Effects of Sediment Content, Flooding, and Drainage Process on Rice Growth and Leaf Physiology of Early Rice During Heading–Flowering Stage

Abstract

In recent years, there has been a notable increase in the frequency and intensity of floods and heavy rains, which has resulted in the frequent inundation of rice-growing areas. Flooding during the heading–flowering stages of early rice can result in significant yield losses. To elucidate the response of rice to sediment content, flooding, and drainage processes and their underlying mechanisms, a pot experiment was conducted to investigate the effects of sediment contents (S₁: 0, S₂: 0.10 kg m⁻³, and S₃: 0.25 kg m⁻³), flooding time (F₁: 3 days and F₂: 6 days), and drainage time (D₁: 3 days and D₂: 6 days) during the heading–flowering stage on the oxidation resistance and grain yield of early rice in the Poyang Lake Region. At the same time, an experimental control group (CK) was set up with no sediment, no flooding, or no drainage treatment. The results showed that the flag leaf area of S₁F₁D₂ treatment was diminished by flooding. The relative chlorophyll content (SPAD) reached its lowest value prior to drainage. The treatment of S₂F₂D₁ showed the greatest decrease in SPAD value of 41.57%, which was only 53.88% of that of the control treatment. The activity of superoxide dismutase (SOD), peroxidase (POD), and the content of malondialdehyde (MDA) were observed to increase during the flooding period in comparison to the control treatment. The maximum values for these parameters were recorded at 5.68, 3.09, and 1.9 times higher than those of the control treatment, respectively. However, a decrease was observed after drainage. Furthermore, the occurrence of flooding during the early rice heading–flowering stage resulted in a notable reduction in the grain number per spike and the fruiting rate, consequently leading to a considerable decline in grain yields, with a decrease ranging from 31.81% to 69.96%. The findings indicate that flooding during the heading–flowering stage resulted in a reduction in early rice grain yield yet enhanced the antioxidant capacity of the leaves. Regression analyses indicated that a prediction model for the actual yield after flooding stress at the heading–flowering stage of early rice could be constructed using SFW as the independent variable. The findings of this study provide a theoretical basis for the formulation of a scientific and reasonable drainage scheme with the objective of reducing yield loss following rice flooding in the southern rice-growing region of China.

1. Introduction

Rice (Oryza sativa L.) is one of the world’s leading food crops, providing food and subsistence for half the global population. The threat of flooding, one of the most hazardous natural disasters, is a major constraint on rice production worldwide [1,2]. Between 1970 and 2019, extreme weather-triggered floods accounted for more than half of all disasters, according to the World Meteorological Organization [3]. As global warming progresses, flooding has become more severe, which has already resulted in reduced yields and huge economic losses in major crops such as rice [4]. The lower reaches of the Yangtze River, especially in the basin of Poyang Lake, is an important grain-producing area in China, with a cultivated area of 1.9 million square kilometers and a total grain output of 17.9 million tons [5]. However, due to the low-lying topography of the area, the confluence of surface and groundwater, and the frequent concentration of precipitation, the area is frequently subject to inland flooding during the tapping stage of early rice growth [6]. Flooding of rice during this period may hamper its growth and development, leading to lower yields with serious implications for food security [7]. Maintaining or improving yields and quality in flood-prone areas can be achieved by understanding how crops respond to flooding.

Various morphological and physiological indicators, as well as rice yield potential, have been studied in field and pot trials conducted by domestic and international scholars. There has been some evidence that flooding at different growth stages and durations can have different effects on the growth of the rice and final yield [8,9,10]. The first thing observed when rice is subjected to flooding stress is an alteration in the morphology of the rice. The study showed that rice plant height increased with a short period of flooding stress, but after a long period of flooding stress, rice plant height decreased [11]. Some researchers found that as the depth of flooding increased, the effective tillering rate of rice decreased by 48 percent [12]. Kato et al. have demonstrated that flooding prevents new leaf formation, reduces total leaf area, and promotes leaf senescence [13]. Singh et al. (2014) observed a drastic decrease of 70% in dry weight of leaves of sensitive varieties under 17 days of flooding stress [14].

The flooding of rice has a significant impact on its physiological characteristics, especially the ability to produce photosynthesis and antioxidants [15]. As a consequence of the flooding, there is a degradation of chlorophyll, a stimulation of stomatal closure, an increase in lipid peroxidation, and a reduction in the concentration of intracellular carbon dioxide, which in turn impairs photosynthesis [16]. Studies have shown that flooding stress leads to a reduction in photosynthetic rate by half after 4 days of hypoxia in rice [17]. Under flooding stress, plants can rely on antioxidant enzyme systems and other active antioxidants to maintain the dynamic balance of ROS, thus reducing the extent of oxidative damage [18,19]. In plants, superoxide dismutase (SOD) is the first system of defense against oxidation, and under continuous flooding stress, SOD activity usually increases as a response to flood-induced oxidative stress to protect plants against oxidative damage [20,21]. Additionally, peroxidase (POD) activity is elevated in response to flooding stress, facilitating the breakdown and scavenging of hydrogen peroxide. This reduces the accumulation of hydrogen peroxide within cells, mitigates oxidative damage, and enhances plant resilience [22]. Malondialdehyde (MDA) is a commonly used indicator of cell membrane damage. MDA levels tend to increase under continuous flooding conditions, with high MDA levels reflecting the extent of damage to cell membranes [23]. The results demonstrated that under flooding conditions, the MDA content in rice exhibited a marked increase, whereas rice plants mitigated ROS toxicity by enhancing the activities of SOD and POD. With the prolongation of the flooding period, the SOD activity in rice displayed a tendency to initially increase and subsequently decline, while the POD activity exhibited a gradual increase [24,25].

