Effects of Heavy Metals and Aluminum on the Root Physiology of Spruce (Picea Abies Karst.) Seedlings

Abstract

The toxicity of the heavy metals Zn, Cd, Hg and Pb and organic complexes of Hg and Pb to four-week old Picea abies seedlings was assessed by estimating the effects of these metals on root elongation. The toxicity of these metals when supplied as inorganic salts was found to differ considerably, the order of toxicity being Hg > Pb > Cd > Zn, with Hg being over a 100 times more toxic than Zn. Methyl-Hg was considerably more toxic than HgCl₂. This was shown to be due to its chemical form and not to a greater uptake. Lead supplied as triethyl, diethyl and dimethyl lead complexes was also more toxic than PbCl₂. However, there was no significant difference between the toxicity of the organic lead complexes. In short-term uptake experiments (5 h), the uptake of Cd into roots of Picea abies was found to be influenced by the ionic composition and pH of the uptake solution. Cd uptake decreased with decreasing pH, and increased with decreasing Ca supply. Al and Mn inhibited Cd uptake, whereas Hg and Zn produced a slight stimulation in uptake.

With the use of X-ray microprobe techniques, the ion contents of vacuoles in root tissue were found to be influenced by both pH and Al. At 2 mM A1 (Ca/Al ratio 0.75) the vacuolar contents of Mg, K and P declined significantly compared to untreated plants. Toxic levels of Cd were found not to influence the K or Mg contents of the roots of Picea abies. Comparison of the metal concentrations shown to inhibit root elongation in nutrient solu- tionto those found in the soil solution fromthehumus layer taken from under trees showing the symptoms of decline, suggests that the levels of Hg, Pb and Zn are sufficiently high to in- fluence root growth.