,低铬处理下,水稻组织中铬以乙醇和去离子水提取态为主,说明硝酸盐、氯化物和氨基酸以及某些水溶性有机酸对铬的吸收和转运具有重要作用；而高铬处理下,乙醇和去离子水提取态比例显著下降,盐酸和残渣提取态转为铬的主要存在形态,表明磷酸盐、草酸盐以及难溶性高分子量化合物在阻止铬在水稻根系中移动、缓解铬毒害上发挥着一定的作用。7.水稻叶、根在不同铬处理下蛋白质差异表达的研究为了深入阐明植物铬毒害和耐性的分子机理,本研究利用双向电泳和MALDI-TOF质谱技术对Cr积累特性不同的两个水稻基因型(秀水113、单K5)叶片和根在0、2、200μM Cr处理条件下蛋白质组表达的变化进行了分析。水稻叶片中鉴定成功41个蛋白点,其中秀水113有21个,单K5有20个,这些蛋白以RuBisco和能量代谢相关蛋白为主。水稻根系中仅14个蛋白点质谱鉴定成功,其中秀水113有4个,单K5有10个,它们大多数与环境胁迫相关。本研究检测到一批对铬胁迫有响应且表达上调的蛋白,如NADP-异柠檬酸脱氢酶、热激蛋白(Hsp90)、乙二醛酶Ⅰ、蛋白质糖基化多肽、S-腺苷甲硫氨酸合成酶、谷氨酰胺合成酶、ATP合成酶以及两个信号分子,G蛋白β亚基和信号识别颗粒54蛋白等。这些蛋白质中,大部分已被证实与盐害、冷害等环境胁迫的耐性有关,但在铬胁迫的耐性研究中涉及非常少,而蛋白质糖基化多肽与重金属等环境胁迫的关系为首次报道。本研究鉴定到的铬胁迫响应蛋白可为深入阐明水稻铬耐性机理提供新的契机。8.外源还原型谷胱甘肽(GSH)对水稻铬毒害的缓解效应本试验通过分析Cr胁迫下不同浓度GSH处理后水稻植株生长、叶绿素和可溶性蛋白质含量、抗氧化酶活性以及膜脂过氧化程度的变化,研究了外源GSH处理对铬胁迫的缓解效应。结果显示,100μM Cr处理下,植株生长、叶绿素和可溶性蛋白含量显著降低,添加外源GSH后,铬胁迫造成的抑制效应得到明显缓解,说明外源GSH可减轻了Cr胁迫对水稻幼苗的毒害。100μM Cr处理导致水稻叶片和根系中MDA含量的急剧增加以及某些抗氧化物酶活性的降低,说明对水稻幼苗产生了严重的氧化胁迫,而外源GSH的加入显著提高叶片和根系的抗氧化物酶活性,降低叶片和根系中的MDA含量,表明GSH可通过增强水稻的抗氧化能力而减轻铬的毒害。此外,外源GSH的加入虽然显著增加根的铬含量,但显著降低水稻地上部组织中的铬含量,即降低了铬由地下部向地上部的转运,说明外源GSH促使更多的铬滞留在水稻根系,这可能与GSH的还原以及对金属离子的螯合区室化效应有关。9.硅营养对水稻铬毒害的缓解效应本试验研究了施硅对铬胁迫下水稻植株生长、可溶性蛋白质含量、抗氧化酶活性以及膜脂过氧化的影响。结果表明,100μM Cr处理显著降低水稻株高和干重、可溶性蛋白含量和根系抗氧化酶活性,而明显增加组织中Cr含量和MDA含量。水培液中添加硅营养,铬胁迫对水稻植株生长造成的抑制效应得到有效缓解。硅营养加入也显著降低水稻植株对铬的吸收及向地上部组织的转运。另外,硅营养可通过增高叶片和根系中抗氧化物酶活性和降低MDA含量缓解Cr毒害引起的氧化胁迫。试验结果还显示,硅营养对氧化胁迫的缓解有一定的基因型效应,单K5叶片中SOD、POD、CAT以及AXP活性在低浓度Si处理(75mg L-1)即可快速增加,而秀水113则需高浓度Si处理(150mg L-1),说明硅营养对单K5氧化胁迫的缓解效应要比秀水113更为有效。更多还原

【Abstract】 Chromium occurs naturally in several oxidation states, with the trivalent (Cr3+) and hexavalent (Cr6+) forms being the most stable and common. Although Cr3+ is considered as an essential element for human and animals, both Cr3+ and Cr6+ are toxic to plants. Hexavalent chromium is known to be much more toxic than trivalent chromium to living organisms, and is easily reduced to Cr3+ by redox reaction. In comparison with some other toxic metals, like Cd and Pb, Cr has received relatively little attention from plant scientists. Its complex electronic chemistry and changeable station have been a major factor affecting the research on its toxicity mechanisms in plants. Chromium phyto-toxicity has been intensively studied from the multiple levels, including plant growth and development, physiological processes, antioxidant system. However, there is little information on the mechanisms and genotypic difference of Cr toxicity and tolerance in plants. The current experiments were carried out to investigate the physiological and molecular mechanisms of Cr toxicity and tolerance in rice plants, based on the research on chromium availability in soil, Cr toxicity in rice plants, Cr kinetic uptake, subcellular distribution and chemical forms of Cr in plant tissues, and the proteomic changes in responses to chromium toxicity. Meanwhile, the possibility of reducing Cr uptake and alleviating Cr stress in rice plants by exogenous application of chemical regulators were also studied. The major results are summarized as follows:1. The influence of pH and organic matter content in paddy soil on heavy metal availability and their uptake by rice plantsSoil pH, organic matter content and EDTA-extractable heavy metal contents in 27 paddy soils from three locations of Zhejiang province and the heavy metal concentrations in rice plants were investigated to elevate the influence of soil properties on heavy metal uptake and translocation in rice plants. The results showed that the soils from Nanhu exhibited the highest soil organic matter content and EDTA-extractable heavy metal contents, but the lowest pH value. Simple linear regression analysis suggested that the EDTA-extractable contents of Cr, Cu, Fe, Mn, Pb and Zn was negatively correlated with soil pH value, but positively correlated with organic matter content. The combination of soil pH and Log10(OM) in the stepwise multiple linear regression analysis could produce a more precise model for estimation of EDTA-extractable Cu, Pb and Zn contents in soils, but not for Cr, Fe and Mn, indicating that availability of heavy metal in soil was a complex parameter, and determined by many soil factors. Rice plants grown in Nanhu soil had the higher straw heavy metal concentrations than those grown, coinciding with the results of soil EDTA-extractable heavy metal content. Simple linear regression analysis suggested that