The Accumulation Pattern of Zn, Cu, Ni, Cd in Wheat Grown in Heavy-metal Enriched Substrate

The better understanding of mineral elements accumulation in plants and distribution in grains is the prerequisite for increasing the concentration of beneficial elements and minimizing the content of toxic heavy metals in edible parts of cereal crops. The effect of elevated concentrations of Zn, Cu, Ni and Cd in a peat substrate on wheat yields and the metal contents of the plants were studied in a pot experiment. The distribution and possible co-localization of the beneficial micronutrients and toxic heavy metals in the wheat grain were examined by laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS). The metals were added jointly to the peat substrate as sulfates. Combined heavy metals added to peat substrate did not affect grain yield, single grain dry weight, straw dry weight and harvest index of wheat at maturity. However, great changes on metals distribution in plants or grains as affected by heavy metal treatments were observed. Zn, Cu, Ni and Cd concentrations in the grain, chaff, leaf, leaf sheath and stem were significantly enhanced by heavy metal addition but the enhancement varied with different elements or plant organs. Elevated heavy metal supply increased the concentrations of Zn, Cu, Ni and Cd in wheat grains by factors of 1.8, 0.5, 48.1 and 45.3, respectively. The distribution pattern of Cu and Cd in different organs of wheat were not affected by heavy metal treatments, but the distribution pattern of Zn and Ni were significantly changed by heavy metal treatments. High percentages of Zn were translocated in grains at normal Zn supply conditions, but more Zn was retained in vegetative organs by heavy metal treatments. The changes in distribution pattern of Ni showed different trends compared to Zn. LA-ICP-MS analysis of wheat grains revealed that Cu, Zn, Ni, and Cd were distributed in a similar way with the high concentrations in the aleurone layer and low concentrations in the starchy endosperm. Elevated heavy metal supply increased the concentrations of beneficial elements Zn and Cu by 78% and 86%, toxic heavy metals Ni and Cd by 30 and 121 folds in aleurone layer, respectively. Elevated heavy metal supply increased the concentrations of Zn and Cu in starchy endosperm by 49% and 48%, respectively. The concentrations of Ni and Cd in starchy endosperm were not detectable under control conditions, but elevated heavy metal supply increased Ni and Cd concentrations to levels far over the maximum permitted levels set by food standards for heavy metals. These results indicated that heavy metals could accumulate in plants to levels that did not impair plant growth but might be detrimental to human health when the plant was consumed. In addition, the co-localization of beneficial micronutrients Zn, Cu and toxic heavy metals Ni, Cd in the wheat grain might imply a possible risk of the micronutrient bio-fortification strategy.