Effects of nitrogen fertilizer application rate on lodging resistance for rice (Oryza sativa L.) stem

Abstract

Rice yield could be increased by apply higher level of nitrogen fertilizer, but excessive use of nitrogen fertilizer will cause plant lodging. This study aimed to investigate the effect of nitrogen application rate on lodging resistance of rice stems. Four japonica rice varieties with different lodging resistance were used, and six nitrogen fertilizer levels were set up to analyze the morphological structure, mechanical properties, and chemical components of rice stems under such treatments. The dynamic changes of lodging resistance of rice stems under different nitrogen fertilizer application rates were exanimated. The study provide valuable insights for improving lodging resistance and subsequently increasing rice yield. The results indicated that WYD4 exhibited the highest yield under the N1 treatment, whereas JYJ, JJ 525, and JND 667 achieved the highest yield under the N2 treatment. The lodging index of rice varieties fluctuated at the filling stage, peaking 30 days after heading. Moreover, the lodging index increased progressively with higher nitrogen fertilizer application rates, reaching its maximum under the N5 treatment, which corresponded to an increased lodging risk. Specifically, the lodging index under the N5 treatment increased by 0.63–1.21 times compared to the optimal nitrogen fertilizer level. Concurrently, the fracture bending point (s) and lodging resistance of the rice stem exhibited a gradual decline. Additionally, plant height, internode length, and barycenter height significantly increased with rising nitrogen application rates and were positively correlated with the lodging index. Conversely, the wall thickness of the second basal node decreased and showed a negative correlation with the lodging index. Furthermore, the contents of lignin, cellulose, soluble sugar, and starch in the second internode diminished with increasing nitrogen rates, and were positively correlated with the breaking moment.

Introduction

Rice (Oryza sativa L.) is a crucial cereal crop globally¹. To meet the demands of a growing population, rice production will need to increase by 20% in 2030 compared to 2017. Maintaining high rice yield on limited arable land is an effective approach to address the increasing food demand². Lodging, a widespread issue in rice production worldwide, poses challenges in harvesting and results in reduced rice yield and quality. It inhibits rice photosynthesis, hampers dry matter accumulation, and impedes the transport of organic matter to grains^3–7. Previous research indicates that plant lodging can cause a decrease in rice yield ranging from 10 to 40%^8–11.

Rice lodging is a complex physiological phenomenon that occurs due to the interaction between the plant itself and external factors. It can be broadly categorized into two types: root lodging and stem lodging¹². Root lodging occurs when the plant has shallow roots or poorly developed lower roots, resulting in the tilting of the entire plant. On the other hand, stem lodging is characterized by bending, breaking, and fracturing of the basal internodes, primarily caused by a slender stem and underdeveloped mechanical tissue at the base¹³. Stem lodging is the more common type of lodging and is mainly influenced by the weight of the upper part of the plant (stem, leaf, and ear) and the mechanical strength of the stem base^14–17.

Nitrogen is an essential nutrient element for plants, including rice. Increasing the application of nitrogen fertilizer in rice production has been found to positively impact rice yield, making it a key strategy for increasing productivity¹⁸. Nitrogen fertilizer plays a significant role in various aspects of plant growth, such as dry matter accumulation^19,20, photosynthetic matter production^21,22, lodging resistance^23–25, and nitrogen uptake and utilization^26,27. The application of nitrogen fertilizer can impact the mechanical strength and lodging resistance of rice stems by influencing morphological characteristics, the composition of structural carbohydrates (lignin and cellulose), and non-structural carbohydrates (soluble sugar and starch) in the stem^{24,25,28–32}.

The current studies on the effect of nitrogen fertilizer on rice stem strength have mainly focused on specific periods, different niches, and different genotypes. However, there has been a lack of systematic research on the effect of nitrogen fertilizer application rate on the commonness of rice varieties and the sensitive period of lodging. Therefore, this study aims to investigate the regulation mechanism of nitrogen fertilizer application rate on rice stem lodging resistance using Jijing 525 (JJ 525), Jinongda 667 (JND 667), Jiyujing (JYJ), and Wuyoudao 4 (WYD 4) as experimental materials. The study will explore the morphology, mechanics, and chemical composition of the rice stems, providing a theoretical basis for improving rice lodging resistance and increasing yield.

Materials and methods

Test materials and locations

The study selected various japonica rice varieties with different levels of lodging resistance, including Jinongda 667 (high lodging resistance), Jijing 525 (high lodging resistance), Jiyujing (moderate lodging resistance), and Wuyoudao 4 (lodging-susceptible). The experiment was conducted at the teaching and scientific research base of Jilin Agricultural University in Changchun, China, from 2021 to 2022. The previous crop grown in the field was rice. Table 1 presents the basic physical and chemical properties of the tested soil. Figure 1 shows the monthly total rainfall and average temperature in Changchun City from 2021 to 2022.

Table 1.

Basic physicochemical properties of the tested soil.

Agrotype	pH	Organic matter (g kg−1)	Total N (g kg−1)	Available N (mg kg−1)	Available P (mg kg−1)	Available K (mg kg−1)
Black calcium soil	6.82	31.51	1.50	121.57	9.48	134.80

Fig. 1.

Monthly total rainfall and monthly mean temperature during 2021–2022.

Experimental design

The experiments were conducted using a split-plot design with three replicates. The plot size was 30 m². The main plots consisted of six nitrogen levels: no nitrogen applied (N0), 100 kg hm⁻² applied (N1), 150 kg hm⁻² applied (N2), 200 kg hm⁻² applied (N3), 250 kg hm⁻² applied (N4), and 300 kg hm⁻² applied (N5). The split-plots were assigned with four varieties. The row spacing was 30 cm, the plant spacing was 13.3 cm, and the variety was considered as the sub-area. Each treatment was replicated three times. Ridges were created between the main treatments and wrapped with plastic films to ensure separate irrigation for each plot.

Nitrogen fertilizer was applied in a ratio of 4:3:3 (mass ratio) for base fertilizer, tillering fertilizer, and panicle fertilizer. A base fertilizer of 50 kg hm⁻² of pure phosphorus was applied once. A total of 75 kg hm⁻² of pure potassium was divided into base fertilizer and applied twice during the panicle stage, with a ratio of 1:1 (Table 2). The nitrogen fertilizer used was urea (N 46%), the phosphate fertilizer used was superphosphate (P₂O₅ 12%), and the potassium fertilizer used was potassium sulfate (K₂O 50%). Throughout the entire growth period, diseases, insects, and weeds were strictly controlled.

Table 2.

The details of nitrogen.

