Carbon–Nitrogen Management via Glucose and Urea Spraying at the Booting Stage Improves Lodging Resistance in Fragrant Rice

Abstract

Rice is an important crop that significantly contributes to food security. Lodging is considered an important factor limiting rice yield and quality. The objective of this study was to investigate the effects of carbon and nitrogen on lodging in fragrant rice. A 2-year field experiment (2021 to 2022) was conducted with the fragrant rice cultivars Meixiangzhan 2 and Xiangyaxiangzhan grown under nine carbon and nitrogen co-application treatments (CK: 0 mg/L glucose + 0 mg/L urea; T1: 0 mg/L glucose + 50 mg/L urea; T2: 0 mg/L glucose + 100 mg/L urea; T3: 150 mg/L glucose + 0 mg/L urea; T4: 150 mg/L glucose + 50 mg/L urea; T5: 150 mg/L glucose + 100 mg/L urea; T6: 300 mg/L glucose + 0 mg/L urea; T7: 300 mg/L glucose + 50 mg/L urea; and T8: 300 mg/L glucose + 100 mg/L urea). The lodging index and stem characteristics of fragrant rice were investigated. Compared with the CK treatment, the T5 and T7 treatments significantly increased the pushing resistance force by 22.22–127.78% and 50.00–77.50%, respectively. Compared with the other fertilization treatments, the T5 treatment kept the lodging index at a low level and reduced the plant height. The stem characteristics were regulated under the carbon and nitrogen co-application treatments, and the internode length and dry weight significantly influenced the plant height and the pushing resistance force and then regulated the lodging index. Structural equation modeling and random forest modeling analyses suggest that carbon and nitrogen co-application treatments may further improve the resistance of rice to lodging by increasing the dry weight of the third and fourth internodes. Overall, optimized carbon and nitrogen co-application could regulate stem internode morphology and improved lodging resistance. Furthermore, the T5 treatment (150 mg/L glucose + 100 mg/L urea) improved lodging resistance. This study provides guidelines for enhancing lodging resistance by regulating internode characteristics via the co-application of carbon and nitrogen at the booting stage in fragrant rice.

1. Introduction

Rice is an important food crop worldwide, meeting a considerable portion of the population’s nutritional needs; approximately 60% of the global population relies on rice as a primary food source [1]. Fragrant rice is a representative high-quality variety that is favored by consumers for its strong aroma during cooking and consumption [2]. However, the production of fragrant rice has disadvantages due to its low yield and poor resistance to adverse conditions [3]. Lodging affects the normal growth of plants, which in turn impacts grain filling and seed-setting rates, affecting both yield and quality [4,5]. Lodging is a critical factor limiting rice yield improvement [6]. In recent years, with the increasing frequency of disastrous weather events, the occurrence of rice lodging has significantly increased, becoming a major challenge for achieving high-yield and high-quality rice and efficient rice production. South China is prone to extreme rainfall and typhoons during the rice growing season; these events have a significant negative impact on rice production resulting from lodging [7]. Extreme rainfall events can also cause field flooding, which can significantly impact on rice stalks and reduce resistance to rice lodging [8]. Lodging can result in restricted rice growth and reduced yields. Improving the resistance of rice to lodging is thus important for food security.

To reduce the impact of lodging on rice, it is important to select rice varieties that are resistant to lodging [9]. Measures used to reduce rice lodging include nutrient and water management, reducing nitrogen fertilizer application, and applying plant growth regulators [10]. Properly coordinating and managing fertilizers can improve rice yield and lodging resistance [11]. Lu et al. [12] reported that nitrogen application mainly affects rice stalk length and the internode cellulose content, which are important for rice regarding resistance to stunting. A previous study reported the use of liquid fertilizers for basal fertilization to control lodging in fragrant rice [13]. A suitable levels of nitrogen, as one of the basic nutrients in rice, and suitable nitrogen levels are important for rice regarding resistance to lodging. In addition, rice stubble resistance also requires attention to rice stem strength.

A previous study has shown that the bending strength of crop straw is influenced by substances such as cellulose [14]. Changes in the chemical composition of rice stalks are related to rice genetics and environmental conditions [15]. Different planting methods are important for improving the efficiency of substance transportation in rice stalks and ensuring the strength of stalks [16]. Rice lodging is closely related to the physical strength of stalks [17]. Stem lodging, the main type of rice lodging, directly affects the morphology and chemical composition of stalks [18]. Various measures are usually taken to reduce rice lodging, including regulating the stem morphological characteristics, stem material accumulation, stem physicochemical properties, and plant metabolic physiology [16,19]. The stem sheath is the primary organ for storing nonstructural carbohydrates (NSCs) during the heading stage, and appropriate nitrogen application can help increase NSC accumulation in the stem sheath and improve its proportion, which can thicken the stem wall and increase elasticity, enhanceing lodging resistance [20]. On the other hand, spraying plant growth regulators such as paclobutrazol can effectively improve the resistance of rice to lodging [21]. It has been reported that plant growth regulators are closely related to carbon regulation in rice [22]. Shimono et al. [23] reported that atmospheric carbon is beneficial for mitigating rice lodging. Research on carbon and nitrogen balance is important for improving the resistance of rice to lodging.

Nitrogen fertilization is good for yield, but excessive fertilization may lead to an increase in rice height and promote the risk of lodging, so it is critical to explore what could be considered an appropriate amount of fertilizer [24]. The excessive application of nitrogen fertilizer leads to an increase in the lodging index of rice plants [25]. Nitrogen management directly affects crop nitrogen uptake, and attention should be given to the balance between yield and resistance to lodging caused by greater plant height [26]. In the above studies, carbon and nitrogen metabolism usually affected rice yield and quality after different experimental treatments where varying amounts of carbon and nitrogen fertilizers were applied. The application of carbon or nitrogen alone to observe the consequent changes in rice yield, quality, and resistance to lodging has been widely reported in previous studies. One previous study reported that carbon and nitrogen interactions can drive improvements in yield improvements and fragrance in fragrant rice [27]. However, there are few reports on the impacts of the co-application of carbon-nitrogen on fragrant rice lodging. Therefore, in this study, different levels of exogenous glucose and urea were used to study the effects of carbon–nitrogen regulation on the lodging and internode characteristics in fragrant rice and to investigate the relationship between lodging and internode characteristics, which is important for future fragrant rice development as it enhances lodging resistance.

2. Materials and Methods

2.1. Experiment Descriptions

This experiment was conducted at the teaching experimental farm of South China Agricultural University from 2021 to 2022. The experiment was conducted on sandy loam soil. Huang et al.’s method of determining soil physicochemical properties was employed [28]. Before transplantation in 2021, the main physicochemical parameters of the experimental field soil were as follows: organic matter, 22.41 g/kg; total nitrogen, 1.82 g/kg; total phosphorus, 0.84 g/kg; and total potassium, 10.32 g/kg, pH 6.2. Before transplantation in 2022, the main physicochemical parameters of the soil were as follows: organic matter, 28.37 g/kg; total nitrogen, 1.79 g/kg; total phosphorus, 0.83 g/kg; and total potassium, 10.06 g/kg, pH 6.61. The monthly average temperature, precipitation, sunlight hours, and relative humidity during the growth period of fragrant rice are shown in

Table 1. Monthly average temperature, precipitation, sunshine hours, and relative humidity during growth period of fragrant rice from 2021 to 2022.

Year	Mouth	Average Air Temperature (°C)	Maximum Temperature (°C)	Minimum Temperature (°C)	Precipitation (mm)	Sunshine Hours (h)	Relative Humidity (%)
2021	July	30.3	38.4	24.4	224.4	212.0	76.1
	August	29.0	36.0	23.6	304.7	154.7	83.1
	September	30.1	36.3	25.0	37.1	208.0	76.3
	October	24.5	35.5	15.0	90.8	145.0	75.2
	November	20.3	31.2	11.6	14.6	168.7	63.9
2022	July	30.5	38.3	24.7	307.7	206.3	75.6
	August	28.8	37.1	23.9	269.1	181.2	81.1
	September	29.5	37.2	24.2	70.0	238.5	66.7
	October	25.8	35.8	18.1	0.6	253.8	57.0
	November	22.4	31.3	15.0	125.1	72.3	84.8

(data from the Guangzhou Statistical Bureau).

