Root damages induced by extended phytotoxic effects of cadmium pre-exposure against subsequent lindane uptake in rice seedlings

Abstract

The heavy metal accumulation in plants via uptake generally poses some phytotoxic effects on plant roots, which may in turn impact the plant uptake process. The present study was conducted to evaluate the phytotoxic effects of cadmium (Cd) pre-exposure (2 and 40 mg L−1) on rice (Oryza sativa) root at organ, tissue and cellular levels, and to investigate the subsequent uptake performance of the affected rice seedlings by re-exposing to lindane (4 mg L−1). Biomass, relative water content (RWC), fatty acid (FA) profiles were quantified as the indicators for phytotoxicity. Following the Cd pretreatment, varying degrees of water deficit and lipid peroxidation of rice seedlings were found compared to the control, as evidenced by the decrease in RWC and the increase in FA saturation degree, with no significant effect on root biomass. Meanwhile, damages to root tissue structure were confirmed microscopically. All these symptoms did not show any signs of recovery after the Cd stress was removed, but even deteriorated especially in the high-level Cd pretreatment. The root function of uptake, which was interrelated to the above indicators, was consequently impaired, verified by the decrease in active root uptake area (ARUA). In addition, pre-exposure to much higher levels of Cd brought about lower lindane uptake in roots and lower lindane root-to-shoot translocation. After 5-d lindane exposure, lindane concentrations decreased by 15.8 ± 0.5 % and 30.3 ± 0.4 % respectively compared to the control in seedling roots of two Cd pretreatments, and 34.1 ± 0.6 % and 59.2 ± 1.2 % respectively in shoots. The lindane proportions in shoots of Cd pretreatment were approximately 20 % and 40 % while that from the control exceeded 45 %. These results demonstrated the “uptake-phytotoxicity” cycle, and indicated that the hampered uptake and translocation progresses in plant appeared to depend on the severity of root damages, which was induced by the extended phytotoxic effects.