Physiological responses of Bituminaria bituminosa to heavy metals

Two hydroponic experiments were performed to study the physiological responses to heavy metals (HMs) of two populations of Bituminaria bituminosa (L.) C.H. Stirton (Fabaceae): one ("C2") from a site contaminated by HMs and one from a non-contaminated site ("LA"). In the first, we studied the effects of elevated concentrations of Zn (12 and 61 μM). Population C2 was more tolerant in terms of root and shoot growth at 61 μM Zn, relative to control plants (1 μM Zn). The similar tissue Zn levels of the two populations suggest that C2 is more tolerant of high tissue Zn. Of the parameters measured that could be related to Zn phytotoxicity (micro and macronutrients, root hydraulic activity, water-extractable Zn and organic acids), none could explain totally the superior tolerance of C2. In the second assay, the effects of Cd (4.4 μM), Cu (7.8 μM) and Zn (76 μM) on plant accumulation of the furanocoumarins (FCs) psoralen and angelicin, which function as feeding deterrents and photo-activated toxins, were assessed. For population C2, all three HMs increased the root FC concentrations, while Cd also raised shoot levels. For LA, Cu raised the root concentrations of both FCs. There was a relationship between plant stress, manifested as proline accumulation and disruption of plant water relations, and increased FC accumulation. Higher tissue levels of FCs likely provide greater protection against bacterial or fungal infection and herbivores.