Germination Biology of Three Cyperaceae Weeds and Their Response to Pre- and Post-Emergence Herbicides in Paddy Fields

Abstract

(1) Background: Cyperaceae weeds have become a major type of weed in local paddy fields in China. (2) Methods: We assessed the impact of environmental factors, including temperature, light, salinity, water stress and soil depth, on the germination and emergence of three dominant Cyperaceae weeds: Cyperus difformis L., C. iria L. and Fimbristylis littoralis Gaudich. Using the dish dipping method, the performances of the pre- and post-emergence herbicides commonly used in paddy fields on three Cyperaceae weeds were evaluated using the pot method. (3) Results: The seeds optimally germinated at 35 °C in constant conditions and 25 °C/40 °C in alternating conditions. The seeds of the three Cyperaceae weeds were sensitive to light and could not germinate under dark conditions. The germination rate of the three weeds decreased with the increase in the NaCl concentration and water potential; the three weeds could not germinate at a 320 mmol·L⁻¹ NaCl concentration and −0.1 MPa water potential. When the pH levels were 4 to 9, the germination rates of the three weeds were all greater than 80%. The burial depths to inhibit 50% of the emergence of C. difformis, C. iria and F. littoralis were 0.27, 1.06 and 0.42 cm, respectively. The control efficacy of the pre-emergence herbicides of pretilachlor, butachlor and oxyfluorfen on the three weeds were all above 90% at the recommended dose in the field. Halosulfuron-methyl, florpyrauxifen-benzyl and bentazone could effectively control the three Cyperaceae weeds; their performances on the three weeds at the 3- to 4-leaf stage were all above 82%. (4) Conclusions: The three Cyperaceae weed seeds have a strong adaptability to temperature, water potential, salinity and soil depth, and these weeds are sensitive to most pre- and post-emergence herbicides. Therefore, taking Cyperaceae weed seeds into the deep soil layer by tillage or selecting appropriate herbicides according to their growth stages can effectively control Cyperaceae weeds in rice fields.

1. Introduction

China is one of the original areas of rice production, accounting for about 20% of the global rice planting area [1]. In recent years, the proportion of rice in China’s grain production has also been increasing. Weeds compete with rice for light, space and nutrients, affecting rice yield and quality, and are important biological factors restricting rice production [2,3]. Cyperaceae, a family of widespread C4 plants comprising annual or perennial herbs, pose a significant challenge to rice cultivation due to their competitive nature. Compared with the C3 rice plant, C4 plants are more likely to absorb CO₂, water, nutrients and light. In the process of competition, rice is weak and Cyperaceae weeds are strong [4,5]. As agricultural practices evolve, so does the composition of weed communities in paddy fields, with Cyperaceae species emerging as dominant threats to rice yields. Fimbristylis littoralis Gaudich., Cyperus iria L., C. difformis L. and other Cyperaceae weeds have risen to become the dominant weeds in many paddy fields; these three Cyperaceae weeds only reproduce by seeds. C. difformis and C. iria are widely distributed and can be seen in almost all parts of China. C. difformis is also distributed in Australia, Russia, India, Africa and America. C. iria can also grow in rice paddies and moist waterside places in Japan, India, Northern Africa and America. F. littoralis can be seen almost everywhere in China except Northeast China, Tibet and Xinjiang. And it is also distributed in Japan, Malaysia, Thailand and other Asian countries, as well as Northern Australia [6]. Many Cyperaceae weeds occur in some fields in Anhui Province, which seriously restricts the high quality and high yield of rice.

