0 引言
【研究意义】镉（Cd）是生物毒性最强的重金属之一[1]。一旦土壤受到Cd污染,就很难消除,只能在各种形态之间相互转化、迁移或富集[2]。当植物体内的Cd浓度增加到某一数值,植物就会受到毒害,造成细胞膜通透性增加,细胞被破坏,呼吸作用和光合作用受到影响,从而影响植物的生长[3,4,5,6,7]。土壤中的Cd由于具有较强的化学活性而易被植物吸收。在植物收获器官中的Cd经过食物链的传递进入人体,在人体组织器官中累积,使其受不同程度的损伤,包括致癌、突变、致畸、动脉硬化等毒性作用,危害人类健康[8,9,10]。【前人研究进展】水稻品质优劣直接关系着人类的健康。FUJIMAKI等[11]利用107Cd同位素的研究表明,水稻灌浆期从土壤中吸收的Cd仅需半天时间就被运送到籽粒,韧皮部运输起到关键作用;而KASHIWAGI等[12]的研究则认为,水稻籽粒中的Cd主要来自抽穗前累积在叶片和茎秆中的Cd,在抽穗后被转运到籽粒中,而抽穗后从根运送到茎秆和穗部的Cd直接进入气生器官（除糙米以外地上部器官）,而不影响糙米中Cd的累积。【本研究切入点】国内外科学工作者在Cd污染水稻生理生化毒害机理方面取得了一定的进展[13,14,15,16,17,18],对水稻籽粒中累积Cd的来源与吸收机理研究较多[19,20,21]。但由于条件不同,结果不一致。进入水稻籽粒中的Cd是由地上部分转移而来,还是从土壤中吸收直接运送而来,或者两者皆有,现有研究报道较少。【拟解决的关键问题】本研究探讨水稻灌浆后籽粒中Cd的来源,为水稻Cd污染防控选用恰当的农艺措施和实施时间提供科学依据。1 材料与方法
1.1 试验材料
供试作物：水稻,品种为Ⅱ优838,由四川省原子核应用技术研究所选育。水稻采样田的位置和与试验研究的相关土壤性质：试验用水稻种植在广汉市连山镇锦花村。土壤为老冲积黄壤与紫色土坡积物混合物发育而成的水稻土。土壤pH 7.1,有机质34.4 g·kg-1,速效氮64.45 mg·kg-1,速效磷19.7 mg·kg-1,速效钾98 mg·kg-1,全镉 0.68 mg·kg-1,有效镉 0.12 mg kg-1,属于Cd污染土壤。水培试验于2015年5月在四川省农业科学院土壤肥料研究所盆栽场进行。
1.2 试验设计
水培试验：设3个Cd浓度处理,分别为0、0.2和0.5 mg·L-1,重复3次。试验用营养液采用国际水稻研究所常规营养液配方[22]。其中,氮、磷、钾浓度分别为100、30、150 mg·L-1,Cd用分析纯CdCl2,在水稻植入时加入营养液。水培盆钵为陶瓷盆（D20 cm×H26 cm）,每盆加营养液5 L。在水稻齐穗期,从广汉市Cd污染稻田取长势一致的水稻植株12窝,带回后用自来水冲洗根部,去除泥污。3窝直接用于水稻齐穗期植株Cd含量分析,然后把9窝分别移入已备好的盛有营养液含3个Cd浓度处理的盆钵中,每个处理1窝,重复3次;培养至成熟期,收获不同处理植株样本作分析用。田间试验：从水稻齐穗期开始至成熟期,在籽粒形成过程中每隔9 d从同一稻田随机取长势一致的水稻植株3窝,连续取样4次,分别是8月1 日（齐穗期）;8月10日（灌浆期）;8月19日（腊熟期）;8月28日（成熟期）。每次取样后,洗净泥土,装入尼龙网袋中,晾干后置于烘箱于105℃杀青30 min,65℃下干燥,分析不同器官中Cd含量。
1.3 测定项目与方法
水稻样品分为根、叶片、茎、谷壳和糙米5部分收集,分别称重。样品经磨碎过40目尼龙筛,测定其Cd含量。采用HNO3-HClO4湿法消煮,同时消煮空白和标准样品（GBW10010）进行质量控制和结果校正,用石墨炉原子吸收光谱仪（novAA400-德国耶拿）测定Cd含量[23]。水稻植株Cd吸收量=植株生物量×植株Cd浓度。1.4 数据处理
采用Microsoft Excel和DPSv6.55进行有关数据的计算和统计检验。2 结果
2.1 不同取样时间水稻器官干重的变化
从水稻齐穗期至成熟期,田间样品的籽粒干重不断增加,在成熟期达到最大值,为35.98 g/ plant（表1）。在此期间,水稻茎秆、叶和根的重量变化趋势与籽粒相反,为不断减少。其中茎秆的重量减少最为显著,从齐穗期至腊熟期其重量差异达到显著水平。而谷壳在不同时期的变化不大,基本保持稳定。这些结果表明,齐穗前累积在这些器官（主要是茎和叶）中的光合产物在齐穗后源源不断地向籽粒运输,而其重量不断减轻。Table 1
表1
表1不同取样时间的水稻器官干重
Table 1Dry weight of different plant parts of rice at different sampling times
| 处理 Treatment | 糙米重 Brown rice wt (g/plant) | 谷壳重 Chaff wt (g/plant) | 茎秆重 Culm wt (g/plant) | 叶重 Leaf wt (g/plant) | 根重 Root wt (g/plant) |
|---|---|---|---|---|---|
| 田间样 Field sample | |||||
| 齐穗期 Full heading stage | - | 8.93a | 35.82a | 15.41ab | 6.35bcd |
| 灌浆期 Milk stage | 9.13c | 8.53a | 31.95b | 13.80ab | 6.29bcd |
| 腊熟期 Dough stage | 27.53b | 8.90a | 28.07c | 13.52ab | 5.19d |
| 成熟期 Mature stage | 35.98a | 8.94a | 27.81c | 12.19b | 5.53cd |
| 水培样(成熟期) Hydroponics sample (Mature stage) | |||||
| Cd 0 | 36.54a | 8.11a | 26.39cd | 15.97a | 8.70a |
| Cd 0.2 | 35.43a | 7.93a | 24.28d | 15.16ab | 7.95ab |
| Cd 0.5 | 34.08a | 7.90a | 24.85cd | 14.18ab | 7.36abc |
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在水培样中,添加浓度为0.2和0.5 mg·L-1的Cd后其糙米、谷壳、茎秆、叶、根重量比CK下降,但差异不显著。除Cd 0.5处理的茎秆重比Cd 0.2处理高0.57 g/plant外,Cd 0.5处理的其他4个组织器官的干重都略低于Cd 0.2处理。0.2和0.5 mg·L-1的浓度对水稻各器官生长的影响不大,可能是因为水稻移至营养液时已至齐穗期,各器官的生长已基本完成,对Cd毒性的敏感性降低之故。
2.2 齐穗后稻谷籽粒中Cd累积与来源
表2表明,在田间条件下,籽粒中Cd浓度随生长趋于成熟期而不断降低,而累积量却增加。浓度降低主要是由于籽粒充实期间淀粉的累积速度大于Cd 的累积速度,产生稀释效应,这一效应在齐穗后的前9—18 d,即腊熟期最为明显（-21.85%）;而Cd累积高峰期也处于齐穗后9—18 d（+145.92%）,腊熟期后淀粉的累积速率降低。Table 2
