Comparison of rice root development and function among different degrees of dry-wet alternative irrigation coupled with nitrogen forms

Objectives Rice (Oryza sativa L.) is one of the main staple food crops in the world. Nitrogen application and irrigation play key roles in rice production. This study investigated the coupling effects of dry-wet alternative irrigation with nitrogen supplying forms on root morphology and physiology of rice, to provide a theoretical basis for efficient water and nutrient management in high-yield production of rice.

Methods Field experiments were conducted with completely 2-factor random design in the Pilot Farm of Henan University of Science and Technology in 2016 and 2017, the tested rice cultivar was Oryza sativa L. cv. Xudao-3 and the soil was clay loam. Three N form treatments of NH₄⁺-N∶NO₃^–-N ratio in 100∶0, 50∶50 (1∶1) and 0∶100 were set up, using ammonium sulfate, ammonium nitrate and sodium nitrate as the required N sources and with three irrigation regimes included shallow submerged irrigation control (CK, 0 kPa), alternative wetting and moderate drying (–20 kPa, WMD) and alternate wetting and severe drying (–40 kPa, WSD). Plant and soil samples were collected at mid-tillering, panicle initiation, heading and maturity stages. In soil sample volume of 20 cm × 20 cm × 30 cm with the plant stem in the center, root length, diameter, surface area, volume and tip number were measured, the oxidative ability and the contents of amino acid, protein and soluble sugar in root bleeding sap were determined. The yield and its composition at maturity were investigated.

Results There was a significant interaction between irrigation regime and nitrogen forms. Grain yield was the highest in the treatment of ammonium nitrate ratio 1∶1 coupling with WMD among all treatments in two years. Under WMD, the ammonium nitrate ratio 50∶50 treatment enhanced root length, average root diameter, root volume and root surface area at the main growth stages with averages of 10.6%–17.0%, 3.98%–25.25%, 5.27%–26.40% and 6.27%–25.19%, respectively, and increased the number of root tips, the contents of amino acids, proteins and soluble sugar in root bleeding fluid, and promoted carbon, nitrogen metabolism and nutrient status, water absorption in the root system. Under WSD, panicle number per unit area and grain number per panicle reduced, hence the rice yield reduced with averages of 38.2% for NH₄⁺, 29.9% for NH₄⁺-N∶NO₃^–-N ratio in 1∶1 and 35.0% for NO₃^–. WSD reduced root length, root volume, root surface area and root tip number, inhibited root activity and material synthesis in bleeding sap, and it was not conducive to the maintenance of root function. Nitrogen forms in different irrigation regimes had different effects on root system. Under CK, root morphology, root activity, root bleeding sap components of amino acid, protein, soluble sugar content were higher under the condition of NH₄⁺ treatment. Under WMD, the NH₄⁺-N∶NO₃^–-N ratio in 1∶1 treatment improved root morphology, increased root activity, and promoted root carbon and nitrogen metabolism. The treatment of NO₃^– reduced root length, average root diameter, root volume and root surface area at the main growth stages, which also decreased the number of root tips, the content of amino acids, proteins and soluble sugar in root bleeding fluid, and inhibited carbon, nitrogen metabolism and nutrient, water absorption in the root system. Besides, grain yield was positively and significantly correlated with root length, root volume, root surface area, number of root tips, and root activity at main growth stages. The NO₃^– treatment reduced rice yield significantly.

Conclusions Moderate wet-dry irrigation coupled with mixed supply of ammonium and nitrate nitrogen fertilizers are inductive to construct satisfactory root morphology and active physiological functions, which is the foundation for high yield of rice. These results suggest that a good root morphology and physiology, improving root of carbon and nitrogen metabolism and nutrient uptake through the appropriate regulation of irrigation regime coupling with ammonium nitrate are much more beneficial to increasing grain yield of rice.