Nitrogen fertilizer management	Total nitrogen (kg hm−2)	Basal fertilizer (kg hm−2)	Tillering fertilizer (kg hm−2)	Panicle fertilizer (kg hm−2)
N0	0	0	0	0
N1	100	40	30	30
N2	150	60	45	45
N3	200	80	60	60
N4	250	100	75	75
N5	300	120	90	90

Measurement index and methods

At the heading stage, rice panicles with uniform growth and heading on the same day were selected from each plot. A total of 15 fresh single stems were taken at the heading stage, as well as at 10, 20, 30, 40, and 50 days after heading. The morphological indexes, mechanical properties, physical, and chemical components were then determined.

Stem morphological index

Plant height and center of gravity height

The plant height (PH) was determined by measuring the distance from the base of the stem to the top of the spike (Fig. 2). To calculate the relative center of gravity height, the underground part of the stem was subtracted, and the distance from the base of the stem to the balance of the stem was measured, representing the height of the center of gravity (GCH).

Fig. 2.

Method for determination of plant height (a), breaking-resistance strength (b), and inner and outer diameters (c).

Internode length and internode dry weight

The experiment involved removing leaves and sheaths from the stem, and then measuring the length and dry weight of the second internode at the base of the stem. The dry weight per length was calculated using this data. The stem dry mass per unit length (mg cm⁻¹) of the second internode of rice base was used as an indicator of its internode fullness.

Dry weight per length in the second internode (mg cm⁻¹) = the 2nd internode length/the 2nd internode dry weight.

Stem diameter (SD) and stem wall thickness (SWT)

To measure the outer and inner diameters of the long and short axes at the broken part, cut off the middle of the second elongation internode (excluding the leaf sheath) at the base. Use a vernier caliper to determine the outer diameter of the long axis (b₁), the outer diameter of the short axis (a₁), the inner diameter of the long axis (b₂), and the inner diameter of the short axis (a₂).

SD (mm) = (a₁ + a₂)/2 where a1 is outer diameter of the minor axis in an oval cross-section (mm), and a2 is inner diameter of the minor axis in an oval cross-section (mm).

SWT (mm) = [(b₁ − b₂) + (a₁ − a₂)]/4 where b1 is the outer diameter of the major axis in an oval cross-section (mm), b2 is inner diameter of the major axis in an oval cross-section (mm).

Stem physical properties

The breaking strength of the second internode, including the leaf sheath, was evaluated using a Plant Stem Strength Testing Machine (model YYD-1, Hangzhou TOP Instrument Co., Ltd., Hangzhou, China). The measurement was conducted by placing two support points 5 cm apart. Additionally, the outer diameter, inner diameter, and culm wall thickness of the basal culm were measured at the second internode. The physical parameter calculations were carried out based on a previous study by Ookawa et al.³³.

Bending moment of the whole plant (BM, g cm) = SL × FW, where SL is the length from broken point to spike top and FW is the fresh weight of the plant material above the broken point (g).

Breaking moment (M, g cm), M = BL × L × 1/4 × 10³, where BL is the force applied to break the stem segment (kg) and L is the distance between two points (cm).

Section modulus (SM, mm³): SM = π/32 × (a1³b1 − a2³b2)/a1, where b1 is the outer diameter of the major axis in an oval cross-section (mm), b2 is inner diameter of the major axis in an oval cross-section (mm), a1 is outer diameter of the minor axis in an oval cross-section (mm), and a2 is inner diameter of the minor axis in an oval cross-section (mm).

Carbohydrate components in stems

The second internode of the base was bagged, deactivated at 105 °C for 30 min, dried to constant weight at 80 °C, and used to determine the content of chemical components after passing through an 80-mesh sieve.

Soluble sugar and starch content

The method of Yoshida and colleagues was followed with slight modifications³⁴. For the extraction of soluble sugar, 0.1000 g of samples were accurately weighed and placed in a 10 ml centrifuge tube. Three replicates were prepared and 10 ml of 80% ethanol solution was added to each tube. The tubes were then placed in a water bath at 80 °C for 60 min. After extraction, the tubes were centrifuged at 10,000 r/min for 10 min and the supernatant was collected. This extraction process was repeated three times for each centrifuge tube and the supernatant was combined in a 50 ml volumetric flask. The volume was adjusted to the mark with 80% ethanol solution.

For the extraction of starch, the remaining residue from the previous extraction was mixed with 3 ml of distilled water and stirred evenly. The residue was then placed in a water bath at 80 °C for 30 min to remove any residual ethanol. Subsequently, the sealed centrifuge tubes were placed in a boiling water bath at 100 °C for 30 min to gelatinize the starch. After gelatinization, the tubes were immediately transferred to an ice water bath to cool completely. Once cooled, 2 ml of 9.2 N perchloric acid was added to each tube and stirred for 15 min, followed by the addition of 4 ml of distilled water. The mixture was then subjected to centrifugation at 10,000 rpm for 10 min and the supernatant was collected. This process was repeated with the addition of 2 ml of 4.6 mol/l perchloric acid and 7 ml of distilled water. The resulting supernatant was collected after centrifugation. Finally, the sample was washed twice with 10 ml of distilled water by mixing evenly and centrifuging at 12,000 rpm for 5 min. The supernatant from each wash was collected and all the supernatant was concentrated in a 50 ml volumetric flask and diluted.

Chromogenic assay: The extracted soluble sugar solution was brought to a constant volume. Then, 1 ml of the solution was transferred into a 10 ml centrifuge tube using a pipette gun. Subsequently, 4 ml of anthrone sulfate reagent was added. The mixture was vigorously shaken and promptly placed in a boiling water bath at 100 °C for 10 min. After the reaction, the test solution was removed and allowed to cool to room temperature naturally. Absorbance values were determined using a spectrophotometer (Persee TU-1900, Beijing, China) at a wavelength of 620 nm, and the amount of soluble sugar in the stem was determined using the standard curve.

Cellulose content

Accurate 0.1 g samples were placed in a clean test tube and 5 ml of acetic acid/nitric acid reagent (100 ml of 80% acetic acid mixed with 10 ml of nitric acid solution) was added. The test tube was then placed in a boiling water bath for 30 min. Afterward, it was taken out and allowed to cool to room temperature. The mixture was centrifuged at 5000 rpm for 15 min, and the supernatant was discarded. These steps were repeated three times until the residue in the test tube turned into white floc. Next, 5 ml of 72% sulfuric acid was added to the test tube, shaken, and left at room temperature for 12 h. Once the white floc completely dissolved, it was transferred to a 50 ml volumetric flask and diluted with distilled water. A 1 ml constant volume solution was then transferred to a 10 ml centrifuge tube using a pipette gun, followed by the addition of 4 ml of anthrone sulfate reagent. The solution was vigorously shaken and quickly placed in a boiling water bath at 100 °C for 10 min. After the reaction, the solution was removed and allowed to cool to room temperature naturally. Absorbance values were determined using a spectrophotometer (Persee TU-1900, Beijing, China) at a wavelength of 620 nm, and the cellulose content was calculated using the standard curve of soluble sugar³⁵.