表2
表2不同处理对齐穗后稻谷籽粒中Cd浓度和累积量的影响
Table 2Cd concentrations and accumulation in rice grain as affected by different treatments after heading stage
| 处理 Treatment | Cd浓度 Cd content | Cd累积量 Cd accumulated | ||
|---|---|---|---|---|
| (mg·kg-1) | (±%*) | (μg/ plant) | (±%) | |
| 田间样 Field sample | ||||
| 齐穗期 Full heading stage | - | - | - | - |
| 灌浆期 Milk stage | 0.1693c | - | 1.49b | - |
| 腊熟期 Dough stage | 0.1323c | -21.85 | 3.67b | +145.92 |
| 成熟期 Mature stage | 0.1233c | -27.17 | 4.40b | +194.28 |
| 水培样(成熟期) Hydroponics sample (Mature stage) | ||||
| Cd 0 | 0.1488c | - | 5.44b | - |
| Cd 0.2 | 1.1999b | +706.38 | 42.89a | +688.97 |
| Cd 0.5 | 1.5925a | +970.23 | 55.24a | +916.11 |
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在水培试验中,水稻籽粒中的Cd浓度和累积量随培养液Cd浓度的增加而显著增加。与Cd 0处理相比较,强制加入速效外源Cd后使稻米中Cd的吸收量成倍增加,Cd 0.2和Cd 0.5处理分别增加了688.97%和916.11%,说明介质中可利用Cd的含量高低对稻米Cd累积的重要作用。在田间自然生长条件下,水稻籽粒的Cd含量和累积量都远远小于水培试验的加Cd处理。产生这种差异的原因可能是田间土壤中的有效镉含量在水稻生长后期很低（试验开始时土壤有效镉仅为 0.12 mg·kg-1）,特别是在根区;而水培试验中的有效镉浓度为0.2—0.5 mg·L-1,并且都是水溶性镉。因此,无论是Cd的供应强度还是容量,水培试验都要远高于田间土壤,自然水培试验就更加有利于水稻对Cd的吸收和累积。
2.3 水稻其他器官中Cd的累积情况
水稻齐穗-成熟期其他器官中的Cd浓度和累积量变化情况如表3所示。在田间条件下,水稻茎秆中的Cd浓度随生育进程不断上升,而累积量却在齐穗期后上升,腊熟期略有下降,成熟期又有所上升。这说明水稻灌浆-蜡熟期茎秆中光合产物向籽粒的转移率高于Cd,才导致Cd浓度升高和Cd累积量基本不变。齐穗-灌浆期茎秆中的累积的Cd从10.32 μg/plant上升到16.83 μg/plant,而根系中Cd的累积量未见等量降低,说明茎秆在向籽粒转移Cd的同时,还接受来自根系和土壤Cd补充,特别是土壤;腊熟期后茎秆中的Cd停止向籽粒转移。水稻叶片中的Cd浓度和累积量从齐穗期至成熟期一直处于下降趋势,其中灌浆期的下降最快,分别比齐穗期下降了8.55%和19.25%,表明累积在叶片的Cd在齐穗后持续向籽粒中转移。谷壳中的Cd也出现类似的趋势。因而,水稻叶片和谷壳在齐穗期前为Cd的库,而在齐穗后变成了籽粒Cd的源;而茎秆和根系起到库、源并重的角色,特别是灌浆-腊熟期,成熟时Cd基本停止转移。Table 3
表3
表3水稻齐穗-成熟期其他器官中Cd的分布
Table 3Cd distribution in the other plant parts of rice from fully heading to mature stage
| 处理 Treatment | 茎秆镉 Cd in culm | 叶片镉 Cd in leaf | 谷壳镉 Cd in chaff | 根部镉 Cd in root | ||||
|---|---|---|---|---|---|---|---|---|
| 浓度 Content (mg·kg-1) | 累积量 Accumulation (μg/plant) | 浓度 Content (mg·kg-1) | 累积量 Accumulation (μg/plant) | 浓度 Content (mg·kg-1) | 累积量 Accumulation (μg/plant) | 浓度 Content (mg·kg-1) | 累积量 Accumulation (μg/plant) | |
| 田间样Field sample | ||||||||
| 齐穗期 Full heading stage | 0.2875c | 10.32c | 0.0912c | 1.41c | 0.0812c | 0.73bc | 3.2637b | 21.05b |
| 灌浆期Milk stage | 0.5260c | 16.83c | 0.0834c | 1.14c | 0.0731c | 0.60bcd | 3.2951b | 20.85b |
| 腊熟期Dough stage | 0.5686c | 16.09c | 0.0835c | 1.13c | 0.0425de | 0.37cd | 3.2845b | 20.39b |
| 成熟期Mature stage | 0.6583c | 18.45c | 0.0791c | 0.97c | 0.0275e | 0.25d | 3.9529b | 22.18b |
| 水培样(成熟期) Hydroponics sample (Mature stage) | ||||||||
| Cd 0 | 0.5040c | 13.27c | 0.3108c | 4.95c | 0.0584cd | 0.48cd | 0.8034b | 6.88b |
| Cd 0.2 | 7.5569b | 182.02b | 1.8003b | 27.23b | 0.1103b | 0.87b | 25.6589a | 203.16a |
| Cd 0.5 | 13.7115a | 337.09a | 4.3433a | 61.56a | 0.1756a | 1.39a | 28.9305a | 211.05a |
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在不同Cd浓度的水培试验中,水稻所有器官中的Cd都随处理Cd浓度增加而增加,其中水稻茎秆和根系是水稻储存Cd的两大主要器官,其次是叶片,谷壳中储存的Cd量非常小。因此,当介质中不存在Cd的情况下,即水培试验的Cd 0处理,水稻齐穗后籽粒中累积的Cd主要来自于水稻根和茎,其次是叶片。当介质中存在较高浓度的有效镉（即Cd 0.2和Cd 0.5处理）时,水稻在齐穗期后会大量吸收Cd,并将其运送到水稻籽粒中。该结果表明,控制土壤溶液中有效镉的浓度是降低水稻吸收Cd的关键,特别是在齐穗-成熟期。
