한국환경농학회지 제28권 제2호 (2009) Korean Journal of Environmental Agriculture Vol. 28, No. 2, pp. 146-157 Lead Induced Organic Acid Exudation and Citrate Enhanced Pb Uptake in Hydroponic System Kwon-Rae Kim 1),2) , Gary Owens 1) , Ravi Naidu 1) , Soon-Ik Kwon 2) , and Kye-Hoon Kim 3)* 1 Centre for Environmental Risk Assessment and Remediation, University of South Australia 2 Department of Agricultural Environment, National Academy of Agricultural Science 3 Department of Environmental Horticulture, University of Seoul (Received April 20, 2009, Accepted June 4, 2009) ABSTRACT: The influence of Pb-citrate complex formation on Pb uptake and the effect of Pb on organic acid exudation were investigated using four plant species, viz., sunflower (Helianthus annuus L), Indian mustard (Brassica juncea), canola (Brassica napus) and vetiver grass (Vetiveria zizanioides) under hydroponic conditions. Seedlings were exposed to different levels of Pb and Pb-citrate for 24 hrs and subsequently Pb distributions in plant shoot, root and hydroponic solution were measured. The dissolved organic carbon (DOC) concentration generally decreased as the concentration of Pb in the hydroponic solution increased. In contrast to DOC, the total organic acid concentrations exuded from Indian mustard roots significantly increased (424 to 6656 mg kg -1 ) with increased Pb treatment, implying that exuding organic acids were involved in Pb accumulation in Indian mustard. The complexation of Pb with citrate enhanced Pb accumulation in the above ground portions. Lead concentration in Indian mustard increased from 2.05 mg kg -1 to 6.42 mg kg -1 when the concentration of citrate in solution increased from 0 to 50 mg L -1 . This result showed enhanced translocation of Pb from root to shoot with observation of transfer coefficient (Kt ) increase from 2.03E-3 to 5.72E-3. Key Words: DOC, Indian mustard, organic acids, Pb, sunflower, canola, transfer coefficient (K t ) 146 * 연락저자: Tel: +82-2-2210 2605 Fax: +82-2-2214 4030 E-mail: johnkim@uos.ac.kr Introduction It is well known that plant roots may exude organic carbon in the form of a wide variety of organic compounds which are synthesized during photosynthesis and translocated into the roots 1,2) . Plant roots may also exude counter ions such as H + and OH – for compensating charge balance in the inner sphere of cells when they take up elemental nutrients, which results in changes of soil solution pH 3) . These root exudates may significantly influence on the properties of the soil environment in the vicinity of the rhizosphere 4-6) including microbial activity, rhizosphere physical properties, pH and metal speciation 6-8) . Numerous researches were conducted to under- stand the role of root exudates in metal translocation and to identify the specific compounds comprising root exudates 4,9-11) . From these studies low molecular weight (LMW) compounds such as amino acids, organic acids, sugars, phenolics, and various other secondary metabolites are believed to comprise the majority of root exudates, with high molecular weight (HMW) compounds including mucilage and proteins being of secondary importance 9,11,12) . The exact constitution of any specific root exudate is variable being highly dependent upon plant species and soil type. The investigation of root exudates can be appro- ached in two different ways 1) alleviation of heavy metal toxicity or 2) increase in heavy metal accessi- bility to plants 13-16) . It is generally accepted that free metal ions are more toxic and more accessible to 연구보문