Plant growth regulators accelerate grain development in rice (Oryza sativa L. cv. OM5451)

Rice grain development is a complex physiological process that strongly determines yield and harvest timing. This study examined morphological, physiological, and biochemical changes during grain development of the OM5451 cultivar, and assessed the effects of plant growth regulators on the duration of grain development. Grain development was divided into four distinct stages: milk (0-5 days after fertilization, (DAF)), soft dough (6-10 DAF), hard dough (11-20 DAF), and maturity (from 21 DAF onward). Morphological observations showed progressive embryo differentiation, while physiological measurements revealed a sharp increase in grain fresh and dry weight until 20 DAF. Biochemical analysis of grains indicated a marked decline in soluble sugar content, from 127.64 mg/g at 5 DAF to 4.83 mg/g at 25 DAF, accompanied by a corresponding increase in starch content, from 194.40 mg/g to 513.26 mg/g at 20 DAF, reflecting a shift in assimilate allocation in grains. Endogenous hormone profiling using high-performance liquid chromatography coupled with diode array detection (HPLC-DAD) revealed decreasing levels of indole-3-acetic acid (IAA), zeatin, and abscisic acid (ABA) between 5 and 15 DAF, highlighting their temporal roles in grain development. Exogenous application of ethephon (25 and 50 mg/L) at the hard dough stage effectively shortened the grain development period by approximately six days without reducing 1000-grain weight. Gibberellic acid (GA3) application at 50 mg/L significantly improved grain filling, increased 1000-grain weight and plant height, and reduced sterility. These findings demonstrate that precise application of plant growth regulators (PGRs) can accelerate ripening and optimize yield, providing practical solutions for improving rice production under climate-induced stress in the Mekong Delta.