2.4 水稻不同器官中Cd的分布
水稻吸收的Cd在不同器官中的分配比例见表4。不难看出水稻根系和茎秆所含的Cd比例最大,在田间样品中二者之和大于86%;在水培样中两个施入外源Cd处理的二者之和大于82%。Cd 0处理的茎秆和根Cd吸收量之和约占65%,糙米和叶片之和占33%。这表明介质中的有效镉含量高低会影响Cd在水稻各器官中的分配比例。当培养介质中有效镉含量较低时（即田间情况）,根是水稻Cd的第一大贮存器官。而当介质中有效镉含量较高时（水培试验的Cd 0.2和Cd 0.5处理）,水稻茎秆则变为第一储存器官。当介质中的Cd含量为0时（水培试验的Cd 0处理）,茎秆为Cd的主要储存器官,并出现糙米中Cd的比例高于任何其他处理的情况。这就解释了为什么在介质中不含Cd（水培试验的Cd 0处理）的情况下成熟期籽粒的Cd浓度和累积量反而高于田间样。产生这种现象的原因可能是因为水培试验的营养和其他生长条件（通透性好,不含有还原性有毒有害物质等）优于田间,有利于光合产物和Cd向籽粒转移,因而籽粒产量和秸秆产量都高于田间样。Table 4
表4
表4不同采样时间水稻不同器官中Cd的分配比例
Table 4Distribution of Cd in different rice organs at different sampling times
| 处理 Treatment | Cd分配比例 Distribution of Cd（%） | ||||
|---|---|---|---|---|---|
| 糙米 Brown rice | 谷壳 Chaff | 茎秆 Culm | 叶 Leaf | 根 Root | |
| 田间样Field sample | |||||
| 齐穗期 Full heading stage | - | 2.19 | 30.79 | 4.19 | 62.83 |
| 灌浆期Milk stage | 3.65 | 1.46 | 41.14 | 2.78 | 50.97 |
| 腊熟期Dough stage | 9.51 | 0.97 | 41.63 | 2.92 | 44.98 |
| 成熟期Mature stage | 9.51 | 0.54 | 39.90 | 2.09 | 47.97 |
| 水培样(成熟期) Hydroponics sample (Mature stage) | |||||
| Cd 0 | 17.53 | 1.54 | 42.80 | 15.95 | 22.18 |
| Cd 0.2 | 9.40 | 0.19 | 39.90 | 5.97 | 44.54 |
| Cd 0.5 | 8.29 | 0.21 | 50.59 | 9.24 | 31.67 |
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3 讨论
3.1 稻谷籽粒中Cd的来源
在田间条件下从土壤吸收并直接运送到籽粒中的Cd量显著少于外源Cd培养条件下从介质中吸收的Cd。有研究认为,Cd是受植物体内的锌铁转运蛋白zip基因调控[24],通过根部吸收进入体内。但这种基因并不是调控Cd吸收的专一基因,而是同时调控Cd和Zn的吸收[25]。被吸收的Cd通过其他重金属转运蛋白卸入木质部,随后迅速被运送到地上部[26]。但是也有研究表明,Cd还有其他的通道进入植物体内,即与OsHMA2转运蛋白相关的木质部装载机制[19]。本试验结果表明,在水稻生长后期,水稻籽粒中的Cd来源于两个途径,一个是来自于土壤中的有效镉,另一个是其他器官中的Cd也会向籽粒中转运,这两种途径同时存在。但在水稻齐穗后土壤/介质中有效镉含量丰富的情况下,前者是主要途径;而在水稻抽穗后土壤中的有效镉含量很低的情况下,两个途径同时存在;当介质中不存在可利用Cd的情况下,后者成为唯一途径。这就很好地解释了FUJIMAKI等[11] 和KASHIWAGI等[12]相互矛盾的研究结果。水稻籽粒中的Cd并不是来源于单一途径,谁主谁辅要视具体情况而定。FUJIMAKI等的107Cd同位素研究结果,即在水稻灌浆期,水稻从土壤中吸收的Cd被运送到籽粒;KASHIWAGI等的研究结论表明也可以来自于抽穗前水稻其他器官积累的Cd。KASHIWAGI等发现被转移到水稻籽粒中的Cd主要来自抽穗前累积在叶片和茎秆中的Cd,对籽粒Cd的富集起主要作用,抽穗后从根运送到茎秆和穗部的Cd直接进入气生器官,不影响糙米中Cd的累积。
FENG等[27]的研究表明,水稻的根和节是Cd进入水稻地上部分的主要障碍,这两个部位的Cd含量最高,当外界有效镉含量较低时,进入水稻体内的大部分Cd被固定在这两个部位,这时水稻其他器官中Cd的再转运成为籽粒中Cd的主要来源。
3.2 水稻Cd的运移特征
从水稻各器官中Cd的累积情况可以得出,叶片是向籽粒净转移Cd的主要器官。叶片衰老期Cd通过韧皮部向籽粒转移是Cd再分配的主要过程[28]。而茎秆和根系则始终为Cd的主要累积器官或库[20]。水稻茎上的节和维管束系统的复杂相互连接能延缓重金属在水稻节中的流动[11],是重金属在水稻叶片、上部茎秆和籽粒中的重新分配的通道[29]。李正翔[30] 的研究表明水稻中Cd在不同器官中的再分配与基因型有关,茎秆在部分基因型水稻中Cd的再分配过程中仅仅起到了通道作用,并无储藏再分配的作用。当水稻齐穗后土壤/介质中存在可利用Cd时,茎秆在向籽粒转移Cd的同时也接受来自根系Cd的补充;而根系在向茎秆转移Cd的同时也接受来自土壤/介质Cd的补充,形成一个动态运移系统。根系对Cd的吸收和运输速度很快,在根系接受Cd处理1 h后便进入木质部汁液和地上组织[31], 穗颈和韧皮部是Cd进入籽粒的路径[32]。在韧皮部汁液中,Cd 与一个未知的13 kDa蛋白和巯基化合物结合[33,34]。由于水稻茎节会阻止Cd的运输,根系在吸收107Cd后1 h到达水稻茎基部和基部分蘖节中,7 h后才到达籽粒,但36 h后仍未测到Cd进入叶片[11]。因此,水稻在抽穗-成熟期吸收的Cd有较大比例滞留在各节间,通过节间的Cd会被优先运送到籽粒。
4 结论
本试验结果明确了水稻齐穗后籽粒中Cd的来源及其与培养介质中Cd供应强度的关系。籽粒中的Cd来自各器官在齐穗前储存Cd的转移和土壤/介质中Cd的吸收和直接运输。在水稻齐穗后生长介质中不存在可利用Cd的情况下,籽粒中累积的Cd则主要来自于根系和茎秆在齐穗前累积的Cd。在田间情况下,水稻齐穗后土壤中有效镉含量较低,籽粒中的Cd同时来自于土壤和水稻体内在齐穗前累积的Cd,而叶片和谷壳是两个向籽粒净转移Cd的器官。而水稻齐穗后土壤/长介质中有效镉含量较丰富的情况下,籽粒中的Cd则主要来自土壤/生长介质,由根系吸收后直接运送到籽粒,由茎、叶等其他器官转移到籽粒中的Cd所占比例很小。水稻茎秆和根系是水稻Cd的两大主要储运器官。水稻抽穗-成熟期,是水稻Cd污染防控的关键时期,采取恰当的农艺措施降低土壤中Cd的有效性以及根系吸收和向籽粒的直接运输量,是实现水稻安全生产的关键。The authors have declared that no competing interests exist.