The heading–flowering stage represents a critical period for the formation of rice yield, with a direct impact on the yield and quality of the final product [26]. The capacity of rice to tolerate flooding is relatively limited during the reproductive growth stage. At this stage, the negative impact of flooding on rice yield is greater than the positive contribution of flooding tolerance to yield, resulting in a relatively greater adverse effect of flooding on rice yield. This results in a more pronounced reduction in yield due to flooding at the jointing–booting stage and the heading–flowering stage than at other stages of rice growth [27]. Flooding that occurs during the heading–flowering stage has a significant impact on the exchange of oxygen and carbon dioxide gases between rice tissues and the atmosphere. This restricts photosynthesis and the accumulation of dry matter and accelerates the depletion of energy reserves [28]. Additionally, it impairs the normal flowering and pollination of rice, increasing the incidence of sterility and empty grains, markedly reducing the fruiting rate of rice, and causing significant yield losses [29].

The majority of previous studies have concentrated on the growth, development, and physiological characteristics of rice under fixed conditions of flooding time and flooding depth. However, the actual processes of flooding and drainage are frequently dynamic and complex. In this study, we set up different flooding times, drainage times, and sediment contents to investigate the response mechanisms of rice physiology to dynamic inundation and to analyze the effects of different treatments on rice growth and development, as well as the interactions of the factors on rice yield and its components. Ultimately, a suitable drainage program is proposed. This study offers a theoretical basis for reducing rice yield loss in Southern Chinese rice-producing regions after flooding.

2. Materials and Methods

2.1. Experimental Site

The rice potting trial was conducted from May to August 2022 at the Jiangxi Irrigation Experiment Central Station in Nanchang County, Jiangxi Province (located at 116°00′ E, 28°26′ N), which is a typical humid monsoon climate zone in the subtropics (Figure 1). Over the preceding 44 years (1978–2021), the research region experienced average annual conditions, including a temperature of 17.6 °C, 1506 mm of rainfall, 1720.8 sunshine hours, and 915.0 mm of pan evaporation. Detailed weather data, including maximum and minimum temperatures and rainfall, were documented by the Research Base of the Jiangxi Provincial Irrigation Experiment Central Station during the study period (Figure 1). Rainfall in the region is seasonally distributed, with approximately 66% occurring between March and July. The average light duration during the potting experiment was 3.58 h, and the relative humidity was 87.5%. The soil used in our study was alluvial yellow soil, sourced from the top 20 cm of a paddy field and exhibiting the following properties: 13.9 g kg⁻¹ of organic matter, 1.2 mg kg⁻¹ of total nitrogen, 0.50 mg kg⁻¹ of total phosphorus, 5.8 mg kg⁻¹ of total potassium, 11.60 mg kg⁻¹ of effective phosphorus, 118.0 mg kg⁻¹ of available potassium, a pH level of 5.2, and a soil density of 1.2 g cm⁻³.

2.2. Experimental Design

The rice flooding and drainage process was conducted in a flooding pool equipped with a lifting platform that can be adjusted within a range of 0 to 120 cm in height (Figure 2). The pool was constructed from reinforced concrete and measured 4.7 m × 3 m × 1.2 m in size. A water suction pipe was located at the upper portion of the pool, while a drainage pipe was situated at the lower end. Up to three pots can be placed on each lifting platform for testing as a group, and each group is equipped with a water pump and is connected to a PVC pipe (drilled holes) to stir and model sediment content. The experiment employed an early rice variety, Xiangzaoxian 45. The flooding stress experiments were conducted at the heading–flowering stage, with due consideration given to the characteristics of flood stress in Southern China. In this study, there were thirteen treatments (Figure 2): three sediment contents (0 kg m⁻³, S₀; 0.10 kg m⁻³, S₁; 0.25 kg m⁻³, S₂), two flooding times (3 days, F₁; 6 days, F₂), two drainage times (3 days, D₁; 6 days, D₂), and one control (CK). The control group consisted of rice plants grown under standard conditions without any sediment content, flooding, or drainage treatment. A total of 78 treatment pots (40 cm × 30 cm × 20 cm) were used for six replications (Figure 2). The three-leaf seedlings were then transplanted into the pots, with six holes in each pot and two seedlings per hole. At the commencement of the rice heading–flowering stage, the F₁ and F₂ treatments were submerged below the water surface, with the distance between the water surface and the last leaf tip measuring approximately 5–10 cm. This indicates that, following the specified period of inundation, a period of three days and six days is required, respectively, to reduce the standing water to a depth below the crop’s tolerance to inundation (10 cm above the soil surface). Once the flooding and drainage processes were complete, the pots were removed from the pools and allowed to recover and grow until the time of harvest. Prior to transplantation, rice was fertilized with basal fertilizer, followed by subsequent fertilizer applications during the tillering and heading–flowering stages. Each treatment received a total of 180 kg ha⁻¹ of nitrogen fertilizer, 67.5 kg ha⁻¹ of phosphorus fertilizer, and 150 kg ha⁻¹ of potassium fertilizer. Pest and weed control measures were executed in accordance with seasonal agricultural practices, while additional management strategies were aligned with local customs.

2.3. Sampling and Measurements

2.3.1. Plant Height and Tillering

At the outset of the experiment, six designated observation points were selected for each treatment, and measurements of plant height and tiller count were systematically documented at three-day intervals.

