In crop improvement programs, hybrid vigour (heterosis) is an important breeding strategy but the molecular mechanisms of hybrid vigour are still unclear. Grain yield declines after F1 generation due to phenotypic segregation. We found that, at the early seedling stage in hybrids derived from the temperate japonica rice varieties ‘Doongara’ and ‘Reiziq’, hybrid vigour was approximately 40% greater than in the better parents. Inbred high-yielding lines (Hybrid Mimics) were developed from the ‘Doongara’ × ‘Reiziq’ F1 by selfing and recurrent selection for F1-like plants in the F2 through to the F5 generation. Grain yields are stable over subsequent generations in the Hybrid Mimic lines. The importance of photosynthesis in early seedling development was demonstrated. Photosynthesis-related genes were expressed in the hybrid earlier than in the parents; physiological evidence using gas exchange indicated the early commencement of photosynthesis. Dark germination experiments supported the requirement for photosynthesis for early vigour in hybrids.