To investigate the interactions between genotype, nitrogen application rate and planting density on the regulation of wheat lodging resistance and grain yield, so as to identify the optimal combination of nitrogen-density that matches the biological characteristics of varieties. The results provide theoretical basis and technical support for stable and abundant winter wheat yield and resistant strain cultivation.

A split-split plot field experiment was conducted in Jiaozuo, Henan Province, China, for two consecutive years from 2020 to 2022. Two wheat varieties Xinhuamai 818 and Xinmai 26 with different lodging resistance were selected in the main plots. The nitrogen fertilizer application rates were used as split-plots, and five levels were set: no N application (N0), 180 kg·hm^-2 (N1), 240 kg·hm^-2 (N2), 300 kg·hm^-2 (N3) and 360 kg·hm^-2 (N4), the planting densities were used as split-split plots, and three levels were set: 2.25 million plants/hm² (D1), 3.75 million plants/hm² (D2) and 5.25 million plants/hm² (D3). The study focused on analyzing the effects of the three-factor combination of variety, nitrogen application and planting density on the anatomical structure of wheat culms, field lodging rate and yield.

The results showed that nitrogen application rate and planting density significantly regulated the vascular bundle structure of both wheat varieties. The number and area of big vascular bundles and the ratio of number and area of big and small vascular bundles were significantly and positively correlated with culm wall thickness and culm breaking strength, while the area of small vascular bundles was significantly and negatively correlated with culm wall thickness. Compared with Xinmai 26, Xinhuamai 818 had more big vascular bundles and larger area, while the number of small vascular bundles was equal and the area was smaller. This may be the anatomical basis for the superiority of Xinhua 818 over Xinmai 26 in terms of lodging resistance. Under the same planting density, the number and area of big vascular bundles of both wheat varieties showed a trend of increasing and then decreasing with the increase of nitrogen application rate, with the largest number and area of big vascular bundles in N3 treatment. The average increase of number and area of big vascular bundles of Xinhuamai 818 and Xinmai 26 under N3 treatment compared with the minimum treatment were 14.61%, 15.80% and 16.18%, 20.10% respectively. The number and area of small vascular bundles showed similar changes. Under the same level of nitrogen application rate, the number and area of big vascular bundles of both varieties were the largest in the low density D1 treatment. Compared with the minimum value of high density D3, the average increase in the number and area of big vascular bundles of Xinhuamai 818 and Xinmai 26 under D1 treatment were 6.14%, 5.20% and 8.95%, 11.42%, respectively.

Nitrogen-density control combination D1N2 with 240 kg·hm^-2 and planting density of 2.25 million plants/hm² can optimize the vascular bundle structure, coordinate the development of big and small vascular bundles. Specifically, the number and area of big vascular bundles and the number ratio and area ratio of two vascular bundles were increased in this treatment. The combination can also increase the thickness of the culm wall between the basal nodes and improve the breaking strength of the plant. These changes realize the synchronous improvement of lodging resistance and yield of wheat. We think this treatment can be used as a suitable nitrogen-density combination pattern for high-yielding and efficient cultivation of winter wheat in high-yielding irrigation areas in northern Henan.

This study aimed to clarify the regulatory effects of exogenous melatonin on floret development and carbon nutrient metabolism in winter wheat under drought stress.

Two soil water conditions (drought stress treatment: D, and normal moisture treatment: W) were set up using multi-spike variety Yumai 49-198 and large-spike variety Zhoumai 22 as experimental wheat materials, with foliar spraying 100 μmol·L^-1 exogenous melatonin (MT) and clear water control (CK) before the peak of floret degradation (about 20 days after jointing) in 2021-2023, focusing on the effects of exogenous melatonin on the number and morphological characteristics of floret development, SPAD value, net photosynthetic rate, sucrose content and its metabolic enzyme activities of top spread leaves, and yield component factors of winter wheat under drought stress.

The drought stress led to an increase in floret degradation and abortion in wheat, while spraying exogenous melatonin could effectively reduce floret degradation and abortion, and increase number of fertile florets of the two varieties, but could not completely counteract the negative effect of drought stress; exogenous melatonin also showed positive regulatory effect on normal water treatments of the two varieties. Spraying exogenous melatonin could effectively increase SPAD value, net photosynthetic rate, carbon metabolism- related enzyme activities of top spread leaves and spike sucrose content of the two varieties under drought stress and normal water treatment, and the increase range was higher in drought treatment than in normal water treatment; exogenous melatonin decreased sucrose content of stem and leaf organs of both varieties under drought stress, but the opposite was true under normal water treatment. Spraying exogenous melatonin significantly increased grain number per spike of two varieties under two moisture treatments, compared with no-spraying MT treatment, the grain number per spike of Yumai 49-198 with spraying MT treatment increased by 19.12% (D) and 6.65% (W), respectively; the grain number per spike of Zhoumai 22 with spraying MT treatment increased by 21.57% (D) and 8.73% (W), respectively; spraying MT showed some regulation effect on spike number and thousand grain weight of the two varieties under the same water treatment, but did not reach a significant level. Compared the differences between two varieties, the regulatory effect of spraying melatonin was overall higher in the large-spike variety Zhoumai 22 than in the multiple-spike variety Yumai 49-198.

Spraying exogenous melatonin before the peak of floret degradation could effectively increase the SPAD value, net photosynthetic rate, and carbon metabolism-related enzyme activities of top spread leaves in wheat, and promote synthesis of photosynthesis products and the distribution and transportation of sucrose from stem and leaf nutrient organs to spike organ, which could provide sufficient nutrient security for the development of florets to increase number of fertile florets, thereby increasing grain number per spike, and the regulating effect on the large-spike variety of Zhoumai 22 was more pronounced. The results of this study provided the theoretical basis and technical support for increasing grain number per spike, stabilizing yield and reducing disaster under drought stress through the application of exogenous melatonin.