Zn tolerance was high in all three studied species
Contrary to expectations, growth rates were not highest in the treatments without the addition of Zn. Low levels of Zn even increased growth rates for two of the species studied (L. gibba andL. minor ). Zn is an essential plant nutrient, and Zn deficiency has been shown to reduced fresh weight production in L. gibba(Vaughan et al., 1982). Zn is present in the Hoagland’s E Medium; nevertheless, it is possible that the Lemna species are Zn-limited in this medium and, therefore, show a lower growth rate as a sign of Zn deficiency. All three Lemnaceae species showed a very high tolerance to Zn. Only L. minor exhibited reduced growth rates in the highly polluted environment (11.35 mg/l). This is in line with previous results showing that Zn at lower concentrations promotes the growth of duckweeds, but inhibits duckweed growth at higher concentrations (Jayasri & Suthindhiran, 2017).
The high Zn tolerance for L. gibba contrasts previous work that found that concentrations of 4 mg/L of Zn inhibited growth by 50% and 10 mg/L of Zn reduced specific biomass growth by 90% (Lahive, O’ Halloran, et al., 2011).
For L. minor, the results partially confirm previous work. Jayasri & Suthindhiran (2017) showed that L. minor increased biomass yield in fronds treated with a lower concentration (0.5 mg/l) of Zn by 30% compared to the control. However, they concluded that 10 mg/l had little effect on growth after 4 days. This contrasts our results that found that 11.35 mg / L reduced growth rates of L. minor(significantly so in comparison with 1.82 mg / L). Lahive et al.(2011) reported that L. minor tolerated Zn concentration above 100 mg/L, but specific biomass growth rate was reduced significantly at low concentrations of 3 mg/L (reduction by 20%).
The duration of our experiment exceeded that of previous experiments, which were conducted over four to seven days (e.g., Jayasri & Suthindhiran, 2017; Lahive, O’ Halloran, et al., 2011; Megateli et al., 2009). We speculated that these previous studies may have underestimated the toxicity of Zn (and overestimated the duckweed Zn tolerance) because the plants may become more sensitive over time, as their tissue accumulates more Zn. However, the overall high tolerance to Zn we found after 17 days suggests that toxicity does not increase over time. The high tolerance of duckweed to polluted environments might be an explanation as to why duckweeds are common all around the world (Landolt, 1986).