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

Abstract

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 (38cm×28cm×12cm) in a greenhouse at photosynthetic photon flux density about 200μmolm−2s−1 (normal light intensity) or 100μmolm−2s−1 (low light intensity) of light intensity. Four ammonium:nitrate ratios were applied: (a) NH4+:NO3−=0:100 (5mM NO3−), (b) NH4+:NO3−=10:90 (0.5mM NH4++4.5mM NO3−), (c) NH4+:NO3−=15:85 (0.75mM NH4++4.25mM NO3−), and (d) NH4+:NO3−=25:75 (1.25mM NH4++3.75mM NO3−). Two weeks after treatments started, under normal light, plants treated with 0.75mM NH4++4.25mM 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 5mM NO3−. Under shading condition, plants fertilized with 0.5mM NH4++4.5mM 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.