Moderate ammonium:nitrate alleviates low light intensity stress in mini Chinese cabbage seedling by regulating root architecture and photosynthesis

•Moderate NH4+:NO3− enhanced chlorophyll contents and photosynthesis of cabbage.•Moderate NH4+:NO3− protected chloroplast from damage under low light stress.•NH4+ triggered lateral root initiation and NO3− promoted root elongation.•Higher NH4+:NO3− ratio was not good for cabbage growth under low light intensity.

The hydroponic experiment was conducted to study the effects of ammonium (NH4+):nitrate (NO3−) ratios on growth, photosynthetic responses, chloroplast ultrastructure and root architecture of mini Chinese cabbage under low light intensity condition. Mini Chinese cabbage (Brassica pekinensis) seedlings were grown in pots (38 cm × 28 cm × 12 cm) in a greenhouse at photosynthetic photon flux density about 200 μmol m−2 s−1 (normal light intensity) or 100 μmol m−2 s−1 (low light intensity) of light intensity. Four ammonium:nitrate ratios were applied: (a) NH4+:NO3− = 0:100 (5 mM NO3−), (b) NH4+:NO3− = 10:90 (0.5 mM NH4+ + 4.5 mM NO3−), (c) NH4+:NO3− = 15:85 (0.75 mM NH4+ + 4.25 mM NO3−), and (d) NH4+:NO3− = 25:75 (1.25 mM NH4+ + 3.75 mM NO3−). Two weeks after treatments started, under normal light, plants treated with 0.75 mM NH4+ + 4.25 mM NO3− exhibited significantly promoted growth. Simultaneously, these plants showed significantly higher degree of granal stacking and photosynthesis, as well as larger absorption area of root system compared with plants fed with 5 mM NO3−. Under shading condition, plants fertilized with 0.5 mM NH4+ + 4.5 mM NO3− resulted in better growth and had intact chloroplast ultrastructure and a higher degree of granal stacking, chlorophyll contents and net photosynthetic rate, as well as larger root system. Our results indicate that compared with nitrate, adding moderate ammonium concentration in the nutrient could alleviate low light intensity stress in mini Chinese cabbage seedling by regulating root architecture and photosynthesis. The challenge is to determine how to manage ammonium:nitrate ratio according to the various light intensities during plant growth process.