2.2. Experimental Design

The rice varieties used in this study were Meixiangzhan 2 and Xiangyaxiangzhan. Both varieties are extensively cultivated in South China. The field trials adopted a randomized block design with 9 carbon–nitrogen co-application treatments (CK: 0 mg/L glucose + 0 mg/L urea; T1: 0 mg/L glucose + 50 mg/L urea; T2: 0 mg/L glucose + 100 mg/L urea; T3: 150 mg/L glucose + 0 mg/L urea; T4: 150 mg/L glucose + 50 mg/L urea; T5: 150 mg/L glucose + 100 mg/L urea; T6: 300 mg/L glucose + 0 mg/L urea; T7: 300 mg/L glucose + 50 mg/L urea; and T8: 300 mg/L glucose + 100 mg/L urea). We chose the above glucose and urea concentrations based on the previous literature and reports [29]. We dissolved a mixture of urea and glucose in water and sprayed this mixture on the leaves at a concentration of 50 ml/m². Spraying was performed at the booting stage, ensuring that the leaf surface was thoroughly wetted, and the process was conducted after 4:00 PM [27]. The experimental plot had an area of 25 m². Rice was planted at a density of 33 cm × 14 cm, with 540 plants per plot. The rice was sprayed once at the booting stage, and spray treatments were applied to the rice plant shoot. Three replicate plots were set up for each treatment. Four replicates were taken from each plot for subsequent indicator measurements. The sowing date for the main trial in 2021 was 13 July, and mechanical transplanting occurred on 1 August. The sowing date for the main trial in 2022 was 14 July, and transplanting occurred on 2 August, with timely checks to ensure that there were enough seedlings in the water field. During the growth period of fragrant rice, a water management strategy involving alternating dry and wet conditions was adopted, maintaining a water layer of approximately 5 cm during the tillering stage and a shallow water layer during the heading stage, with field drying occurring one week before the rice ripening stage. Other management practices consistent with local rice cultivation measures were applied in the field were consistent with local rice cultivation measures, with timely prevention and control of pests and weeds.

2.3. Sampling and Measurements

2.3.1. Determination of the Lodging Index

The lodging characteristics of the fragrant rice were measured following the methods of Hong et al. [13]. Ten days before harvest, eight representative rice plants were randomly selected from each treatment. The strength of the rice stem was measured at a height of 20 cm above the ground (H) using a plant stem strength meter (YYD-1A, Zhejiang Top). Measurements were accurately performed at a 20 cm height of the rice from the ground using a ruler. The rice plants were pushed until the angle between the stem and the ground was 45°, and the peak bending resistance of single plants was recorded in Newtons (N). Moreover, the fresh weight, plant height, and total number of tillers per hill were measured to calculate the lodging index of fragrant rice.

Lodging index = (Plant height × fresh weight × 9.8)/Pushing resistance force × 100

2.3.2. Measurement of Internode Morphology and Dry Weights

At the maturity stage, samples were taken from each plot, and six uniformly growing rice plants were selected per sample. Three representative single stems were collected from each plant to measure the length, dry weight, outer diameter, and stem thickness of the rice stem nodes. The nodes were defined by the stem segment connecting the rice panicle base and upper segment, where scissors were used to cut each segment from top to bottom. An internode is defined as a section of stem between two stem nodes after plucking the leaf sheath. Each segment was recorded as the first, second, third, fourth, or fifth segment. A scale was used to measure the length of each segment in centimeters (cm). An electronic caliper was used to measure, in millimeters (mm), the outer and inner diameters at the midpoint of each segment, termed the stem thickness and inner wall thickness. Each segment was placed in an envelope, marked, stored in a drying oven at 105 °C for 30 to kill it, and then oven-dried at 80 °C until a constant weight was achieved (WGL-230D, Tianjin TAISITE, Tianjin, China). The dry weight of each segment was measured and recorded in g.

2.4. Statistical Analysis Methods

The experimental data were processed using Microsoft Excel 2013. For data analysis, Statistix 8.0 (Tallahassee, FL, USA) was used for multiple comparisons, and the average values of different treatments were compared using the least significant difference (LSD) method (p < 0.05 for significance). The data were analyzed via ANOVA after an equal variance test using the Levene test. The R software package (version: 4.2.1) was used to analyze correlations, and structural equation modeling and random forests were used to investigate the associations between lodging and internode development in fragrant rice.

3. Results

3.1. The Plant Height, Pushing Resistance Force, and Lodging Index

Xiangyaxiangzhan exhibited a higher plant height and a greater lodging index but lower pushing resistance force compared to Meixiangzhan 2. Variations in meteorological data during the rice growth periods of 2021 and 2022 contributed to differences in plant height, pushing resistance force, and lodging index among fragrant rice varieties across different years (Table 1 and

Table 2. Effects of co-application of glucose and urea at booting stage on plant height, pushing resistance force, and lodging index of fragrant rice.

Year	Cultivar	Treatment	Plant Height (cm)	Pushing Resistance Force (N)	Lodging Index
2021	Meixiangzhan 2	CK	114.00 ± 1.72 c	0.36 ± 0.05 e	46.16 ± 3.87 b
		T1	119.18 ± 1.07 a	0.61 ± 0.06 bcd	38.34 ± 4.46 b
		T2	118.23 ± 0.53 a	0.42 ± 0.04 de	55.20 ± 7.61 ab
		T3	117.88 ± 0.87 ab	0.43 ± 0.02 cde	73.88 ± 17.27 a
		T4	114.33 ± 1.01 bc	0.66 ± 0.12 ab	54.31 ± 2.4 ab
		T5	113.38 ± 1.17 c	0.82 ± 0.08 a	35.29 ± 3.22 b
		T6	118.50 ± 0.79 a	0.49 ± 0.05 bcde	71.79 ± 6.37 a
		T7	112.75 ± 1.09 c	0.61 ± 0.02 bc	42.03 ± 1.48 b
		T8	111.58 ± 2.18 c	0.46 ± 0.09 cde	54.37 ± 13.98 ab
	Xiangyaxiangzhan	CK	122.98 ± 1.7 abcd	0.40 ± 0.03 bc	54.36 ± 14.30 c
		T1	125.73 ± 2.27 ab	0.38 ± 0.06 bc	95.81 ± 2.52 ab
		T2	127.40 ± 1.71 a	0.55 ± 0.06 ab	71.13 ± 8.01 bc
		T3	125.48 ± 1.45 ab	0.28 ± 0.03 c	129.44 ± 15.98 a
		T4	122.80 ± 0.86 bcd	0.47 ± 0.07 bc	47.72 ± 8.40 c
		T5	119.40 ± 1.49 cd	0.40 ± 0.01 bc	79.78 ± 9.47 bc
		T6	125.03 ± 1.73 ab	0.34 ± 0.06 bc	118.43 ± 26.90 a
		T7	118.88 ± 0.59 d	0.71 ± 0.19 a	45.23 ± 3.69 c
		T8	123.65 ± 1.42 abc	0.41 ± 0.02 bc	49.17 ± 2.89 c
2022	Meixiangzhan 2	CK	105.25 ± 2.43 bc	0.31 ± 0.02 ab	70.42 ± 4.73 e
		T1	104.75 ± 1.25 c	0.17 ± 0.01 d	159.48 ± 10.35 a
		T2	109.00 ± 1.73 abc	0.21 ± 0.01 cd	131.44 ± 7.65 b
		T3	108.25 ± 2.56 abc	0.25 ± 0.03 bc	91.43 ± 12.38 cde
		T4	109.00 ± 0.41 abc	0.25 ± 0.02 bc	80.29 ± 7.47 de
		T5	108.75 ± 2.50 abc	0.25 ± 0.02 bc	91.25 ± 6.14 cde
		T6	111.00 ± 1.08 ab	0.25 ± 0.02 bc	97.43 ± 7.14 cd
		T7	107.25 ± 1.89 abc	0.33 ± 0.04 a	69.20 ± 9.26 e
		T8	112.25 ± 3.09 a	0.19 ± 0.02 cd	111.98 ± 10.55 bc
	Xiangyaxiangzhan	CK	111.00 ± 0.71 abc	0.18 ± 0.01 b	135.12 ± 8.00 abc
		T1	108.00 ± 0.71 bcd	0.19 ± 0.01 b	137.75 ± 7.47 ab
		T2	109.50 ± 0.87 bcd	0.23 ± 0.04 ab	106.41 ± 15.99 bc
		T3	109.75 ± 2.50 bc	0.22 ± 0.02 ab	111.69 ± 6.32 bc
		T4	107.00 ± 0.91 cd	0.20 ± 0.01 b	111.07 ± 6.75 bc
		T5	111.25 ± 1.03 abc	0.22 ± 0.02 ab	104.29 ± 8.71 c
		T6	112.00 ± 0.91 ab	0.18 ± 0.01 b	147.72 ± 15.36 a
		T7	105.25 ± 2.69 d	0.27 ± 0.03 a	104.11 ± 12.05 c
		T8	114.75 ± 1.49 a	0.25 ± 0.03 ab	121.36 ± 15.09 abc

