桑树（Morus alba）可与丛枝菌根（AM）真菌形成互利共生体，丛枝菌根真菌能对桑树的重金属元素吸收产生积极的响应。然而，这种响应随环境条件的变化而有所不同。贵州荔波和黄平两地种植养蚕桑树，提高桑叶质量对养蚕业具有重要意义。荔波和黄平两地均处于喀斯特地区，土壤pH因石灰岩和砂岩交叉分布而有所差异。本文将揭示土壤因素是如何影响桑树与丛枝菌根真菌互利共生体的形成从而影响桑树对重金属元素的吸收。实验结果表明，荔波桑地平均土壤 pH （4.92±1.03）明显低于黄平（5.96±1.08）。土壤酸性直接影响 AM 真菌的分布，荔波的偏酸性土壤环境有利于真菌生长，且有利于桑树与丛枝菌根真菌共生关系的形成。此外，偏酸性土壤条件有利于增加重金属元素的生物可利用性，从而加强植物体对重金属元素的吸收，包括有毒元素 Cd。与黄平相比，生长在荔波的桑树叶片具有较高含量的重金属元素。荔波桑树叶片中的糖类含量明显低于黄平的，分别为（67±27）mg·g-1、（105±57）mg·g-1；而荔波桑树根系中的糖类含量明显高于黄平，分别为（125±43）mg·g-1、（91±43）mg·g-1。该结果说明，与黄平（具有较高的土壤pH）相比，荔波（具有较低的土壤pH）桑树叶片中的光合作用产物将更多地被输入到桑树的根系中。真菌是专性共生物，如果没有植物所供给的光合产物，真菌就不能生存。根系分泌物的另一个重要作用是溶解重金属，使它们具有较强的移动能力，以便被宿主植物所吸收。这就解释了为什么在较低土壤pH环境条件下生长的桑树的叶片含有较高含量的重金属元素。因此，菌根植物根系的分泌作用是一个非常重要的过程。尽管桑树具有耐干旱贫瘠的能力，且能够适应于喀斯特环境，但桑树喜好的是环境仍是偏酸性且养分充足的土壤。%Mulberry is colonized by Arbuscular mycorrhizal (AM) fungi. The AM symbiosis is important for heavy metal absorption by host mulberry. However, the symbiont of mulberry with AM fungi should change with various soil conditions. Mulberry is usually established for sericulture in Libo and Huangping areas of southwest China. Therefore, the leaf quality is important for sericulture. Libo and Huangping are located in limestone areas, among where sandstone distributed, resulting in different soil pH in collected samples in both Libo and Huangping. Our results showed that average soil pH of Libo soils (4.92±1.03) was significantly lower than that of Huangping soils (5.96±1.08). Soil acidity directly affects the distribution of AM fungi. Relatively lower soil pH of Libo soils should be favorable to the growth of fungi, therefore, to the formation of symbionts, resulting in higher bioavailability of heavy metals in soils, and higher absorption of heavy metals by mulberry. In addition, sugar concentration of mulberry foliage from Libo (67±27) mg·g-1 was significantly lower than that from Huangping (105±57) mg·g-1, but sugar concentration of mulberry roots from Libo (125±43) mg·g-1was significantly higher than that from Huangping (91±43) mg·g-1 Those results indicate that compared with Huangping (with higher soil pH), more products of photosynthesis enter to roots from Libo (with lower soil pH). Fungi are obligated symbionts and cannot survive without photosynthate supply from plants. Another role of root exudates is the solubility of heavy metals, resulting in higher ability of movements, and easily absorption by host plants. Therefore, it is why mulberry foliagein from Libo with lower soil pH having higher concentrations of heavy metal. Although mulberry can adapt to Kast areas with drought and poor nutrients of environments, their favorable soil is relatively acidified with rich nutrients.
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