2.3.2. Leaf SPAD Value and Leaf Area

From the outset of the submergence, measurements of the relative chlorophyll content (SPAD value) in the rice leaves were initiated using a chlorophyll meter (SPAD-502 Plus, Konica Minolta Optics, Inc., Tokyo, Japan). For each treatment, six representative rice upper fully expanded flag leaves (the first leaf from the top) were selected, and SPAD readings were obtained from the top, middle, and bottom sections of each leaf with great care and precision. The mean SPAD value for each leaf was recorded at regular three-day intervals using the chlorophyll meter. In the meantime, the leaf length and width of flag leaves were also measured to determine leaf area, which was calculated as leaf length × width × 0.75. When performing data measurement, the potted plants were lifted out of the water and immediately sunk back after measurement.

2.3.3. Leaf Antioxidant Properties

Five to seven representative leaf samples were selected from each experimental group with great care and on the same day as the SPAD measurements. Concurrent measurements of malondialdehyde (MDA), peroxidase (POD), and superoxide dismutase (SOD) activities in these leaves were carried out. MDA was measured using a thiobarbituric acid (TBA) colorimetric assay; POD was assessed by POD-catalyzed hydrogen peroxide (H₂O₂) oxidation of specific substrates; and SOD was determined using the water-soluble tetrazolium monosodium salt (WST-8) method [30]. To ensure accuracy, measurements for each treatment were repeated on three occasions, and the mean values were calculated.

2.3.4. Aboveground Biomass (AGB) and Yield

At maturity, the aboveground parts of rice plants were cut and washed in clean water and divided into stems, leaves, and grains for dry matter determinations. The fresh plants were dried at 105 °C for 30 min and then kept at 85 °C until constant weight. Nine plant samples were collected for each treatment to determine the average, grain number per spike, effective number of spikes, fruiting rate, and thousand-grain weight. The remaining plants in the pots for each treatment were taken for determination of the actual yield. Grain yield was determined after the grains were air-dried and adjusted to the standard moisture content of 13.5%.

2.3.5. Statistical Analysis

All statistical analyses were conducted using the SPSS 23.0 software package (SPSS Inc., Chicago, IL, USA). The significance of the difference analysis was determined by means of one-way analysis of variance (ANOVA). The means that were found to be significantly different were then compared using the least significant difference (LSD) test. Multiple comparisons were conducted using Duncan’s new multiple range test (MRT). The correlation was analyzed using Pearson’s dual test. Figures were plotted using Figdraw (www.figdraw.com, accessed on 31 August 2024), Origin 2021 software (Northampton, MA, USA), and Microsoft Excel 2016 (Microsoft Cooperation, Redmond, WA, USA).

3. Results

3.1. Effect of Sediment Content, Flooding, and Drainage Process on Plant Height and Tillering

Differences in the effects of different sediment contents, flooding time, and drainage time on rice plant height and tillering were not significant in most treatments (

Table 1. Rice plant height under different treatments.

Treatment	Plant Height
	Before Flooding (cm)	Before Drainage (cm)	After Drainage (cm)	Average Growth Rate (cm d−1)
CK	69.97 ± 1.29 ab	72.73 ± 1.42 abc	74.57 ± 1.08 a	0.38 ± 0.15 a
S0F1D1	67.20 ± 2.31 b	70.93 ± 2.45 c	73.00 ± 3.60 ab	0.97 ± 0.55 a
S0F1D2	71.03 ± 2.94 ab	75.00 ± 3.18 abc	76.97 ± 3.33 ab	0.66 ± 0.14 a
S0F2D1	70.63 ± 2.59 ab	76.57 ± 2.31 ab	77.90 ± 1.90 ab	0.81 ± 0.39 a
S0F2D2	71.67 ± 1.88 ab	77.07 ± 1.15 ab	76.77 ± 0.65 ab	0.43 ± 0.17 a
S1F1D1	70.07 ± 3.61 ab	74.77 ± 1.72 abc	76.90 ± 0.85 ab	1.14 ± 0.47 a
S1F1D2	70.70 ± 2.82 ab	73.67 ± 0.58 abc	76.60 ± 2.62 b	0.66 ± 0.46 a
S1F2D1	68.10 ± 0.52 b	73.40 ± 2.99 abc	73.43 ± 3.79 ab	0.59 ± 0.40 a
S1F2D2	69.30 ± 1.91 b	73.77 ± 3.50 abc	76.13 ± 2.61 ab	0.57 ± 0.37 a
S2F1D1	68.20 ± 0.36 b	70.80 ± 2.51 c	72.97 ± 6.55 ab	0.79 ± 1.09 a
S2F1D2	70.07 ± 2.80 ab	72.23 ± 3.23 bc	76.50 ± 2.18 ab	0.71 ± 0.14 a
S2F2D1	71.03 ± 2.37 ab	75.47 ± 1.75 abc	76.20 ± 1.47 ab	0.57 ± 0.10 a
S2F2D2	71.57 ± 0.29 a	77.37 ± 1.36 a	78.87 ± 3.84 ab	0.61 ± 0.10 a

Data are the mean of three replicates; different letters represent statistically significant differences within one column (p < 0.05).

and

Table 2. Rice tillering under different treatments.