CK: 0 mg/L glucose + 0 mg/L urea; T1: 0 mg/L glucose + 50 mg/L urea; T2: 0 mg/L glucose + 100 mg/L urea; T3: 150 mg/L glucose + 0 mg/L urea; T4: 150 mg/L glucose + 50 mg/L urea; T5: 150 mg/L glucose + 100 mg/L urea; T6: 300 mg/L glucose + 0 mg/L urea; T7: 300 mg/L glucose + 50 mg/L urea; T8: 300 mg/L glucose + 100 mg/L urea. Different lowercase letters denote significant differences according to LSD test (p < 0.05).

). In 2021, only the T3 and T6 treatments for both varieties, and the T2 treatment for Xiangyaxiangzhan, showed significantly higher lodging indices compared to the CK. In 2021, for Meixiangzhan 2, the plant height was the highest under treatment T1 compared to the CK treatment; the single-plant bending resistance achieved with the various fertilization treatments was ranked as T5 > T4 > T7. Rurthermore, this treatment resulted in a lodging index lower than those obtained with other fertilization treatments. The T5 treatment significantly increased the single-plant bending resistance by 127.78% compared to the CK, while the lodging index decreased significantly by 23.55%. For Xiangyaxiangzhan, the plant height was the greatest under treatment T2, and the single-plant bending resistance under treatment T7 was significantly greater than that under CK, with an increase of 77.50% and the lowest lodging index. In 2022, for Meixiangzhan 2, the greatest plant height was observed under treatment T8 compared to the CK, and the T7 treatment significantly increased the single-plant bending resistance by 6.45%, with the lowest lodging index. For Xiangyaxiangzhan, the plant height was the greatest under treatment T8, while the single-plant bending resistance increased by 22.22% and 50.00% under the T5 and T7 treatments, respectively, with corresponding decreases of 22.82% and 22.95% in the lodging index. Treatments T5 and T7 significantly increased the single-plant bending resistance while reducing the lodging index compared to the other fertilization treatments (Table 2). These results indicate that carbon and nitrogen treatments can increase the plant height and improve the pushing resistance force, helping to keep the lodging index at a low level. Overall, the T5 treatment was the most effective in improving rice resistance to lodging.

3.2. The Stem Internode Length

The co-application of glucose and urea treatments resulted in various changes in the internode length. The level of significant difference in the results was evaluated with the LSD test (p < 0.05), and Meixiangzhan 2 exhibited shorter internodes at the base compared to Xiangyaxiangzhan. In 2021, for Meixiangzhan 2, there were no significant differences in the length of the first internode among all the treatments. The lengths of the second, third, and fourth internodes were the greatest under the T4 and T7 treatments, whereas the fifth internode length was the greatest under T2. In Xiangyaxiangzhan, the lengths of the first and second internodes were reduced compared to those under CK, but there was no significant difference for the internode length under the T7 treatment compared to the CK. The lengths of the third and fourth internodes were significantly higher under the T4, T5, and T7 treatments compared to the CK, while the fifth internode’s length significantly increased under treatment T6. In 2022, for Meixiangzhan 2, there were no significant differences in the lengths of the first and second internodes compared to those under CK. The third internode length was the lowest under treatment T2, while the fourth and fifth internode lengths were the lowest under treatment T4 and the highest under treatment T8. For Xiangyaxiangzhan, the lengths of the first internode under the T3 and T5 treatments significantly increased compared to those under CK. The second internode length was the greatest under treatment T3, and the lengths of the third, fourth, and fifth internodes were significantly greater under treatments T3 and T4 compared to the CK. The variations in internode length at the base are linked to differences in lodging resistance among the experimental varieties (

Table 3. Effects of co-application of glucose and urea at booting stage on internode length in fragrant rice.

