Humic acids affect root characteristics of fodder radish (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 kg−1 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.