Use of Δ15N value to estimate the relationship between inorganic nitrogen supply and nitrogen demand for Morus alba seedlings under different nitrate nitrogen concentrations

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.

Keywords

• nitrate nitrogen
• morus alba
• carbon and nitrogen metabolism
• stable nitrogen isotope fractionation
• nitrogen demand