参考文献 原文顺序
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| [1] | . Abstract BACKGROUND: Cadmium (Cd) is a heavy metal particularly hazardous for human health, as it is highly diffused and, therefore, a ubiquitous environmental toxicant. In fact, in the general population, the main sources of exposure are food, cigarette smoking, inhalation of ambient air, drinking water, contaminated soil or dust. Furthermore, an occupational exposure usually involves human during mining, fume inhalation or manufacturing nickel-cadmium battery, electroplating and paint pigments that utilize Cd. METHODS: We undertook a structured search in literature about Cd. This metal is noxious on the cells of many organs, among which the kidney, the testis and the brain will be considered in this review. RESULTS: The toxic effects induced by Cd include many specific mechanisms, such as the oxidative stress, cellular death and inflammation. As no specific therapy for the prevention or treatment of the morbidity and mortality associated with Cd exposure is available, the state of the art of the therapeutic approaches is illustrated. CONCLUSION: Nowadays, a therapy able to counteract Cd toxicity is still lacking and the development of new therapeutic agents is requested. Copyright脗漏 Bentham Science Publishers; For any queries, please email at epub@benthamscience.org. |
| [2] | . Cadmium (Cd) contamination of soil and food crops is a ubiquitous environmental problem that has resulted from uncontrolled industrialization, unsustainable urbanization and intensive agricultural practices. Being a toxic element, Cd poses high threats to soil quality, food safety, and human health. Land is the ultimate source of waste disposal and utilization therefore, Cd released from different sources (natural and anthropogenic), eventually reaches soil, and then subsequently bio-accumulates in food crops. The stabilization of Cd in contaminated soil using organic amendments is an environmentally friendly and cost effective technique used for remediation of moderate to high contaminated soil. Globally, substantial amounts of organic waste are generated every day that can be used as a source of nutrients, and also as conditioners to improve soil quality. This review paper focuses on the sources, generation, and use of different organic amendments to remediate Cd contaminated soil, discusses their effects on soil physical and chemical properties, Cd bioavailability, plant uptake, and human health risk. Moreover, it also provides an update of the most relevant findings about the application of organic amendments to remediate Cd contaminated soil and associated mechanisms. Finally, future research needs and directions for the remediation of Cd contaminated soil using organic amendments are discussed. |
| [3] | . The aim of this study was to determine important metal pools for bioaccumulation by the earthworms Lumbricus rubellus and Aporrectodea caliginosa in soils with high binding capacity. Cd, Cu and Zn concentrations in soil, pore water and CaCl 2 extracts of soil, in leaves of the plant species Urtica dioica and in earthworms were determined at 15 field sites constituting a gradient in metal pollution. Variations in the Cu and Cd concentrations in L. rubellus and Cu concentrations in A. caliginosa were best explained by total soil concentrations, while variation in Cd concentration in A. caliginosa was best explained by pore water concentrations. Zn concentrations in L. rubellus and A. caliginosa were not significantly correlated to any determined variable. It is concluded that despite low availability, earthworms in floodplain soils contain elevated concentrations of Cu and Cd, suggesting that uptake takes place not only from the soluble metal concentrations. |
| [4] | . Cadmium (Cd) is a heavy metal that is of great concern in the environment, because of its toxicity to animals and humans. This article reviews recent papers showing how soil factors (such as pH, phosphate, zinc, and organic matter), Cd hyperaccumulation, and soil amendments affect Cd availability. The studies confirm that the pH of the soil is usually the most important factor that controls uptake, with low pH favoring Cd accumulation, and that phosphate and zinc decrease Cd uptake. The work reveals that the availability of Cd is increased by the application of chloride and reduced by application of silicon. The most striking result of this review is the elevated levels of Cd in plants that are being reported in recent studies. Data for concentrations of Cd in soils and plants under variously polluted conditions are presented in a table and show that all plants have Cd concentrations ≥ 0.102mg/kg, the normal concentration in plants. Concentrations ranged from two low concentrations of 0.102mg/kg Cd (in grain of corn, Zea mays, on an abandoned sludge disposal site that had not received sludge for 1002years, and in roots of hybrid poplar, Populus deltoides x P. nigra, at a 25-year old active sludge farm) to 38002mg/kg Cd in leaves of penny-cress ( Thlaspi caerulescens). Plants that hyperaccumulate Cd (i.e., have 10002mg/kg Cd in the tissue or more) belong to the genus Thalspi, the only known Cd hyperaccumulator. Of particular concern for humans are the high concentrations of Cd in rice grain and tobacco leaves. Even if Cd availability is decreased by adding amendments, it is still in the soil and a potential hazard. The best solution for maintaining non-contaminated soils and plants is to remove the sources of Cd in the environment. Given that that is essentially impossible at this time, further research needs to determine how soil and plant factors affect Cd availability on polluted soils. |
| [5] | . 利用水培试验方法研究了Hg对小麦种子萌发、幼苗生长及生理生化特性的影响.结果表明,随 Hg浓度的增加,Hg对小麦种子萌发、芽和根的伸长的抑制作用加强.高浓度的Hg胁迫下,幼苗叶片内的叶绿素含量、根系活力显著下降,丙二醛和游离脯氨酸 含量大量增加,说明膜脂过氧化作用加强;随胁迫强度增加,POD和CAT活性呈现先上升后下降的趋势;叶片净光合速率和单株生物量显著降低,特别是在 50.0 μmol·L-1Hg2+浓度下,植株生长受到极大抑制.相关分析表明,净光合速率与叶绿素含量、干物质积累显著正相关. . 利用水培试验方法研究了Hg对小麦种子萌发、幼苗生长及生理生化特性的影响.结果表明,随 Hg浓度的增加,Hg对小麦种子萌发、芽和根的伸长的抑制作用加强.高浓度的Hg胁迫下,幼苗叶片内的叶绿素含量、根系活力显著下降,丙二醛和游离脯氨酸 含量大量增加,说明膜脂过氧化作用加强;随胁迫强度增加,POD和CAT活性呈现先上升后下降的趋势;叶片净光合速率和单株生物量显著降低,特别是在 50.0 μmol·L-1Hg2+浓度下,植株生长受到极大抑制.相关分析表明,净光合速率与叶绿素含量、干物质积累显著正相关. |
