Metal distribution and interaction in plant cultures on artificial soil

Abstract

Rye-grass, Lolium multiflorum, cultures on perlite, an artificial soil, were carried out with a basic nutrient solution (modified in each experiment), adding double distilled water when necessary to restore evaporation losses. Cultures were collected at different times, separated into roots and tops and controlled by moisture, dry weights and metallic element contents measurements. Plant chemical analyses were performed after HF-HN03-HClO4 digestion by AAS (Mg, Ca, Fe, Mn, Cu, Zn, Co, Ni, Cd, As, Pb) and flame emission (Na and K). Cultures with changes in basic nutrient solution component concentrations, with additions of potentially toxic elements (Co, Ni, Cd, As, Pb, Cu, Zn, Na) and with different periods of culture (7 to 49 days after 6 days for germination) were carried out. Measurements of metallic elements dissolution and diffusion within a perlite layer were also made. For discussion of results ionic impulsions are introduced as I = c 1/n , where c is the considered metallic element concentration and n its oxidation number. Root or top uptakes of a metallic element follow linear relationships with their ionic impulsions in nutrient solution or root, respectively. Roots and tops seem to keep constant the total ionic impulsions, sums of individual metallic elements ionic impulsions. Toxic effects were evident for total ionic impulsions greater than average values. Results are further discussed through NUTRIENT SOLUTION ⇔ PERLITE ⇔ ROOT ⇔ TOP equilibria that suggest a single assimilation mechanism for roots, and two different ones for ROOT ⇒ TOP step: (i) an osmotic permeation with root accumulation of some elements (Cu, Zn, Fe, Co, Ni, Cd, As, Pb) and (ii) a DONNAN permeation leading to top accumulation of other metals (Mg, K, Ca, Na, Mn).