Lignin content

The determination of lignin is based on the methods of Morrison³⁶ and Hatfield³⁷. To determine the lignin content of the stem, 10 mg samples were accurately weighed and dissolved in 2 ml of a 30% bromoacetyl glacial acetic acid solution. The reaction was halted by adding 0.9 ml of 2 mol/l NaOH at 70 °C for 30 min. Subsequently, 10 ml of glacial acetic acid and 0.1 ml of hydroxylamine hydrochloride were added. The resulting solution was diluted to 20 ml with glacial acetic acid and centrifuged at 4000 r/min for 5 min. The supernatant was collected and the OD value at a wavelength of 280 nm was measured using a spectrophotometer (Persee TU-1900, Beijing, China). By utilizing the standard curve, the lignin content of the stem was calculated.

Yield and yield components

For each plot, the plants occupying a 3 m² area were harvested at maturity. Furthermore, 9 holes were specifically chosen for indoor testing, where measurements were taken for panicle weight, panicle number per hole, grain number per panicle, and 1000-grain weight.

Statistical analysis

Statistically significant differences (P < 0.05) were determined by analyses of variance (ANOVA) based on the appropriate F-tests, and the differences between means were compared utilizing the Fisher’s least significant difference (LSD) test.

Table and figures were prepared in Microsoft Excel 2021 software for Windows.Origin 2019 software (Origin Lab, Northampton, MA, USA) was used to plot the data.

ANOVA, Pearson correlation analysis and Path analysis were performed using IBM SPSS statistics 26 Statistical Software. Tables and figures were processed with Microsoft Excel 2019 and Origin 2019.

Results

Grain yields and yield components

The yields of WYD 4, JYJ, JND 667, and JJ 525 ranged from 6.56 to 9.03 t hm⁻², 6.61 to 9.38 t hm⁻², 6.50 to 10.56 t hm⁻², and 6.57 to 10.24 t hm⁻², respectively. The application of nitrogen fertilizer significantly influenced the yield and yield components of these rice varieties (Table 3).

Table 3.

Yield and yield components as affected by nitrogen rates.

Cultivars	Treatment	Yield (t hm−2)		Panicle (× 104 hm−2)		Spikelets per panicle		1000-grain weight (g)
		2021	2022	2021	2022	2021	2022	2021	2022
WYD 4	N0	6.35d	6.77d	311.61d	337.72d	85.85c	86.69d	26.94a	26.68a
	N1	8.95a	9.10a	414.37c	425.44c	105.43a	108.99a	26.79a	26.45ab
	N2	7.94b	8.17b	431.91b	430.45bc	102.59ab	104.00b	26.65a	26.24ab
	N3	7.80bc	7.96b	441.94ab	431.70bc	99.44b	102.33b	26.37a	26.09b
	N4	7.71bc	7.45c	446.95a	441.10ab	98.80b	101.55bc	26.18a	26.01b
	N5	7.49c	7.23c	450.29a	450.50a	97.95b	98.19c	26.04a	25.96b
JND 667	N0	6.48d	6.51d	242.27d	246.87d	122.91c	121.52c	23.96a	24.04a
	N1	8.13c	9.05c	292.40c	325.81c	132.32abc	134.07b	23.63ab	23.76ab
	N2	10.37a	10.74a	363.41b	363.41b	138.84a	144.07a	23.56abc	23.56bc
	N3	9.45b	9.87b	366.75ab	370.93b	133.89ab	135.95b	23.23bcd	23.42bc
	N4	9.21b	9.07c	376.78ab	380.95ab	128.17bc	125.07c	23.09 cd	23.24 cd
	N5	9.17b	8.95c	384.29a	392.23a	127.64bc	122.62c	22.91d	22.86d
JYJ	N0	6.68c	6.53d	295.74d	303.26d	97.51d	94.18d	25.26a	24.83a
	N1	8.05b	8.02c	357.56c	362.16c	104.21c	104.07c	24.82ab	24.70a
	N2	9.28a	9.48a	370.93bc	379.07b	116.02a	118.47a	24.79ab	24.67a
	N3	8.46b	8.55b	377.61ab	384.71ab	111.61ab	111.48b	24.66ab	24.46a
	N4	8.24b	8.44b	382.62ab	390.35ab	109.17b	110.00bc	24.50b	24.34a
	N5	8.06b	8.12c	389.31a	395.36a	107.67bc	107.59b	24.32b	24.16a
JJ 525	N0	6.51d	6.63e	248.12e	259.40d	126.41c	125.72c	22.96a	22.82a
	N1	8.88c	9.06bc	335.84d	338.35c	131.35b	133.67b	22.74a	22.65a
	N2	9.86a	10.61a	367.59c	369.67b	143.14a	146.27a	22.72a	22.52a
	N3	9.32b	9.18b	370.09bc	381.58	139.20a	129.29bc	22.35a	22.34a
	N4	8.90c	8.89 cd	385.96ab	396.62a	129.17bc	126.02c	22.26a	22.29a
	N5	8.67c	8.76d	390.98a	402.26a	126.54c	125.06c	22.11a	22.22a

The response of different rice varieties to nitrogen fertilizer varied significantly. The yield of WYD 4 initially increased with nitrogen application before decreasing, reaching its peak with the N1 treatment. In contrast, the yields of the JYJ, JND 667, and JJ 525 varieties peaked with the N2 treatment, subsequently declining with further increases in nitrogen fertilizer. Compared to the N0 treatment, the yield of WYD 4 increased by 37.65%, 22.87%, 20.12%, 15.55%, and 12.20% for the respective treatments. For JYJ, the increases were 21.63%, 41.91%, 28.74%, 26.17%, and 22.39%. The yield of JJ 525 showed increases of 36.53%, 55.86%, 40.79%, 35.46%, and 32.72%. JND 667 exhibited production increases of 32.15%, 62.46%, 48.62%, 40.62%, and 39.38% over a two-year average. From the perspective of yield components, the effective panicle number for the four rice varieties exhibited an increasing trend with higher nitrogen fertilizer application, whereas the 1000-grain weight demonstrated a decreasing trend.

The number of grains per panicle for all varieties initially increased and subsequently decreased. WYD 4 exhibited the highest number of grains per panicle under the N1 treatment (100 kg hm⁻² nitrogen application), whereas the other three varieties reached their maximum under the N2 treatment (150 kg hm⁻² nitrogen application), which was significantly greater than the number observed under the N0 treatment.

Lodging index and its related mechanical parameters

LI

From the heading stage to 50 days post-heading, the lodging index (LI) of all rice varieties initially increased and subsequently decreased, reaching its peak value approximately 30 days after heading (Fig. 3). Notable variations in the lodging indices were observed among the different cultivars. Specifically, the lodging index of WYD 4 was 32.76%, 35.34%, and 39.9% higher than those of JYJ, JJ 525, and JND 667, respectively. The application of nitrogen fertilizer significantly influenced the lodging index, with a marked increase observed at higher nitrogen fertilizer levels. Although the degree of response varied among the four rice varieties, the overall trend remained consistent, with the lodging index following the sequence N0 < N1 < N2 < N3 < N4 < N5 (Fig. 3 and Table 4).