Year	Cultivar	Treatment	First Internode (cm)	Second Internode (cm)	Third Internode(cm)	Fourth Internode (cm)	Fifth Internode (cm)
2021	Meixiangzhan 2	CK	32.88 ± 2.48 a	20.43 ± 1.05 ab	14.45 ± 1.82 b	4.7 ± 0.54 ef	1.18 ± 0.13 c
		T1	35.63 ± 1.65 a	18.33 ± 0.6 bc	14.80 ± 0.53 b	6.93 ± 0.25 cd	1.5 ± 0.31 bc
		T2	36.50 ± 0.79 a	19.63 ± 0.64 abc	13.60 ± 0.83 b	10.75 ± 0.54 b	4.25 ± 0.74 a
		T3	35.65 ± 1.48 a	17.45 ± 0.85 cd	12.13 ± 0.82 bc	7.48 ± 0.82 c	1.28 ± 0.15 bc
		T4	37.15 ± 1.14 a	21.93 ± 1.43 a	14.93 ± 0.87 ab	13.20 ± 0.70 a	1.73 ± 0.31 bc
		T5	34.78 ± 2.06 a	14.83 ± 0.69 e	12.98 ± 0.81 bc	6.48 ± 0.35 cde	2.33 ± 0.4 b
		T6	35.23 ± 1.99 a	15.70 ± 0.68 de	10.63 ± 0.53 c	5.43 ± 0.43 def	1.45 ± 0.21 bc
		T7	36.53 ± 1.42 a	19.83 ± 0.71 abc	17.75 ± 1.21 a	9.40 ± 1.19 b	1.58 ± 0.23 bc
		T8	32.75 ± 1.04 a	15.63 ± 0.25 de	12.80 ± 0.88 bc	4.53 ± 0.41 f	1.7 ± 0.38 bc
	Xiangyaxiangzhan	CK	36.95 ± 0.06 a	20.75 ± 0.26 ab	14.95 ± 0.46 bcd	7.85 ± 0.54 de	1.48 ± 0.24 ab
		T1	35.10 ± 0.99 ab	17.55 ± 0.51 de	14.43 ± 1.49 cd	11.93 ± 0.77 b	0.68 ± 0.11 c
		T2	33.48 ± 1.35 bc	16.58 ± 0.84 e	13.85 ± 1.79 d	7.22 ± 0.87 de	1.3 ± 0.32 abc
		T3	35.70 ± 0.94 ab	21.40 ± 0.64 a	16.63 ± 0.72 abc	5.88 ± 0.89 e	1.05 ± 0.34 bc
		T4	33.48 ± 0.80 bc	20.98 ± 0.37 ab	17.65 ± 0.8 ab	14.60 ± 0.59 a	1.38 ± 0.21 abc
		T5	33.10 ± 0.29 bc	18.60 ± 0.57 cd	17.28 ± 0.47 ab	12.80 ± 0.39 ab	1.63 ± 0.31 ab
		T6	34.13 ± 1.25 b	17.40 ± 0.72 de	17.05 ± 0.48 abc	8.68 ± 0.32 cd	1.95 ± 0.45 a
		T7	34.48 ± 1.55 ab	20.68 ± 0.52 ab	18.03 ± 0.81 a	10.83 ± 1.23 bc	1.03 ± 0.15 bc
		T8	30.93 ± 0.26 c	19.68 ± 0.41 bc	15.63 ± 0.18 abcd	11.38 ± 0.77 b	0.93 ± 0.09 bc
2022	Meixiangzhan 2	CK	33.73 ± 1.20 ab	15.63 ± 0.44 ab	10.43 ± 1.16 d	5.58 ± 0.44 c	2.08 ± 0.19 b
		T1	32.85 ± 0.46 b	17.25 ± 1.05 a	11.78 ± 0.6 abcd	7.00 ± 0.56 b	2.1 ± 0.54 b
		T2	35.63 ± 0.42 a	16.60 ± 0.98 ab	9.60 ± 0.19 d	4.83 ± 0.19 cd	1.25 ± 0.15 cd
		T3	34.55 ± 0.12 ab	15.33 ± 0.60 abc	13.35 ± 0.94 ab	4.55 ± 0.32 d	1.76 ± 0.23 bc
		T4	34.90 ± 0.98 ab	14.50 ± 0.67 bcd	10.85 ± 0.62 cd	3.83 ± 0.32 d	0.95 ± 0.05 d
		T5	34.90 ± 0.51 ab	13.30 ± 1.05 cd	10.83 ± 0.52 cd	5.78 ± 0.41 c	2.38 ± 0.38 b
		T6	34.35 ± 0.57 ab	17.38 ± 0.71 a	11.18 ± 0.82 bcd	4.08 ± 0.21 d	1.85 ± 0.26 bc
		T7	35.18 ± 1.03 a	15.80 ± 0.65 ab	12.85 ± 0.46 abc	7.00 ± 0.25 b	2.25 ± 0.19 b
		T8	34.53 ± 0.54 ab	12.70 ± 0.61 d	14.05 ± 1.39 a	8.02 ± 0.28 a	3.43 ± 0.17 a
	Xiangyaxiangzhan	CK	30.20 ± 0.80 d	17.63 ± 0.15 bc	14.90 ± 0.29 cd	7.33 ± 0.17 e	2.75 ± 0.23 bcd
		T1	31.08 ± 0.69 cd	18.78 ± 0.44 a	15.15 ± 0.29 bc	10.18 ± 0.39 b	2.38 ± 0.19 d
		T2	29.75 ± 0.13 d	17.43 ± 0.15 c	15.45 ± 0.38 bc	9.28 ± 0.15 bc	2.43 ± 0.34 d
		T3	33.88 ± 0.91 a	18.60 ± 0.37 ab	16.48 ± 0.28 a	8.03 ± 0.51 de	3.55 ± 0.61 ab
		T4	32.80 ± 0.29 abc	17.75 ± 0.57 abc	15.95 ± 0.49 ab	11.40 ± 0.14 a	4.03 ± 0.37 a
		T5	34.30 ± 1.11 a	16.93 ± 0.33 cd	13.70 ± 0.10 ef	6.18 ± 0.24 f	2.83 ± 0.41 bcd
		T6	33.50 ± 0.61 ab	17.33 ± 0.13 c	14.18 ± 0.51 de	9.73 ± 0.19 b	2.55 ± 0.3 cd
		T7	31.73 ± 0.43 bcd	15.00 ± 0.54 e	13.05 ± 0.15 f	8.03 ± 0.34 de	3.4 ± 0.07 abc
		T8	31.13 ± 0.89 cd	16.25 ± 0.10 d	14.20 ± 0.08 de	8.4 ± 0.47 cd	2 ± 0.19 d

CK: 0 mg/L glucose + 0 mg/L urea; T1: 0 mg/L glucose + 50 mg/L urea; T2: 0 mg/L glucose + 100 mg/L urea; T3: 150 mg/L glucose + 0 mg/L urea; T4: 150 mg/L glucose + 50 mg/L urea; T5: 150 mg/L glucose + 100 mg/L urea; T6: 300 mg/L glucose + 0 mg/L urea; T7: 300 mg/L glucose + 50 mg/L urea; T8: 300 mg/L glucose + 100 mg/L urea. Different lowercase letters denote significant differences according to LSD test (p < 0.05).

).

3.3. The Diameter of Stem Internodes

The simultaneous application of glucose and urea led to consistently high stem internode diameters compared to the CK treatment. The base diameter of the stem internodes in Meixiangzhan 2 was smaller than that in Xiangyaxiangzhan. The level of significant difference in the results was evaluated using the LSD test (p < 0.05). In 2021, for Meixiangzhan 2, there were no significant differences in the diameters of the first, second, third, or fifth internodes among all the treatments. Except for T5, the diameter of the fourth internode in all treatments significantly increased in all treatments compared to the CK. For Xiangyaxiangzhan, there were no significant differences in the diameters of the first, second, third, or fifth internodes among the treatments; the diameter of the fourth internode under the T3 treatment decreased slightly compared to that under CK, but the difference was not significant. In 2022, for Meixiangzhan 2, the diameters of the first and second internodes under treatment T7 significantly increased compared to those under CK, while there were no differences in the diameters of the fifth internode among all the treatments compared to the CK. For Xiangyaxiangzhan, the diameters of all the internodes (first, second, third, fourth, and fifth) were significantly greater than those in the CK, with the greatest diameter observed under treatments T5 and T8 (

Table 4. Effects of co-application of glucose and urea at booting stage on culm diameter of internode in fragrant rice.