The germination and emergence of weed seeds is a key stage for the successful invasion of the species into farmland. This process is affected not only by the conditions of the weed seeds, but also by external environmental conditions such as temperature, light, water, soil salinity, sowing depth and other factors [7,8,9,10]. Temperature and light are the key factors that directly affect germination. The suitable germination temperature and light range of different weeds are often different, which leads to differences in weed species in the field during the same period [11,12]. Excessive soil salinity and extreme water potential conditions will inevitably reduce the germination of some weed seeds. Populations that can tolerate these extreme conditions can continue to grow and cause an infestation. Therefore, varying salinity and water potential will also affect the distribution and expansion of weed populations in the field [13,14]. The burial depth is the decisive factor for the successful emergence of germinated seeds. In production, the main tillage methods are deep tillage, shallow tillage and non-tillage. Studies have shown that tillage depth is negatively correlated with soil weed seed density [15]. With the increase in tillage depth, the seed density of C. rotundus L., Ammannia baccifera L., Leptochloa chinensis (L.) Nees and other weeds gradually decreases. Paddy tillage and transplanting can also effectively control weeds in paddy fields and improve rice productivity [16,17]. The systematic study of the effects of external environmental factors on the germination and emergence of Cyperaceae seeds is conducive to an understanding of the occurrence and infestation of Cyperaceae weeds. At present, chemical herbicides are still an important way to control weeds in rice fields. Pyrazosulfuron-ethyl, florpyrauxifen-benzyl, etc., are commonly used herbicides in paddy fields in China. However the relevant studies show that C. difformis has evolved a resistance to pyrazosulfuron-ethyl and other acetohydroxyacid synthase (AHAS) inhibitors in paddy fields in China [18,19]. There are also studies show that quintrione can effectively control C. difformis [20]. Florpyrauxifen-benzyl has a good control effect on C. iria; however, there are few reports on the activity of pre-emergence herbicides in paddy fields and the effects of post-emergence herbicides on Cyperaceae weeds [21]. Therefore, it is of great significance to clarify the biological characteristics of the germination and emergence of Cyperaceae weeds and to screen herbicides that can prevent them.

In this study, the effects of environmental factors such as temperature, photoperiod, pH, water potential, salinity and burial depth on seed germination and the emergence characteristics of Cyperaceae weeds were studied using F. littoralis, C. iria and C. difformis collected from paddy fields in Anhui Province as target materials. Therefore, the germination characteristics of Cyperaceae weed seeds and the potential factors affecting their dispersal were clarified, which provided a basic theoretical knowledge for their prevention and control. In addition, 21 herbicides commonly used in paddy fields were selected, to evaluate the sensitivity of the three Cyperaceae weeds; the aim was to screen effective herbicides for scientific and rational control. By studying their biological characteristics and exploring effective chemical control methods, rice field control measures with Cyperaceae weeds as the dominant weeds can be formulated.

2. Materials and Methods

2.1. Test Materials

2.1.1. Tested Weed Seeds

F. littoralis was collected in October 2021 from Shucha New Village, Shucha Town, Shucheng County, Lu’an City, Anhui Province (117.05° E, 31.35° N); C. iria was collected in September 2022 from the Planting Science and Technology Park of Anhui University of Science and Technology (117.57° E, 32.87° N); and C. difformis was collected from Fangqiuhu Farm (117.56° E, 32.90° N) in Fengyang County, Chuzhou City, Anhui Province in October 2021. The above weed seeds were dried under natural conditions and stored at 0 °C~5 °C.

2.1.2. Test Herbicides

Ten pre-emergence herbicides and eleven post-emergence herbicides commonly used in paddy fields were selected, to screen the weed control agents of Cyperaceae according to the recommended doses in the field. The herbicide information and treatment doses are shown in

Table 1. Detailed information of herbicides and treated doses.

