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• DHEAP is a novel plant growth regulator. • DHEAP improved maize lodging resistance. • DHEAP shaped a compact maize plant type. • DHEAP is helpful to increases summer maize planting density in NCP. Increasing the planting density (PD) of maize (Zea mays L.) crops is the most efficient method for enhancing maize grain yield, but supra-optimum PD leads to an increase in lodged maize, which alters the maize canopy structure. In this study, N, N-Diethyl-2-hexanoyl oxygen radicals-ethyl amine (2-ethyl chloride) phosphonic acid salt (DHEAP, a single compound), a novel plant growth regulator, was sprayed at the seven-expanded-leaf stage under two different PDs (75,000, and 90,000 plants ha-1) using two maize hybrids, ZD958 (lodging-resistance) and XY335 (lodging-susceptible), to reduce maize lodging rates and improve the maize canopy structure under high PD. After DHEAP treatment, the maize lodging rate of ZD958 and XY335 decreased by 70.87% and 75.22%, respectively, through a change in the morphological structure (i.e., reduced plant and ear heights and increased mean tilt angle) and an improvement in the basal internode quality (i.e., increased diameter, dry weight per unit length, crushing strength, cellulose content, lignin content, and vascular area, and reduced length). In addition, DHEAP treatment significantly reduced gibberellin (GA 3) contents in maize basal internodes. Moreover, the increase in the mean tilt angle (MTA) above the maize ear leaf after DHEAP treatment improved the light penetration in the middle leaf strata, which increased the chlorophyll content (SPAD), net photosynthetic rate (P n), and Rubisco enzyme activities. Finally, DHEAP treatment increased the grain yield of ZD958 and XY335 under 90,000 PD by 22.28% and 43.14%, respectively. Thus, it was concluded that the spraying of DHEAP effectively reduced maize lodging rates and improved the photosynthetic performance of the maize ear leaf, which increased maize grain yield under high PDs. [ABSTRACT FROM AUTHOR]