Year	Cultivar	Treatment	First Internode (mm)	Second Internode (mm)	Third Internode (mm)	Fourth Internode (mm)	Fifth Internode (mm)
2021	Meixiangzhan 2	CK	2.49 ± 0.19 a	3.57 ± 0.47 a	3.91 ± 0.33 ab	4.22 ± 0.18 b	5.34 ± 0.3 a
		T1	2.50 ± 0.18 a	3.35 ± 0.25 a	4.29 ± 0.25 a	4.92 ± 0.21 a	4.98 ± 0.18 a
		T2	2.21 ± 0.17 a	2.96 ± 0.26 a	3.58 ± 0.14 ab	4.65 ± 0.1 ab	4.69 ± 0.15 a
		T3	2.58 ± 0.2 a	3.01 ± 0.37 a	4.10 ± 0.32 a	4.68 ± 0.19 ab	5.34 ± 0.21 a
		T4	2.25 ± 0.21 a	3.21 ± 0.07 a	4.14 ± 0.29 a	4.50 ± 0.24 ab	5.00 ± 0.45 a
		T5	2.29 ± 0.14 a	2.86 ± 0.23 a	3.27 ± 0.18 b	4.32 ± 0.21 b	5.17 ± 0.17 a
		T6	2.56 ± 0.08 a	3.48 ± 0.04 a	4.35 ± 0.36 a	4.71 ± 0.12 ab	4.74 ± 0.33 a
		T7	2.40 ± 0.22 a	3.33 ± 0.24 a	3.78 ± 0.14 ab	4.51 ± 0.12 ab	4.67 ± 0.22 a
		T8	2.36 ± 0.17 a	3.51 ± 0.16 a	4.3 ± 0.29 a	4.87 ± 0.16 a	5.18 ± 0.18 a
	Xiangyaxiangzhan	CK	2.21 ± 0.11 a	3.13 ± 0.27 ab	3.82 ± 0.34 a	4.22 ± 0.37 bc	5 ± 0.21 a
		T1	2.42 ± 0.13 a	3.63 ± 0.08 ab	4.31 ± 0.43 a	4.59 ± 0.15 abc	5.32 ± 0.2 a
		T2	2.48 ± 0.17 a	3.67 ± 0.1 a	4.19 ± 0.28 a	4.9 ± 0.19 a	5.3 ± 0.22 a
		T3	2.38 ± 0.11 a	3.22 ± 0.17 ab	3.87 ± 0.35 a	4.06 ± 0.26 c	5.19 ± 0.21 a
		T4	2.57 ± 0.27 a	3.1 ± 0.11 ab	4.27 ± 0.17 a	4.74 ± 0.13 ab	5.47 ± 0.19 a
		T5	2.62 ± 0.21 a	3.05 ± 0.05 b	4.32 ± 0.24 a	4.73 ± 0.12 ab	5.55 ± 0.19 a
		T6	2.62 ± 0.16 a	3.4 ± 0.38 ab	3.65 ± 0.19 a	4.56 ± 0.27 abc	5.2 ± 0.07 a
		T7	2.62 ± 0.07 a	3.48 ± 0.19 ab	4.1 ± 0.16 a	4.68 ± 0.2 abc	5.16 ± 0.28 a
		T8	2.44 ± 0.2 a	3.26 ± 0.29 ab	4.12 ± 0.26 a	4.56 ± 0.17 abc	5.44 ± 0.17 a
2022	Meixiangzhan 2	CK	2.58 ± 0.13 abc	3.41 ± 0.17 ab	3.78 ± 0.12 bcd	4.61 ± 0.2 ab	5.26 ± 0.09 a
		T1	2.76 ± 0.08 a	3.55 ± 0.14 ab	4.56 ± 0.24 a	4.77 ± 0.14 a	5.25 ± 0.15 a
		T2	2.71 ± 0.06 ab	3.46 ± 0.21 ab	4.09 ± 0.2 abc	4.49 ± 0.17 ab	5.23 ± 0.15 a
		T3	2.55 ± 0.07 abc	3.67 ± 0.11 a	3.61 ± 0.13 cd	4.43 ± 0.22 ab	5.41 ± 0.19 a
		T4	2.6 ± 0.2 abc	3.1 ± 0.21 b	3.37 ± 0.32 d	4.3 ± 0.15 ab	5.48 ± 0.18 a
		T5	2.48 ± 0.11 abc	3.56 ± 0.14 ab	3.7 ± 0.12 cd	4.29 ± 0.12 b	5.28 ± 0.1 a
		T6	2.41 ± 0.07 bc	3.85 ± 0.12 a	3.91 ± 0.08 bcd	4.3 ± 0.2 ab	5.14 ± 0.44 a
		T7	2.77 ± 0.05 a	3.85 ± 0.23 a	4.38 ± 0.26 ab	4.49 ± 0.13 ab	5.56 ± 0.18 a
		T8	2.38 ± 0.06 c	3.5 ± 0.14 ab	3.97 ± 0.28 abcd	4.58 ± 0.15 ab	5.19 ± 0.36 a
	Xiangyaxiangzhan	CK	2.23 ± 0.07 b	2.23 ± 0.07 c	3.51 ± 0.14 bc	4.38 ± 0.12 c	4.65 ± 0.25 b
		T1	2.62 ± 0.19 a	3.33 ± 0.23 ab	4.1 ± 0.29 ab	4.55 ± 0.19 bc	5.27 ± 0.09 a
		T2	2.41 ± 0.11 ab	3.52 ± 0.17 a	4.27 ± 0.12 a	4.57 ± 0.28 bc	5.24 ± 0.27 ab
		T3	2.27 ± 0.09 ab	3.32 ± 0.17 ab	3.16 ± 0.29 c	4.57 ± 0.11 bc	5.12 ± 0.27 ab
		T4	2.51 ± 0.13 ab	3.54 ± 0.1 a	3.87 ± 0.23 ab	4.50 ± 0.21 bc	5.19 ± 0.24 ab
		T5	2.41 ± 0.21 ab	3.71 ± 0.2 a	4.35 ± 0.24 a	5.02 ± 0.32 ab	5.53 ± 0.17 a
		T6	2.44 ± 0.07 ab	3.02 ± 0.22 b	4.01 ± 0.11 ab	4.58 ± 0.18 bc	5.23 ± 0.24 ab
		T7	2.53 ± 0.13 ab	3.01 ± 0.13 b	3.87 ± 0.11 ab	4.23 ± 0.1 c	5.29 ± 0.11 a
		T8	2.52 ± 0.14 ab	3.53 ± 0.08 a	4.36 ± 0.21 a	5.55 ± 0.29 a	5.54 ± 0.17 a

CK: 0 mg/L glucose + 0 mg/L urea; T1: 0 mg/L glucose + 50 mg/L urea; T2: 0 mg/L glucose + 100 mg/L urea; T3: 150 mg/L glucose + 0 mg/L urea; T4: 150 mg/L glucose + 50 mg/L urea; T5: 150 mg/L glucose + 100 mg/L urea; T6: 300 mg/L glucose + 0 mg/L urea; T7: 300 mg/L glucose + 50 mg/L urea; T8: 300 mg/L glucose + 100 mg/L urea. Different lowercase letters denote significant differences according to LSD test (p < 0.05).

).

3.4. The Wall Thickness of Stem Internodes

The co-application of glucose and urea resulted in stem internodes having a consistently high wall thicknesses compared to that under CK treatment. In 2021, for Meixiangzhan 2, the inner wall thickness of the first internode significantly increased under all the treatments compared with that under the CK, with the thickness being the greatest under treatment T4. There were no significant differences in the thickness of the second internode among the treatments; the inner wall thickness of the third internode significantly increased compared with that in the CK treatment, with the greatest value observed under treatment T1. The thickness of the fourth internode in the T5 treatment significantly increased compared to that under CK, and the fifth internode’s thickness also significantly increased in all the treatments except for T6. For Xiangyaxiangzhan, the inner wall thickness of the first internode significantly increased under all the treatments compared to CK. There were no significant differences in thickness between treatments for the second and third internodes; the thickness was the lowest under treatments T3 and T4, respectively. Compared with that in the CK treatment, both values were greater in the T5 treatment and the inner wall thickness of the fourth internode significantly increased, whereas the thickness of the fifth internode also significantly increased compared with that in the CK treatment across all treatments. In 2022, for Meixiangzhan 2, the inner wall thickness of all the internodes significantly increased compared to that under CK. The greatest thicknesses of the first and second internodes were observed under treatments T7 and T1, respectively. The thicknesses of the third and fourth internodes were the greatest under treatment T6, whereas the thickness of the fifth internode was the greatest under treatment T7. For Xiangyaxiangzhan, no significant differences were found between the T8 treatment and the CK; however, the inner wall thickness of the first internode significantly increased in all the other treatments compared with CK. The thickness of the second internode significantly increased across all treatments compared to the CK, while no significant differences were found among the treatments excluding the CK. The thickness of the third internode decreased slightly under treatment T5 but without significant differences compared to the CK. The highest thickness of the third internode was observed under treatment T7, and the fourth internode’s thickness was also the highest under treatment T7 (

Table 5. The effects of the co-application of glucose and urea at the booting stage on the wall thickness of the internode in fragrant rice.