Treatment	Tillering
	Before Flooding (cm)	Before Drainage (cm)	After Drainage (cm)	Spike Rate (%)
CK	17.67 ± 1.15 abc	17.78 ± 2.04 ab	18.00 ± 2.00 abc	88.89
S0F1D1	17.33 ± 0.58 bc	17.00 ± 1.00 ab	17.00 ± 1.00 abc	57.39
S0F1D2	17.67 ± 1.15 abc	17.33 ± 1.15 ab	18.33 ± 1.53 abc	76.75
S0F2D1	18.33 ± 1.15 abc	19.67 ± 0.58 a	18.67 ± 0.58 abc	62.32
S0F2D2	19.00 ± 1.00 ab	18.67 ± 0.58 ab	19.00 ± 1.00 abc	59.63
S1F1D1	17.67 ± 0.58 abc	17.00 ± 1.00 ab	17.67 ± 0.58 abc	69.78
S1F1D2	18.67 ± 0.58 abc	18.67 ± 1.53 ab	19.33 ± 1.53 ab	65.00
S1F2D1	19.67 ± 1.53 a	19.67 ± 2.52 a	20.00 ± 3.00 a	76.65
S1F2D2	17.67 ± 0.58 abc	17.33 ± 0.58 ab	15.67 ± 1.53 c	65.92
S2F1D1	17.33 ± 0.58 bc	17.67 ± 0.58 ab	17.67 ± 1.15 abc	51.83
S2F1D2	17.00 ± 1.00 bc	15.67 ± 1.53 b	16.33 ± 2.08 bc	64.71
S2F2D1	17.00 ± 1.73 bc	17.67 ± 1.53 ab	18.33 ± 1.53 abc	83.93
S2F2D2	16.67 ± 1.15 c	17.00 ± 1.73 ab	19.00 ± 1.00 abc	73.68

Data are the mean of three replicates; different letters represent statistically significant differences within one column (p < 0.05).

). The rice plant height results showed that the S₁F₁D₂ treatment significantly (p < 0.05) increased in height after drainage compared to the control. This suggests that the specific combination of sediment content and partial flooding in this treatment may have a positive effect on plant height. In all other treatments, plant height was not significantly different (p < 0.05) from the control.

In terms of tillering, the data showed no significant difference from the control before and after drainage, except for the S₁F₂D₂ treatment, which showed a negative effect after drainage. The results showed that tillering changes in all treatments were similar to the control, indicating that the experimental conditions did not significantly affect tillering in rice plants at the tassel flowering stage. However, spike formation was lower in all treatments compared to the control, suggesting that sediment content, flooding, and drainage process had a negative effect on the formation of effective spikes.

3.2. Effect of Sediment Content, Flooding, and Drainage Process on Leaf Area and SPAD Value

The effects of varying sediment content, flooding time, and drainage time on rice leaf area were not statistically significant in the majority of treatments (

Table 3. Rice leaf area under different treatments.

Treatment	Leaf Area
	Before Flooding (cm2)	Before Drainage (cm2)	After Drainage (cm2)	Average Growth Rate (cm2 d−1)
CK	31.59 ± 0.86 a	35.12 ± 1.04 a	37.60 ± 0.80 a	0.50 ± 0.03 a
S0F1D1	29.62 ± 2.73 a	33.08 ± 2.09 abc	34.00 ± 1.54 ab	0.53 ± 0.13 a
S0F1D2	31.04 ± 2.77 a	32.64 ± 0.58 abc	33.32 ± 1.07 ab	0.42 ± 0.30 a
S0F2D1	30.13 ± 0.75 a	33.84 ± 2.20 ab	36.02 ± 3.66 ab	0.53 ± 0.08 a
S0F2D2	30.58 ± 1.72 a	33.48 ± 1.67 abc	35.84 ± 2.16 ab	0.44 ± 0.32 a
S1F1D1	30.28 ± 2.53 a	32.93 ± 1.40 abc	33.12 ± 1.04 ab	0.58 ± 0.19 a
S1F1D2	29.80 ± 0.84 a	30.62 ± 0.48 c	31.85 ± 2.00 b	0.51 ± 0.17 a
S1F2D1	31.55 ± 2.05 a	34.28 ± 0.90 ab	34.44 ± 3.24 ab	0.38 ± 0.05 a
S1F2D2	29.68 ± 1.81 a	33.14 ± 0.88 abc	34.19 ± 1.11 ab	0.37 ± 0.23 a
S2F1D1	30.55 ± 0.86 a	32.37 ± 0.86 abc	34.03 ± 2.26 ab	0.47 ± 0.26 a
S2F1D2	29.49 ± 1.25 a	33.37 ± 1.05 abc	33.51 ± 1.08 ab	0.44 ± 0.05 a
S2F2D1	28.95 ± 2.09 a	32.29 ± 2.75 abc	33.40 ± 1.91 ab	0.43 ± 0.11 a
S2F2D2	29.22 ± 0.67 a	31.69 ± 0.47 bc	33.80 ± 1.29 ab	0.38 ± 0.09 a

Data are the mean of three replicates; different letters represent statistically significant differences within one column (p < 0.05).

). The S₁F₁D₂ and S₂F₂D₂ treatments exhibited a notable decline in rice leaf growth prior to drainage when compared to the CK treatment (p < 0.05). A significant difference in leaf area was observed between the S₁F₁D₂ treatment and the CK treatment following drainage (p < 0.05). The impact of varying sediment content, flooding time, and drainage time on the average growth rate of rice exhibited no significant differences in all treatments. However, following drainage at the rice heading–flowering stages, it was observed that the leaf area was smaller in all treatments than in the CK treatment.

The effects of different sediment content, flooding time, and drainage time on rice SPAD value exhibited notable differences (Figure 3). After the beginning of the experiment, the SPAD value of the leaves exhibited a gradual increase in the CK treatment, whereas a decreasing trend was observed in the other treatments. The greatest decline in SPAD value was observed in S₂F₂D₁, with a reduction of 41.57% on the sixth day following continuous submergence, which was only 53.88% of that of the control treatment. The SPAD values of all treatments that were subjected to complete flooding and without drainage were found to be significantly lower (p < 0.05) than those of the CK treatment (p < 0.05). When there was prolonged flooding (six days), SPAD values were significantly lower in the S₂ treatment than in the S₀ and S₁ treatments, indicating that the high sediment content disrupted the physiological activities of rice and further reduced photosynthesis.