heavy metal concentrations in rice straw and grains were negatively correlated with soil pH value, but positively correlated with soil organic matter content, except grain Pb and Zn concentrations. Stepwise multiple linear regression analysis indicated that soil pH played an important role in predicting heavy metal concentrations in rice plants, suggesting that it is possible to reduce the heavy metal accumulation in rice plants by regulating the pH level in paddy soil through agronomic approaches.2. Genotypic and environmental variation in chromium, cadmium and lead concentrations in rice grainsGenotypic and environmental variation in Cr, Cd and Pb concentrations of rice grains and the interaction between these metals were investigated by using 138 rice genotypes grown in three contaminated soils. There are significantly genotypic differences in the three heavy metal concentrations of rice grains, with the absolute difference among 138 rice genotypes in grain Cr, Cd and Pb concentrations being 24.5,9.1 and 23.8 folds under the slightly contaminated soil (Cr, Cd and Pb content was 4.61,1.09 and 28.28 mg kg-1, respectively), respectively. Some genotypes, such as Xiushui 113, Zhongjian 9836 and Yongdan 24 etc. which showed consistently low grain Cr, Cd or Pb concentration under the 3 contaminated soils were identified. There was a highly significant interaction between genotype and environment in the 3 heavy metal concentration of the rice grains, suggesting the importance of cultivar choice for a given environment. Correlation analysis showed that Cr concentration in rice grains was not correlated with Cd and Pb concentration in the three contaminated soils. However, there was a significant correlation between Cd and Pb in slightly and highly contaminated levels. The results indicated that interaction between heavy metals in their availability in soil and accumulation in plant is complex.3. The influence of chromium toxicity on antioxidant system and nutrient uptake and accumulation in rice plantsThe effect of chromium (Cr) on the lipid peroxidation, activities of antioxidant enzymes, and the uptake and accumulation of nutrient were studied in two rice genotypes, Xiushui 113 and Dan K5, differing in grain Cr accumulation. The treatment with low Cr level (10μM) showed little influence on lipid peroxidation but increased the activities of SOD and POD. However, Cr stress with high level (100μM) significantly increased MDA content and decreased the activities of SOD and POD in both leaf and root, suggesting that high chromium level in medium would induce serious oxidative to rice plant. Furthermore, chromium toxicity significantly decreased the uptake and distribution of nutrients in rice plants. Maximum nutrient accumulation occurred at level of 10μM Cr, while the minimum accumulation occurred at level of 100μM Cr, indicating more plant growth at 10μM Cr than the control and other Cr treatments. It may be assumed that there might be a synergistic effect of Cr on the plant growth in micro doses. Chromium accumulation was significantly and negatively correlated with the accumulation of each nutrient, suggesting that increasing Cr level may create nutrient deficiencies or imbalance in rice.4. Changes of organic acid exudation and rhizosphere pH in the rice plants under chromium stressThe changes of rhizosphere pH, organic acid exudation of roots under chromium stress and their effects on chromium uptake and accumulation were studied using two rice genotypes though determining organic acid contents and checking the solution pH values. The results showed that rhizosphere pH increased with Cr level in the culture solution and the exposed time. The effects of Cr level and exposed time on organic acid exudation varied with the species of organic acids as well as genotypes. Among the 6 organic acids examined in this experiment, oxalic and malic acid contents were much higher than citric, latic, acetic and succinic acid, and had significantly positive correlation with rhizosphere pH, indicating that the exudation of these organic acids might play an important role in the change of rhizosphere pH. In addition, Cr accumulation in rice plants showed significantly positive correlations with rhizosphere pH, oxalic, malic and citric acid contents, respectively. It may be suggested