根系分泌物及其在植物修复中的作用

 近年来环境污染日益严重，污染物在土壤植物中的行为引起了人们的高度关注。利用植物去除土壤水体等介质中污染物的植物修复是近10年来兴起的一项安全、廉价的技术，已成为污染生态学和环境生态学的研究热点，它通过植物吸收、根滤、稳定、挥发等方式清除环境中的重金属和有机污染物。国内外有关植物修复的研究报道和概述很多, 但对植物根系分泌物在植物修复中所起的作用及其机理少有述评。 本文从根系分泌物对土壤重金属和土壤有机污染物的去除作用出发,对根系分泌物的种类、数量及其在去除环境污染物中的作用机理和功能地位进行了总结,并借助研究事例对影响植物根系分泌的内外因子，如植物种类、营养胁迫、重金属胁迫、根际环境的理化性质、土壤微生物及其它环境因子进行了讨论。概言之，根系分泌物在修复污染土壤中的重金属途径是多种多样的，主要是通过调节根际pH值、与重金属形成螯合物、络合反应、沉淀、提高土壤微生物数量和活性来改变重金属在根际中的存在形态以及提高重金属的生物有效性，从而减轻它对环境的危害。在清除有机污染物时，根系分泌物中的酶可以对有机污染物进行直接降解，根系分泌物影响下的微生物也可以对有机污染物进行间接降解，且被认为是主要的降解途径。根系分泌物在植物修复过程中确实起着某些重要作用，今后应将这方面的研究重点放在某些特异性根系分泌物植物，尤其是某些重金属超富集植物资源的寻找、筛选上，通过室内实验和野外研究确定其根系分泌物对清除重金属和有机污染物的效率，证实超富集植物根系分泌物的特异性与污染物超富集的内在联系，找到污染土壤生态恢复和治理的有效方法并加以推广应用，如针对性地在被污染地大面积种植此类具特异性根分泌物植物，并辅以营林措施如修剪等，加快生物修复进程，提高修复效率。植物根系分泌物在植物修复过程中所具有的重要生态意义和可能应用前景，为污染生态学和化学生态学之间的联合研究开拓了全新的领域，今后将取得新的突破和重要进展。

ROOT EXUDATES AND THEIR ROLES IN PHYTOREMEDIATION

Environmental pollution has become more and more serious in recent years, and much attention has been paid to its effects in soil-plant systems. Phytoremediation, a technique using plants to remove contaminants from soil and water, has become a hot topic in the last decade due to its environmental safety and potential cheapness. It can clean up heavy metals and organic contaminants in the environment in four major ways: phytoextraction, rhizofiltration, phytostabilization and phytovolatilization. Numerous studies and several reviews have been made on phytoremediation of contaminated environments, but reviews on the role and the mechanism of root exudates in phytoremediation of contaminants are few. Here we sum up the varieties of root exudates and discuss the internal and external factors influencing the exudation of plant roots. These factors include plant species, nutrient and heavy metal stresses, rhizosphere, soil microbes and others. The roles of root exudates in phytoremediation of heavy metals and organic pollutants are also reviewed and illustrated by some case-study examples. In general, root exudates can affect metal absorption by plants through changing the physical and chemical characters of rhizospheres. The modes of root exudates in removing heavy metals vary in a broad range, for example, by modifying the pH of the rhizosphere, chelating, complexting and depositing with heavy metals, altering the numbers and the activity of rhizospheric microbes. Through these ways, root exudates can change the chemical existence of heavy metals, increase their bio-availability, activate soil microbes and reduce the environmental dangers. While removing organic contaminants in the soil, the ecotoenzymes in the root exudates may degrade pollutants directly, or degrade them indirectly by stimulating the activity of rhizospheric microbes (which is considered to be the dominant process). Root exudates do play a key role in phytoremediation on contaminated sites. Future research should focus on seeking and screening certain specific plants with high bio-production and excellent efficacy, especially developing hyper-accumulators, and testing their efficiency in removing heavy metals or organic pollutants by lab and/or field trials. Emphasis should also be laid on process-oriented correlations between contaminants and root exudates of these plants. If so, more effective approaches to ecological restoration of polluted soils can be developed and put into practice. For instance, we can attempt to plant various kinds of plants, especially already-known hyper-accumulators on a large scale on contaminated sites, and supplement treatment with additional forestry practices such as coppicing, to enhance the efficiency of phytoremediation by speeding up the processes of bioremediation. The important ecological contribution and wide perspectives in field application of root exudates provides an entirely new research area. A combined and integrated study of pollution ecology and chemical ecology, and new findings and breakthroughs, are to be expected .