Research on cadmium toxicity mechanism during maize germination

To explore the mechanism of heavy metal metabolism in plants under heavy metal stress, maize tolerance to cadmium (Cd) was investigated as a test case. Maize seedling was germinated under different Cd concentrations (5~100 μmol·L^-1) and the processing time was studied. The parameters of maize seedling, Cd content and distribution in different tissues, High Performance Liquid Chromatography-Mass Spectrometry spectra (HPLC-MS), Nuclear Magnetic Resonance spectra (1H NMR) and amino acids contents were investigated. The results showed that under Cd stress <50 μmol·L^-1, maize germination was significantly enhanced. When, however, Cd stress exceeded 50 μmol·L^-1, maize germination was inhibited. Cd was unevenly distributed in the fresh endosperm, root, stem and bud tissues in the order of endosperm > radicle > stem > burgeon. HPLC-MS and ¹H NMR analyses indicated Cd chelation in maize in the form of phytochelatin (PC). Total protein amino acid content analysis showed that nutrient absorption and synthesis were to some extent affected by Cd stress. Cd accumulation and transmission process in maize seedling was such that while a large amount of Cd was adsorbed on maize endosperm, limited Cd was directly adsorbed on radicle surface. Cd existed in both free and phytochelatin forms in the maize plant. Cd interacted with various cell proteins and was transmitted among cytosol, vacuole, cell plasmodesmata and other seedling organelles. Cd was transmitted to burgeon through the radicle and stem and the transfer process was driven mainly by concentration gradient. The process likely followed along a concentration gradient facilitated by diffusion, and the rationality of which was proven by the experiment.