Leaf width expansion and biomass allocation, rather than photosynthetic rate, drive early vigor in newly developed rice lines

Early seedling vigor is a key determinant of rapid canopy establishment and early biomass production in rice, yet its physiological and morphological drivers remain incompletely understood. Rapid growth is often assumed to result from enhanced leaf-level photosynthetic capacity, but this assumption has rarely been tested explicitly. Here, we examined growth dynamics, biomass allocation, leaf morphology, and photosynthetic traits in newly developed rice lines exhibiting strong early vigor and compared them with the elite cultivar HHZ. The new lines accumulated biomass more rapidly than HHZ and exhibited non-exponential growth patterns, with higher absolute and relative growth rates. Despite lower leaf photosynthetic rates and lower leaf nitrogen content, the new lines showed significantly greater early biomass accumulation and larger canopy leaf area than HHZ. Canopy expansion was driven mainly by increased leaf width, rather than leaf length or leaf number per tiller, together with a greater allocation of biomass to leaves relative to roots during early growth. These results demonstrate that rapid early growth can be achieved through a canopy expansion strategy that prioritizes leaf morphological development and biomass allocation over photosynthetic efficiency per unit leaf area. This observation entails clear trade-offs, including nitrogen dilution and reduced photosynthetic biochemical capacity, yet it results in greater whole-plant biomass accumulation during early establishment. Our findings challenge the conventional assumption that early vigor is necessarily associated with higher leaf photosynthesis and highlight alternative physiological pathways for improving early growth and competitiveness in rice.