关键词: 杨树, 外生菌根, 氮吸收, 非损伤微测, 酶活性

Abstract:

Objective: This study aims to analyze the effects of Paxillus involutus on the absorption, assimilation and metabolism of ammonium and nitrate in Populus tremula × Populus alba, and reveal the physiological characteristics of P. involutus in promoting the nitrogen absorption and utilization of P. tremula × P. alba. Method: In this paper, the sand-cultivated P. tremula × P. alba seedlings were inoculated with ectomycorrhizal fungi, P. involutus, to form ectomycorrhiza. Then, normal roots, ectomycorrhiza, and their leaves were used as research objects. The absorption rates of ammonium and nitrate were determined using a non-invasive micro-test technique. The contents of ammonium and nitrate and the activities of nitrogen assimilation enzymes were determined with a spectrophotometer. The transcription levels of nitrogen transporter encoding genes were analyzed by a method of qRT-PCR. Result: After 16 weeks of inoculation with P. involutus, the ectomycorrhiza was emerged in the poplar roots, with the mycorrhizal colonization rate of 55.5%. The net photosynthetic rate of mycorrhizal (M) poplars was significantly higher than that of non-mycorrhizal (NM) poplars. However, there was no significant difference in the total biomass between M poplars and NM poplars. The net influxes of NH₄⁺ and NO₃^- in the root tips of M poplars were increased compared to those of NM poplars. It is worth noting that the inoculation of P. involutus led to change in spatial characteristics of NO₃^? fluxes on poplar roots. The maximum NO₃^? influx point on the poplar roots was shifted from 900 μm to 300 μm due to P. involutus colonization. In line with the results of the ion fluxes, the expression levels of genes involved in NH₄⁺ transport, such as AMT1;1, AMT2;1, and AMT3;2, and genes related to NO₃^? transport, such as NPF1.2F, NPF2.11A, and NPF6.3, were higher in the roots of M poplars than those in NM poplars. Meanwhile, the increase in transcript levels of genes involved in NH₄⁺ transport was stronger than that of genes related to NO₃^? transport in the roots of M poplars. Correspondingly, the contents of NH₄⁺ and NO₃^? in the roots and leaves of M poplars were higher than those of NM poplars. In addition, the activities of enzymes including glutamine synthase (GS), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH), which are involved in NH₄⁺ assimilation, were increased significantly in the roots and leaves of M poplars in comparison with NM poplars. Inoculation with P. involutus caused a significant increase in the total nitrogen in the roots and a significant decrease in the total carbon in the leaves of M poplars, resulting in significantly lower C/N levels in the roots and leaves of M poplars than those of NM poplars. Conclusion: These results suggest that the ectomycorrhizal fungus, P. involutus, enhances the nitrogen nutrition of its host P. tremula × P. alba by mainly increasing the absorption, assimilation and metabolism of ammonium nitrogen.

Key words: poplar, ectomycorrhiza, nitrogen absorption, non-invasive micro-test technology, enzymatic activity