镉胁迫下铵态氮对红树植物秋茄(Kandelia obovata)生理生态特征的影响

城市红树林湿地面临重金属污染和营养元素过量输入的环境问题。其中，镉（Cd）污染具有较高的生态风险，而铵态氮（NH₄⁺-N）是红树林湿地沉积物中氮素的主要赋存形态。本论文采用盆栽实验研究了镉（Cd）胁迫下铵态氮（NH₄⁺-N）对红树植物秋茄（Kandelia obovata）生理生态特征的影响，包括生物量、叶光合参数（净光合速率P_n，气孔导度G_s，胞间CO₂浓度C_i和蒸腾速率E）、叶丙二醛（MDA）和可溶性糖含量以及根系活力。结果表明：（1）在单Cd胁迫下，秋茄生物量和蒸腾速率（E）显著降低，但根系活力在较高的单Cd胁迫下显著增加；（2）Cd胁迫下，低浓度NH₄⁺-N显著增加秋茄根生物量，但对地上生物量（Cd3处理的叶和Cd1处理的茎除外）和光合作用（P_n、G_s和E）的促进作用不显著；高浓度NH₄⁺-N显著抑制了生物量和光合作用（P_n、G_s和E）；（3）在Cd1处理下，根系活力随NH₄⁺-N胁迫的增强呈先升高后降低的趋势；在Cd2和Cd3处理下，NH₄⁺-N显著降低了根系活力；（4）Cd胁迫下，低浓度NH₄⁺-N对叶MDA（表征氧化损伤）和叶可溶性糖含量（表征渗透调节）的抑制不显著，高浓度NH₄⁺-N显著降低了MDA和可溶性糖的含量。因此，较低浓度的NH₄⁺-N能够缓解Cd对秋茄地下根的毒害，对地上部分的缓解作用有限；高浓度NH₄⁺-N会与Cd产生协同的复合胁迫，加重对秋茄的毒害。

Effects of Ammonium Nitrogen on Physiological and Ecological Characteristics of Kandelia obovata under Cadmium Stress

The heavy metal pollution and excess input of nutrient elements become two important environmental problems in mangrove forest. The ecological risk of cadmium(Cd) is relatively high, and ammonium nitrogen(NH₄⁺-N) is the main speciation of nitrogen in sediment of mangrove. In order to clarify the growth and physiological characteristics of Kandelia obovata under combined stresses of Cd and NH₄⁺-N, pot experiments were conducted. Plant growth, leaf photosynthetic parameters(photosynthetic rate(P_n), stomatal conductance(G_s), intercellular CO₂ concentration(C_i) and transpiration rate(E)), leaf malondialdehyde(MDA), leaf soluble sugar, and root activity were determined. The results showed that:(1)Plant growth and leaf transpiration rate(E) of K.obovata decreased under single Cd stress, with root activity to be reduced under higher single Cd stress; (2)Under Cd stress, low concentration of NH₄⁺-N significantly improved growth of root, but did not significantly improved growth of aboveground part, and leaf photosynthesis(P_n, G_s and E); while high concentration of NH₄⁺-N significantly reduced plant growth, and leaf photosynthesis(P_n, G_s and E); (3)Under Cd1 treatment, root activity first increased and then decreased with increasing NH₄⁺-N concentration; while reduced root activity were shown under Cd2 and Cd3 treatments with NH₄⁺-N supply; (4)Under Cd stress, low concentration of NH₄⁺-N did not significantly reduce content of malondialdehyde(indicative of oxidative damage) and soluble sugar(indicating osmotic adjustment), while high concentration of NH₄⁺-N significantly reduced the content of MDA and soluble sugar. Therefore, the lower concentration of NH₄⁺-N can alleviate the Cd toxicity to root, with limited emission on aboveground part; while high concentration of NH₄⁺-N and Cd combined stress would have a synergistic effect to aggravate Cd toxicity to K.obovata.