Alternate wetting and moderate drying irrigation harmonize rice root and shoot growth, improves grain yield and nitrogen use efficiency

Objectives Alternate wetting and drying (AWD) irrigation, one of the most efficient water use technologies, is widely adopted in rice production worldwide. However, its effects on grain yield and N-use efficiency varies with cultivation method, soil and climate. In this study, we investigate rice root and shoot trait response to AWD to understand the mechanism underlying the beneficial effect of the technology.

Methods A pot experiment was conducted using a mid-season japonica rice cultivar of Xudao 3 in 2018 and 2019. Three irrigation regimes were set up, namely, conventional irrigation (CI), alternate wetting and moderate drying (−20 kPa, AWMD) and alternate wetting and severe drying (−40 kPa, AWSD). Rice root morphology and physiological indexes were recorded.

Results AWMD significantly increased root length, root weight, root-shoot ratio, root bleeding sap, and cytokinin content by 13.3%, 6.7%, 10.8%, 8.1%, and 7.4% at heading stage, respectively. AWMD increased the dry matter accumulation from heading to maturity, promoted NSC transport from stem and sheath to grains, and improved grain ATPase activity by 16.3%−18.4%. Also, leaf nitrate reductase (NR) activity increased by 10.9%−44.0% in the main growth stages, N uptake of plants at maturity increased, and the grain filling rate, 1000-grain weight, and grain yield increased as well. Meanwhile, the N absorption and utilization efficiency, agronomic utilization efficiency, and partial productivity increased by 17.2%, 21.6%, and 8.4%. AWSD significantly reduced root length by 30.1%, root weight by 20.8%, inhibited root activity by 40.5%, and the ability to synthesize cytokinins by 34.4% at heading stage. Aboveground growth and leaf photosynthesis rate across the main growth stages were thus decreased, such as reduction of 26.4% at heading stage in photosynthesis rate. Although pre-stored NSC remobilization from stem to grains was enhanced, less post-anthesis dry matter accumulation and grain sink activity resulted in lower rice yield. The plant N uptake (P < 0.05) reduced under AWSD, and NR activity of rice leaf decreased by 19.2%. The N use efficiency (P < 0.05) decreased compared with conventional irrigation. The N use efficiency, agronomic use efficiency, and partial productivity decreased by 18.0%, 34.7%, and 31.8%.

Conclusions Alternate wetting and moderate drying irrigation effectively improves root morphology, increases root metabolism, promotes aboveground growth, and enhances rice yield and N use efficiency. Alternate wetting and severe dying irrigation is not recommended.