Effects of Cadmium and Zinc Interactions on Growth and Chemical Composition of Corn (Zea mays cv. single cross)

Heavy metal contamination not only adversely affects the chemical properties, availability of nutrients and biological activity of the soils, but also causes serious risk to the human health from entering the food chain. Cadmium as an unnecessary heavy metal is highly toxic to plants. Cadmium toxicity inhibits plant growth and even death. Metabolic processes such as photosynthesis and cellular respiration are disturbed due to cadmium toxicity. Among the heavy metals, zinc is an important nutrient in many biological processes such as photosynthesis, activity of antioxidant enzymes, proteins, hormones and other activities. Because of the similar chemical behavior of cadmium and zinc, interaction between the two metals is of interest to many researchers.
In this study, to investigate the interaction effects of cadmium and zinc on shoot and root dry matter and chemical composition of corn (Zea mays cv. single cross 704), a factorial experiment as a randomized complete block design in triplicate with eight levels of cadmium (zero, 0.5, 2.5, 5, 10, 20, 40 and 80 mg Cd kg-1) and eight levels of zinc (zero, 5, 25, 50, 100, 200, 400 and 800 mg Zn kg-1) was conducted in a loamy sand soil under greenhouse conditions. After 60 days, the plants were harvested and dry weights of shoots and roots were determined. Moreover, after wet digestion, the concentrations of cadmium, zinc, iron, manganese and copper in these tissues were determined by flame atomic absorption spectrometry (Shimadzu-6300).
Treatments with 800 mg Cd kg-1 showed symptoms of cadmium and or zinc toxicity at early stages of the growth. These plants died after 10 to 20 days of germination. The results showed that the cadmium and zinc interactions on shoot and root dry weights were significant. At 0.5, 2.5, 20 and 80 mg Cd kg-1 (except for Cd0.5-Zn25), application of 5 to 50 mg Zn kg-1 increased shoot dry weight. Higher levels of zinc supplementation exhibited adverse effects. At 5 and 10 mg Cd kg-1, supply of 5 to 100 mg Zn kg-1 was associated with an increase in shoot dry weight, but shoot growth was reduced at higher zinc levels. At 40 mg Cd kg-1, application levels of 5 to 200 mg Zn kg-1 increased shoot dry weight, whereas 400 mg Zn kg-1 showed adverse effects. Moreover, the cadmium and zinc interactions on chemical composition of corn were significant. Based on the results, at low levels of cadmium, zinc supplementation at each level increased the shoot and root cadmium concentrations, while at high levels of cadmium, low and high zinc supply caused a decrease and increase in the shoot as well as root cadmium concentrations, respectively. The concentration of a particular trend was observed on the shoot and root. Supplementation of zinc at each level of cadmium (except for 80 mg Cd kg-1), first increased and then decreased the iron concentration of shoots and roots. Application of zinc at each level of cadmium decreased manganese and copper concentrations in shoots and roots.
According to the results, the ecological tolerance of corn to zinc was found to be 800 mg Zn kg-1. Also, the application level of zinc with positive effect on shoot dry weight increased with an increase in cadmium level. At all levels of cadmium, supplementation of zinc at medium levels prevented the accumulation of cadmium in shoots, while high and low levels of zinc intensified the cadmium accumulation. The highest accumulation of cadmium in roots was occurred at highest level of zinc. Zinc supplementation at each level of cadmium first increased and then decreased iron concentration in shoots and roots. However, zinc supply at each level of cadmium decreased copper and manganese concentrations in shoots and roots. As a conclusion, zinc at low levels diminished toxic effects and accumulation of cadmium, meanwhile high levels of zinc not only did not control cadmium but showed deleterious effects. The critical level of poisoning for cadmium in aerial parts of both plants in lower density in the soil (up to 90 mg kg-1 ) showed very little changes, but in higher density, it decreased in brassica napus and increased in zea mays. The critical level of poisoning on the aerial parts of both plants showed very little change with increasing the total density of cadmium in the soil.