Cadmium impact on root exudates of sorghum and maize plants: A speciation study

The effect of cadmium (Cd) on root exudates of sorghum and maize was investigated in order to get further insight into the mechanisms of plant tolerance to Cd. Plants were grown hydroponically and supplemented with: 0, 0.5, and 5.0 mg Cd L-1. Hydroponic solutions containing exudates were analyzed by high performance liquid chromatography (HPLC). The results showed different exudation patterns by sorghum and maize with cadmium supply. While sorghum enhanced malate exudation over the entire range of applied Cd in the uptake solutions, maize increased mainly citrate. Moreover, malate concentration exuded in sorghum rhizosphere presented higher values than citrate (from maize). With the aid of the HYPERQUAD speciation program, a significant decrease in the bioavailable Cd (free Cd plus Cd chloro-complexes) was found due to the increase of Cd organic complexation in the hydroponic solution. Furthermore, similar metal organic complex concentrations were obtained for both plants, which turned the maize and sorghum overall detoxification process equivalent. Exudation of malate and citrate should contribute to tolerance mechanisms of these plants, reducing deleterious effects of free Cd on root growth. These findings support the idea that the metal-binding capabilities of root exudates may be an important mechanism for stabilizing metals in soil.