Fig. 3.

Changes of lodging index under different nitrogen.

Table 4.

Analyses of variance of lodging index and its related physical parameters.

Treatment	LI (%)	BM (g cm)	M (g cm)	SL (cm)	FW (g)	BS (g mm−2)	SM (mm3)
Y	**	**	**	**	**	**	NS
C	**	**	**	**	**	**	**
N	**	**	**	**	**	**	**
Y × C	**	**	NS	**	**	NS	NS
Y × N	*	NS	NS	**	NS	NS	NS
C × N	**	**	**	**	**	**	**
Y × C × N	*	NS	NS	**	NS	NS	NS

Y Year, C Cultivar, N Nitrogen application rate. NS mean not significant at P = 0.05. *F-value is significant at P < 0.05. **F-value is significant at P < 0.01. The same as following.

Bending moment and breaking moment

This study analyzed the bending moment, which reflects the weight exerted by the entire plant on the second internode at its base. The results indicated a gradual increase in the stem bending moment from the heading stage to 30 days post-heading, culminating in a peak at 30 days. However, a slight decrease in the bending moment was observed from 40 to 50 days after heading, a trend that was consistent across the four varieties examined. Specifically, the bending moment of JYJ was found to be 20.85% lower than that of WYD 4, whereas JJ 525 and JND 667 exhibited bending moments that were 51.43% and 57.09% higher than WYD 4, respectively. Furthermore, the application of nitrogen fertilizer led to a progressive increase in the rice stem bending moment. When compared to the N0 treatment, the bending moment of WYD 4 increased by factors of 0.49, 0.60, 0.68, 0.73, and 0.81 under various nitrogen application treatments. Similarly, the bending moment for JYJ rose by 0.39, 0.69, 0.82, 0.95, and 1.07 times, while JND 667 showed increases of 0.33, 0.62, 0.76, 0.84, and 0.95 times. On average over two years, JJ 525’s bending moment increased by 0.46, 0.76, 0.84, 0.92, and 1.00 times. It is important to note that inter-annual differences were also observed.

The bending moment, indicative of the stem strength of rice, exhibited a decreasing trend from the heading stage to 30 days post-heading. However, a slight increase was observed from 40 to 50 days after heading. Consequently, the rice stem breaking moment reached its lowest point at 30 days post-heading (Fig. 4). In comparison to WYD 4, the bending moments for JYJ, JJ 525, and JND 667 increased by 0.23, 1.44, and 1.77 times, respectively, based on a two-year average. As the nitrogen fertilizer application rate increased, the breaking moment of the second internode at the base of the rice stalk consistently decreased across all four rice varieties. Compared to the control treatment (N0), the bending moment of WYD 4 decreased by 5.95%, 13.18%, 18.87%, 25.39%, and 29.47% under subsequent nitrogen application rates. Similarly, the bending moment of JYJ decreased by 6.37%, 11.49%, 22.61%, 29.49%, and 40% with the other nitrogen treatments. For JND 667, the bending moment decreased by 9.41%, 13.92%, 27.26%, 35.93%, and 43.12%, while JJ 525 exhibited decreases of 8.68%, 13.54%, 22.96%, 29.89%, and 43.76%, respectively, based on the two-year average.

Fig. 4.

Changes of breaking strength under different nitrogen.

Length from broken point to spike top (SL) and fresh weight of the plant material above the broken point (FW)

The length from the broken part of the rice stem to the spike top, along with its fresh weight, can significantly influence the bending moment of the stem. Figures 5 and 6 illustrate the effect of nitrogen (N) fertilizer on both the length from the broken site to the spike top and the fresh weight. The results indicate that from the heading stage to 50 days post-heading, the length from the broken site to the spike top remained relatively stable. In contrast, the fresh weight from the broken site to the spike top exhibited an initial increase followed by a decrease, peaking on day 30 after heading before experiencing a slight decline. This pattern was consistent across all four varieties studied. Furthermore, as the nitrogen fertilizer application rate increased, both the length from the broken part to the spike top and the fresh weight also increased, following the trend N0 < N1 < N2 < N3 < N4 < N5. This trend was consistent across all four varieties, with no significant differences observed among the years.

Fig. 5.

Effects of different nitrogen treatments on the fresh weight from breaking point to spiketop (FW).

Fig. 6.

Changes of the length from the breaking point to spike top (SL) under different nitrogen treatments.

Section modulus and bending stress

The bending moment of the stem is determined by its section modulus and bending stress. In comparison to WYD 4, the sectional modulus of JYJ, JJ 525, and JND 667 increased by 0.17, 0.87, and 1.52 times, respectively (refer to Figs. 7 and 8). As the nitrogen fertilizer application rate increased, the section modulus of the second internode at the base initially increased and subsequently decreased. Furthermore, the bending stress decreased with the increase in nitrogen application. Compared to N0, the bending stress of WYD 4 decreased by 14.25% to 24.87%, JYJ decreased by 12.93% to 27.29%, JND 667 decreased by 20.58% to 34.65%, and JJ 525 decreased by 13.6% to 25.87%.

Fig. 7.

Changes of section modulus (SM) under different nitrogen treatments at 30 days after heading.

Fig. 8.

Changes of bending stress (BS) under different nitrogen treatments at 30 days after heading.

Correlation analysis and path analysis

Table 5 presents a correlation analysis between the lodging index of rice stalks and lodging-related mechanical indices. The findings reveal a significant negative correlation between the lodging index of the four varieties and the breaking moment. Additionally, the bending moment, the length from the broken site to the top of the ear, and the fresh weight from the broken site to the top of the ear exhibit a significant positive correlation with the lodging index, as well as a significant negative correlation with the breaking moment. Furthermore, the section modulus and bending stress demonstrate a negative correlation with the lodging index, while positively correlating with the breaking moment. Figure 9 illustrates the path analysis of stalk lodging-related indices and the lodging index of rice. The results indicate that the bending moment has a highly significant positive regulatory effect on the lodging index. Moreover, it exhibits a remarkably negative regulatory effect on the lodging index, with the diameter coefficient of the bending moment to the lodging index being greater than that of the breaking moment to the lodging index. This suggests that the breaking moment has a greater influence on the lodging index compared to the bending moment.

Table 5.

Correlation analysis between lodging index and its related physical parameters in rice.

