Response of wheat tillering and spike formation to nitrogen rate under supplementary irrigation based on soil moisture content

To elucidate the physiological mechanisms by which nitrogen rate regulates wheat tillering and spike formation under water-saving irrigation with supplemental irrigation guided by soil moisture measurements, we evaluated four nitrogen rates (0, 130, 180, and 230 kg hm⁻²) in wheat cultivar Jimai 22 from 2020 to 2022. We assessed the cross-sectional area of the tillering node, the photosynthetic capacity of the uppermost fully expanded leaves on the main stem, and ¹³C-assimilate partitioning among stems and tillers during tillering and spike formation. Compared with 0 and 130 kg hm⁻², 180 kg hm⁻² significantly increased tillering-node cross-sectional area and trans-zeatin (tZ) content, decreased indole-3-acetic acid (IAA) and abscisic acid (ABA) contents, and enhanced the photosynthetic capacity of the uppermost fully expanded leaves at the overwintering and regreening stages. Total tillers per population at overwintering increased by 35.1% and 14.3% under 180 kg hm⁻² relative to 0 and 130 kg hm⁻², respectively; at jointing, the corresponding increases were 80.6% and 22%. The allocation of ¹³Cassimilates to tillers I, II, and III under 180 kg hm⁻² was significantly higher than under 0 kg hm⁻², and the mean grain yield across the two years was 47.9% and 19.2% higher than that under 0 and 130 kg hm⁻², respectively. Increasing the nitrogen rate to 230 kg hm⁻² did not further improve these traits compared with 180 kg hm⁻², but significantly reduced nitrogen partial factor productivity. Correlation analysis showed that tillering-node cross-sectional area, trans-zeatin content, and net photosynthetic rate of the uppermost fully expanded leaves were significantly and positively correlated with total tillers per population and spike number, and that ¹³C-assimilate allocation to tillers was significantly and positively correlated with spike number. These results indicate that increasing tillering-node cross-sectional area and trans-zeatin content, enhancing leaf photosynthetic capacity, and promoting assimilate allocation to tillers provide key physiological bases for improving spike number and grain yield. Overall, under the soil-moisture-guided water-saving irrigation regime used here, 180 kg hm⁻² was the optimal nitrogen rate for achieving high yield and high nitrogen-use efficiency.