In the present study, single and mixture effects of cadmium (Cd) and zinc (Zn) on Lemna gibba were analyzed and compared using growth parameters, based on frond number and fresh weight, and biochemical parameters, such as pigment, protein content and activity of antioxidant enzymes. Plants were exposed for 7 days to these metals in nutrient solution. Single and mixture exposures affected plant growth and the biomarkers of the antioxidant response. Considering the growth parameters, Cd was found to be much more toxic than Zn. IC50-7d, based on growth rate calculated on frond number, were 17.8 and 76.73 mg/L, and on fresh weight were 1.08 and 76.93 mg/L, for Cd and Zn respectively. For Cd, LOEC values were obtained at 2.06 and 1.03 mg/L, for frond number and fresh weight respectively; while for Zn, at 20.1 and 74.6 mg/L. A high toxicity effect, considering the same response variables, was observed in plants exposed to the mixtures. Three fixed ratios, based on toxic units (TU) were assayed, ratio 1: 2/3 Cd-1/3 Zn, ratio 2: 1/2 Cd-1/2 Zn and ratio 3: 1/3 Cd-2/3 Zn. Ratio 3 (where Zn was added in higher proportion) was the less toxic. All concentrations of Ratio 1 and 2 significantly inhibited plant growth, showing a 100% inhibition of growth rate at the highest concentrations when based on frond number. Catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APOX; EC 1.11.1.11) and guaiacol peroxidase (GPOX; EC 1.11.1.7) activities in single metals assays were higher than controls. In mixture tests, the activity of APOX and GPOX was significantly stimulated in plants exposed to all evaluated combinations, while CAT was mainly stimulated in Ratio 3. It was observed that the activity of the enzymes was increased in the mixtures compared with similar concentrations evaluated individually. APOX activity was observed to fit the CA model and following a concentration-response pattern. The response of this antioxidant enzyme could serve as a sensitive stressor biomarker for Cd-Zn interactions. Frond number in Cd-Zn mixtures was not well predicted from dissolved metal concentration in solution using concentration addition (CA) as reference model, as results showed that toxicity was more than additive, with an average of ΣTU = 0.75. This synergistic effect was observed up to 50 mg Zn/L in the mixture, but when it was present in higher concentrations a less than additive effect was observed, indicating a protective effect of Zn. A synergistic and dose-ratio deviations from CA model were also observed.

中文翻译：

---

镉和锌的单一和二元混合物暴露对Lemna gibba生长和生物标志物的比较生态毒性。

在本研究中，使用生长参数（基于叶数和鲜重）以及生化参数（例如色素，蛋白质含量和活性），分析并比较了镉（Cd）和锌（Zn）对Lemna gibba的单一和混合效应抗氧化酶。使植物在营养液中暴露于这些金属7天。单一和混合暴露影响植物的生长和抗氧化剂反应的生物标志物。考虑到生长参数，发现镉的毒性比锌大得多。Cd和Zn的IC50-7d（基于叶数计算的增长率）分别为17.8和76.73 mg / L，而鲜重分别为1.08和76.93 mg / L。对于镉，叶重和鲜重的LOEC值分别为2.06和1.03 mg / L。而锌的浓度为20.1和74.6 mg / L。毒性高，考虑到相同的响应变量，在暴露于混合物的植物中观察到。测定了三种基于毒性单位（TU）的固定比例：比例1：2/3 Cd-1 / 3 Zn，比例2：1/2 Cd-1 / 2 Zn和比例3：1/3 Cd-2 / 3锌。比例3（其中锌的添加比例较高）的毒性较小。比率1和2的所有浓度均显着抑制植物生长，当基于叶数时，在最高浓度下显示100％的生长速率抑制。单金属测定中的过氧化氢酶（CAT; EC 1.11.1.6），抗坏血酸过氧化物酶（APOX; EC 1.11.1.11）和愈创木酚过氧化物酶（GPOX; EC 1.11.1.7）的活性均高于对照。在混合物测试中，暴露于所有评估组合的植物均显着刺激了APOX和GPOX的活性，而CAT主要以比率3刺激。观察到与单独评估的相似浓度相比，混合物中酶的活性增加。观察到APOX活性适合CA模型并遵循浓度-响应模式。这种抗氧化酶的反应可以作为Cd-Zn相互作用的敏感应激生物标记。使用浓度添加（CA）作为参考模型不能很好地从溶液中的溶解金属浓度预测Cd-Zn混合物中的前叶数，因为结果表明毒性大于添加剂，平均ΣTU= 0.75。在混合物中高达50 mg Zn / L时观察到了这种协同作用，但是当它以较高的浓度存在时，观察到的协同作用要小得多，表明Zn的保护作用。还观察到与CA模型的协同和剂量比偏差。