Optimizing root morphology is a key to improving maize yield under nitrogen reduction and densification cultivation

Context

The optimal allocation of planting density and nitrogen (N) fertilization is crucial for enhancing maize yield. However, the mechanisms involved in increasing planting density and optimizing N fertilizer application on regulating root morphology and hormone secretion, coordinating root-shoot relationships to improve yield remain unclear.

Objective

Herein, we aimed to investigate the relationship between yield and root N utilization, architecture, and physiological and biochemical traits under the optimal density-N combination.

Methods

A split-split-plot experiment of two planting densities (D1, 6.75 ×10⁴ plants ha⁻¹ and D2, 8.25 ×10⁴ plants ha⁻¹) and five N application rates (N0, 0 kg ha⁻¹; N160, 160 kg ha⁻¹; N220, 220 kg ha⁻¹; N280, 280 kg ha⁻¹; N340, 340 kg ha⁻¹) was conducted using the maize variety Xianyu 335 during 2021–2022.

Results

With increasing planting density and N application rate, root gibberellic acid contents, glutamine synthetase, and glutamate synthase activities were increased. These root hormone contents and enzyme activities enhanced root length density, dry matter, and N accumulation in both root and shoot. As planting density increased, indole acetic acid (IAA) content and nitrate reductase (NR) activity decreased, meanwhile the root morphology as root volume and root surface area per plant and root configuration as root angle ratio, root floor area, and root to shoot ratio per plant decreased. However, IAA content and NR activity were significantly enhanced as the N application rate increased, which promoted the development of root morphology and configuration. Structural equation modeling suggested that the main pathway for increasing maize yield was through root gibberellic acid, which promoted the development of root morphologies, thereby enhancing shoot dry matter accumulation and further increasing yield. The increase of planting density and N application rate improved the yield and the number of effective ears, and N application effectively alleviated the problem of yield reduction caused by the decrease of 100-kernel weight, kernel number per ear, and yield per plant due to the increase of density. The average maize yield under N160 and N220 increased by 16.1 % compared to N280 and N340 over two years, and the yield under D2 was 15.1 % higher than D1. The high-yield treatments (D2N160 in 2021 and D2N220 in 2022) averaged a yield increase of 15.6 % over two years compared to other treatments.

Conclusions

A planting density of 8.25 × 10⁴ plants ha⁻¹ and N application rates of 160–220 kg ha⁻¹ reached the highest yield compared to other treatments in the semi-arid region of the Guanzhong Plain, China. Optimizing planting density and N fertilizer application increases yield through enhanced root architecture, improved shoot dry matter accumulation, and further coordinated root–shoot development. This cultivation model provides a theoretical basis and practical guidance for achieving high maize yields.