that increase in rhizosphere pH, and oxalic, malic and citric acid exudation enhances Cr accumulation in rice plants.5. Kinetic characteristics of chromium uptake by rice rootThe kinetic characteristics of Cr uptake by rice roots for both Cr3+ and Cr6+ were studied with hydroponic culture and different Cr supply concentrations and exposure time. The results showed that the uptake rate of both Cr3+ and Cr6+ by rice root increased with the Cr supplying level, but they were saturated at high Cr level (Cr3+,100μM; Cr6+ 400μM). When fitted to Michaelis-Menten equation (V= Vmax* C/(Km+C)), the kinetic uptake of Cr3+ and Cr6+ for two rice genotypes could be illustrated as the following equations:for Xiuhui 113, V(Cr3+)= 2295.99* C / (34.78+C), R2= 0.9785**; V(Cr6+)= 853.24* C/(273.36+C), R2= 0.9701**; for Dan K5, V(Cr3+)= 2777.83* C (38.60+C), R2= 0.9438**; V(Cr6+)= 1232.00* C/(550.09+C), R2= 0.9915**. Hexavalent chromium showed much higher Km value but much lower Vmax than Cr3+, suggesting that Cr3+ had much more affinity with rice roots than Cr6+. In addition, both Km and Vmax values of Dan K5 were much higher than those of Xiushi 113, indicating that Dan K5 had lower affinity with Cr but higher potentiality to uptake Cr than Xiushui 113. During 0.5-48h with supplying 100μM Cr3+ and Cr6+, Cr concentration in rice root increased with the exposure time. However, Cr3+ concentration was saturated at t=12h, while Cr6+ was not saturated at that time. Furthermore, the uptake of Cr3+ was more rapid than of that of Cr6+. For an example, at 0.5h after treatment, Cr concentration in rice roots rose up to 400μg g-1 DW when supplied with Cr3+, while only 20μg g-1 DW when supplied with Cr6+. The experiment of metabolic inhibitors and low temperature showed that the uptake Cr6+ was energy dependent, but Cr3+ required much less energy. It may be assumed that that Cr6+ uptake is an active process, while Cr3+ uptake is passive one.6. Subcellular distribution and chemical forms of chromium in rice plants under different chromium stressThe subcellular distribution and chemical forms of different heavy metals in rice is correlated with their bio-toxicity. An experiment was conducted to investigate the subcellular distribution and chemical forms of chromium (Cr) in two rice genotypes (Oryza sativa L. cvs. Xiushui 113 and Dan K5) differing in Cr accumulation, to understand the mechanism of Cr toxicity and tolerance in rice plants. The results of microbeam synchrotron radiation X-ray fluorescence (μ-SRXRF) showed that Cr in rice tissues mainly distributed in epidermis and vascular bundles, indicating that the epidermis and xylem play important roles in Cr fixation and translocation, respectively. Furthermore, it was found that Cr in the root cells of rice plants exposed to Cr stress was mainly localized in cell walls, whereas Cr in leaf or stem cells were mainly present in both cell walls and vacuoles, suggesting that both compartments act as important protective barriers against Cr toxicity in rice cells. Although Cr ions in all plant tissues exist predominantly in the forms extracted by 80% ethanol and d-H2O, the amount of Cr in the chemical forms extracted by 2% HAc,0.6M HCl and in residues was significantly increased under the highest Cr level (100μM Cr) compared to the plants grown under lower Cr levels. These results also indicate that excess Cr accumulated in rice plants under Cr stress is bound to undissolved or low bioavailable compounds, such as undissolved phosphate and oxalate, being beneficial for rice plants to alleviate Cr toxicity. In addition, under the highest Cr level (100μM), Dan K5 had a higher percentage of Cr in the chemical forms extracted by 2% HAc,0.6μM HCl and in residues compared to Xiushui 113 in both stems and leaves, indicating that more Cr ions in shoots of Dan K5 were bound to undissolved or low bio-available compounds, in comparison with those of Xiushui 113. It is evident that the low bioavailability of Cr in the shoots of Dan K5 is related to a high Cr accumulation.7. Proteomic changes in response to chromium treatments in riceThe mechanisms of chromium toxicity and tolerance still remain to be illustrated. In the present study, an investigation aimed at understanding molecular mechanisms of chromium toxicity was carried out using proteomic