Cellular localization of cadmium and structural changes in maize plants grown on a cadmium contaminated soil with and without liming

The effects of different concentrations of soil cadmium (0, 1, 3, 5, 10, and 20 mg kg−1) on growth, structural changes and cadmium cellular localization in leaves of maize plants (Zea mays L.) were investigated in a pot experiment. The results showed that the structural changes observed in maize leaves were not only a response to the Cd-induced stress but also a cellular mechanism to reduce the free Cd+2 in the cytoplasm. However, this mechanism seems to be efficient only up to a Cd concentration in leaves between 27 and 35 mg kg−1 for soils without and with liming, respectively. The cellular response varied with both the Cd concentration in soil and liming. For limed soil, Cd was preferentially accumulated in the apoplast while for unlimed soils Cd was more evenly distributed into the cells. The ability of Cd accumulation depended on the leaf tissue considered. The apoplast collenchyma presented the highest Cd concentration followed by the endodermis, perycicle, xylem, and epidermis. On the other hand, symplast Cd accumulated mainly in the endodermis, bundle sheath cells, parenchyma, and phloem. Based on the structural changes and growth reduction, the critical toxic concentration of soil Cd to maize plants is between 5 and 10 mg kg−1.