Indicators	WYD 4		JYJ		JND 667		JJ 525
	LI (%)	M (g cm)	LI (%)	M (g cm)	LI (%)	M (g cm)	LI (%)	M (g cm)
M (g cm)	− 0.905**	–	− 0.926**	–	− 0.954**	–	− 0.959**	–
BM (g cm)	0.927**	− 0.741**	0.930**	− 0.825**	0.923**	− 0.861**	0.912**	− 0.835**
SL (cm)	0.830**	− 0.685**	0.907**	− 0.828**	0.843**	− 0.801**	0.833**	− 0.773**
FW (g)	0.908**	− 0.715**	0.889**	− 0.788**	0.908**	− 0.848**	0.896**	− 0.818**
SM (mm3)	− 0.348*	0.446*	− 0.481*	0.513*	− 0.461*	0.490*	− 0.551**	0.586**
BS (g mm− 2)	− 0.554**	0.557**	− 0.630**	0.671**	− 0.719**	0.738**	− 0.624**	0.635**

Fig. 9.

Path analysis of lodging index and its related physical parameters.

The determination of the bending moment involved measuring the length from the break site to the top of the ear (SL) and the fresh weight from the break site to the top of the ear (FW). Path analysis reveals that both SL and FW exert a highly significant positive regulatory effect on the bending moment, with the path coefficient of FW to the bending moment being larger than that of SL. These results indicate that the fresh weight from the breaking point to the top of the spike is the primary factor influencing the bending moment.

The bending moment of the stem is determined by its section modulus and bending stress. Path analysis shows that both the section modulus and bending stress significantly positively affect the breaking moment, with bending stress contributing more to the breaking moment. The section modulus and bending stress of the JYJ, JND 667, and JJ 525 varieties exhibited a negative correlation. Similarly, the section modulus and bending stress of WYD 4 were significantly negatively correlated, and this trend persisted over two years. Moreover, the bending moment of all four varieties displayed a highly significant negative correlation over the same period. Additionally, a strong positive correlation was observed between the length from the break site to the top of the ear and its fresh weight.

Morphological traits of stem

Plant height, center of gravity

The figure below (Fig. 10 and Table 6) present the plant height and center of gravity height for different rice varieties. The findings indicate that nitrogen application resulted in an increase in plant height across all four rice varieties. Compared to the control treatment (N0), the plant height under varying nitrogen treatments showed average increases of 16.99%, 22.31%, 24.54%, 25.49%, and 27.50%, averaged across the four varieties over two years. Additionally, the application of nitrogen fertilizer led to an increase in the length of the basal internode (I1 + I2) and its proportion to culm length for the other varieties. The four varieties exhibited the following trend: N5 > N4 > N3 > N2 > N1 > N0 (Table 6), with the ranking being WYD4 > JYJ > JND 667 > JJ 525.

Fig. 10.

Changes of center of gravity height under different nitrogen.

Table 6.

Effects of nitrogen rates on plant morphological characters at 30 days after heading.

Cultivars	Treatment	PH (cm)		SD (cm)		The ratio of basal internode (I1 + I2) length to culm length (%)		RCGH (%)
		2021	2022	2021	2022	2021	2022	2021	2022
WYD 4	N0	96.35c	95.68d	4.35c	4.33b	24.41b	23.55b	45.41b	42.50b
	N1	121.53b	118.05c	4.62ab	4.66a	26.94ab	26.01ab	45.79ab	45.37a
	N2	124.28a	121.85b	4.6a	4.62ab	27.99ab	28.77a	46.05a	45.92a
	N3	124.43a	123.93ab	4.55b	4.58ab	28.35a	29.06a	46.59a	46.21a
	N4	125.08a	124.93ab	4.51b	4.54ab	28.48a	28.93a	46.84a	46.5a
	N5	125.9a	126.78a	4.5bc	4.51ab	28.72a	28.83a	46.9a	46.93a
JYJ	N0	84.63c	87.75d	4.71b	4.9ab	26.68a	24.99b	41.64c	40.38d
	N1	96.4c	98.23c	5.06a	5.07ab	27.41a	26.4ab	42.9bc	42.75 cd
	N2	102.18b	103.23b	5.12a	5.14a	28.36a	28.222ab	44.56ab	43.6c
	N3	103.6b	106.4ab	5.05a	4.96ab	28.44a	29.61ab	45.48a	45.2bc
	N4	107.1ab	107.6a	4.88ab	4.76bc	28.58a	30.35a	45.95a	46.71ab
	N5	109.78a	109.08a	4.67b	4.54c	28.76a	30.47a	46.53a	47.96a
JJ 525	N0	90.90e	92.80d	5.67bc	5.45ab	14.12d	14.58b	41.83d	41.32c
	N1	107.13d	104.60c	5.97a	5.67ab	14.19d	14.63b	42.57d	43.45c
	N2	111.75c	111.55b	6.09a	5.83a	14.87c	14.82ab	45.35c	46.42b
	N3	112.18c	115.48ab	5.82ab	5.59ab	16.00b	14.93ab	47.94b	46.94ab
	N4	114.00b	117.70a	5.64bc	5.34b	16.84ab	15.32a	49.74a	49.08ab
	N5	115.58a	119.13a	5.47c	5.28b	17.06a	15.81a	51.01a	49.66a
JND 667	N0	94.20c	90.25d	6.06c	6.07c	11.86b	11.58b	40.62b	41.05c
	N1	106.20b	105.80c	6.53ab	6.52ab	15.15ab	15.33a	42.47ab	40.33c
	N2	112.40a	109.43bc	6.80a	6.71a	16.38a	16.21a	43.8ab	42.94bc
	N3	114.10a	112.75ab	6.44b	6.49ab	16.50a	16.24a	45.23a	44.08ab
	N4	116.05a	114.65a	6.40b	6.43ab	16.96a	17.00a	46.34a	45.07ab
	N5	118.85a	116.15a	6.31bc	6.35b	17.12a	15.55a	47.73a	46.82a

Regarding the barycenter height of rice (Fig. 10), it was observed that the barycenter height of WYD 4, JYJ, JND 667, and JJ 525 gradually increased from the heading stage to 50 days after heading, reaching its peak at that time. Furthermore, the experiment revealed a positive correlation between nitrogen application and the height of the center of gravity, with the overall trend as follows: N0 < N1 < N2 < N3 < N4 < N5. This trend was consistent across all four varieties.

Internode morphology

The internode morphology of the second internode at the base of rice includes stem thickness and stem wall thickness. The stem wall thickness gradually decreased from the heading stage to 50 days after heading, and this trend was consistent among the four varieties. Additionally, the results indicated that nitrogen application had a significant negative effect on stem wall thickness (Table 7). Compared to the N0 treatment, the other nitrogen treatments resulted in a reduction in stalk wall thickness. The overall trend of the nitrogen treatments was as follows: N5 < N4 < N3 < N2 < N1 < N0. This trend was observed in both years and was consistent across the four varieties (Fig. 11).

Table 7.

Analyses of variance of morphology indicators.

Treatment	PH (cm)	GCH (cm)	SD (mm)	SWT (mm)
Y	**	NS	*	**
C	**	**	**	**
N	**	**	**	**
Y*C	**	*	*	*
Y*N	**	NS	NS	*
C*N	**	**	*	**
Y*C*N	*	NS	NS	NS

Fig. 11.

