Bioactive levels of Zn, Pb, Cu, Cd and Mg, Fe in pollution sensitive and tolerant Scots pines needles – Is survival mineral-dependent?

Abstract

Pine species are considered worldwide as developing various mechanisms counteracting industrial pollution specifically with Zn, Pb, Cu, and Cd. We assumed that the survival of plants should involve also the active concentrations of plant-born heavy metals as well as the mineral elements (Mg, Fe) crucial for normal photosynthetic activity of needles. The needles were collected from 100 randomly selected trees, which were divided into 50 specimens of sensitive (S) and 50 specimens of tolerant (T) species. In the study, the needles were also separated by age into 1 and 2 year-old needles. Two solutions of pH 3.5 and 6.0, mimicking acid rains (AR) were applied for extracting the bioactive forms of Zn, Pb, Cu, Cd and Mg and Fe. The elaborated mineral photosynthetic indices (12 in total), SPI (Single Phytotoxicity Indices), IPI (Integrated Phytotoxicity Indices) and TIPI (Total Integrated Phytotoxicity Indices) revealed that bioactive concentrations of Mg and Fe should be specifically regulating Zn, Pb, Cu, Cd fluxes in Scots pine needles. Such mechanism appears crucial for the survival of pine needles, both 1 and 2-years old, since Mg and Fe are directly activated in the photosynthesis process. The concentrations of bioactive Mg and Fe fractions were higher in sensitive (S) as compared to tolerant (T) pines, explaining thus the basis of their response - stronger for S and weaker for T - to adverse effects induced by the particular heavy metals. We stipulated that magnesium and iron should emerge promptly and permanently as toxicity mitigation factors, acting as a Mineral Toxicity Fence (MTF), supporting thus the survival mineral-dependent hypothesis.