Improving photosynthetic efficiency of flag leaves at anthesis stage and nitrogen utilization of winter wheat by deep placement of common and control-released urea mixture

Objectives Nitrogen supply has important impact on photosynthetic characteristics, photosystem Ⅱ (PS Ⅱ) performance and nitrogen utilization as well. The effects of urea type and application depth on the nitrogen utilization and yields were studied from the aspect of photosynthesis.

Methods A field experiment with a split-plot randomized block design was conducted using urea type as main plot and fertilization depth as subplot in 2015–2017. The urea types included normal urea (T1) and the mixture of normal urea, sulfur coated urea and polymer coated urea at 4∶3∶3 (T2); the two fertilization depth was 5 cm (D1) and 10 cm (D2). The normal urea was applied in basal / topdressing ratio of 4∶6 in T1 and the mixed urea T2 was once applied as basal fertilizer. The gas exchange parameters and fast chlorophyll fluorescence induction kinetics of main stem flag leaves after anthesis, nitrogen accumulation of aboveground plant at different stages and the final yield and its components were monitored. The fertilizer nitrogen efficiencies were calculated.

Results Urea type and fertilization depth affected grain yields significantly. Treatment T2D2 got the highest yield in 2 years; T2 increased spike number per hectare and the weight of 1000 grains, and D2 increased the weight of 1000 grains in 2016–2017. In contrast to spike number per hectare and the weight of 1000 grains, no significant difference between T2 and T1 was found for grain number per spike. Similarly, there were no significant differences in spike number per hectare and grain number per spike between D1 and D2 treatments. The gas exchange parameters of flag leaves were remarkably different under different treatments. Under the same fertilization depth, mixed urea (T2) led to a significantly higher net photosynthetic rate (P_n) with a significantly higher stomatal conductance (G_s), but significantly decreased the intercellular CO₂ concentration (C_i). With the same type of urea, the P_n and G_s of wheat flag leaves were significantly increased in D1 than in D2, but C_i decreased. The treatment of T2D2 increased photosynthetic capacity of flay leaves as the prolonged duration of the stead photosynthesis at late stage as well. T2D2 obtained higher maximal photochemical efficiency (φ_Po) and quantum yield of electron transfer (φ_Eo) of photosystem Ⅱ. Moreover, the increase of φ_Po was higher than φ_Eo, resulting in improved fluorescence photochemical quenching coefficient (ψ_o). Meanwhile, fluorescence at K-step (W_k) of PS Ⅱ electron donor side and the fluorescence at J-step (V_j) of acceptor side were declined in treatment T2D2, which significantly improved the performance of PS Ⅱ reaction center, and enhanced the photosynthetic capacity of flay leaves after anthesis as well. Compared with T1, T2 increased the nitrogen accumulation of aboveground in later stage. Nitrogen accumulation in D2 was increased in all the growth stages than those in D1 treatment. Besides, the final results in yield of T2 and D2 were higher than T1 and D1, which increased nitrogen agronomic efficiency (NAE), nitrogen partial productivity (PEP_N) and nitrogen apparent recovery rate (NRE) in maturity stage.

Conclusions The mixed application of normal and control-released urea at 10 cm depth of soil could effectively stimulate the photosynthetic rate after anthesis, which showed significant priority to conventional fertilization and shallow fertilization in nitrogen utilization and wheat yield, being a labour-saving and high efficient measurement in winter wheat production.