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 Chi-nese 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 den-sity about 200 mol m−2
s
−1
(normal light intensity) or 100 mol 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 photo-synthesis, 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