Effects of seedling age at transplanting on root-shoot traits and yield in direct-seeded maize

To address weak growth and low yield of spring maize under transplanting and seedling replacement, this study evaluated the effects of transplanting at different seedling ages on dry matter production, with the aim of identifying the optimal transplanting stage. Field experiments were conducted at two sites using a randomized block design. Three transplanting stages—2-leaf 1-heart (2L1H), 3-leaf 1-heart (3L1H), and 4-leaf 1-heart (4L1H)—were compared with direct seeding (CK). Root shoot traits, dry matter accumulation, and ear characteristics were assessed. Compared with direct seeding, transplanting shortened the jointing-to-silking period but extended both the transplanting-to-jointing and silking-to-maturity phases. Transplanting significantly reduced root traits, including 0–20 cm root dry mass(RDM), root length(RL), root surface area(RS), and number of forks (NF), as well as leaf traits such as leaf area(LA), specific leaf weight (SLW), and SPAD values. Transplanted plants exhibited physiological stress, reflected by decreased leaf instantaneous spectral use efficiency (SUE) and increased water use efficiency (WUE) and instantaneous carboxylation rate (Ce). These changes limited dry matter accumulation and its translocation to grains, resulting in reduced yield. Among the treatments, transplanting at the 3-leaf 1-heart stage (3L1H) showed the smallest deviation from direct seeding in root traits and dry matter accumulation, thereby minimizing yield loss.