The stem wall thickness of the second internode in different nitrogen treatments.

The stem diameter of the four varieties initially increased and then decreased with increasing nitrogen fertilizer dosage (Table 6). Each variety responded differently to nitrogen application. For WYD 4, the order of response was N1 > N2 > N3 > N4 > N5 > N0, with the maximum value obtained at a nitrogen application rate of 100 kg hm^-2. JYJ showed a response order of N2 > N1 > N3 > N4 > N0 > N5, JJ 525 responded in the order of N2 > N1 > N3 > N0 > N4 > N5, and JND 667 responded in the order of N2 > N1 > N3 > N4 > N5 > N0. The maximum stem diameter for these three varieties was observed at a nitrogen fertilizer application rate of 150 kg hm⁻².

Dry weight per length

From the heading stage to 50 days post-heading, a noticeable trend in stalk fullness was observed. Initially, stalk fullness decreased before beginning to increase. At 30 days after heading, the stalk fullness in the second segment reached its lowest point, but it subsequently increased. This trend was consistent across all four varieties. With the increase in nitrogen fertilizer application, the fullness of the second internode at the rice stalk base decreased. Compared to the N0 treatment, the second internode fullness of WYD 4 decreased by 0.19, 0.23, 0.36, 0.46, and 0.49 times, respectively. Similarly, the internode fullness of JYJ decreased by 0.15, 0.19, 0.21, 0.39, and 0.47 times, respectively. The internode fullness of JND 667 decreased by 0.19, 0.26, 0.30, 0.50, and 0.54 times, while the internode fullness of JJ 525 decreased by 0.09, 0.12, 0.23, 0.43, and 0.45 times, respectively (Fig. 12).

Fig. 12.

Dry weight per cm in the second internode at the base as affected by nitrogen rates (DW: dry weight). Vertical bars above mean values indicate standard deviations. Means with different alphabetical letters show significant differences between treatments according to the LSD (P < 0.05). The same as following.

Correlation analysis

Correlation analysis revealed positive correlations between plant height, center of gravity, relative center of gravity, and the ratio of basal node to stalk length of each variety with the lodging index. Conversely, these factors were negatively correlated with the breaking moment (Table 8). Additionally, wall thickness and internode fullness exhibited negative correlations with the lodging index, while showing positive correlations with the breaking moment.

Table 8.

Correlation analysis between physical parameters and morphological indicators in rice.

Morphological indicators	WYD 4		JYJ		JND 667		JJ 525
	LI (%)	M (g cm)	LI (%)	M (g cm)	LI (%)	M (g cm)	LI (%)	M (g cm)
M (g cm)	− 0.905**	–	− 0.926**	–	− 0.954**	–	− 0.959**	–
PH (cm)	0.797**	− 0.667**	0.766**	− 0.791**	0.820**	− 0.788**	0.855**	− 0.803**
GCH (cm)	0.838**	− 0.747**	0.867**	− 0.792**	0.877**	− 0.892**	0.802**	− 0.786**
CWT (mm)	− 0.713**	0.749**	− 0.823**	0.774**	− 0.787**	0.822**	− 0.798**	0.804**
DW (mg cm−1)	− 0.959**	0.931**	− 0.971**	0.919**	− 0.943**	0.960**	− 0.922**	0.944**
RCGH (%)	0.630**	− 0.578**	0.506**	− 0.443**	0.817**	− 0.791**	0.868**	− 0.844**
The ratio of basal internode (I1 + I2) length to culm length (%)	0.586**	− 0.559**	0.449**	− 0.517**	0.481**	− 0.465**	0.448**	− 0.431**

Carbohydrates in the second internode

Soluble sugar and starch content

In this experiment, the soluble sugar content in the stem of the second internode at the base was measured, with results presented in Fig. 13. From the heading stage to 30 days after heading, a decline in soluble sugar content in the stems was observed. However, from 30 to 50 days after heading, there was a slight increase in soluble sugar content. When compared to the heading stage, the soluble sugar content in the stems of all varieties decreased by approximately 40.71%, 37.51%, 37.12%, and 39.55%, respectively, 30 days after heading. Furthermore, when compared to the control group with no nitrogen (N) application, the soluble sugar content in the second internode of the rice base was significantly reduced under all other N treatments. Specifically, compared to N0, the soluble sugar content in the stem of the second internode at the base of variety JYJ decreased by 15.08%, 16.70%, 22.15%, 26.72%, and 32.10%, respectively. For variety JJ 525, the decrease was 15.84%, 20.06%, 22.65%, 29.46%, and 34.40%, respectively. WYD 4 exhibited decreases of 8.76%, 10.87%, 13.99%, 18.71%, and 22.06%, respectively, while JND 667 showed decreases of 17.40%, 19.46%, 25.52%, 27.80%, and 31.88%, respectively (averaged over 2 years).

Fig. 13.

The stem soluble sugar contents of the second internode at the base under different nitrogen treatments.

The figure below illustrates the starch content of the stem at the second internode from the base (Fig. 14). From the heading stage to 30 days post-heading, there was a notable decrease in starch content within the stems. At 30 days after heading, the starch content in the stalks of all varieties (JYJ, JJ 525, WYD 4, and JND 667) decreased by 29.45%, 24.97%, 33.38%, and 25.15%, respectively, compared to the heading stage. Nitrogen application was found to negatively regulate the starch content in the second basal internode; as nitrogen application increased, the starch content in the stems decreased. Compared to the control (N0), the starch content in the stem of the second internode at the base of JYJ decreased by 5.76%, 10.7%, 18.75%, 22.49%, and 26.08%, while that of JJ 525 decreased by 17.36%, 19.7%, 21.78%, 23.68%, and 25.03%. For WYD 4, the decreases were 9.85%, 12.64%, 16.4%, 19.18%, and 22.45%, and for JND 667, the corresponding decreases were 10.58%, 15.4%, 18.79%, 23.77%, and 26.18%.

Fig. 14.

The stem starch contents of the second internode at the base under different nitrogen treatments.

Cellulose and lignin content

The lignin content in the second internode of the base was measured at the heading stage, 30 days after heading, and 50 days after heading (Fig. 15). It was observed that the lignin content in the second internode of JND 667 and JJ 525 was higher compared to JYJ and WYD 4. From the heading stage to 50 days after heading, there was a progressive increase in lignin content in the second basal node. Notably, the increase in lignin content from the heading stage to 30 days after heading was greater than the increase observed from 30 to 50 days after heading. Furthermore, the application rate of nitrogen fertilizer significantly influenced the lignin content in the second internode of the base. The effect of nitrogen fertilizer application rate on cellulose content in the second internode of the base is illustrated in Fig. 16. From the heading stage to 50 days after heading, the cellulose content in the second internode of the base increased under each treatment. The increase in cellulose content from the heading stage to 30 days after heading was greater than that from 30 to 50 days after heading. However, nitrogen application negatively impacted cellulose content in the stems; as the nitrogen fertilizer application increased, the cellulose content in the second internode of the rice base decreased. This trend was consistent across all varieties.