| [6] | . <p>采用盆栽试验，研究了不同镉（Cd）浓度胁迫下忍冬科荚蒾属珊瑚树(<em>Viburnum odoratissimum</em>)和地中海荚蒾(<em>V. tinus</em>)2种园林树种3年生苗木叶片受伤害形态特征及生理生化指标（叶绿素、丙二醛、可溶性糖及可溶性蛋白含量、电解质渗透率、过氧化物酶活性）的变化，并分析了植株各营养器官（根、茎、叶）Cd的富集水平。结果表明：Cd对珊瑚树叶片生长及生理生化指标均未产生显著影响；地中海荚蒾在Cd胁迫浓度为100 mg·kg<sup>-1</sup>时出现伤害症状，电解质渗透率增加，丙二醛含量增加，叶绿素含量减少；而当胁迫浓度提高到400 mg·kg<sup>-1</sup>时，植株叶片枯黄，植株萎蔫，生长势差；2种植物地下部富集量均大于地上部，可以推断珊瑚树和地中海荚蒾对土壤中Cd具有一定的富集能力，在土壤镉污染严重地区栽植具有重要意义。</p> . <p>采用盆栽试验，研究了不同镉（Cd）浓度胁迫下忍冬科荚蒾属珊瑚树(<em>Viburnum odoratissimum</em>)和地中海荚蒾(<em>V. tinus</em>)2种园林树种3年生苗木叶片受伤害形态特征及生理生化指标（叶绿素、丙二醛、可溶性糖及可溶性蛋白含量、电解质渗透率、过氧化物酶活性）的变化，并分析了植株各营养器官（根、茎、叶）Cd的富集水平。结果表明：Cd对珊瑚树叶片生长及生理生化指标均未产生显著影响；地中海荚蒾在Cd胁迫浓度为100 mg·kg<sup>-1</sup>时出现伤害症状，电解质渗透率增加，丙二醛含量增加，叶绿素含量减少；而当胁迫浓度提高到400 mg·kg<sup>-1</sup>时，植株叶片枯黄，植株萎蔫，生长势差；2种植物地下部富集量均大于地上部，可以推断珊瑚树和地中海荚蒾对土壤中Cd具有一定的富集能力，在土壤镉污染严重地区栽植具有重要意义。</p> |
| [7] | . The tolerance to increasing doses of lead and cadmium salts on the growth and survival of barley ( Hordeum vulgare L.) and wheat ( Triticum aestivum L.) seedlings were studied. Seedlings grown under controlled conditions were treated with dilute (0.001–0.005 mM) solutions of either lead nitrate or cadmium bromide for 1, 4, or 7 days. Subsequently, they were incubated for 7 days in solutions of the same compounds, but at sublethal or lethal concentrations (0.05–10 mM). Plant pretreatment with low concentrations of heavy metals induced an increase in their tolerance to the metals, because pretreated plants could tolerate heavy metals at high concentrations. It is concluded that plant tolerance to increasing concentrations of heavy metals is related to the activation of protective and adaptive processes in their tissues. |
| [8] | . The efficacy of a chelating agent in binding a given metal in a biological system depends on the binding constants of the chelator for the particular metals in the system, the concentration of the metals, and the presence and concentrations of other ligands competing for the metals in question. In this study, we make a comparison of the in vitro binding constants for the chelator, ethylenediaminetetraacetic acid, with the quantitative urinary excretion of the metals measured before and after EDTA infusion in 16 patients. There were significant increases in lead, zinc, cadmium, and calcium, and these increases roughly corresponded to the expected relative increases predicted by the EDTA-metal-binding constants as measured in vitro. There were no significant increases in urinary cobalt, chromium, or copper as a result of EDTA infusion. The actual increase in cobalt could be entirely attributed to the cobalt content of the cyanocobalamin that was added to the infusion. Although copper did increase in the post-EDTA specimens, the increase was not statistically significant. In the case of magnesium, there was a net retention of approximately 85% following chelation. These data demonstrate that EDTA chelation therapy results in significantly increased urinary losses of lead, zinc, cadmium, and calcium following EDTA chelation therapy. There were no significant changes in cobalt, chromium, or copper and a retention of magnesium. These effects are likely to have significant effects on nutrient concentrations and interactions and partially explain the clinical improvements seen in patients undergoing EDTA chelation therapy. |
| [9] | . . |
| [10] | . . |
| [11] | . |
| [12] | . pAbstract/p pBackground/p pCadmium (Cd) translocation and accumulation in the grain and aerial plant parts of rice (itOryza sativa /itL.) is an important aspect of food safety and phytoextraction in areas with contaminated soil. Because control of Cd translocation and accumulation is likely to be determined by the plants genetics, the Cd contents of grain and the aerial parts of rice may be manipulated to improve food safety and for phytoextraction ability. This study studied Cd translocation and accumulation and their genetic control in aerial parts of rice to provide a starting point for improving food safety and phytoextraction in Cd-contaminated soils./p pResults/p pIn the itjaponica /itrice cultivar Nipponbare, Cd accumulated in leaves and culms until heading, and in culms and ears after heading. Two quantitative trait loci (QTLs) from itindica /itcv. Kasalath, itqcd4-1 /itand itqcd4-2/it, affect Cd concentrations in upper plant parts just before heading. Three near-isogenic lines (NILs) with itqcd4-1 /itand itqcd4-2 /itwere selected from the Nipponbare background, and were analyzed for the effects of each QTL, and for interactions between the two QTLs. From the results compared between Nipponbare and each NIL, neither QTL influenced total Cd accumulation in aerial parts at 5 days after heading, but the interaction between two QTLs increased Cd accumulation. At 35 days after heading, itqcd4-2 /ithad increased Cd accumulation in the aerial plant parts and decreased translocation from leaves other than flag leaf, but interaction between the two QTLs increased translocation from leaves. NILitqcd4-1,2 /itaccumulated higher concentrations of Cd in brown rice than Nipponbare./p pConclusion/p pThree types of Cd translocation and accumulation patterns demonstrated by NILs suggested that the accumulation of Cd in leaves and culms before heading, and translocation from them after heading are responsible for Cd accumulation in grain. Cd translocation from roots to culms and ears after heading may direct Cd to the aerial organs without influencing brown rice accumulation./p |
| [13] | . . |
| [14] | . 61Radial oxygen loss controls Fe plaque formation on rice roots and affects Cd uptake.61Flooding before and after heading is effective for reducing Cd uptake by rice.61Inoculation with suitable AMF can be applied to reduce Cd accumulation in rice.61Intercropping and rotation systems can decrease Cd uptake by rice. |
| [15] | . A field-based study was undertaken to analyze the effects of soil bioavailable heavy metals determined by a sequential extraction procedure, and soil microbial parameters on the heavy metal accumulation in rice grain. The results showed that Cd, Cr, Cu, Ni, Pb and Zn concentrations in rice grain decreases by 65.9%, 78.9%, 32.6%, 80.5%, 61.0% and 15.7%, respectively in the sites 3 (far-away), compared with those in sites 1 (close-to). Redundancy analysis (RDA) indicated that soil catalase activity, the MBC/MBN ratio, along with bioavailable Cd, Cr and Ni could explain 68.9% of the total eigenvalue, indicating that these parameters have a great impact on the heavy metal accumulation in rice grain. The soil bioavailable heavy metals have a dominant impact on their accumulation in rice grain, with a variance contribution of 60.1%, while the MBC/MBN has a regulatory effect, with a variance contribution of 4.1%. Stepwise regression analysis showed that the MBC/MBN, urease and catalase activities are the key microbial parameters that affect the heavy metal accumulation in rice by influencing the soil bioavailable heavy metals or the translocation of heavy metals in rice. RDA showed an interactive effect between Cu, Pb and Zn in rice grain and the soil bioavailable Cd, Cr and Ni. The heavy metals in rice grain, with the exception of Pb, could be predicted by their respective soil bioavailable heavy metals. The results suggested that Pb accumulation in rice grain was mainly influenced by the multi-metal interactive effects, and less affected by soil bioavailable Pb. |