Year	Cultivar	Treatment	First Internode (mm)	Second Internode (mm)	Third Internode (mm)	Fourth Internode (mm)	Fifth Internode (mm)
2021	Meixiangzhan 2	CK	0.59 ± 0.02 c	0.51 ± 0.04 b	0.32 ± 0.02 d	0.31 ± 0.05 a	0.21 ± 0.04 c
		T1	0.7 ± 0.03 abc	0.57 ± 0.04 ab	0.52 ± 0.01 a	0.41 ± 0.05 a	0.27 ± 0.02 ab
		T2	0.69 ± 0.06 abc	0.48 ± 0.02 b	0.35 ± 0.02 d	0.32 ± 0.06 a	0.25 ± 0.02 abc
		T3	0.79 ± 0.02 a	0.57 ± 0.02 ab	0.51 ± 0.03 ab	0.36 ± 0.02 a	0.26 ± 0.01 abc
		T4	0.77 ± 0.08 a	0.58 ± 0.01 ab	0.46 ± 0.01 abc	0.36 ± 0.02 a	0.3 ± 0.01 a
		T5	0.7 ± 0.04 abc	0.65 ± 0.06 a	0.45 ± 0.01 bc	0.35 ± 0.02 a	0.25 ± 0.02 abc
		T6	0.62 ± 0.01 bc	0.51 ± 0.06 b	0.49 ± 0.04 abc	0.41 ± 0.02 a	0.21 ± 0.01 c
		T7	0.7 ± 0.04 abc	0.56 ± 0.03 ab	0.45 ± 0 bc	0.37 ± 0.01 a	0.23 ± 0.01 bc
		T8	0.73 ± 0.01 ab	0.58 ± 0.02 ab	0.45 ± 0.02 c	0.34 ± 0.06 a	0.26 ± 0.02 abc
	Xiangyaxiangzhan	CK	0.66 ± 0.05 b	0.51 ± 0.04 b	0.45 ± 0.01 ab	0.35 ± 0.02 ab	0.16 ± 0.05 b
		T1	0.71 ± 0.04 ab	0.54 ± 0.02 ab	0.46 ± 0.01 ab	0.27 ± 0.02 b	0.26 ± 0.02 a
		T2	0.75 ± 0.02 ab	0.61 ± 0.02 a	0.46 ± 0.02 a	0.38 ± 0.02 a	0.22 ± 0.01 a
		T3	0.68 ± 0.04 ab	0.51 ± 0.02 b	0.45 ± 0.01 ab	0.17 ± 0.08 c	0.26 ± 0.02 a
		T4	0.72 ± 0.03 ab	0.54 ± 0.01 ab	0.4 ± 0.02 c	0.37 ± 0.03 a	0.26 ± 0.01 a
		T5	0.67 ± 0.04 b	0.51 ± 0.02 b	0.47 ± 0.02 a	0.38 ± 0.02 a	0.26 ± 0.01 a
		T6	0.77 ± 0.05 a	0.57 ± 0.03 ab	0.42 ± 0.01 bc	0.41 ± 0.01 a	0.25 ± 0.01 a
		T7	0.67 ± 0.02 b	0.54 ± 0.05 ab	0.42 ± 0.02 bc	0.34 ± 0.01 ab	0.27 ± 0.01 a
		T8	0.73 ± 0.01 ab	0.57 ± 0.01 ab	0.47 ± 0.01 a	0.38 ± 0.01 a	0.24 ± 0.02 a
2022	Meixiangzhan 2	CK	0.49 ± 0.03 a	0.45 ± 0.05 d	0.29 ± 0.04 d	0.20 ± 0.03 d	0.16 ± 0.03 d
		T1	0.77 ± 0.03 ab	0.7 ± 0.03 ab	0.62 ± 0.03 ab	0.47 ± 0.06 a	0.23 ± 0.03 abc
		T2	0.76 ± 0.05 ab	0.52 ± 0.05 cd	0.6 ± 0.06 ab	0.34 ± 0.08 abcd	0.28 ± 0.03 ab
		T3	0.73 ± 0.02 ab	0.57 ± 0.01 c	0.39 ± 0.03 c	0.23 ± 0.07 cd	0.19 ± 0.03 cd
		T4	0.79 ± 0.05 ab	0.72 ± 0.02 a	0.63 ± 0.02 ab	0.29 ± 0.03 bcd	0.28 ± 0.03 ab
		T5	0.79 ± 0.05 ab	0.77 ± 0.04 a	0.54 ± 0.02 b	0.28 ± 0.03 bcd	0.23 ± 0.02 abcd
		T6	0.83 ± 0.02 ab	0.73 ± 0.05 a	0.69 ± 0.02 a	0.41 ± 0.06 ab	0.21 ± 0.03 bcd
		T7	0.77 ± 0.02 b	0.6 ± 0.02 bc	0.39 ± 0.04 c	0.37 ± 0 abc	0.3 ± 0.01 a
		T8	0.82 ± 0.03 c	0.54 ± 0.02 cd	0.42 ± 0.03 c	0.47 ± 0.05 a	0.25 ± 0.02 abc
	Xiangyaxiangzhan	CK	0.7 ± 0.02 bcd	0.61 ± 0.08 ab	0.46 ± 0.05 bcd	0.34 ± 0.02 b	0.22 ± 0.03 c
		T1	0.68 ± 0.04 cd	0.52 ± 0.02 b	0.48 ± 0.06 abcd	0.41 ± 0.04 ab	0.31 ± 0.04 ab
		T2	0.6 ± 0.06 d	0.55 ± 0.05 b	0.55 ± 0.04 abc	0.37 ± 0.03 ab	0.29 ± 0.02 abc
		T3	0.79 ± 0.03 abc	0.7 ± 0.03 a	0.58 ± 0.04 ab	0.46 ± 0.02 a	0.26 ± 0.02 bc
		T4	0.66 ± 0.06 d	0.59 ± 0.03 ab	0.44 ± 0.04 cd	0.45 ± 0.02 a	0.35 ± 0.01 a
		T5	0.81 ± 0.03 ab	0.62 ± 0.03 ab	0.42 ± 0.07 d	0.42 ± 0.07 ab	0.24 ± 0.05 bc
		T6	0.62 ± 0.04 d	0.54 ± 0.05 b	0.46 ± 0.04 bcd	0.38 ± 0.01 ab	0.27 ± 0.04 abc
		T7	0.82 ± 0.03 ab	0.7 ± 0.06 a	0.59 ± 0.01 a	0.44 ± 0.01 a	0.28 ± 0.03 abc
		T8	0.83 ± 0.03 a	0.58 ± 0.03 ab	0.56 ± 0.03 abc	0.45 ± 0.02 a	0.22 ± 0 c

CK: 0 mg/L glucose + 0 mg/L urea; T1: 0 mg/L glucose + 50 mg/L urea; T2: 0 mg/L glucose + 100 mg/L urea; T3: 150 mg/L glucose + 0 mg/L urea; T4: 150 mg/L glucose + 50 mg/L urea; T5: 150 mg/L glucose + 100 mg/L urea; T6: 300 mg/L glucose + 0 mg/L urea; T7: 300 mg/L glucose + 50 mg/L urea; T8: 300 mg/L glucose + 100 mg/L urea. Different lowercase letters denote significant differences according to LSD test (p < 0.05).

).

3.5. The Dry Weights of the Stem Internodes

The dry weights of the various internodes of fragrant rice under different concentrations of carbon–nitrogen were statistically analyzed, as shown in

Table 6. The effects of the co-application of glucose and urea at the booting stage on the dry weight of the internodes in fragrant rice.

