Effect of inorganic and organic germanium treatments on the growth of lettuce (Lactuca sativa)

Germanium (Ge) is a rare heavy metal and is known to toxic to plants at high level. However, there is little evidence about the Ge effect on plant growth. Here, we investigated the effect of inorganic (GeO2) and organic (Ge-132) germanium on lettuce growth by treatment with various concentrations of GeO2 and Ge-132. Under GeO2 treatment, lettuce growth was not much inhibited at 2.5 mg/L concentration and then significantly inhibited at 5 mg/L concentration. However, under Ge-132 treatment, lettuce growth was not much inhibited by concentrations up to 10 mg/L. Relative fresh weight of lettuce at 2.5, 5, 10 and 25 mg/L concentrations was 99, 76, 65 and 35% in GeO2 treatments and was 105, 99, 97 and 75% in Ge-132 treatments, respectively. In GeO2 treatments, Ge was highly accumulated in the roots at concentration below 10 mg/L and in the shoots at concentration above 25 mg/L. However, Ge was primarily accumulated in the roots at all Ge-132 concentrations. Accumulated Ge amounts of plants under GeO2 treatment were 0.72 mg/ g DW in roots and 0.27 mg/ g DW in shoots at a 10 mg/L concentration. At a 50 mg/L concentration of GeO2, the Ge content was 0.77 mg/g DW in roots and 1.58 mg/g DW in shoots, respectively. Based on our results, inorganic germanium is more toxic for lettuce growth than organic germanium. Upper critical toxic levels for lettuce growth were 2.5 to 5 mg/L concentrations in GeO2 treatments and 10 to 25 mg/L concentration in Ge-132 treatments, respectively.