Objectives Oilseed rape (Brassica napus L.) is the third largest oil crop in the world. The growth of oilseed rape is strongly inhibited by the pure NH₄⁺-N supply. We explored the effects of nitrate to ammonium ratios on the photosynthetic capacity of oilseed rape, to optimize nitrogen forms and increase yield of winter oilseed rape.

Methods A hydroponic culture method was used for the research. High and low N nutrition solution were prepared by regulating total N concentrations at 8 mmol (HN) and 1 mmol (LN). The N source treatments in the nutrient solutions included total nitrate (TN), total ammonium (TA), and nitrate to ammonium ratio of 3∶1 (3N1A), 1∶1 (1N1A), and 1∶3 (1N3A), respectively. At the 15, 20 and 25 days of treatment, the photosynthetic parameters of seedling leaves were monitored, and the rapeseed seedlings were harvested for the investigation of N content and biomass. The limitation of N forms was expatiated by photosynthetic restriction model.

Results The seedlings in TA treatment started to wither and became yellowing since 5 days under HN and 7 days under LN treatment, and died completely at 18 days of treatment. Compared with 3N1A treatment, 1N1A and 1N3A treatments decreased the biomass by 5.6%−61.3% and 6.9%−110.6%, respectively. Under the same N level, the nitrate to ammonium ratio treatments were close in leaf total N content, but significantly different in NO₃⁻-N and NH₄⁺-N contents. The leaves contained 91.7%−292.7% and 130.0%−1255.0% higher NO₃⁻-N content in 3N1A treatment than those in 1N1A and 1N3A treatment. The leaves of 1N3A treatment contained 52.6−53.3% and 61.1-76.9% higher NH₄⁺-N content than in 3N1A and TN treatment. The supply of NO₃⁻-N enhanced the net photosynthetic rate (A) significantly, which in 3N1A treatment was 16.7%−50.3% higher than in 1N1A and 1N3A treatment. 3N1A treatment significantly increased the maximum net photosynthetic rate (A_max), maximum carboxylation rate (V_cmax), maximum electron transfer rate (J_max) and mesophyll conductance (g_m), and the values were 6.8%−107.4%, 9.2%−79.5%, 50.5%−115.8% and 8.6%−134.8% higher than those of 1N1A and 1N3A treatment, respectively. The leaf photosynthetic rate did not stop rising with enhanced NO₃⁻-N and NO₃⁻-N to NH₄⁺-N ratio until that the leaf NO₃⁻-N content exceeded 120.2−151.2 μg/g, and the NO₃⁻-N to NH₄⁺-N ratio in leaf exceeded 2.6−3.2. The photosynthetic restriction model showed that the addition of NO₃⁻-N promoted A by reducing biochemical and mesophyll conductance limitation.

Conclusions Relatively high NO₃⁻-N to NH₄⁺-N ratio in leaves accelerates g_m and V_cmax, promotes net photosynthetic rate, and biomass. High ammonium fraction inhibits the growth, and pure ammonium supply may even leads to death of the seedlings. And pure nitrate source is not good for the efficient photosynthesis as well.