Evaluation of Pb resistance in seedlings of four common aquatic Iris species

In this study, we assessed and compared the changes in physiological, growth, and stress indices in the seedlings of four common aquatic Iris species (i.e., I. sibirica, I. ensata var. hortensis, I. pseudacorus, I. ensata). The nutrient solution culture method was used and seedlings were exposed to 1 200 mg·L⁻¹ of lead (Pb) stress for 20 d. The results showed large differences in the degree of Pb-induced damage to the different Iris species. The malondialdehyde (MDA) content stress index had a range of 125.39%～748.00%; the soluble protein (SP) and proline (pro) content stress indices had ranges of 104.77%～143.99% and 77.49%～340.20%, respectively, while the superoxide dismutase (SOD) and catalase (CAT) activity stress indices had ranges of 57.61%～106.15% and 26.85%～97.11%, respectively. Furthermore, the degree of Pb-induced damage to Iris ensata var. hortensis was mild, while the Pb enrichment, bioconcentration factor, and transport coefficient of Iris ensata var. hortensis was the largest. The principal component analysis classified twelve indices into three groups, namely the extent of injury, Pb-resistant ability, and growth status; moreover, the contribution of the first principal component was relatively high (55.02%). The contributions of twelve Pb resistance indices, including leaf length (LL), leaf width (LW), root fresh weight (RFW), stem and leaf fresh weight (SFW), and root dry weight (RDW), were higher in the first principal component, indicating that these indices may reflect the Pb resistance found in aquatic Iris species. Differences in the comprehensive score and subordinate function value of the twelve Pb resistance indices of each Iris species were large, with I. ensata var. hortensis and I. sibirica displaying higher values. Ultimately, the ranking of Pb tolerance in the four aquatic Iris species was: I. ensata var. hortensis > I. sibirica > I. pseudacorus > I. ensata. Finally, through this comprehensive analysis, we suggest that the Pb resistance, Pb enrichment, and transport coefficient of I. ensata var. hortensis and I. sibirica are superior to the other two species assessed here, suggesting that these two Iris species could be used for the phytoremediation of Pb-polluted water.