为了探究大气CO2浓度升高背景下，水蕴草〔Elodea densa (Planch.) Casp.〕对水体NH⁺₄胁迫的响应机制，设置2个NH⁺₄质量浓度水平（1和6 mg·L^-1）和3个CO2流量水平（0.0、0.4和1.0 m3·h^-1，分别对应环境CO₂、低流量CO₂和高流量CO₂），对胁迫0、3、10和17 d时6个处理组的水体物理化学指标及水蕴草的生长量和生理生化指标〔包括叶绿素含量、抗氧化酶活性、丙二醛（MDA）含量和碳氮代谢关键酶活性〕进行了比较和双因素方差分析。结果显示：与胁迫0 d和NH⁺₄单一胁迫处理组相比，胁迫3、10和17 d时，4个复合胁迫处理组水体的溶解氧含量和pH值显著（P＜0.05）下降，而电导率、氧化还原电位、盐度、NH⁺₄含量和HCO^-₃含量多显著升高，并且，CO₂流量对上述水体指标的影响以及NH⁺₄质量浓度对水体NH⁺₄含量的影响在0.001水平具有统计学意义，二者交互作用对水体NH⁺₄含量的影响在0.05水平具有统计学意义。6个处理组的水蕴草生长量均为负值；在同一CO₂流量下，1和6 mg·L^-1NH⁺₄处理组间的水蕴草生长量差异较小，而在同一NH⁺₄质量浓度下，低流量和高流量CO₂处理组的水蕴草生长量显著低于环境CO₂处理组；并且，仅CO₂流量对水蕴草生长量的影响在0.001水平具有统计学意义。4个复合胁迫处理组水蕴草的叶绿素含量均随胁迫时间延长呈现先升高后降低的变化趋势；与环境CO₂处理组相比，整体而言，4个复合胁迫处理组水蕴草的超氧化物歧化酶和过氧化物酶活性在胁迫3 d时显著下降、在胁迫10和17 d时显著升高，过氧化氢酶活性在胁迫3、10、17 d时升高；在同一NH⁺₄质量浓度下，高流量CO₂处理组水蕴草的MDA含量在胁迫10和17 d时显著低于低流量CO₂处理组。总体来看，在同一NH⁺₄质量浓度下，低流量和高流量CO₂处理组水蕴草的谷氨酰胺合成酶（GS）、谷氨酸合酶和谷氨酸脱氢酶（GDH）活性在胁迫3 d时高于环境CO₂处理组，而GS、GDH、核酮糖-1,5-二磷酸羧化酶和碳酸酐酶活性在胁迫10和17 d时低于环境CO₂处理组。以上研究结果显示：CO₂浓度升高可促进水蕴草的碳氮协同代谢，从而缓解水中NH⁺₄对水蕴草的毒性效应，但随着胁迫时间延长，溶解在水中的HCO^-₃不断累积，导致水体酸化，反而加剧了NH⁺₄对水蕴草的毒性效应。

To explore the response mechanism of Elodea densa (Planch.) Casp. to NH⁺₄ stress of water body under the background of elevating atmospheric CO₂ concentration, two NH⁺₄ mass concentration levels (1 and 6 mg·L^-1) and three CO₂ flow rate levels (0.0, 0.4, and 1.0 m3·h^-1, corresponding to ambient CO₂, low flow rate of CO₂, and high flow rate of CO₂, respectively) were set, the physical and chemical indexes of water body and growth increment and physiological and biochemical indexes 〔including chlorophyll content, antioxidant enzyme activities, malonaldehyde (MDA) content, and activities of key enzymes in carbon and nitrogen metabolisms〕 of E. densa in six treatment groups at 0, 3, 10, and 17 d of stress were compared and analyzed by using two-factor analysis of variance. The results show that, compared with 0 d of stress and NH⁺₄ single stress treatment group, at 3, 10, and 17 d of stress, the dissolved oxygen content and pH value of the water body in the four combined stress treatment groups significantly (P<0.05) decrease, while the electric conductivity, oxidation-reduction potential, salinity, NH⁺₄ content, and HCO^-₃ content mostly increase significantly, in addition, the effects of CO₂ flow rate on the above indexes of water body and the effect of NH⁺₄  mass concentration on the NH⁺₄ content in the water body are statistically significant at the 0.001 level, and the effect of their interaction on the NH⁺₄ content in the water body is statistically significant at the 0.05 level. The growth increments of E. densa in the six treatment groups are all negative; under the same CO₂ flow rate, the difference in growth increment of E. densa between 1 and 6 mg·L^-1 NH⁺₄ treatment groups is relatively small, while under the same NH⁺₄ mass concentration, the growth increments of E. densa in the low and high CO₂ flow rate treatment groups are both significantly lower than that in the ambient CO₂ treatment group; moreover, only the effect of CO₂ flow rate on the growth increment of E. densa is statistically significant at the 0.001 level. The chlorophyll content of E. densa in the four combined stress treatment groups show a variation tendency of first increase and then decrease with the elongation of stress time; compared with the ambient CO₂ treatment group, on the whole, the activities of superoxide dismutase and peroxidase of E. densa in the four combined stress treatment groups significantly decrease at 3 d of stress and significantly increase at 10 and 17 d of stress, while the catalase activity increases at 3, 10, and 17 d of stress; under the same NH⁺₄ mass concentration, the MDA content of E. densa in the high CO₂ flow rate treatment group is significantly lower than that in the low CO₂ flow rate treatment group at 10 and 17 d of stress. Overall, under the same NH⁺₄ mass concentration, the activities of glutamine synthetase (GS), glutamate synthase, and glutamate dehydrogenase (GDH) of E. densa in the low and high CO₂ flow rate treatment groups are higher than those in the ambient CO₂ treatment group at 3 d of stress, while the activities of GS, GDH, ribulose-1,5-bisphosphate carboxylase activity, and carbonic anhydrase are lower than those in the ambient CO₂ treatment group at 10 and 17 d of stress. The above results suggested that elevated CO₂ concentration can promote the synergistic carbon-nitrogen metabolism of E. densa, thereby alleviating the toxic effect of NH⁺₄ in water on E. densa, but with the elongation of stress time, the dissolved HCO^-₃ in water accumulates, leading to water body acidification, which in turn exacerbates the toxic effect of NH⁺₄ on E. densa.