Document Type : Research Article
Ferdowsi University of Mahhad
Introduction: Among the heavy metals, cadmium, because of high mobility and bioavailability in soil and also toxicity at low concentrations is very important. Cadmium (Cd) is known as carcinogen and can induce many types of cancers. Human activities (metallic industries, contaminated fertilizer, herbicides or insecticides, irrigation with contaminated groundwater, and use of contaminated sewage sludge) are largely responsible for accumulation of different levels of Cd in soil. Saint John’s Wort (Hypericum perforatum L.) is a medicinal plant and belongs to the family Hypericaceae which its extract, one of the best-characterized herbal medicines, known as the Cd-hyperaccumulator, is widely sold for the treatment of depression. Hyperaccumulator plants are species able to accumulate high amounts of heavy metals in their tissue at concentrations of 10 to 100 times higher than tolerated by crop plants. Zinc (Zn) is an essential element occurring in several enzymes, where it plays a catalytic or structural role. Cadmium and zinc have similar electron configuration, valence state as well as affinity to S, N and O donor ligands and thus their geochemical and environmental properties are comparable. The antagonism activity between Cd and Zn in the environment and their chemical similarity can lead to interactions between Cd and Zn during plant uptake, transport from roots to shoots, or accumulation in edible tissues. Ion exchange is one of the methods used for the removal of several toxic substances. In recent years, natural amendments, such as zeolite have been widely used to address trace metals contamination. Therefore, adding zinc and zeolite to the growth medium of plant can be moderating the toxic effects of cadmium.
Materials and Methods: This pot experiment was conducted at the Experimental Field of Ferdowsi University of Mashhad (FUM) from September 2013 to June 2014. Treatments consisted of three levels of cadmium (0, 10 and 20 mg Cd kg-1 soil), three levels of zinc (0, 25 and 50 mg Zn kg-1 soil) and three levels of Zeolite (Clinoptilolite) (0, 5 and 10g zeolite kg-1 soil) arranged in a factorial based on Randomized Complete Block Design (RCBD) with three replications. Cadmium sulfate (CdSO4·4H2O) and zinc sulfate (ZnSO4·7H2O) were purchased from MERCK company. The zeolite used in this research was collected from the Semnan mine, located in South of Semnan Province. After preparing the soil, the pots (30 cm in diameter with 18 kg capacity) were filled with 18 kg soil and thoroughly mixed with appropriate amounts of cadmium, zinc and zeolite diluted in distilled water. Seeds of H. perforatum L. cv. “Topaz” were obtained from the FUM Research Field. After 24 hours of soaking in tap water, the seeds were sown on the surface of moistened soil in each pot and germinated after 10 days. Following germination, the seedlings (about 1.0 cm high) were thinned and only 10 seedlings in each pot were kept. At the full flowering stage, these plants were harvested after 9 months. Plant height, root length, flowering stem and flower number, leaf area, relative chlorophyll and number of black nodules was measured. Chlorophyll (Chl) content were determined by leaf area meter (Li-Cor-1300,USA) and SPAD chlorophyll meter. Data were analyzed statistically by using JMP 8 and Excel software. The differences between averages were tested by Tukeyʼs test at P < 0.05.
Results and Discussion: The results showed that increasing cadmium levels lead to significant decrease of 6.28, 3.45, 2.04, 5.49, 0.82, 15.71 percents in plant height, root length, flowering stem, flower number, leaf area and relative chlorophyll, respectively compared with control. In comparison to control, the number of black nodules in leaf significantly increased by 4.23 percent. Combined application of cadmium and zinc could significantly decline the effects of cadmium on plant height and root length. Meanwhile, interaction effect of cadmium and zeolite was detected significantly on root length and number of flowers. The results of three-way interactions for root length were superior in Zn0Cd0Z10 treatment. Cadmium is an inhibitor of uptake and accumulation of essential mineral nutrients, reduces conductivity of stomata andwater potential of cells and damaged photosystems; therefore, can decrease biomass production in stem, flower and root. The reduction of Chl content could lead to enzymatic degradation of these pigments or inhibition of their biosynthesis, which could be connected with Cd-induced deficiency of iron and zinc, decrease of magnesium content or cadmium bond to essential thiol groups in both the protochlorophyllide reductase protein and other enzymes involved in the light dependent synthesis of 5-aminolevulinic acid. Heavy metal stress with impact on biosynthetic pathways of pharmacologically active molecules can either increase or decrease them. Changes in black nodules number, containing hypericin and other secondary metabolites in H. perforatum in response to heavy metals can help us understand the role of this material in stress conditions. It could be assumed that high metal accumulating ability of plants producing specific secondary metabolites in H. perforatum could be also connected with chelation of toxic metals with mentioned substances.
Conclusions: Our results confirmed that the addition of zeolite and zinc presumably improved plant growth, because of increased availability of essential nutrient elements such as K, Mg, Ca, NH4, and micronutrients in presence of zeolite or decreased Cd sorption and transmission in presence of zinc.