Influence of Ca/Mg ratio and Cd2+ and Pb2+ elements on low molecular weight organic acid secretion by Salix viminalis L. roots into the rhizosphere

A modified hydroponic experiment was focused on the influence of mutual changes in the Ca/Mg ratio and trace elements (Cd and Pb) concentration in solution on the efficiency of low molecular weight organic acid (LMWOA) formation in the Salix viminalis rhizosphere. The study showed that changes in the LMWOA contents were caused by the Ca/Mg ratio as well as the kind and concentration of trace element (Cd or Pb) in hydroponic solution. Under 0.5 mM solution of Cd, the total concentration of LMWOAs—depending on the Ca/Mg ratio—decreased according to the formula: 20:1 > 4:1 > 1:10 > 1:¼, while under 0.5 mM of Pb, it was as follows: 20:1 ≈ 4:1 > 1:¼ > 1:10. When comparing all the experimental systems, it is important to underline that regardless of the Ca/Mg ratio, the concentration of LMWOAs in the rhizosphere was higher under 0.5 mM Cd than Pb, with the exception of 1:¹/₄ Ca/Mg ratio, where the opposite observation was made and both of these values were lower than in the control. In addition, accumulation of Cd and Pb in Salix roots and young shoots was dependent on Ca/Mg ratio only and decreased according to the formula 1:10 > 4:1 > 20:1 > 1:¹/₄, regardless of Cd or Pb supplementation.