| [16] | . 比较水稻亲本材料的镉耐性差异, 筛选镉低积累水稻种质资源, 为水稻镉安全品种（Cd-safe cultivars, CSCs）的培育提供遗传材料。以收集的具有明显遗传差异的145种水稻亲本材料为研究对象, 通过水培试验, 研究水稻植株生长性状和镉积累特征, 比较不同材料的镉耐性和镉积累差异, 并以耐性指数和镉含量为指标, 筛选镉低积累种质资源。结果表明： （1）在镉胁迫条件下, 水稻生物量和株高受到不同程度的抑制, 根长和根冠比呈不同程度增加。（2）恢复系各材料间镉含量和积累量最大值分别为最小值的2.79倍和6.45倍, 保持系各材料间镉含量和积累量最大值分别为最小值的2.00倍和2.98倍。（3）根据耐性指数差异将恢复系和保持系各分成耐性不同的5类, 并将耐性较强材料进行镉积累差异分类, 得到恢复系镉低积累种质资源13种, 分别是＂MR183＂、＂MR86＂、＂R047＂、＂R364＂、＂泸恢602＂、＂泸恢615＂、＂泸恢17＂、＂GR548/M63//527_2＂、＂R18＂、＂成恢838＂、＂GR548/M63//M63_5＂、＂GRL17/IRBN95-199_3＂和＂GRL17/ATTP//L17_3＂; 保持系镉低积累种质资源2种, 分别是＂玉香B＂和＂D62B＂。（4）镉耐性较强材料中, 高积累材料的镉含量和镉积累量表现为恢复系中分别为低积累材料的1.97倍和2.03倍, 保持系中分别为低积累材料的1.43倍和1.40倍; 镉含量和镉积累量在两系的低积累材料间无明显差异。筛选镉低积累材料培育镉安全品种将成为解决镉安全威胁的关键。 . 比较水稻亲本材料的镉耐性差异, 筛选镉低积累水稻种质资源, 为水稻镉安全品种（Cd-safe cultivars, CSCs）的培育提供遗传材料。以收集的具有明显遗传差异的145种水稻亲本材料为研究对象, 通过水培试验, 研究水稻植株生长性状和镉积累特征, 比较不同材料的镉耐性和镉积累差异, 并以耐性指数和镉含量为指标, 筛选镉低积累种质资源。结果表明： （1）在镉胁迫条件下, 水稻生物量和株高受到不同程度的抑制, 根长和根冠比呈不同程度增加。（2）恢复系各材料间镉含量和积累量最大值分别为最小值的2.79倍和6.45倍, 保持系各材料间镉含量和积累量最大值分别为最小值的2.00倍和2.98倍。（3）根据耐性指数差异将恢复系和保持系各分成耐性不同的5类, 并将耐性较强材料进行镉积累差异分类, 得到恢复系镉低积累种质资源13种, 分别是＂MR183＂、＂MR86＂、＂R047＂、＂R364＂、＂泸恢602＂、＂泸恢615＂、＂泸恢17＂、＂GR548/M63//527_2＂、＂R18＂、＂成恢838＂、＂GR548/M63//M63_5＂、＂GRL17/IRBN95-199_3＂和＂GRL17/ATTP//L17_3＂; 保持系镉低积累种质资源2种, 分别是＂玉香B＂和＂D62B＂。（4）镉耐性较强材料中, 高积累材料的镉含量和镉积累量表现为恢复系中分别为低积累材料的1.97倍和2.03倍, 保持系中分别为低积累材料的1.43倍和1.40倍; 镉含量和镉积累量在两系的低积累材料间无明显差异。筛选镉低积累材料培育镉安全品种将成为解决镉安全威胁的关键。 |
| [17] | . Accumulation of cadmium and nonprotein thiols (NPT) in rice (Oryza sativa L.) seedling as affected by short-term treatment of different combinations of sulfate and cadmium was studied.The results showed that short-term(6 days) Cd stress did not cause significant decline of the seedling biomass,but the interaction between cadmium and sulfate did affect cadmium accumulation in rice seedlings.It was observed that increasing sulfate supply could considerably enhance cadmium transport from root to shoot,thus resulting in higher accumulation of cadmium in leaf.Both of cadmium only stress and higher level of sulfate supply increased NPT content in seedlings,particularly in roots,although level of the phosphate buffer-extractable proteins in plants was kept on changed.Supply of cadmium and sulfate together could further increase NPT content in seedlings,indicating a positive effect of the interaction on NPT.It was concluded that sufficient supply of sulfate could improve rice tolerance to cadmium stress but also enhance cadmium accumulation in the leaf. . Accumulation of cadmium and nonprotein thiols (NPT) in rice (Oryza sativa L.) seedling as affected by short-term treatment of different combinations of sulfate and cadmium was studied.The results showed that short-term(6 days) Cd stress did not cause significant decline of the seedling biomass,but the interaction between cadmium and sulfate did affect cadmium accumulation in rice seedlings.It was observed that increasing sulfate supply could considerably enhance cadmium transport from root to shoot,thus resulting in higher accumulation of cadmium in leaf.Both of cadmium only stress and higher level of sulfate supply increased NPT content in seedlings,particularly in roots,although level of the phosphate buffer-extractable proteins in plants was kept on changed.Supply of cadmium and sulfate together could further increase NPT content in seedlings,indicating a positive effect of the interaction on NPT.It was concluded that sufficient supply of sulfate could improve rice tolerance to cadmium stress but also enhance cadmium accumulation in the leaf. |
| [18] | . <p>为揭示水稻镉抗性的分子机理，以抗镉水稻品种PI312777和镉敏感水稻品种IR24为材料，在镉离子浓度为0（对照）、50和100 μmol·L<sup>-1</sup>条件下水培处理7 d，应用蛋白质组学方法分析了2种水稻叶片对镉胁迫响应的蛋白质差异表达.结果表明：镉胁迫下水稻PI312777叶片中共检测到差异表达蛋白质点31个，通过MALDI-TOF/MS分析，鉴定了其中的24个蛋白质（包括20个不同蛋白质，4个重复检出蛋白质）；IR24叶片中共检测到差异表达蛋白质点19个，其中15个蛋白质得到鉴定.PI312777叶片鉴定出的20个蛋白质覆盖了IR24叶片鉴定的15个蛋白质，前者有4个与光合作用相关，11个与细胞防御代谢相关，3个与其他代谢相关，2个为功能未知蛋白.与对照相比，不同浓度镉胁迫下，抗镉水稻PI312777叶片中热激蛋白、谷胱甘肽还原酶、蛋白酶体α亚基6型、果糖1,6-二磷酸醛缩酶、硫氧还蛋白和DNA重组修复蛋白均上调表达；镉敏感水稻IR24叶片中热激蛋白、谷胱甘肽还原酶、蛋白酶体α亚基6型的表达无显著差异，果糖1,6-二磷酸醛缩酶和硫氧还蛋白则下调表达.此外，DNA重组修复蛋白仅在镉胁迫的PI312777叶片中表达.水稻PI312777比IR24具有更强的镉抗性与这些差异表达的蛋白质密切相关.</p> <p>为揭示水稻镉抗性的分子机理，以抗镉水稻品种PI312777和镉敏感水稻品种IR24为材料，在镉离子浓度为0（对照）、50和100 μmol·L<sup>-1</sup>条件下水培处理7 d，应用蛋白质组学方法分析了2种水稻叶片对镉胁迫响应的蛋白质差异表达.结果表明：镉胁迫下水稻PI312777叶片中共检测到差异表达蛋白质点31个，通过MALDI-TOF/MS分析，鉴定了其中的24个蛋白质（包括20个不同蛋白质，4个重复检出蛋白质）；IR24叶片中共检测到差异表达蛋白质点19个，其中15个蛋白质得到鉴定.PI312777叶片鉴定出的20个蛋白质覆盖了IR24叶片鉴定的15个蛋白质，前者有4个与光合作用相关，11个与细胞防御代谢相关，3个与其他代谢相关，2个为功能未知蛋白.与对照相比，不同浓度镉胁迫下，抗镉水稻PI312777叶片中热激蛋白、谷胱甘肽还原酶、蛋白酶体α亚基6型、果糖1,6-二磷酸醛缩酶、硫氧还蛋白和DNA重组修复蛋白均上调表达；镉敏感水稻IR24叶片中热激蛋白、谷胱甘肽还原酶、蛋白酶体α亚基6型的表达无显著差异，果糖1,6-二磷酸醛缩酶和硫氧还蛋白则下调表达.此外，DNA重组修复蛋白仅在镉胁迫的PI312777叶片中表达.水稻PI312777比IR24具有更强的镉抗性与这些差异表达的蛋白质密切相关.</p> |
| [19] | . Among cereals, rice has a genetic propensity to accumulate high levels of cadmium (Cd) in grains. Xylem-mediated root-to-shoot translocation rather than root uptake has been suggested as the main physiological factor accounting for the genotypic variation observed in Cd accumulation in shoots and grains. Several evidence indicate OsHMA2 鈥 a putative zinc (Zn) transporter 鈥 as the main candidate protein that could be involved in mediating Cd- and Zn-xylem loading in rice. However, the specific interactions between Zn and Cd in rice often appear anomalous if compared to those observed in other staple crops, suggesting that root-to-shoot Cd translocation process could be more complex than previously thought. In this study we performed a complete set of competition experiments with Zn and Cd in order to analyze their possible interactions and reciprocal effects at the root-to-shoot translocation level. The competition analysis revealed the lack of a full reciprocity when considering the effect of Cd on Zn accumulation, and vice versa, since the accumulation of Zn in the shoots was progressively inhibited by Cd increases, whereas that of Cd was only partially impaired by Zn. Such behaviors were probably dependent on Cd-xylem loading mechanisms, as suggested by: i) the analysis of Zn and Cd content in the xylem sap performed in relation to the concentration of the two metals in the mobile fractions of the roots; ii) the analysis of the systemic movement of107Cd in short term experiments performed using a positron-emitting tracer imaging system (PETIS). Our results suggest that at least two pathways may mediate root-to-shoot Cd translocation in rice. The former could involve OsHMA2 as Zn2+/Cd2+xylem loader, whereas the latter appears to involve a Zn-insensitive system that still needs to be identified. The online version of this article (doi:10.1186/s12284-016-0088-3) contains supplementary material, which is available to authorized users. |
