Abstract: Nitrate is predominant in the karst soils. However, the temporal and spatial heterogeneities are observed for the nitrate content in the soils. Hence, the Morus alba seedlings grown in karst regions may suffer from low nitrogen stress. In order to provide a scientific management of inorganic nitrogen for M. alba seedlings grown in karst regions, the M. alba seedlings were used as the experimental materials in this study. The M. alba seedlings were cultured hydroponically with a modified Hoagland solution. NaNO₃, with a δ¹⁵N of 22.35‰, was employed as the sole nitrogen source at three concentrations(0.5, 2.0, and 8.0 mmol·L^-1)in this study. The photosynthetic characteristics, dry weights, carbon contents, nitrogen contents and δ¹⁵N values of the leaves, stems and roots of the M. alba seedlings were measured in this study. The physiological responses to different nitrogen supply levels were analyzed for M. alba seedlings. The relationship between internal nitrogen demand and external nitrogen supply of M. alba seedlings was estimated based on the stable nitrogen isotope fractionation values at the whole-plant scale. The carbon-nitrogen coupling relationship was studied based on the nitrogen accumulation amount(NAA)and carbon accumulation amount(CAA)in the whole plant. The results were as follows:(1)When the nitrate nitrogen concentrations at 0.5, 2.0 mmol·L^-1, increasing nitrate nitrogen concentrations significantly increased the chlorophyll content and net photosynthetic rate of the M. alba seedlings, which in turn significantly promoted the biomass accumulation. However, when the nitrate nitrogen concentration exceeded 2.0 mmol·L^-1, more nitrate nitrogen supply(8.0 mmol·L^-1)did not lead to a significant increase in the chlorophyll content, net photosynthetic rate and biomass.(2)Increasing the nitrate nitrogen supply could promote the nitrogen assimilation in M. alba seedlings. The NAA in M. alba seedlings gradually increased with increasing nitrate nitrogen supply. However, the CAA in M. alba seedlings did not change significantly at nitrate nitrogen concentrations of 2.0 mmol·L^-1 and 8.0 mmol·L^-1.(3)The stable nitrogen isotope fractionation values of the nitrate nitrogen assimilates in the whole M. alba seedlings reached the minimum value at 2.0 mmol·L^-1. Therefore, the inorganic nitrogen supply at the nitrate nitrogen concentration of 2.0 mmol·L^-1 is close to the internal nitrogen demand of M. alba seedlings, and the close balance between external nitrogen supply and internal nitrogen demand of the plants means an effective coordination of carbon and nitrogen metabolism in plants, thus achieving a simultaneous increase in carbon and nitrogen assimilates.

Key words: nitrate nitrogen, Morus alba, carbon and nitrogen metabolism, stable nitrogen isotope fractionation, nitrogen demand

摘要： 喀斯特地区土壤中的硝态氮占主导地位,但土壤中的硝态氮含量存在时间和空间上的异质性。因此,种植在喀斯特地区的桑树幼苗可能会遭受低氮胁迫。为了给种植在喀斯特地区的桑树幼苗提供科学的无机氮管理,该研究以桑树幼苗为材料,采用水培试验,以改进的霍格兰(Hoagland)营养液为培养基质,以δ¹⁵N值为22.35‰的硝酸钠提供唯一氮源,设置3个硝态氮浓度梯度(0.5、2.0、8.0 mmol·L^-1),测定桑树幼苗的光合特征以及叶、茎和根的干重、碳含量、氮含量和δ¹⁵N值,分析不同供氮水平下桑树幼苗的生理响应,通过整个植株尺度的稳定氮同位素分馏值评估桑树幼苗的氮需求与氮供应的关系,通过植株的氮积累量与碳积累量研究碳氮耦合关系。结果表明:(1)当硝态氮浓度在0.5、2.0 mmol·L^-1时,增加硝态氮的浓度能显著提高桑树幼苗的叶绿素含量和净光合速率,进而显著促进生物量的积累。然而,当硝态氮浓度超过2.0 mmol·L^-1时,更多的硝态氮供应(8.0 mmol·L^-1)并没有带来叶绿素含量、净光合速率和生物量的显著增加。(2)增加硝态氮的供应量能促进桑树幼苗的氮同化,桑树幼苗的氮积累量随着硝态氮供应量的增加而逐渐增加,然而,桑树幼苗的碳积累量在硝态氮浓度为2.0 mmol·L^-1和8.0 mmol·L^-1时无明显变化。(3)桑树幼苗的硝态氮同化产物的稳定氮同位素分馏值在硝态氮浓度为2.0 mmol·L^-1时达到最小。综上所述,硝态氮浓度为2.0 mmol·L^-1时的无机氮供应量接近桑树幼苗的无机氮需求量,外部氮供应量与植株氮需求量接近平衡意味着植物体内的碳氮代谢能够有效协调,进而实现了碳氮同化产物的同步增长。

关键词: 硝态氮, 桑树, 碳氮代谢, 稳定氮同位素分馏, 氮需求