Lead Induced Organic Acid Exudation and Citrate Enhanced Pb Uptake in Hydroponic System

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.