Abstract: INTRODUCTION: In agricultural production, the utilization of heterosis has brought significant benefits to society and economic development by markedly increasing crop yield, stress resistance, and quality. Foxtail millet (Setaria italica var. germanica), as an important coarse grain crop in the arid and semi-arid regions of northern China, holds a significant position in dryland ecological agriculture. However, the slow increase in the yield per unit area of foxtail millet has limited the further realization of its production potential. Utilizing heterosis has thus become one of the effective ways to enhance the yield of foxtail millet. Nevertheless, research on the physiological and molecular mechanisms of its heterosis is still relatively weak, and the formation mechanism remains unclear. Therefore, a deep understanding of the physiological mechanisms of heterosis in foxtail millet is of great importance for improving the yield of hybrid varieties.  RATIONALE: The Changzagu series of foxtail millet hybrids (Changzagu 466, Changzagu 2922, and Changzagu 333) exhibit significant over-parent heterosis in yield. To elucidate the formation mechanism of yield heterosis in foxtail millet, we systematically analyzed the yield advantages of these hybrids and their influencing factors by measuring yield-related traits and key physiological indicators of the hybrids and their parental lines.  RESULTS: Throughout the entire growth period, the chlorophyll content of the three hybrid varieties was higher than that of their parents. Among them, Changzagu 466 exhibited the highest chlorophyll content at the jointing stage, reaching 13.86 mg∙g⁻¹ FW. During the seedling and jointing stages, the root activity of the three hybrids was significantly higher than that of their parents. Specifically, Changzagu 466 showed the highest root activity at the seedling stage, measuring 1.76 mg∙g⁻¹∙h⁻¹, which was 7.8 times and 5.5 times higher than its female and male parents, respectively. Changzagu 2922 had root activity values at the seedling stage that were 0.38 and 0.66 mg∙g⁻¹∙h⁻¹ higher than its female and male parents, respectively. Meanwhile, Changzagu 333 displayed more pronounced advantages at the jointing stage, with root activity values 0.31 and 0.62 mg∙g⁻¹∙h⁻¹ higher than its female and male parents, respectively. In terms of yield-related traits, compared to their parents, the hybrids showed significant improvements in both grain filling rate and spikelet number. Changzagu 466 reached its maximum grain filling rate of 1.58 g∙d⁻¹ per panicle at 19 days after flowering. Both Changzagu 466 and Changzagu 333 had significantly higher spikelet numbers than their parents, while Changzagu 2922 also showed a significant increase in spikelet numbers. In addition, the hybrid varieties also demonstrate certain advantages in root nitrogen accumulation and nitrogen translocation efficiency. Among them, Changzagu 2922 exhibits the strongest root nitrogen accumulation advantage and the highest nitrogen translocation efficiency (nearly 56%), which is significantly higher than that of its male parent line M22.  CONCLUSION: The Changzagu series of foxtail millet hybrids effectively enhances photosynthetic capacity and nutrient absorption and utilization efficiency by significantly increasing chlorophyll content, root activity during the early growth stages, and root nitrogen accumulation. Meanwhile, the significant increase in grain filling rate and spikelet number of the hybrids further enhances both grain weight and grain number per panicle, ultimately achieving high yield.

Key words: hybrid millet, physiological index, grouting rate, agronomic traits, yield