植物根系形态对镉的可塑性响应机制：避逆还是觅食？

Cadmium (Cd) contamination has become a major problem that threats ecological environment security and restricts the sustainable development of agriculture. The plant root, which as the principal organ for the absorption of water and mineral elements from the rhizosphere, is extremely sensitive to Cd exposure. Root system was demonstrated to show marked developmental plasticity in response to Cd stress, differing among species, ecotypes, or cultivars. However, the eco-physiological significance of root morphological plasticity to Cd exposure is unclear. In the present project, we hypothesized that the well-developed root system may be responsible for high capability of Cd accumulation of a given plant species, ecotype or cultivar. Hyperaccumulators exhibit positive responses to Cd in the root system architecture which enables them to take up more Cd from the rhizosphere. By contrast, root systems of excluders show adverse responses, leading to a limitation of Cd uptake and accumulation in plants. Moreover, we conceived that there are more species existing between the hyperaccumulators and excluders, and together with the former two, forming a "hyperaccumulator-excluder continuum". To our knowledge, this hypothesis has never been tested experimentally by a large number of species or cultivars. Therefore, the purpose of this project is to (1) test the "hyperaccumulator- excluder continuum" hypothesis, (2) reveal the rule of the morphological response of plant roots to Cd, and (3) clarify the eco-physiological mechanism of Cd uptake, translocation and accumulation by means of root morphological methods. For the purpose mentioned above, the response of root system to Cd (homogeneous and heterogeneous) in plant species or cultivars, as well as the relationship of root morphology and its plasticity to Cd accumulation in plants will be investigated via a series of hydroponic and pot experiments, at both the species and cultivar levels. Thirty angiosperm species, belonging to 21 families and 28 genera and differing in Cd accumulation capacities, will be selected for the inter-species experiments. For the intra-species experiments, 15-20 pakchoi cultivars or genotypes with different capability of Cd accumulation will be selected. The implementation of the project is expected to confirm and improve the "hyperaccumulator-excluder continuum hypothesis", which will lay an experimental foundation for fully understanding the physiological mechanism of Cd uptake and accumulation in plants. Meanwhile, the results of this project will also provide a theoretical basis for screening Cd hyperaccumulators or Cd pollution-safe cultivars by assessing the root morphological parameters and their plasticity.

镉(Cd)污染已成为威胁生态环境安全和制约农业可持续发展的重要因素。根系作为植物吸收水和矿质元素的重要器官，对镉极为敏感并能通过发育可塑性作出响应，且在不同物种、生态型或品种之间存在差异。迄今，关于植物根系响应镉胁迫的生理生态意义尚不清楚。本项目提出了"超积累-排斥型植物连续统"假说，拟通过水培和盆栽试验，从物种（隶属21科28属的30种不同镉积累型的被子植物）和品种（15～20个不同镉积累能力的小白菜品种或基因型）两个水平，分析不同镉积累型植物或品种的根系形态对镉的可塑性响应，探讨植物根系形态及其可塑性与镉积累之间的关系，揭示植物根系响应镉胁迫的规律，从根系形态学的角度阐明植物吸收和积累镉的生理生态机制。本项目的实施，有望证实和完善"超积累-排斥型植物连续统"假说，为系统阐明植物吸收和积累镉的生理机制奠定基础，同时为通过根系形态及其可塑性筛选镉超积累植物或镉污染预防品种提供理论依据。

本项目旨在从物种和品种两个水平，探讨植物根系形态对镉的可塑性响应及其与镉积累之间的关系，从根系形态学的角度阐明植物吸收和积累镉的生理生态机制。主要研究结果如下：（1）搞清了镉积累型植物（Cd-accumulators）和非积累型植物（Cd non-accumulators）在镉积累特性和抗镉机制上的差异，首次针对非积累型植物提出了“敏感-避性连续统”概念，进一步在物种和品种水平上证实了非积累型植物镉积累的个体大小依赖假设（individual size-dependent hypothesis），并发现生物量对镉的稀释效应在这一过程中起重要作用。（2）在物种和品种两个水平均证实，在镉胁迫下，根系通过缩短根长和增加直径减少对镉的吸收并增加根系对镉的截留能力。植物根系形态与镉积累之间的存在一定的相关性，根系发达且细根较多的品种对镉具有较强的吸收能力。（3）在异质镉环境下，根系通过避性机制来适应高镉斑块引起的胁迫，并通过无镉斑块的生长得到补偿。地上部分镉含量受加镉斑块大小和镉含量的共同影响。无论是高镉还是低镉处理，地上部分镉含量均随着根际镉斑块的增大而增大。在镉斑块大小相同的情况下，地上部分镉含量则随着根际镉浓度的升高而增大。根系镉含量受其根际斑块中镉浓度的影响，并随着根际斑块中镉浓度的升高而升高，而与斑块面积无关。无论根际镉斑块的浓度和面积如何，生长在无镉斑块中的根系的镉含量均很低，说明镉斑块中的根系吸收的镉只能通过木质部向地上部分运输，而不能运输到根系的其它部分。（4）干旱诱导蓖麻对Cd吸收的减少可能是由于其抑制根系生长并改变了根系形态，而不是降低蒸腾速率。本项目的实施，为系统阐明植物吸收和积累镉的生理机制奠定基础，同时为通过根系形态及其可塑性筛选镉超积累植物或镉污染预防品种提供了理论依据。发表SCI 论文11 篇； CSCD论文1篇；会议论文2篇；申请发明专利1件（专利号：ZL201410833936.2）。培养硕士研究生3名。