Trace element accumulation and distribution in sunflower plants at the stages of flower bud and maturity

The aim of this study was to analyze the accumulation and distribution of cadmium (Cd), zinc (Zn) and copper (Cu) in different portions of plants of sunflower (Helianthus annuus L., cv. Oleko) grown in soil with contaminants (5, 300, 400 mg kg–1 of Cd, Zn and Cu, respectively) and without (untreated soil as a control) from the emergence of cotyledon leaves until to two phenological stages: flower bud (R-1) and maturity (R- 9). Sunflower accumulated considerable amounts of heavy metals in both phenological stages showing slight reductions of dry matter production. At R-1 stage, Cd, Zn and Cu were accumulated mainly in the roots with concentrations respectively up to 5.4, 233 and 160 mg kg^–1 of dry matter with a low translocation from roots to the aerial part. Yet at the R-1 stage, the bioconcentration factor (BCF) of Cd showed a significantly higher value in the Cd-Zn-Cu treatment (0.27) with respect to the untreated control (0.02), vice versa was observed for Cu, whereas no significant difference between treatments was observed for Zn (0.12 on average). However among metals, Cd showed the highest value of BCF. Referring only to the epigeous portion, differences in the accumulation and distribution of the three metals in the treated plants were found in both phenological stages; indeed passing from flower bud to the maturity stage, Cd, Zn and Cu concentrations increased in the stems and leaves, particularly in the old ones, whereas decreased in the heads. Metal accumulation in the achenes was very low and never exceed the toxicity threshold value considered for livestock. The high storage of heavy metals in roots and the probable re-translocation of the three metals along the plant during the growing cycle could be considered as a strategy of sunflower to preserve young metabolically-active leaves and reproductive organs from toxic metal concentrations.