Abstract: Nitrogen and magnesium are two essential elements for plant photosynthesis. In this paper, annual Conyza japonica plants were hydroponically cultured in three climate chambers to study the nitrogen and magnesium absorption by the plants under three illumination levels (high, (168 ± 3) μmol·m^-2·s^-1; medium, (66 ± 1) μmol·m^-2·s^-1; and low, (23 ± 2) μmol·m^-2·s^-1), aimed to explore the nitrogen and magnesium absorption strategies of the plants under different illumination conditions. Illumination level had significant effects on the nitrogen and magnesium absorption by the plants. With the increasing level of illumination, the mass-based nitrogen absorption increased significantly, while the mass-based magnesium absorption was in adverse (P<0.05). The leaf area-based chlorophyll content increased with the decrease of illumination level,but the leaf mass-based chlorophyll content was the highest under medium illumination level and the lowest under low illumination level (P<0.05). These results suggested that under high illumination level, the plants increased their nitrogen absorption to synthesize more photosynthetic enzymes to improve electron transfer and carboxylation efficiency, whereas under low illumination level, they increased their magnesium absorption to synthesize more chlorophyll to capture more light energy. The differential absorption of nitrogen and magnesium by the C. japonica plants under different illumination levels could be explained by  their different requirements of nitrogen and magnesium for the photosynthesis under different illumination conditions.

Key words: Mountain forest landscape, Landscape pattern, Landscape indices, Qilian Mountains, Xishui Natural Reserve