High temperature and heat damage frequently occur in the summer maize production areas of the Huang-Huai-Hai area, which severely inhibits the growth and development and impairs functions such as water and nutrient uptake and storage of maize roots. Elucidating the effects of the ethephon-glycine betaine-salicylic acid (EGS) mixture on root system architecture and yield formation under heat stress could provide the technical support and theoretical basis for establishing chemical regulation strategies for heat resistance and yield increase in summer maize cultivation in the Huang-Huai-Hai area.

Field experiments were conducted at the Xinxiang Experimental Station of the Chinese Academy of Agricultural Sciences in 2022 and 2023. Using Yudan 9953 (YD9953) and Zhengdan 958 (ZD958) as test materials, chemical regulation treatment and heat str ess treatment were established. For the chemical regulation treatment, the EGS mixture was sprayed on the leaves at the 6 th-leaf stage (V6), while an equal amount of water was applied to the control group. Heat stress treatments were implemented for 4 days at the 9 th-leaf stage (V9) and tasseling stage (VT), respectively, with field heating treatment (H) and normal temperature control (CK). The study aimed to investigate the effects of EGS treatment on root system architecture, physiological functions, and yield of maize under heat stress during different growth stages.

Under heat stress treatment in V9 and VT stages, compared with normal temperature control, in terms of root system architecture, the root dry weight, root-shoot ratio, the number of roots, root length, root surface area and root volume of YD9953 and ZD958 were significantly decreased; in terms of root physiological functions, root activity and activities of root antioxidant enzymes (SOD, POD, and CAT) were decreased significantly, while malondialdehyde (MDA) content increased significantly. Consequently, yield components and final output were severely compromised, with significant reductions in kernel number per ear, 100-kernel weight, and grain yield. Compared with heat stress treatment, EGS-H treatment improved root system architecture, alleviated the inhibition effect of heat stress on the number of roots, especially the number of aerial roots, significantly increased the root dry weight, root length, root surface area and root volume, and root volume was significantly positively correlated with yield. EGS-H treatment enhanced root physiological functions, significantly increased root activity and root antioxidant enzymes (SOD, POD, and CAT), while membrane peroxidation degree was significantly decreased. EGS-H treatment significantly increased the kernel number per ear and yield. After heat stress was applied at the V9 and VT stages, compared with heat stress treatment, EGS-H treatment increased the yield of YD9953 by an average of 19.42% and 19.56% in 2022, respectively, and by 14.40% and 17.95% in 2023, respectively. For ZD958, the yield increased by 9.81% and 13.02% in 2022, respectively, and by 7.68% and 7.78% in 2023, respectively.

The EGS mixture could regulate the root system architecture of summer maize under heat stress, promote root growth and development, and increase maize yield under heat stress.

Root is an important organ for maize to obtain water and nutrients, and a developed root structure is the key to maximum maize yield potential, which is currently an important issue to be solved in maize cultivation research. Ethylene- chlormequat-potassium and nitrogen application can affect the development of the maize root system. The aim of this study was clarify the effects of ethylene-chlormequat-potassium on the construction of summer maize root morphology and yield under different nitrogen application rates, so as to provide the theoretical and technical basis for the improvement of high yield and efficient cultivation management and rational fertilization of maize.

In 2019 and 2020, the field experiments were carried out in Daliudian village, Yanjiao town, Langfang city, Hebei province, and Shunyi Experimental Base, Chinese Academy of Agricultural Sciences, Shunyi district, Beijing, respectively, using maize single cross Yudan 9953 as experimental material. A split-zone experimental design was used, with the ethylene-chlormequat-potassium treatment (ECK) and the clear water control (CK) as the main zones, and the six nitrogen levels of 0 (N0), 96 (N96), 132 (N132), 168 (N168), 204 (N204) and 240 kg·hm^-2 (N240) as the secondary zones, aiming to analyze the effects of ECK on root morphology and yield of summer maize at different nitrogen application rates.

The nitrogen application significantly increased root dry weight, number of aerial roots, root length, root surface area and root volume. Compared with no nitrogen application, root dry weight, the number of aerial roots, root length, root surface area and root volume increased by 15.0%-25.2%, 31.7%-71.7%, 15.5%-30.8%, 19.0%-40.9% and 28.8%-54.0% on average with different nitrogen application rates, respectively. Compared with CK, ECK treatment increased root dry weight, number of root layers, number of roots in 1 to 2 layers and the number of aerial roots in summer maize with different nitrogen application rates by 10.4%-17.0%, 5.8%-12.6%, 10.8%-33.9% and 12.5%-79.6%, respectively; On the construction of root morphology, compared with CK, ECK treatment significantly increased the total root length, root surface area and root volume of summer maize with different nitrogen application rates by 7.5%-21.0%, 8.4%-29.3% and 14.3%-38.8%, respectively, and the root length with root diameter > 1.0 mm was significantly increased at medium and high nitrogen levels (N≥N204). Compared with CK, ECK treatment had no significant effect on summer maize yield per unit area in 2019 and 2020 under N0-N168, but significantly increased summer maize yield in 2019 and 2020 under N204 and N240, which increased by an average of 6.3% with N204 and 3.2% with N240. Correlation analysis showed that kernel number, 1000-kernel weight, root length, root surface area and root volume were positively correlated with summer maize yield, and the correlation coefficient between yield and root length was the highest.

ECK and nitrogen could synergistically promote maize root development and increase summer maize yield under high nitrogen conditions. In the current study, spraying ECK at the V6 growth stage combined with 204 kg·hm^-2 N fertilizer was a suitable cultivation technique and N fertilizer management practice for high-yielding summer maize in the Beijing-Tianjin area.