Accumulation of risk metals into faba bean in two growth phases

Legumes are rich and inexpensive source of proteins, carbohydrates, dietary fibres to millions of peoples. In addition, legumes are also reported to be a good source of minerals (K, P, Ca, Mg) and trace elements. Metals, such as iron, zinc and manganese are essential metals, since they play an important role in biological systems,  are essential micronutrients, they can be toxic when taken in excess. Zinc was added to the soil to reduce the intake of other heavy metals especially of Cd.

In model conditions of the vegetation pot experiments we investigated the effect of addition of increasing rates of selected heavy metals (zinc and copper, separately) into the soil and its consecutive accumulation in two cultivars of faba bean Saturn and Zobor. Four variants in each of the experiments were realised: A: control (without Zn addition), B: 40 mg Zn.kg^-1 of soil (hygienic limit), C: 250 mg Zn.kg^-1 of soil (half dose of analytically significant contamination, D: 500 mg Zn.kg^-1 of soil (analytically significant contamination). Faba beans matter (seeds, biomass) were harvested at milk as well as in full ripeness of seeds.

Trace elements (Zn, Cu, Co, Ni, Cr, Pb, Cd) – in soil and matter - were determined using a Varian AA240FS atomic absorption spectrometer equipped with a D2 lamp background correction system, using an air–acetylene flame. Pseudototal content of risk metals including all of the forms besides residual metal fraction was assessed in soil extract by aqua regia and content of mobile forms of selected heavy metals in soil extract by NH₄NO₃ (c =1 mol.dm^-3) and in the solution of 2 M HNO₃ was determined. The sample of faba bean after their drying, regulation and decomposition by HNO₃ on the microwave digestion were used. The results were evaluated according to the legislative valid in the Slovak Republic as well as threshold values proposed by European Commission (2006) and the content of risky elements in legume seeds were evaluated according to the Food Codex of the Slovak Republic as well as according to Commission Regulation 1881/2006.  

The soil from Cakajovce locality used in the pot trial was uncontaminated. The strong statistical relationship between soil Zn content and Zn amount in seeds of both of investigated faba bean cultivars (in milk as well as full ripeness) was confirmed. Despite of high Zn doses applied into the soil in model conditions, in all variants the determined Zn amount in seeds of faba bean cv. Saturn was just below the maximal allowed content in foodstuffs given by the legislative. In cv. Zobor the determined Zn content was in C as well as in D variant higher than maximal allowed amount (by 2% and 12%, respectively). With increased Zn doses applied into the soil a lower Zn content was in biomass of investigated legumes in A and B variants and higher in C and D variants than those in comparison to seeds.

Fresh matter. Faba bean seeds cvs. Saturn and Zobor in milk ripeness accumulated in control variant high amounts of Ni, Pb and Cd. In variants with Zn addition the Zn, Pb and Cd contents were increased (Saturn), while the contents of Cu, Ni and Cr (Saturn) and the Ni and Cd contents (Zobor) were decreased, but also the contents of Cu, Cr and Pb were changed very slightly (Zobor) in comparison to the control variant. In all variants of Zn application into soil the determined Pb and Cd values in biomass of both faba beans were significantly lower in comparison to seeds.

Dry matter. Faba bean seeds cvs. Saturn and Zobor in full ripeness accumulated (in all variants higher than hygienic limits) high amounts of Pb and Cd (Zn application into soil) and amounts of Pb and Ni (Cu application). The contents of all other heavy metals were lower than hygienic limits. In variants with Zn application the determined Zn and Cu contents in seeds were increased, Cd content was decreased and contents of Ni, Cr and Pb were changed only slightly. In all variants of Zn application into soil the determined Pb and Cd values in the biomass of both faba beans were higher in comparison to seeds.