BACKGROUND: As a grain and oil crop, soybean presents a much lower yield than other staple crops. However, crop yields can be improved by applying modern agricultural technology, such as diethylaminoethyl hexanoate (DA-6) and mepiquat chloride (MC), which are important plant-growth regulators that substantially affect crop growth and yield. METHODS: This study examined the effects of DA-6 (30, 60, or 90 mg L RESULTS: The results showed that DA-6 effectively promoted the growth of soybean and increased parameters such as plant height, leaf area, and leaf dry weight in different growth stages by 21.0%, 18.7%, and 66.4%, respectively. In contrast, MC inhibited the growth and other parameters, decreasing the plant height, leaf area, and leaf dry weight in different growth stages by up to 15.7%, 11.9%, and 10.1%, respectively. Both DA-6 and MC promoted root development by increasing the dry weight, length, surface area, volume, tip number, branch number, and cross number. In terms of physiology, DA-6, and MC increased photosynthetic parameters, such as steady-state fluorescence (Fs), maximum fluorescence (Fm'), and photosynthetic system II (Phi2), increased the soluble protein contents, with maximum increases of 27.7% and 28.1% at different periods, and increased the soluble sugar contents by 38.2% and 58.3%. Regarding yield characteristics, DA-6 and MC considerably increased the yield, 100-grain weight, and number of effective pods. DA-6 increased the number of two- and three-seed pods, whereas MC increased the number of one-, two-, and three-seed pods. MC performed better than DA-6
however, they exerted different effects on the two varieties and at different concentrations. DA-6 was most effective at 30-60 mg L CONCLUSIONS: This study revealed the effects of DA-6 and MC on soybean morphology, physiology, and yield characteristics and the appropriate concentrations for application in soybean productions. Thus, these findings provide guidance for the rational application of the two regulators for soybean high-yield cultivation and stress resistance.