3.3. Effect of Sediment Content, Flooding, and Drainage Process on Antioxidant Activity

3.3.1. Activity of SOD

The variation of SOD activity in CK rice leaves during the heading–flowering stage was small, fluctuating around 300 U g⁻¹ (Figure 4). From the overall trend, leaf SOD activities increased and then decreased with the flooding and drainage process, reaching the maximum value when flooding lasted for 3 days. The S₁F₂D₂ treatment showed the greatest increase in leaf SOD activity at 3 days of flooding, which was 5.68 times higher than that of the CK treatment. Regarding the sediment content, the SOD activity of the S₂ treatment was significantly lower than that of the S₀ and S₁ treatments at a certain time of flooding and drainage, indicating that the high sediment content disturbed the dynamic balance of the rice enzyme system and affected the production of SOD.

3.3.2. Activity of POD

The variation in POD activity in CK rice leaves at the heading–flowering stage was minimal, exhibiting fluctuations around 5000 U g⁻¹ (Figure 5). The overall trend in POD activity exhibited a decrease initially, followed by a sharp increase and subsequent decrease in the progression of the flooding and drainage process. The maximum value was reached on the sixth day after the commencement of the experiment (22 June). The leaf POD activity of S₁F₁D₁ treatment decreased to the lowest value on the sixth day of drainage, which was only 39.41% of that of CK in the same period. The maximum increase in leaf POD activity was observed in the S₀F₂D₁ treatment on the sixth day of flooding, with a value 3.09 times higher than that of the CK treatment. In terms of sediment content, higher POD activities were observed in the S₀ and S₁ treatments than in the S₂ treatment in F₁, and lower POD activities were observed in the S₀ and S₁ treatments than in the S₂ treatment in F₂ when the flooding and drainage times were constant. This suggests that POD activity is less responsive to sediment content and more sensitive to the duration of flooding.

3.3.3. MDA Content

The MDA content of the CK in this experiment fluctuates around 15 µmol g⁻¹ (Figure 6). The general trend indicates that the MDA content initially increased and then declined with the progression of the flooding and drainage process, reaching a maximum value after the flooding (without drainage). The greatest increase in MDA content was observed in the leaves of the S₂F₂D₁ treatment, which occurred during the sixth day of flooding. When the sediment content was held constant, the MDA content of the leaves increased by 65.4% in the F₁ treatment after the flooding compared to the CK. Following the conclusion of the flooding period in the F₂ treatment, the MDA content exhibited a 91.1% increase in comparison to the CK. When the receding time was held constant, the MDA content of the S₂ treatment was observed to be higher than that of the S₀ and S₁ treatments. The leaf MDA content at the conclusion of the six-day flooding in treatment D₁ was higher than that observed in treatment D₂. The MDA content of the S₁F₁D₂ treatment was the highest at the conclusion of the experiment, exhibiting a ratio of 1.2 times that of CK.

3.4. Effect of Sediment Content, Flooding, and Drainage Process on Aboveground Biomass (AGB) and Yield

The aboveground biomass exhibited a decline in all treatments except the S₀F₂D₁ treatment following the implementation of distinct sediment content, flooding, and drainage process during the early rice heading–flowering stage (Figure 7). The S₁F₁D₁ treatment showed the lowest total accumulation, 31.09% lower than the CK treatment. After a three-day flooding period, the total bioaccumulation for sediment content S₀, S₁, and S₂ decreased by 11.28%, 24.89%, and 18.83%, respectively, compared to CK. After a six-day flooding period, the total accumulation for S₀, S₁, and S₂ decreased by 5.23%, 11.95%, and 27.75%, respectively, compared to CK. This indicates that bioaccumulation is inversely proportional to flooding duration, with six-day flooding showing the greatest reduction. After the flooding and drainage process, spike and leaf biomass decreased, while stem biomass increased. The AGB increase in S₀F₂D₁ was due to a significant stem expansion. The stem biomass of S₀F₂D₁ and S₀F₂D₂ increased by 114% and 80.38%, respectively, compared to CK (p < 0.05). Similarly, S₁F₂D₁ and S₁F₂D₂ increased by 54.13% and 71.16%, respectively. Leaf biomass in S₀F₁D₁, S₁F₁D₂, and S₂F₂D₁ treatments decreased significantly by 42.67%, 40.64%, and 42.37%, respectively, compared to CK. Spike biomass, except for S₀F₁D₁ and S₀F₁D₂, decreased significantly compared to CK (p < 0.05). Among these, S₀F₂D₂ exhibited the lowest spike accumulation, which was found to be significantly reduced by 60.85% in comparison to CK.

With regard to rice yield and yield components, the grain yield of early rice at the heading–flowering stage was found to decrease significantly in the presence of different sediment contents, flooding, and drainage processes in comparison with the CK treatment (

Table 4. Rice yield and yield components under different treatments.

