Chin J Plan Ecolo ›› 2003, Vol. 27 ›› Issue (5): 709-717.DOI: 10.17521/cjpe.2003.0103

• Research Articles • Previous Articles    

Root Exudates and Their Roles in Phytoremediation

KUANG Yuan-Wen, WEN Da-Zhi, ZHONG Chuan-Wen, ZHOU Guo-Yi   

  • Online:2003-05-10 Published:2003-05-10

Abstract:

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 bioproduction 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.