Year	Cultivar	Treatment	First Internode (g)	Second Internode (g)	Third Internode (g)	Fourth Internode (g)	Fifth Internode (g)
2021	Meixiangzhan 2	CK	0.71 ± 0.19 cd	0.73 ± 0.09 ab	0.66 ± 0.04 abc	0.52 ± 0.07 ab	0.18 ± 0.03 bc
		T1	0.67 ± 0.09 d	0.67 ± 0.08 abc	0.65 ± 0.04 abcd	0.40 ± 0.06 b	0.27 ± 0.05 abc
		T2	1.06 ± 0.04 a	0.83 ± 0.07 a	0.78 ± 0.12 ab	0.62 ± 0.07 a	0.27 ± 0.03 abc
		T3	0.78 ± 0.1 bcd	0.78 ± 0.08 ab	0.84 ± 0.13 a	0.60 ± 0.10 a	0.31 ± 0.06 ab
		T4	0.86 ± 0.04 abcd	0.80 ± 0.04 a	0.58 ± 0.02 bcd	0.35 ± 0.08 b	0.27 ± 0.05 abc
		T5	0.83 ± 0.02 abcd	0.60 ± 0.02 bc	0.45 ± 0.01 d	0.40 ± 0.01 b	0.2 ± 0.04 bc
		T6	0.97 ± 0.08 abc	0.79 ± 0.05 ab	0.74 ± 0.06 ab	0.38 ± 0.08 b	0.37 ± 0.08 a
		T7	1.02 ± 0.07 ab	0.83 ± 0.10 a	0.70 ± 0.06 ab	0.51 ± 0.04 ab	0.34 ± 0.01 a
		T8	0.63 ± 0.02 d	0.52 ± 0.04 c	0.48 ± 0.08 cd	0.34 ± 0.01 b	0.16 ± 0.02 c
	Xiangyaxiangzhan	CK	0.75 ± 0.02 d	0.64 ± 0.02 e	0.54 ± 0.01 e	0.41 ± 0.01 e	0.16 ± 0 bcd
		T1	0.88 ± 0.04 c	0.74 ± 0.05 cde	0.75 ± 0.03 ab	0.49 ± 0.04 ab	0.24 ± 0.02 b
		T2	0.95 ± 0.04 c	0.88 ± 0.03 ab	0.65 ± 0.01 cd	0.52 ± 0.02 cd	0.21 ± 0.02 bc
		T3	1.13 ± 0.02 a	0.94 ± 0.02 a	0.80 ± 0.03 a	0.61 ± 0.02 a	0.45 ± 0.05 a
		T4	0.91 ± 0.07 c	0.70 ± 0.04 de	0.58 ± 0.06 de	0.49 ± 0.00 de	0.15 ± 0.01 cd
		T5	1.01 ± 0.02 abc	0.76 ± 0.01 cd	0.73 ± 0.01 abc	0.38 ± 0.01 abc	0.12 ± 0.02 d
		T6	0.97 ± 0.05 bc	0.79 ± 0.04 bcd	0.62 ± 0.05 de	0.50 ± 0.02 de	0.23 ± 0.03 bc
		T7	0.95 ± 0.05 c	0.76 ± 0.02 cd	0.66 ± 0.03 bcd	0.40 ± 0.02 bcd	0.22 ± 0.02 bc
		T8	1.09 ± 0.05 ab	0.82 ± 0.07 bc	0.66 ± 0.03 bcd	0.43 ± 0.03 bcd	0.18 ± 0.03 bcd
2022	Meixiangzhan 2	CK	0.81 ± 0.04 c	0.66 ± 0.09 e	0.58 ± 0.11 b	0.38 ± 0.04 c	0.10 ± 0.03 d
		T1	1.13 ± 0.05 ab	0.67 ± 0.04 e	0.83 ± 0.05 a	0.48 ± 0.06 abc	0.18 ± 0.04 cd
		T2	1.22 ± 0.07 a	0.88 ± 0.04 bcd	0.88 ± 0.03 a	0.54 ± 0.11 abc	0.22 ± 0.03 bc
		T3	0.98 ± 0.13 bc	0.77 ± 0.11 de	0.78 ± 0.15 ab	0.47 ± 0.1 abc	0.27 ± 0.04 bc
		T4	1.25 ± 0.12 a	1.09 ± 0.03 a	0.77 ± 0.03 ab	0.51 ± 0.08 abc	0.26 ± 0.05 bc
		T5	1.02 ± 0.1 abc	0.99 ± 0.05 ab	0.94 ± 0.04 a	0.63 ± 0.08 ab	0.60 ± 0.05 a
		T6	0.87 ± 0.05 c	0.80 ± 0.05 cde	0.78 ± 0.09 ab	0.41 ± 0.02 bc	0.26 ± 0.05 bc
		T7	0.92 ± 0.03 bc	0.92 ± 0 bcd	0.89 ± 0.04 a	0.37 ± 0.03 c	0.19 ± 0.03 cd
		T8	1.14 ± 0.06 ab	0.95 ± 0.02 abc	0.95 ± 0.08 a	0.65 ± 0.12 a	0.32 ± 0.03 b
	Xiangyaxiangzhan	CK	0.91 ± 0.16 a	0.87 ± 0.08 abc	0.81 ± 0.05 a	0.59 ± 0.12 ab	0.31 ± 0.12 a
		T1	0.93 ± 0.04 a	0.89 ± 0.06 abc	0.87 ± 0.09 a	0.66 ± 0.05 ab	0.22 ± 0.04 a
		T2	0.93 ± 0.01 a	0.8 ± 0.04 bc	0.82 ± 0.18 a	0.65 ± 0.12 ab	0.21 ± 0.02 a
		T3	0.87 ± 0.02 a	0.74 ± 0.02 c	0.72 ± 0.06 a	0.66 ± 0.08 ab	0.33 ± 0.10 a
		T4	0.93 ± 0.02 a	0.90 ± 0.03 ab	0.88 ± 0.01 a	0.48 ± 0.08 b	0.40 ± 0.18 a
		T5	0.99 ± 0.09 a	0.98 ± 0.05 a	0.85 ± 0.05 a	0.74 ± 0.09 a	0.41 ± 0.06 a
		T6	0.99 ± 0.08 a	0.99 ± 0.09 a	0.81 ± 0.03 a	0.73 ± 0.07 a	0.29 ± 0.05 a
		T7	0.94 ± 0.01 a	0.8 ± 0.04 bc	0.84 ± 0.08 a	0.70 ± 0.09 ab	0.30 ± 0.05 a
		T8	0.91 ± 0.02 a	0.88 ± 0.02 abc	0.71 ± 0.04 a	0.48 ± 0.03 b	0.25 ± 0.02 a

CK: 0 mg/L glucose + 0 mg/L urea; T1: 0 mg/L glucose + 50 mg/L urea; T2: 0 mg/L glucose + 100 mg/L urea; T3: 150 mg/L glucose + 0 mg/L urea; T4: 150 mg/L glucose + 50 mg/L urea; T5: 150 mg/L glucose + 100 mg/L urea; T6: 300 mg/L glucose + 0 mg/L urea; T7: 300 mg/L glucose + 50 mg/L urea; T8: 300 mg/L glucose + 100 mg/L urea. Different lowercase letters denote significant differences according to LSD test (p < 0.05).

. The dry weight of the stem internodes remained high when treated with the co-application of glucose and urea compared to the CK treatment. In 2021, for Meixiangzhan 2, the dry weight of the first internode significantly increased compared to that under CK under all the treatments except T6 and T7, with the highest weight being observed under treatment T4. The dry weights of the second and third internodes significantly increased compared to those under CK, with the highest values observed under treatments T5 and T1, respectively. There were no significant differences in the dry weight of the fourth internode among the treatments. For Xiangyaxiangzhan, the first and second internodes’ dry weights significantly increased compared to those under CK in all the treatments except for T7, with the highest values being observed in treatments T6 and T2, although there were no significant differences compared with the values found in T4. The dry weights of the third and fourth internodes were the highest under treatments T5 and T8. In 2022, in Meixiangzhan 2, treatment T6 did not significantly increase the dry weight of the first internodes compared to that under CK. The dry weights of the first and second internodes significantly increased in the other fertilization treatments compared to the CK, with the highest weight being observed under treatment T4. The dry weights of the third internodes significantly increased compared to that under CK across all the treatments, and there were no significant differences among the fertilization treatments. There was no significant difference in the dry weight of the fourth internode between T7 and the CK; however, other treatments significantly increased the dry weight of the fourth internode compared to the CK. For Xiangyaxiangzhan, there were no significant differences in the dry weights of the first and third internodes among the treatments; however, the dry weights of the second and fourth internodes decreased under treatments T3 and T4 compared to those under the CK, with the highest dry weights of the second and fourth internodes being observed under treatment T5 (Table 6). In 2021, the T5 treatment resulted in the greatest increase in the stem dry weight of Meixiangzhan 2, and the T6 treatment achieved the greatest increased in the stem dry weight of Xiangyaxiangzhan. In 2022, the T4 treatment showed the greatest increase in stem dry weight for Meixiangzhan 2, and the T5 treatment led to the greatest increase in stem dry weight for Xiangyaxiangzhan.

3.6. Correlation Analysis

A correlation analysis revealed that the first internode correlation index was significantly correlated with the lodging index and the culm diameter of the fifth internode. The second internode index was significantly correlated with the length of the third internode, the culm diameter of the fifth internode, and the pushing resistance force. Interestingly, the fifth internode index was significantly correlated with the length of the first internode, first internode culm diameter, and the second internode length (Figure 1a).

