Effects of Zinc Supply Levels and Application Patterns on Carbon and Nitrogen Uptake and Utilization in Malus baccata. Seedlings

Objectives  The photosynthetic characteristics, ¹³C assimilation, ¹⁵N uptake and utilization of Malus baccata seedlings were evaluated under different zinc levels and application methods, aiming to provide a theoretical foundation for improving nitrogen nutrition and ensure a stable zinc supply in apple production.

Methods Using seedlings of apple rootstock Malus baccata as experimental materials, this study investigated the effects of five zinc (Zn) supply treatments under sand culture conditions over a 30-day period. The treatments included: LZ, continuous supply of low Zn (0.015 mg L⁻¹); LHZ, initial low Zn (0.015 mg L⁻¹, 15 days) followed by high Zn (5 mg L⁻¹, 15 days); AZ, continuous appropriate Zn (0.3 mg L⁻¹); HLZ, initial high Zn (5 mg L⁻¹, 15 days) followed by low Zn (0.015 mg L⁻¹, 15 days); HZ, continuous high Zn (5 mg L⁻¹). Parameters of root morphology, photosynthetic fluorescence parameters (P_n, g_s, F_v/F_m), activities of carbon-nitrogen metabolism enzymes (Rubisco, NR, SPS), and isotopic tracing of ¹³C/¹⁵N allocation were analyzed.

Results The AZ treatment yielded the highest seedling biomass and optimal root morphology, whereas LZ showed the lowest value. Under AZ, net photosynthetic rate (P_n), stomatal conductance (g_s), PSII photochemical efficiency (F_v/F_m), and enzyme activities (Rubisco, NR, SPS) were significantly elevated. Isotopic tracing revealed that ¹³C accumulation peaked under AZ (HLZ > LZ), and ¹⁵N utilization efficiency was highest under AZ (HZ > LZ). Both Zn supply levels and application patterns differentially regulated ¹³C/¹⁵N allocation in seedling organs.

Conclusions Both inappropriate zinc (Zn) supply levels and unstable Zn application patterns impaired seedling growth and nitrogen uptake. In contrast, appropriate and stable Zn supply levels enhanced net photosynthetic rate (P_n), increased the activity of key carbon metabolism enzymes, promoted carbon fixation in leaves, and enhanced the translocation of photosynthetic products. These improvements facilitated photo-assimilate transport to roots, improved root vitality, optimized root architecture, and enhanced seedling nitrogen uptake.