Treatment	Effective Spike Number	Grain Number per Spike	Fruiting Rate (%)	Thousand-Grain Weight (g)	Yield (g pot−1)	Relative Yield (%)
CK	13.00 ± 1.00 abcd	106.40 ± 3.79 a	87.87 ± 1.79 a	24.26 ± 0.50 a	128.70 ± 3.49 a	—
S0F1D1	10.33 ± 2.31 cd	86.55 ± 18.92 b	67.33 ± 7.18 bc	22.94 ± 0.36 ab	87.77 ± 19.22 b	68.19
S0F1D2	14.33 ± 1.53 abc	82.80 ± 2.93 b	60.97 ± 4.56 bc	22.93 ± 1.26 ab	76.80 ± 22.36 bc	59.67
S0F2D1	12.67 ± 1.53 abcd	79.96 ± 2.63 b	54.66 ± 3.08 cd	22.43 ± 0.49 abc	58.14 ± 7.08 de	45.18
S0F2D2	11.33 ± 3.79 abcd	82.86 ± 8.69 b	40.46 ± 11.00 de	22.26 ± 0.30 abc	38.67 ± 3.68 g	30.04
S1F1D1	12.33 ± 2.31 abcd	83.62 ± 4.50 b	55.08 ± 1.06 c	22.15 ± 0.47 abc	61.14 ± 9.30 de	47.5
S1F1D2	13.00 ± 1.53 abcd	80.25 ± 2.93 b	57.96 ± 4.56 ac	23.31 ± 1.26 ab	63.91 ± 6.68 d	49.66
S1F2D1	15.33 ± 1.00 ab	82.64 ± 12.92 b	32.64 ± 14.64 e	20.32 ± 0.27 cd	67.45 ± 8.34 cd	52.41
S1F2D2	10.33 ± 1.53 cd	76.94 ± 12.82 b	37.26 ± 4.10 e	18.86 ± 1.46 d	49.69 ± 11.89 efg	38.61
S2F1D1	9.33 ± 0.58 d	78.42 ± 7.08 b	68.94 ± 6.73 bc	23.62 ± 2.34 ab	49.25 ± 5.13 efg	38.27
S2F1D2	11.00 ± 0.58 bcd	89.13 ± 9.52 ab	58.40 ± 11.32 b	22.18 ± 1.51 abc	51.06 ± 11.99 ef	39.68
S2F2D1	15.67 ± 4.00 a	81.51 ± 8.30 b	54.98 ± 0.64 c	23.03 ± 0.59 ab	44.34 ± 10.50 fg	34.45
S2F2D2	14.00 ± 2.52 abc	81.95 ± 3.40 b	39.80 ± 6.38 e	21.88 ± 1.01 bc	44.93 ± 5.42 fg	34.91
S	ns	ns	**	**	**	—
F	ns	*	**	**	**	—
D	ns	ns	*	ns	*	—
S × F	*	ns	ns	*	**	—
S × D	ns	*	**	ns	*	—
F ×D	**	ns	*	ns	*	—
S × F × D	ns	*	*	ns	ns	—

Data are the mean of three replicates; different letters represent statistically significant differences within one column (p < 0.05); “**” indicates significance at the 1% probability level, “*” indicates significance at the 5% probability level, and “ns” indicates non-significance. S: sediment content; F: flooding time; and D: drainage time.

). The sediment content and flooding time had a significant impact on the fruiting rate, thousand-grain weight, and yield. With the exception of S₁F₁D₂, the fruiting rate of the remaining treatments was found to be significantly lower in comparison to CK. The fruiting rate demonstrated a decline with an increase in sediment content and flooding time. In this experiment, the S₀F₁D₁ treatment exhibited the highest fruiting rate and actual yield at 67.3% and 87.8 g per pot, respectively. However, its yield reduction exceeded 30%. The yield of the S₀F₁D₁, S₀F₁D₂, and S₁F₂D₁ treatments was reduced by less than 50%, whereas the remaining treatments exhibited a significantly more pronounced reduction in yield. The lowest yield was observed for the S₀F₂D₂ treatment, which was 30.04% of that of CK.

The effects of sediment content (S) and flooding time (F) on fruiting rate (FR), thousand-grain weight (TGW), and yield (Y) were statistically significant (p < 0.01). The effects of drainage time on fruiting rate and yield were statistically significant (p < 0.05). The interactive effects of sediment content and flooding time (S × F) on effective spike number (ESN) and thousand-grain weight (TGW) were statistically significant (p < 0.05), while the effects on yield (Y) were highly statistically significant (p < 0.01). The interactive effects of sediment content and drainage time (S × D) on grain number per spike (GNPS) and yield (Y) were statistically significant (p < 0.05), while the interactive effects on fruiting rate (FR) were statistically significant (p < 0.01). The interactive effects of flooding time and drainage time (F × D) on fruiting rate (FR) and yield (Y) were significant (p < 0.05), as were the interactive effects on effective spike number (ESN) (p < 0.01). The interactive effects of the three factors (S × F × D) on GNPS and FR reached a level of significance (p < 0.05).

3.5. Correlation Analysis

After the end of flooding at the early rice heading–flowering stage, the actual yields at maturity of the treatments were correlated with the indicators, where the values of SOD activity, POD activity, and MDA content were used after the complete drainage of water on the twelfth day (Figure 8). Analysis showed that actual yield was positively correlated with leaf biomass, POD activity, effective spike number, and thousand-grain weight; negatively correlated with stem biomass, SOD activity, and MDA content; significantly positively correlated with fruiting rate (p < 0.05); and highly significantly positively correlated with spike biomass and grain number per spike (p < 0.01). In terms of the correlation between yield components and other indexes, the grain number per spike was significantly negatively correlated with stem biomass (p < 0.05) and positively correlated with spike biomass (p < 0.05); and fruiting rate was significantly positively correlated with spike biomass and grain number per spike (p < 0.05) and negatively correlated with stem biomass (p < 0.05).

These results suggest that yield reductions due to decreased grain number per spike and fruiting rate are linked to increased stem biomass allocation. Additionally, leaf SOD activity was significantly negatively correlated (p < 0.05) with leaf biomass, indicating a close relationship between increased SOD activity and reduced leaf biomass allocation after different sediment content, flooding, and drainage treatments.

