Abstract
Increasing grain weight under dense planting conditions can further improve maize yield. 6-BA is known to be involved in regulating grain development and influencing grain weight. Maize grain development is closely linked to starch accumulation and hormone levels. In this work, the effects of applying 6-BA at the flowering stage under high density on the grain filling characteristics, starch content, starch synthesis critical enzyme activity, and endogenous hormones levels of maize grains (including inferior grains (IGs) and superior grains (SGs)) of two high-yielding summer maize varieties widely cultivated in China were investigated. The findings indicated that applying 6-BA significantly improved maize yield compared to the control, mainly as a result of increased grain weight due to a faster grain filling rate. Additionally, the activities of enzymes associated with starch synthesis, including sucrose synthase (SuSy), ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS), soluble starch synthase (SSS), and starch branching enzyme (SBE), were all increased following 6-BA application, thus facilitating starch accumulation in the grains. Applying 6-BA also increased the zeatin riboside (ZR), indole-3-acetic acid (IAA), and abscisic acid (ABA) levels, and reduced the gibberellin (GA3) level in the grains, which further improved grain filling. It is worth noting that IG had a poorer filling process than SG, possibly due to the low activities of critical enzymes for starch synthesis and imbalanced endogenous hormones levels. However, IG responded more strongly to exogenous 6-BA than SG. It appears that applying 6-BA is beneficial in improving filling characteristics, promoting starch accumulation by enhancing the activities of critical enzymes for starch synthesis, and altering endogenous hormones levels in the grains, thus improving grain filling and increasing the final grain weight and yield of maize grown under crowded conditions. These results provide theoretical and technical support for the further utilization of exogenous hormones in high-density maize production.