Optimum combination of irrigation and nitrogen supply form achieving high photosynthetic and nitrogen utilization efficiency

Objectives Water and nitrogen supplying has interaction on the rice growth and production efficiency. We studied the appropriate nitrogen supplying forms under different water conditions from the activities of nitrogen assimilation enzymes of root and the photosynthetic efficiencies of leaves.

Methods Cement pits of 9 m × 1.5 m × 0.4 m were constructed under a rainproof shed, and used for a 2 factor three level of complete plot treatment, using rice cultivar of Xudao3 as the tested materials. The three levels of factor of water consisted of constant shallow water layer throughout rice lifecycle (CK), alternate drying and wetting under light drought (–20 kPa) and heavy drought (–40 kPa). The three nitrogen forms were 100%NH₄⁺-N (NH), 50%NH⁺-N+50%NO₃^–-N (1/2NH+1/2NN), 100%NO₃^–-N (NN). At the stages of full tellering, initial panicle differentiation, heading and maturing, rice plant samples were collected for the analysis of root assimilation enzyme activities and leaf photosynthetic indexes, and the N contents.

Results With the same N supplying form, the activities of NR, GS, GOGAT and GDH in roots, the photosynthetic rate and the maximum photochemical efficiency were increased in treatment of light drought, while the lowest results were all observed under the heavy drought treatments, in which the rice could not grow properly at all. Under different watering treatments, the suitable N forms were different. Under CK, 100% ammonium supplying treatment achieved the highest activities of GS, GOGAT and GDH in roots, the highest leaf photosynthetic rate and the maximum photochemical efficiency at the main growth stages, which resulted the increased rice yield and photosynthetic nitrogen utilization efficiencies (PNUE) and nitrogen agronomy efficiency (AE_N). Under light drought condition, 1/2NH+1/2NN treatment achieved the highest enzyme activities, leaf photosynthetic rates and the maximum photochemical efficiencies, and the highest rice yield and N use efficiencies. Correlation analysis indicated that the yields and N efficiencies were positively correlated with the activities of GS, GOGAT and GDH in root, the leaf photosynthetic rate, the maximum photochemical efficiency at main growth stages at the 0.05 and even 0.01 probability levels, while negatively correlated with the non-photochemical quenching coefficient.

Conclusions When a shallow water layer is kept throughout the rice growing, ammonium nitrogen is recommended, and in alternative wetting and moderate drying condition, ammonium and nitrate N should be supplied in the form of 50%NH₄⁺-N and 50% NO₃^–-N. This study indicated that suitable combination of water and N forms could promote the activity of nitrogen metabolic enzymes in roots, increase the photosynthetic rate and maximum photochemical efficiency of leaves, and be conducive to high yield and efficient utilization of nitrogen of rice.