(Raphanus sativus L. var. oleiformis Pers.) in metal-polluted wastes

In a series of pot trials, fodder radish (Raphanus sativus L. var. oleiformis Pers.) was cultivated in pyrite cinders heavily polluted by various heavy metals (HM). The effects of application of humic acids (HA) to the cultures were evaluated at various concentrations (0.1 and 1 g kgÀ1 of cinders) and for different methods of application (mixed with cinders before sowing; applied on the top of substrate after sowing; foliar spraying), in comparison with untreated controls. HA increased the bioavailability of various HM, especially when applied at high concentrations. Low concentrations of HA, either mixed with cinders or applied on foliage, slightly enhanced shoot growth. High levels of HA were generally phytotoxic, probably because of the ensuing higher heavy metal concentrations in the shoots. Foliar spraying was partially able to attenuate the phytotoxicity of high dosages of HA, as revealed by the increased rate of leaf expansion. A negative correlation was found between shoot biomass and concentration of HM, and higher removals were generally associated with smaller doses of HA and foliar application. Within the range of concentrations tested, humic substances led to a reduction in root diameter and an increase in specific root length (SRL, length per unit weight of roots), significant only at the highest dosage, together with a tendency towards enhancement of translocation of HM to the shoot – both positive traits for phytoremediation purposes.

The use of biomass plants for phytoremediation is of great interest, as their seeds are easily available and there is extensive information on their cultivation with respect to hyperaccumulators, but there is an urgent need to improve remediation efficiency through agronomic practices. Although data refer to relatively early growth stages in pots, this experiment provides useful information for in situ cultivation of fodder radish, as preliminary results showed the best growth and removal capacities among various herbaceous crop species (i.e. sunflower, alfaalfa, Italian ryegrass, barley) in metal-polluted pyrite cinders. In fodder radish, application of low doses of HA had a slight growth-promoting effect on the shoot, whereas high doses were phytotoxic, although with partial mitigation of these negative effects by additional foliar application. Due to their chelating effect, HA reduced root growth and diameter, and increased SRL, especially at high doses. Among tested treatments, small amounts of HA (0.1 g kgÀ1) increased the amount of HM taken up by the plant and their translocation to the shoot, with major benefits for Cu and Pb, thus suggesting their potential for use in phytoremediation. The greater root growth associated with this treatment, with respect to controls, may also allow plants to withstand periods of drought better when applied to our particular polluted site in which fodder radish showed a maximum root depth of 0.3 m [17]. For practical applications, it may be possible to apply HA by spraying on the surface and promptly incorporating them into cinders by harrowing, but the cost of treatment should be carefully considered, as a 0.15-m deep interval, added at a dosage of 0.1 g kgÀ1, requires more than 200 kg of HA per hectare.