| [20] | . |
| [21] | . Iron (Fe) is essential for rice growth and humans consuming as their staple food but is often deficient because of insoluble Fe(III) in soil for rice growth and limited assimilation for human bodies, while cadmium (Cd) is non-essential and toxic for rice growth and humans if accumulating at high levels. Over-accumulated Cd can cause damage to human bodies. Selecting and breeding Fe-rich but Cd-free rice cultivars are ambitious, challenging and meaningful tasks for researchers. Although evidences show that the mechanisms of Fe/Cd uptake and accumulation in rice are common to some extent as a result of similar entry routes within rice, an increasing number of researchers have discovered distinct mechanisms between Fe/Cd uptake and accumulation in rice. This comprehensive review systematically elaborates and compares cellular mechanisms of Fe/Cd uptake and accumulation in rice, respectively. Mechanisms for maintaining Fe homeostasis and Cd detoxicification are also elucidated. Then, effects of different fertilizer management on Fe/Cd accumulation in rice are discussed. Finally, this review enumerates various approaches for reducing grain Cd accumulation and enhancing Fe content in rice. In summary, understanding of discrepant cellular mechanisms of Fe/Cd accumulation in rice provides guidance for cultivating Fe-fortified rice and has paved the way to develop rice that are tolerant to Cd stress, aiming at breeding Fe-rich but Cd-free rice. |
| [22] | |
| [23] | |
| [24] | . Participation of the intervascular transport system within the rice stem during cadmium (Cd) partitioning was investigated by characterizing109Cd behaviour in the shoot. In addition,45Ca,32P, and35S partitioning patterns were analysed for comparison with that of109Cd. Each tracer was applied to the seedling roots for 15min, and the shoots were harvested either at 15min (i.e. immediately after tracer application) or at 48h. Distribution patterns of each element at 15min were studied to identify the primary transport pathway before remobilization was initiated.32P was preferentially transported to completely expanded leaf blades having the highest transpiration rate. The newest leaf received minimal amounts of32P. In contrast, the amount of35S transported to the newest leaf was similar to that transported to the other mature leaf blades. Preferential movement towards the newest leaf was evident for109Cd and45Ca. These results directly indicate that elemental transport is differentially regulated in the vegetative stem as early as 15min before the elements are transported to leaves. Cd behaviour in the stem was investigated in detail by obtaining serial section images from the bottom part of shoots after109Cd was applied to a single crown root. At 30min, the maximum amount of109Cd was distributed in the peripheral cylinder of the longitudinal vascular bundles (PV) and, interestingly, some amount of109Cd was transported downwards along the PV. This transport manner of109Cd provides evidence that Cd can be loaded on the phloem at the stem immediately after Cd is transported from the root. |
| [25] | . Highlights? The ABC transporters Abc2 (S. pombe), ABCC1 (A. thaliana), and ABCC2 (A. thaliana) have been identified as the long-sought vacuolar phytochelatin transporters. ? The vacuolar ATPase HMA3 affects shoot-root partitioning of cadmium. ? Cadmium in seeds is co-ordinated by thiol-containing compounds. ? Heavy metals induce transcriptional responses that allow plants to preserve metal homeostasis. ? The transcriptional regulators required for Cd-induced and As-induced gene expression remain unknown. |
| [26] | Zinc (Zn) and iron (Fe) are essential but are sometimes deficient in humans, while cadmium (Cd) is toxic if it accumulates in the liver and kidneys at high levels. All three are contained in the grains of rice, a staple cereal. Zn and Fe concentrations in rice grains harvested under different levels of soil/hydroponic metals are known to change only within a small range, while Cd concentrations show greater changes. To clarify the mechanisms underlying such different metal contents, we synthesized information on the routes of metal transport and accumulation in rice plants by examining metal speciation, metal transporters, and the xylem-to-phloem transport system. At grain-filling, Zn and Cd ascending in xylem sap are transferred to the phloem by the xylem-to-phloem transport system operating at stem nodes. Grain Fe is largely derived from the leaves by remobilization. Zn and Fe concentrations in phloem-sap and grains are regulated within a small range, while Cd concentrations vary depending on xylem supply. Transgenic techniques to increase concentrations of the metal chelators (nicotianamine, 2鈥-deoxymugineic acid) are useful in increasing grain Zn and Fe concentrations. The elimination of OsNRAMP5 Cd-uptake transporter and the enhancement of root cell vacuolar Cd sequestration reduce uptake and root-to-shoot transport, respectively, resulting in a reduction of grain Cd accumulation. |