We found a significant correlation between the internode length and the fourth-order internode culm diameter. In addition, the internode culm diameter was significantly correlated with the length of the first and second internodes and the thickness of the fifth internode. Internode thickness was significantly correlated with the third internode culm diameter and fifth internode dry weight. Interestingly, dry weight was significantly and positively correlated with the lodging index, whereas it was significantly and negatively correlated with the pushing resistance force (Figure 1b).

3.7. Structural Equation Modeling and Random Forest Analysis

Structural equation modeling indicated that internode length and dry weight significantly influenced plant height and the pushing resistance force. Compared with plant height, the pushing resistance force significantly influenced the lodging index. Different internode lengths and dry weights may have indirect effects on the lodging index by affecting the pushing resistance force (Figure 2).

A random forest plot revealed that the pushing resistance force (PRF) and third internode dry weight (I3DW) had highly significant effects on the fall index of rice. The dry weight of the first and fourth internodes (I1D2 and I4DW) and the length of the second internode (I2L) had a significantly affected the rice lodging index (Figure 3).

4. Discussion

Lodging is a common problem in agricultural production, presenting a long-running challenge to agronomists. Lodging not only limits high, stable, and good-quality rice production, but also reduces the efficiency of mechanized harvesting. Many scholars believe that suitable optimization management techniques can improve plant traits, thus reconciling the conflict between high rice yield and lodging resistance [30]. Numerous factors that cause lodging in rice, including external factors such as natural environmental conditions and cultivation management practices, and internal factors, such as the lodging resistance traits of the variety itself [31]. The lodging index is an important indicator of the lodging resistance of rice; the higher it is, the greater the likelihood of lodging [11]. The nitrogen level is a critical factor affecting rice lodging. The excessive application of nitrogen fertilizer can reduced the resistance of rice to lodging and limit rice yields [32]. It has been reported that the application of glucose and urea improves the aroma and yield of aromatic rice [27]. The spraying of glucose is an effective measure for improving fragrant rice yield and quality. Currently, applying nutritional and regulatory substances through spraying is an important measure to ensure high and stable rice yields. Therefore, a case of carbon and nitrogen supply resistance was provided in this study. Maintaining a good carbon and nitrogen balance in rice production helps ensure stable rice yields. The supplementation of nitrogen and carbon sources via spraying is easier than traditional fertilization.

This study revealed that, overall, during two-year field trials, the lodging index of fragrant rice increased with the application of nitrogen at levels without carbon, indicating that the application of nitrogen fertilizer decreases the lodging resistance of fragrant rice [33]. At the same nitrogen application level, the addition of carbon tended to reduce the lodging index of both fragrant rice varieties, suggesting that nitrogen application can alleviate the decline in lodging resistance caused by nitrogen fertilizer (Table 2). However, under T6, the lodging index significantly increased for both rice varieties, indicating that enhancing the lodging resistance with carbon fertilizer also requires a certain level of nitrogen application (Table 2). This result may be attributed to the high concentration of carbon application, leading to carbon and nitrogen imbalance, the weakening of rice’s resistance to lodging, and the increase in the lodging index. For Meixiangzhan 2, increasing nitrogen at both low and high carbon levels did not significantly increase lodging resistance. For Xiangyaxiangzhan, particularly under treatments T5 and T7, the reduction in the lodging index was significant (Table 2). A trial revealed that the lodging index for Xiangyaxiangzhan was generally higher than that for Meixiangzhan 2, which is in line with real observations that Xiangyaxiangzhan is more prone to lodging than Meixiangzhan 2 is, which is consistent with a previous study [34]. A previous study showed that the ability of carbon to improve the resistance of rice to lodging may be related to its ability to alter the physical properties of rice [35]. This study revealed similar results: carbon application affected the morphological characteristics of rice, such as plant height and internode length. Plant height is closely related to lodging resistance throughout the growth period of rice [8]. Previous reports have shown that plant growth regulators can enhance the strength of rice stalks by increasing the carbohydrate content to improve the resistance of rice to lodging [36]. The spraying of glucose and urea conducted in this study may have been able to similarly elevate stem thickness and improve the resistance of rice to lodging by increasing the carbohydrate content in the rice stem sheaths. Okuno et al. [37] reported that a reduction in rice stem height could lower the plant’s center of gravity and reduce the load on the base of the rice stem, thereby improving the inherent tolerance of rice to lodging. Moreover, single-plant bending resistance intuitively reflects the strength of rice stems and is a key factor affecting the lodging resistance of fragrant rice. In this trial, treatment T5 achieved a more significant enhancement in single-plant bending resistance in both fragrant rice varieties, resulting in the lowest lodging index among all fertilizations, indicating that enhancing single-plant bending resistance can improve lodging resistance in fragrant rice (Table 2). Heavier and thicker stems grant rice stronger lodging resistance, and the diameter is closely related to the bending resistance of the three lowest internodes in rice [38]. Lodging in rice commonly occurs at the second and third internodes. Therefore, an analysis of internode morphological indicators was conducted in this study. The dry weights of the internodes of both fragrant rice varieties under carbon–nitrogen treatment generally showed no significant differences but were mostly greater than the dry weight under the CK, indicating a trend of increasing in stem thickness after carbon–nitrogen treatment, although the increase was not large (Table 5 and Table 6). The length of the second internode significantly decreased under treatment T5, potentially because carbon–nitrogen treatment promotes the lateral thickening of rice stems without promoting internode elongation, thereby indirectly improving the lodging resistance of fragrant rice (Table 3 and Table 4).

Further correlation analysis revealed that the first internode was significantly correlated with the lodging index, whereas the second internode was significantly correlated with the pushing resistance force (Figure 1a). In addition, the dry weight was significantly correlated with the lodging index and pushing resistance force (Figure 1b). Interestingly, the internode dry weight was positively correlated with the lodging index and negatively correlated with the pushing resistance force. It is possible that as the dry weight of the internode increases, the height of the center of gravity of the rice changes and the strength of the upper stalks of the plant decreases, leading to a consequently increasing in the lodging index. The random forest model analysis revealed that the dry weights of internodes 1, 3, and 4 significantly affected the lodging index (Figure 3). The results of this study are similar to those of previous reports indicating that the strength of the grain between the third and fifth internodes is an important factor in the resistance of the grain to lodging [39]. Further structural equation modeling analysis revealed that the pushing resistance force directly and significantly impacted the lodging index, whereas the dry weight significantly affected the pushing resistance force (Figure 2). We thus inferred that the internode dry weight may further significantly affect the lodging index by influencing the pushing resistance force. This study examined the regulatory effects of various carbon and nitrogen interactions on rice internodes, focusing on their physical properties. We discovered that the interaction of 150 mg/L of glucose and 100 mg/L of urea (treatment T5) yielded the most significant improvements in the thickness and dry weight of critical rice internodes. This combination effectively enhanced the resistance to lodging in Meixiangzhan2 and Xiangyaxiangzhan varieties, thereby reducing the incidence of lodging. However, the carbon and nitrogen spraying concentrations used in this study were set at a low gradient, and the carbon and nitrogen intercropping measures that are most suitable for rice resistance to failure need to be studied in greater depth. Further in-depth studies are also needed on the physiological mechanisms by which carbon and nitrogen balancing promotes the improvement of rice’s resistance to lodging.

5. Conclusions

In summary, different carbon-nitrogen treatments affect the lodging resistance characteristics of fragrant rice. The 150 mg/L glucose + 100 mg/L urea (treatment T5) and 300 mg/L glucose + 50 mg/L urea (treatment T7) treatments lowered the lodging index of fragrant rice to some extent compared to the other fertilization treatments. This was primarily because of the different inner wall thicknesses of the stem internodes and higher single-plant bending resistance during the maturity stage of fragrant rice, along with a reduced plant height, thereby increasing the lodging resistance of the rice varieties. Overall, spraying 150 mg/L glucose + 100 mg/L urea during the elongation stage increased the lodging resistance of fragrant rice.