Fig. 15.

Lignin contents in the second internode at the base as affected by nitrogen rates.

Fig. 16.

Cellulose contents in the second internode at the base as affected by nitrogen rates.

Correlation between physicochemical composition and lodging index

The results indicated a negative correlation between soluble sugar, starch, lignin, and cellulose content with the lodging index, while showing a positive correlation with the breaking moment. Specifically, the soluble sugar, lignin, and cellulose contents at the base of WYD 4 exhibited a significant negative correlation with the lodging index and a positive correlation with the breaking moment. However, the starch content did not show a significant correlation with the lodging index (although it was negatively correlated) or the breaking moment (although it was positively correlated). Similarly, the other three varieties also demonstrated a significant negative correlation between the contents of soluble sugar, starch, lignin, and cellulose with the lodging index, while positively correlating with the breaking moment.

Discussion

The sensitive period of rice lodging occurrence

Previous studies have demonstrated that the physical strength of rice stems, the maximum load of panicles, and the bending resistance of each internode undergo changes during the grain filling period. Additionally, variations in the lodging index have been observed. Research has shown that as grain filling progresses, the storage substances in the stem sheath are continuously transported to the panicle, leading to a reduction of these substances in the stem and a corresponding decrease in lodging resistance^23,38–40. The wax maturity stage, occurring approximately 24 days after heading, is considered a critical point for lodging in japonica rice varieties cultivated in cold regions. In this experiment, the lodging index of the four rice varieties initially increased and then decreased during the grain filling process, peaking at 30 days after heading before declining. The breaking bending moment, bending stress, and internode plumpness of the rice exhibited a decreasing trend followed by an increasing trend, with the lowest values recorded at 30 days after heading. This sensitive period for lodging, around 30 days after heading, aligns with Xu’s findings³⁹. Therefore, implementing appropriate fertilizer management measures prior to 30 days after heading can effectively enhance the lodging resistance of rice. This study also explores the mechanism by which nitrogen application rates affect the lodging resistance of different rice varieties.

Effect of nitrogen application rate on mechanical characteristics of rice stem

The application of nitrogen fertilizer significantly influences the growth and development of plants, particularly in relation to rice lodging. An appropriate amount of nitrogen fertilizer can enhance the lodging resistance of rice; however, excessive application may lead to reduced light penetration and ventilation, stem thinning, decreased stem strength, and an increased risk of lodging^41,42. Previous studies have shown that as the rate of nitrogen fertilizer application increases, the stem strength of rice decreases while the lodging index rises²⁸. Furthermore, some researchers suggest that different rice varieties may exhibit varying responses to different rates of nitrogen fertilizer application⁴³.

Previous studies have demonstrated that lodging-resistant varieties exhibit higher bending and breaking moments, with a more pronounced increase in breaking moment, which in turn reduces the lodging index. The breaking moment of the stem is regarded as the primary index for assessing the lodging resistance of rice^23,38. The findings of this study (Figs. 3, 4, and 17) indicate that the lodging-resistant variety JYJ had a lower bending moment compared to WYD 4; however, its breaking moment was greater, leading to a reduced stem lodging index. Furthermore, the bending and breaking moments of the lodging-resistant varieties JJ 525 and JND 667 were higher than those of WYD 4, but the increase in breaking moment for these two varieties was significantly greater than that of the bending moment, thereby enhancing the lodging resistance of the stems. Consequently, the lodging index for these two varieties was lower than that of WYD 4. In this study, as the nitrogen application rate increased, the bending moment of the stem also increased, while both breaking resistance and breaking moment decreased, resulting in an elevated lodging index and increased lodging risk. The lodging index for all four varieties peaked at a nitrogen application rate of 300 kg hm⁻², which was significantly higher than that of rice stems without nitrogen application. Path analysis (Fig. 9) revealed that the path coefficient from the breaking moment to the lodging index was considerably larger than that from the bending moment, and the breaking moment exhibited a significant negative correlation with the lodging index. This indicates that the breaking moment is the key index for determining the lodging resistance of the stem.

Fig. 17.

Changes of bending moment of the whole plant under different nitrogen.

The application of nitrogen fertilizer has an impact on the bending moment (BM) by influencing stalk length (SL)and fresh weight (FW). The bending moment (M) for stalk breakage is influenced by the section modulus (SM) and bending stress (BS). This study (Figs. 5, 6, 7, and 8) demonstrated that an increase in nitrogen fertilizer did not significantly alter SL, but resulted in an increase in FW, leading to an increase in stem bending moment and lodging index. Additionally, SM initially increased and then decreased with the increase in nitrogen fertilizer, while BS decreased. This trend was consistent across the four varieties studied. Path analysis (Fig. 9) revealed that nitrogen fertilizer primarily regulated the bending moment by controlling the fresh weight from the broken part to the top of the ear, and by regulating the breaking moment of the stem through the section modulus and bending stress.

Effect of nitrogen application rate on morphological characteristics of rice stem

The morphological characteristics of stems, such as plant height, center of gravity, and internode length, have an impact on the lodging resistance of stems. Previous studies have indicated that plant height and center of gravity height are major factors that influence crop lodging resistance, showing a strong positive correlation with the lodging index^44–47. Wen et al. demonstrated that varieties with lower plant height were more favorable for lodging resistance⁴⁸. The correlation analysis of this experiment revealed that the lodging index of rice stem exhibited a positive correlation with plant height, center of gravity height, and basal internode length, while it showed a negative correlation with wall thickness⁴⁹. Additionally, the plant height (Table 6) was ranked as follows: WYD 4 > JND 667 > JJ 525 > JYJ. The height of the center of gravity (Fig. 10) followed the order: WYD 4 > JJ 525 > JND 667 > JYJ. Moreover, the trend of breaking bending moment was as follows: JND 667 > JJ 525 > JYJ > WYD 4. Although the plant height and center of gravity of JND 667 and JJ 525 were higher than those of JYJ, their stem breaking bending moment was also higher. It should be noted that the relationship between plant height, center of gravity, and rice lodging resistance was influenced by the rice varieties, and these factors alone cannot be used as the sole criteria for determining stem lodging. The plant height is determined by the length and number of internodes. The results of the study conducted by Zhu et al.⁵⁰ indicated a positive correlation between the length of basal internodes and the stem lodging index. Moreover, the study also observed that an increase in nitrogen fertilizer application led to an increase in the length of basal internodes. Additionally, there was a growing proportion of basal internodes in relation to culm length. This suggests that the elongation of basal internodes contributes significantly to the reduction in mechanical strength of rice stems under high nitrogen treatment. Consequently, reducing nitrogen fertilizer application appropriately to decrease the length of basal internodes can enhance rice lodging resistance.