3.6. Study of Drainage Programs for Rice After Flooding

The flooding time (F) and drainage time (D) can be transformed into the cumulative value of flooding depth (SFW). The regression equation parameterized by relative yield (Ry) was able to express the relationship between Ry and flooding time (F), drainage time (D), and Ry and SFW under different sediment contents very with a high degree of significance. The correlations of F, D, and SFW with yield and its components at different sediment contents after different flooding and drainage treatments at the early rice heading–flowering stage are shown in

Table 5. Correlation between F, D, and SFW at heading–flowering stage of early rice and yield and its components.

Sediment Content	Impact Factor	Effective Spike Number	Grain Number Per Spike	Fruiting Rate (%)	Thousand-Grain Weight (g)	Yield (g pot−1)
S0	F	−0.660	−0.447	−0.720 *	−0.447	−0.872 **
	D	0.265	−0067	−0.446	−0.067	−0.397
	SFW	0.059	−0.430	−0.830 **	−0.430	−0.958 **
S1	F	0.038	−0.125	−0.836 **	−0.765 **	−0.230
	D	−0.500	−0.264	0.145	−0.037	−0.436
	SFW	−0.189	−0.230	−0.683 *	−0.701 *	−0.401
S2	F	0.717 **	−0.149	−0.836 **	−0.187	−0.971 *
	D	0.003	0.406	−0.206	−0.545	0.211
	SFW	0.642 *	0.048	−0.840 **	−0.411	−0.408

Data are the mean of three replicates; “**” indicates significance at the 1% probability level, “*” indicates significance at the 5% probability level. S: sediment content; F: flooding time; and D: drainage time.

. Additionally, the regression equations and correlation coefficients between F and D, SFW, and yield are shown in

Table 6. Regression model between heading–flowering stage F, D, SFW, and yield of early rice.

Sediment Content	Influencing Factor	Regression Equation	R	F Value	Sig
S0	F and D	Ry = 69.42 − 0.872F − 0.397D	0.901	50.894 **	0.000 **
	SFW	Ry = 50.108 + 0.302SFW2 − 0.711SFW	0.929	58.752 **	0.000 **
S1	F and D	Ry = 59.593 − 0.275F − 0.542D	0.892	2.930	0.294
	SFW	Ry = −7.272 + 20.572SFW2 − 1.773SFW	0.863	3.141	0.371
S2	F and D	Ry = 74.868 − 0.971F − 0.891D	0.965	42.817 *	0.007 *
	SFW	Ry = 39.380 + 0.299SFW2 − 0.106SFW	0.611	0.921	0.224

Data are the mean of three replicates; “**” indicates significance at the 1% probability level, “*” indicates significance at the 5% probability level. S: sediment content; F: flooding time; and D: drainage time.

. The regression models of Ry with F and D (Ry = 69.42 − 0.872F − 0.397D) and Ry with SFW (Ry = 50.108 + 0.302SFW² − 0.711SFW) for the S₀ treatment and the regression model of Ry, F, and D (Ry = 74.868 − 0.971F − 0.891D) for the S₂ treatment were the most highly correlated with the significance of the highest, which can be used as a model for the relationship drainage after flooding during the early rice heading–flowering stage. The study revealed that flooding at the early rice heading–flowering stage resulted in an average yield reduction of 55.12%. Therefore, when flooding occurs at the heading–flowering stages of early rice, with a drainage target of a 50 percent reduction in rice yield, flooded fields should be drained to a normal storage depth (about 10 cm) 5–8.5 days after flood recession if the duration of flooding is 0–3 days, and 2–5 days after flood recession if the duration of flooding is 3–6 days.

4. Discussion

The heading–flowering stage of rice represents a pivotal phase of reproductive growth, during which the plants exhibit heightened sensitivity to water availability. According to the analysis of rice growth (Table 1), after drainage, the S₀F₁D₁ and S₂F₁D₁ treatments showed a positive effect on plant height (p < 0.05). This finding is consistent with the results of previous studies [31,32]. A substantial body of prior research has demonstrated that flooding treatment can facilitate rice growth, primarily due to the fact that rice produces elevated levels of ethylene and gibberellins in order to adapt to flooding in a waterlogged environment. This will stimulate cell division and elongation, promoting the growth of leaf sheaths, leaves, and stem segments [33]. According to the analysis of rice tillering (Table 2), the results of the study showed that there was no significant change in the number of tillers in most of the treatments that were flooded at the heading–flowering stage, which is inconsistent with the results of a previous study [27]. This is because the number of tillers is essentially fixed at the heading–flowering stage. In this study, it was observed that ineffective tillers exhibited progressive wilting in the later stages of growth, resulting in a reduction in the effective spike number in the treatments. The presence of an excess of ineffective tillers results in greater consumption of carbon, which in turn leads to a reduction in the fruiting rate and thousand-grain weight, thus negatively impacting the formation of yield [34].

Leaf area and SPAD value are important parameters to measure crop photosynthesis [15]. These factors influence the absorption and conversion of light energy and aboveground biomass and are therefore closely related to crop yield [35,36]. According to the analysis of leaf area and SPAD value (Table 3) (Figure 3), rice leaf growth in S₁F₁D₂ and S₂F₂D₂ treatments was significantly reduced before drainage, whereas SPAD values were also reduced after different flooding and drainage treatments, which is in agreement with previous studies [14,37]. The results showed that the SPAD value of rice under S₂ treatment was significantly smaller, exhibiting the most pronounced decline in comparison to the CK. This was because high sediment content could result in a reduction in light transmittance of the waterlogged pond water, and a layer of mud was attached to the surface of the rice leaf, resulting in the hindrance of the chlorophyll synthesis process. Moreover, we observed that the S₂F₂D₁ treatment had the largest decrease in the SPAD value after a six-day flooding period.