| [27] | . To identify the key barrier parts and relevant elements during Cd/As transport into brown rice, 16 elements were measured in 14 different parts of 21 rice genotypes; moreover, transcriptomic of different nodes was analyzed. Cd/As contents in root and nodes were significantly higher than those other parts. Node I had the highest Cd content among nodes, leading an increase in gene expressions involved in glycolytic and Cd detoxification. The Cu/Zn/Co distribution and transport to various parts was similar to that of Cd, and Fe/Sb distribution and transport to various parts was similar to that of As. Moreover, Cu/Zn/Co/Mg was correlated with Cd in root and nodes, as well as Fe with As. Besides, the ionomic profile showed the different parts of an organ are closely related, and the spatial distribution of different organs is consistent with the growth morphology of rice. Therefore, root and nodes are two key barriers to Cd/As transport into brown rice. Moreover, Node I has the highest Cd accumulation capacities among nodes. The ionomic profile reflect relationships among plant parts and correlations between the elements, suggesting that nodes are hubs for element distribution, as well as the correlation between Cd with Zn/Cu/Co/Mg, between Fe with As. |
| [28] | . Cadmium (Cd) is absorbed by rice root and transferred into the other rice organs including grain. A solution-culture experiment was conducted to investigate the absorption and distribution of Cd supplied at different growth stages of rice. Two rice cultivars, a japonica ‘Chucheong’ and a tongil-type ‘Milyang23’ that exhibit high and low ability of Cd absorption by root and accumulation in grain were grown in culture solution and subjected to 2 ppm CdCl2 treatment for 2 weeks at four different growth stages: before panicle initiation stage (BPI), after panicle initiation stage (API), early ripening stage (ER), and mid-ripening stage (MR). Cd concentration and accumulation in rice organs were measured at harvest. The two rice cultivars accumulated two to three times greater amounts of Cd in grain in the two Cd treatments before heading (BPI and API treatments) than in the Cd treatment after heading (ER and MR treatment). The higher grain Cd accumulation in BPI and API treatments was not attributed to the higher Cd uptake but to the higher translocation from root to shoot and the higher redistribution from shoot to grain than ER and MR treatments These results imply that the remobilization of Cd through phloem during leaf senescence is the major process for Cd accumulation in rice grain rather than direct transport of absorbed Cd through the xylem-phloem transfer to grain. ‘Milyang23’ absorbed significantly smaller amount of Cd than ‘Chucheong’. However, ‘Milyang23’ accumulated more than a three times larger amount of Cd in grain compared to ‘Chucheong’ as the former exhibited the higher root-shoot translocation and shoot-grain remobilization as well. It indicates that the greater Cd translocation from root to shoot and subsequent higher Cd remobilization from shoot to grain, not the higher absorption ability, have led to the higher Cd accumulation and concentration in grain of ‘Milyang23’. |
| [29] | |
| [30] | . . |
| [31] | . Physiological properties involved in divergent cadmium (Cd) accumulation among rice genotypes were characterized using the indica cultivar 'Habataki' (high Cd in grains) and the japonica cultivar 'Sasanishiki' (low Cd in grains). Time-dependence and concentration-dependence of symplastic Cd absorption in roots were revealed not to be responsible for the different Cd accumulation between the two cultivars because root Cd uptake was not greater in the Cd-accumulating cultivar 'Habataki' compared with 'Sasanishiki'. On the other hand, rapid and greater root-to-shoot Cd translocation was observed in 'Habataki', which could be mediated by higher abilities in xylem loading of Cd and transpiration rate as a driving force. To verify whether different abilities in xylem-mediated shoot-to-root translocation generally account for the genotypic variation in shoot Cd accumulation in rice, the world rice core collection, consisting of 69 accessions which covers the genetic diversity of almost 32,000 accessions of cultivated rice, was used. The results showed strong correlation between Cd levels in xylem sap and shoots and grains among the 69 rice accessions. Overall, the results presented in this study revealed that the root-to-shoot Cd translocation via the xylem is the major and common physiological process determining the Cd accumulation level in shoots and grains of rice plants. |
| [32] | The contribution of the phloem in translocation of cadmium (Cd) to grains of rice plants (Oryza sativa L. cv. Kantou) was estimated. We measured Cd concentrations in phloem sap collected from the uppermost internode of rice plants at the grain-filling stage. Cadmium concentration was 17.802±024.50208mol02L611 in plants treated with a nutrient solution containing 100208mol02L611 Cd for 202days. Subsequently, distribution ratios of 109Cd between the grains and the glumes were determined after feeding of 109Cd to the cut ends of the uppermost internodes or onto the surface of the flag leaves or to the roots. The results suggested that 91–100% of the Cd in the grains was deposited from the phloem. To our knowledge, this is the first determination of Cd concentrations in phloem sap transported to grains, and the first estimation of the contribution of the phloem in Cd transport to rice grains. |
| [33] | . |
| [34] | . In rice (Oryza sativa L.), cadmium (Cd), a toxic heavy metal, is found in phloem sap and eventually accumulates in the grains. To further characterize phloem-transported Cd, the chemical forms of Cd and other metals and varietal differences in phloem sap Cd concentrations were investigated in young rice plants. The size-exclusion chromatography elution times for Cd-bound compounds indicated that phloem Cd in cv. Nipponbare exists mainly as an approximately 13 kDa complex. Protease digestion of rice phloem sap reduced the bound Cd content from 92 to 19%. The remaining Cd may bind to low-molecule SH compounds. An experiment examining in vitro addition of Cd2+ to phloem sap from non-Cd-treated plants revealed that rice phloem sap constitutively contains Cd chelators. The major Cd peak is distinguishable from those of Fe, Zn, Cu, Mn, Ni and Co, which probably bind to nicotianamine, 2芒聙虏-deoxymugineic acid, citrate and histidine. The Cd concentrations of the phloem saps in three varieties (Milyang 23, LAC 23 and Koshihikari) grown under the same soil conditions were correlated with their grain Cd concentrations, which had been reported previously, whereas the concentrations of the xylem saps were not. In conclusion, rice phloem sap Cd differs from other metals as it may bind to a novel approximately 13 kDa protein and SH compounds, and the concentration of Cd in rice phloem sap may be a key determinant of its grain content. |