Stem diameter and wall thickness are important morphological indices that characterize the robustness of stems and are closely related to their mechanical strength. Previous studies have indicated that excessive nitrogen application can decrease the thickness of stem base internodes, stem wall thickness, and internode plumpness, while significantly increasing the lodging rate⁵¹. The results of this experiment (Figs. 10, 11, Table 6) demonstrated that as the nitrogen application rate increased, the wall thickness and stem base internode plumpness decreased in the four varieties, while the stem diameter initially increased and then decreased. WYD 4 achieved the maximum at a nitrogen application rate of 100 kg hm⁻², while the other three varieties reached their maximum at a nitrogen application rate of 150 kg hm⁻². Correlation analysis revealed a positive relationship between stem wall thickness and breaking moment. This indicates that the amount of nitrogen applied can influence the stem strength of rice by affecting stem diameter and wall thickness, thereby impacting the lodging resistance of plants. Additionally, this experiment highlighted that lodging-resistant varieties exhibit characteristics such as short basal internodes, thick stems, thick stem walls, and high internode plumpness compared to lodging-prone varieties.

Effect of nitrogen application rate on chemical composition of rice stem

The carbohydrates found in stems are categorized into non-structural carbohydrates (NSC) and structural carbohydrates (SC). NSC encompasses soluble sugars and starch, whereas SC includes lignin and cellulose. The storage levels of these carbohydrates in rice stems are closely associated with the resistance of stems to lodging^52–54.

This study revealed that nitrogen fertilizer has a significant negative impact on the levels of starch and soluble sugar in the second basal internode. As the application of nitrogen fertilizer increases, the content of soluble sugar and starch in rice stems decreases^55,56. Cellulose and lignin are the primary constituents of the cell wall, and their levels are closely associated with stem lodging resistance. A decrease in lignin and cellulose in rice stems can lead to an increased rate of rice lodging, while an increase in their content significantly enhances the mechanical strength and lodging resistance of the stem. Varieties with strong lodging resistance exhibit higher lignin content compared to lodging-sensitive rice varieties⁵⁷. Previous studies have demonstrated that appropriate nitrogen application can effectively increase the levels of lignin and cellulose in stems, thereby improving lodging resistance. However, this study found that the lignin and cellulose contents of the four varieties decreased with increasing nitrogen application rates^31,57,58. This suggests that the use of nitrogen fertilizer reduces stem strength and increases the risk of rice lodging by diminishing the content of stem lignin and cellulose⁵⁹.

In this experiment, the lodging resistant varieties JJ 525 and JND 667 exhibited higher levels of soluble sugar, starch, lignin, and cellulose compared to the other two varieties. Furthermore, JJ 525 had higher soluble sugar and starch contents than JND 667, but lower lignin and cellulose contents. Interestingly, the lodging index of JND 667 was lower than that of JJ 525.

Correlation analysis (Table 9) revealed significant or extremely significant negative correlations between starch, soluble sugar, lignin, cellulose, and the lodging index in the four rice varieties (except for starch content in WYD 4). These findings suggest that the levels of structural and non-structural carbohydrates in the stems can influence the lodging resistance. Increasing the content of soluble sugar, starch, lignin, and cellulose in the stems may enhance the lodging resistance of rice stems to some extent.

Table 9.

Correlation analysis of breaking moment and bending moment of the whole plant with chemical composition of stem at 30 days after heading.

Indicator	WYD 4		JYJ		JND 667		JJ 525
	LI (%)	M (g cm)	LI (%)	M (g cm)	LI (%)	M (g cm)	LI (%)	M (g cm)
SS (%)	− 0.845**	0.900**	− 0.892**	0.940**	− 0.915**	0.923**	− 0.919**	0.904**
S (%)	− 0.520	0.497	− 0.855**	0.845**	− 0.944**	0.952**	− 0.855**	0.812**
C (mg g−1DW−1)	− 0.949**	0.913**	− 0.986**	0.965**	− 0.842**	0.834**	− 0.961**	0.971**
L (mg g−1DW−1)	− 0.976**	0.970**	− 0.979**	0.954**	− 0.956**	0.950**	− 0.968**	0.964**

SS soluble sugar content, S starch content, C cellulose content, L lignin content.

Conclusion

Excessive nitrogen fertilizer can have negative effects on the carbohydrate content of the second internode at the base, leading to reduced stem plumpness and bending moment, as well as increased risk of lodging. On the other hand, increasing the length of basal internodes and reducing the thickness of the stem wall can decrease the lodging resistance of rice stems and change the stem section modulus. In this study, WYD 4 showed high yield and lodging resistance with a nitrogen application rate of 100 kg hm⁻², while the other three varieties achieved high yield and lodging resistance with a nitrogen application rate of 150 kg hm⁻².

The lodging index of the lodging-resistant varieties JND 667 and JJ 525 was found to be lower compared to the lodging-prone variety WYD 4 and the lodging-resistant variety JYJ. This can be attributed to the higher breaking bending moment and lower proportion of basal internodes in culm length of JND 667 and JJ 525. The higher fracture bending moment is primarily due to the larger section modulus of JND 667 and JJ 525 compared to WYD 4 and JYJ, resulting in higher bending stress. The section modulus is mainly influenced by the stem’s morphological characteristics, and in this case, the lodging-resistant varieties have higher stem diameter and wall thickness than the other two varieties.

Research on rice lodging resistance is a long-term and challenging endeavor. Since the "Green Revolution," lodging has consistently hindered the high-yield, stable, and high-quality development of rice. The amount of nitrogen fertilizer applied plays a crucial role in determining the lodging resistance of rice. The plant’s absorption of nitrogen can influence the root-to-shoot ratio and subsequently affect lodging resistance. Key factors such as the number of vascular bundles, the quantity of thick-walled cell layers, the degree of woodiness at the base stem, and the thickness of the cortical fiber tissue are all closely related to lodging resistance. Therefore, future research on rice lodging resistance should focus on optimizing nitrogen fertilizer application patterns based on the nutrient requirements of rice, exploring the anatomical structure of the base stem, and utilizing molecular biology to investigate the regulatory mechanisms of woodiness and fiber tissue-related genes. These areas will represent new research hotspots and priorities.

Author contributions

Y.L.: Conceptualization, methodology, software, data curation, writing—original draft preparation. J.C., S.B., W.L., X.L.: Conceptualization, methodology, visualization, investigation. Y.G., S.L., L.G.: Reviewing and editing, supervision. X.S.: Reviewing, methodology and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Key Research and Development Program of China (2022yfd1500501) and the Major Special Project of Jilin Provincial Department of Science and Technology (20230302008NC).

Data availability

Data is provided within the manuscript or supplementary information files.

Competing interests

The authors declare no competing interests.

Ethics approval and consent to participate

All methods were performed in accordance with the relevant guidelines and regulations. We have obtained permission to collect plant material and seedlings.