Adverse stress increases the production of reactive oxygen species (ROS) in rice [23,38]. SOD and POD activities are key indicators of a plant’s capacity to eliminate excess ROS and resist aging, reflecting crop sensitivity to stress. MDA content serves as an indicator of stress-induced damage [39,40]. It has been demonstrated that the activities of SOD and POD and the content of MDA in rice increase with the duration of flooding, up to a point. However, when flooding persists for a length of time, the antioxidant enzyme system in the leaf is compromised, and the activities of SOD, POD, and the content of MDA decrease with increasing flooding time. The results of this experiment on SOD activity and MDA content were found to be consistent with those of previous studies [41]. The activity of SOD reached its maximum on the third day of flooding but subsequently declined on the sixth day (Figure 4). This is likely due to the adverse effects of flooding on rice growth and development, which disrupt the dynamic equilibrium of the enzyme system. A slight decline in POD activity was observed on the third day of flooding, which may be attributed to the decline in temperature that occurred when the rice was completely immersed in water. Subsequently, due to the continuous accumulation of ROS, the activity of POD reached its maximum before the conclusion of the flooding (Figure 5). The reduction in sediment levels within the treatment resulted in elevated SOD and POD activities, a more rapid response to flooding stress, and enhanced antioxidant capacity. Conversely, the MDA content was observed to be higher in S₁ and S₂ treatments in comparison to S₀, indicating that the presence of sediment intensifies ROS accumulation and membrane lipid peroxidation, leading to cellular damage and accelerated aging. As flooding continues, the plant’s response to abiotic stress slows. By the end of the experiment, MDA content in all treatments exceeded that of CK (Figure 6), showing that damaged cells cannot quickly recover after flooding ceases, significantly impacting yield.

The experiment (Figure 7) showed that the aboveground biomass (AGB) of rice decreased after flooding and drainage during the heading–flowering stage, consistent with previous studies [13,14]. Furthermore, it was observed that there was an increase in stem biomass as the sediment content, flooding time, and drainage time increased. This trend can be attributed to modifications in organ partition coefficients, which have the potential to augment the dry matter mass of rice stems and leaves during flooding. This phenomenon is primarily attributable to the fact that rice produces elevated levels of ethylene and gibberellin in conditions of waterlogging, which stimulate cell division and elongation, thereby promoting stem segment growth. However, this increased dry matter accumulation necessitates additional carbon assimilation, which ultimately results in a reduction in rice yield [3,42,43].

Previous studies proved that submergence of rice at the tillering stage primarily results in a reduction in effective spike number and a reduction of grain number per spike, while at the heading–flowering stage, it primarily affects the fruiting rate and thousand-grain weight [10,44]. The present study demonstrates that the dynamic flooding and drainage process of early rice at the heading–flowering stage mainly resulted in a decrease in grain number per spike and fruiting rate, leading to a decrease in yield. The reason for this is that the heading–flowering stage is the vegetative growth stage of early rice, and sediment content and flooding stress may cause spikelet degeneration by affecting spikelet development, pollen fertility, stigma activity, and assimilative transporting [29,45,46]. In addition, the anaerobic respiration of rice in water increased, and it continuously consumed organic matter, which ultimately led to a decline in thousand-grain weight and affected grain yield [47,48]. The effects of sediment content and flooding time on fruiting rate were found to be statistically significant. With regard to actual yield, it can be stated that sediment content, flooding time, and drainage time exert a varying degree of influence on rice yield. However, the interaction between sediment content and submergence duration on yield reaches an extremely significant level, indicating that sediment content and flooding time have a more significant effect on yield.

It should be noted that the experiment was conducted in pots with controlled experimental conditions and that only sediment content, flooding time, and drainage time were considered as influencing factors. The experiment was conducted for only one year; therefore, the effect under field test conditions requires further determination and verification. Furthermore, the experiments were conducted using a single rice variety. Different rice varieties can exhibit varying responses to flooding and drainage conditions due to genetic differences. Therefore, the results obtained in this study may not be universally applicable to all rice varieties. Future research should aim to validate these findings using multiple rice varieties to ensure the generalizability of the results.

5. Conclusions

This study shows that after flooding, the leaf area and growth rate of rice decreased. The SPAD value of leaves dropped to its lowest point before drainage and recovered afterward. Total above-ground biomass declined, with reduced accumulation of leaf and spike biomass, while the proportion of stem biomass increased. Activities of SOD and POD and MDA content in rice leaves initially rose and then fell under different treatments. SOD activity responded quickly to stress, peaking on the third day of continuous flooding, whereas POD activity and MDA content peaked on the sixth day. Rice’s antioxidant resistance increased post-submergence and responded swiftly to stress with lower sediment content. However, prolonged flooding (six days) caused cellular damage, hindering recovery post-drainage. Dynamic flooding and drainage during the heading–flowering stage led to a decrease in grain yield due to fewer grains per spike and a lower fruiting rate. Sediment content and flooding duration had a more significant impact on yield than drainage time. The combined effects of these three factors on the grain number per spike and fruiting rate were statistically significant. Flooding stress significantly affected actual yield, with notable differences (p < 0.05) between all treatments and the control (CK), increasing with longer flooding and drainage times. Regression modeling indicated that if flooding occurs during the heading–flowering stages of early rice and the duration of flooding is 0–6 days, the inundated field should be drained to a normal depth 2–5 or 5–8.5 days after the floodwaters recede. Understanding the effects of sediment, flooding, and drainage on antioxidant enzymes and rice yield enhances productivity, quality, and flood resistance and supports effective drainage strategies and climate change mitigation.