Effects of increased planting density on carbon metabolism, starch accumulation, and yield formation of maize grains at different ear positions

Abstract Background High planting density can increase maize yield per unit area, but it may also intensify competition for assimilates during grain filling. Whether this effect differs between grains at different ear positions, and how such differences relate to sugar availability, starch biosynthetic capacity, and starch accumulation, remains insufficiently resolved. Results A two-year field trial was conducted in Heilongjiang Province, China. Three maize hybrids were grown under three planting densities: D1 (82,500 plants·hm − 2 ), D2 (93,800 plants·hm − 2 ), and D3 (105,000 plants·hm − 2 ). Grains were separated into apical and basal-middle groups according to their position on the ear. We quantified early-stage sucrose and soluble sugar levels, activities of key starch biosynthetic enzymes, starch accumulation kinetics, and harvest traits. Increasing planting density generally decreased early sugar availability, reduced the activity-based capacity for starch biosynthesis, and lowered starch accumulation potential and rate during grain filling. Across densities and cultivars, basal-middle grains consistently showed higher early sugar supply, higher ADP-glucose pyrophosphorylase (AGPase), starch synthase (SS), and starch branching enzyme (SBE) activities, and stronger starch accumulation than apical grains. Logistic analysis indicated that higher density mainly reduced the intensity of starch accumulation rather than markedly shifting its temporal pattern. Piecewise structural equation modeling (SEM) further showed that early sugar supply, enzyme activity, and starch accumulation kinetics were closely associated with harvest-stage traits. Conclusion These results indicate that dense planting suppresses maize grain filling in part through coordinated reductions in early sugar availability and starch biosynthetic capacity, with stronger effects in apical grains than in basal-middle grains. Cultivar-specific responses suggest that tolerance to higher density is associated with the maintenance of grain-filling physiology across ear positions. The study provides a physiological framework for understanding ear-position-dependent variation in starch accumulation under dense planting.