analysis. The seedlings of two rice genotypes (Oryza sativa L. cvs. Xiushui 113 and Dan K5) differing in Cr accumulation were exposed to three Cr treatments from 0 to 200μM. Proteins were extracted from the leaves and roots collected from both control and stressed seedlings. Two-dimensional gel electrophoresis (2-DE) coupled with mass spectrometry was adopted to investigate the protein expression patterns of rice leaves and roots in responses to Cr stress. A total of 55 proteins (41 in leaves and 14 in roots) were identified in response to chromium stress by MALDI-TOF mass spectrometry analysis, containing RuBiCo, proteins involved in energy and material metabolism, and the potential proteins involved in Cr tolerance. A group of novel proteins were identified to up-regulate in responding to Cr stress, including Hsp90, NADP-isocitrate dehydrogenase, S-adenosylmethionine synthetase, glyoxalase I, reversibly glycosylated polypeptide, Glutamine synthetase, ATP synthase, Guanine nucleotide-binding protein beta subunit-like protein and Signal recognition particle 54 kDa. These proteins are involved in several cellular processes, including cell wall synthesis, energy production and metabolism, electron transport and detoxification. A protein, named reversibly glycosylated polypeptide, which is involved in cell wall synthesis, was the first time being found associated with heavy metal stress.8. Effect of glutothione in alleviating chromium toxicity to rice plantsThe effect of exogenous glutathione (GSH) in alleviating chromium stress was estimated through examining plant growth, chlorophyll and soluble protein contents, antioxidant enzyme activity and lipid peroxidation in rice seedlings exposed to chromium stress. The results showed that plant growth, and chlorophyll and soluble protein contents were severely reduced when the rice plants were exposed to 100μM Cr. Addition of GSH in the culture solution dramatically alleviated the reduction of plant growth, and chlorophyll and soluble protein contents. The activities of some antioxidant enzymes, including SOD, CAT and GR in rice leaves and CAT and GPX in rice roots were increased under Cr stress, which is attributed to defending response to oxidative stress in plants. Addition of GSH reduced the MDA accumulation and increased the activities of antioxidant enzymes in both leaves and roots, suggesting that GSH may enhance antioxidant capacity in Cr-stressed plants. Furthermore, GSH addition significantly decreased Cr uptake and root-to-shoot transport in rice plants exposed to Cr stress. It can be assumed that GSH is involved in Cr compartmentalization in root cells.9. Effect of silicon in alleviating chromium toxicity to rice plantsThe alleviatory effect of Si on Cr toxicity to rice was investigated using a hydroponic experiment with 12 factorial treatments of two Cr levels (0 and 100μM), three Si levels (0,75 and 150mg L-1) and two rice genotypes (Dan K5 and Xiushui113). The results showed that 100μM Cr markedly decreased plant height, dry biomass, soluble protein content, and root antioxidant enzyme activity, whereas significantly increased Cr concentration and MDA content. However, the reduction of plant height, dry biomass and soluble content was greatly alleviated from Si addition to the hydroponic solution. Compared with the plants treated with Cr alone, Si addition significantly reduced Cr uptake and translocation in rice plants. No significant difference was observed between the two Si treatments (75 or 150mg L-1) in shoot Cr concentration and Cr translocation factor. Si addition also alleviated the reduction of anti-oxidative enzymes SOD and APX in leaves; CAT and APX in roots and the increase of MDA content in the Cr-stressed plants. Furthermore, the beneficial effects of Si on activities of anti-oxidative enzymes under Cr stress were genotype dependent. The highest activities of SOD, POD, CAT and APX in leaves occurred in the treatment Cr+Si 150 for Xiushui113 and in the treatment Cr+Si 75 for Dan K5, respectively. It may be suggested that the beneficial effect of Si on alleviating oxidative stress was much more pronounced in Dan K5 than in Xiushui113. The current results showed that Si alleviates Cr toxicity through lowering Cr uptake and translocation and enhancing the activities of anti-oxidative enzymes.更多还原