Micro-sprinkler irrigation induces synergistic source–sink regulation: A promising strategy for boosting wheat grain weight in the Huaibei Plain, China

• MI increased the endosperm cells number in grain by regulating hormone metabolism. • MI enhanced sucrose synthesis capacity after anthesis, thereby promoting grain filling. • MI improved the TGW and GY when it obtained the highest SN and GN at maturity. Enhancing grain weight of wheat is a crucial strategy for improving yields in the Huaibei Plain (HP). However, the impacts and regulatory mechanisms of different irrigation regimes on wheat grain formation in the HP remained poorly understood. Therefore, a two-year field experiment was conducted to explore three treatments on wheat's source-sink relationship and grain formation: rain-fed (RI, no irrigation, 202.5 kg ha -1 N applied at sowing), conventional flood irrigation (CI, 60 mm irrigation at jointing stage, 112.5 kg ha -1 N at sowing+90 kg ha -1 N with irrigation), and micro-sprinkler irrigation (MI, irrigation based on 0–40 cm soil layer water deficit at jointing, booting and anthesis stages, 112.5 kg ha -1 N at sowing+30 kg ha -1 N at each irrigation). The results indicated that, compared with RI and CI, MI significantly increased the chlorophyll content and enhanced the activity of sucrose phosphate synthase (SPS) in flag leaf at 4 days after anthesis (DAA 4), and these parameters in CI were higher than those in RI. The sucrose and soluble sugar content in grain of MI were the highest at DAA 4. Additionally, at DAA 4, compared with RI, both CI and MI significantly elevated the content of indole propionic acid+zeatin nucleoside (IPA+ZR) and gibberellin (GA) in grain, while reducing the content of auxin (IAA) and abscisic acid (ABA). And the highest endosperm cells number was observed in MI. At the grain filling stage, MI exhibited the slowest chlorophyll degradation rate and the highest activities of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) and SPS in flag leaf, resulting in more sugar accumulation in the leaf and grain. Moreover, MI showed the highest IAA and lowest ABA levels in grain, and maintained the highest starch synthase activity during the filling stage, promoting the starch accumulation. Compared to CI and RI, MI significantly increased 1,000-grain weight by 4.99–5.55% and 7.33–11.51%, and grain yield by 4.99–11.60% and 15.60–39.14% over the two years, respectively. Overall, micro-sprinkler irrigation can optimize the water and nitrogen supply for wheat, effectively enhancing the source capacity in the early stage and the sink capacity in the late stage of grain development, thereby increasing grain weight and achieving high yield in the HP.