Mode of Action	Herbicide and Formulation			Manufacturer	Dose/(g·ai hm−2)
HRAC Group	Concentration	Active Ingredient	Formulation		1/9 X	1/3 X	1 X
Pre-emergence herbicide
5	40%	prometryne	WP	Anhui Jiuyi Agriculture Co., Ltd., Hefei, China	58.33	175.00	525.00
3	33%	pendimethalin	EC	Zhejiang Tianfeng Bioscience Co., Ltd., Jinhua, China	96.25	288.75	866.25
2	10%	bensulfuron-methyl	WP	Jiangsu Kuaida Agrochemical Co., Ltd., Nantong, China	3.75	11.25	33.75
14	240 g/L	oxyfluorfen	EC	Nutrichem Co., Ltd., Hangzhou, China	6.00	18.00	54.00
14	35%	oxadiazon	SC	Bayer AG, Leverkusen, Germany	46.67	140.00	420.00
14	80%	oxadiargyl	WP	Bayer AG, Leverkusen, Germany	8.00	24.00	72.00
15	30%	anilofos	EC	Zhejiang Tianfeng Bioscience Co., Ltd., Jinhua, China	30.00	90.00	270.00
15	300 g/L	pretilachlor	EC	Syngenta Group Co., Ltd., Suzhou, China	47.25	141.75	425.25
15	60%	butachlor	EW	Zhejiang Tianfeng Bioscience Co., Ltd., Jinhua, China	120.00	360.00	1080.00
15	90%	thiobencarb	EC	Jiangsu Agro Farm Chemical Co., Ltd., Lianyungang, China	206.25	618.75	1856.25
Post-emergence herbicide
27	25%	benzobicyclon	SC	SDS BIOTECH K.K., Tokyo, Japan	20.83	62.50	187.50
6	40%	bentazone	AS	Jiangsu Sword Agrochemical Co., Ltd., Yancheng, China	140.00	420.00	1260.00
4	20%	quintrione	SC	Dingyuan Jiahe Crop Protection Co., Ltd., Chuzhou, China	83.33	250.00	750.00
4	3%	florpyrauxifen-benzyl	EC	Corteva, Inc., Wilmington Delaware, USA	3.00	9.00	27.00
4	56%	MCPA-sodium	SP	Zhejiang Tianfeng Bioscience Co., Ltd., Jinhua, China	58.33	175.00	525.00
0	15%	oxaziclomefone	OD	Zhejiang Tianfeng Bioscience Co., Ltd., Jinhua, China	5.00	15.00	45.00
2	10%	pyrazosulfuron-ethyl	WP	Anhui Guoxing Biochemical Co., Ltd., Maanshan, China	2.50	7.50	22.50
2	10%	bispyribac-sodium	SC	Zhejiang Tianfeng Bioscience Co., Ltd., Jinhua, China	2.92	8.75	26.25
2	25 g/L	penoxsulam	OD	Corteva, Inc., Wilmington Delaware, USA	2.50	7.50	22.50
2	60%	bensulfuron-methyl	WG	Zhejiang Tianfeng Bioscience Co., Ltd., Jinhua, China	3.75	11.25	33.75
2	75%	halosulfuron-methyl	WG	Jiangsu Agrochem Laboratory Co., Ltd., Changzhou, China	4.19	12.56	37.69

The underlined number indicates the recommended doses of the pesticide in the field. WP—wettable powder, EC—emulsifiable concentrate, SC—suspension concentrate, EW—emulsion in water, AS—aqueous solution, SP—soluble powder, OD—oil dispersion, WG—water-dispersible granule.

.

2.2. Test Methods

2.2.1. Seed Germination

In this experiment, the dish dipping method was employed [22]. Seeds of a uniform size were selected and soaked in 0.1% HgCl₂ solution for 10 min [23], washed with distilled water 3 times and dried with filter paper. A dish with a diameter of 9 cm was lined with two layers of filter paper, then wetted with distilled water or test solution. Fifty seeds were placed in each dish; 5 mL of distilled water or the liquid to be measured was added with a pipette gun; then, the dishes were placed in a light incubator, with each treatment replicated four times and each experiment repeated twice. The seed germination was observed and recorded daily for 15 days, with the radicle breaking through the seed coat at a length of 2 mm considered as germination.

• Effect of temperature on germination.

The temperature was set to 15, 20, 25, 30, 35 and 40 °C under constant temperature conditions. To evaluate the effect of alternating temperatures on germination, temperatures were set to 15/25, 20/30, 20/35, 25/35 and 25/40 °C night and day with 12 h dark and 12 h light.

2.

Effect of photoperiod on seed germination.

The photoperiod was set to 0/24, 6/18, 12/12, 18/6 and 24/0 (dark/light, unit: h), with the temperature set at 35 °C for 15 days.

3.

Effect of salt stress on seed germination.

NaCl solutions with concentrations of 10, 20, 40, 80, 160 and 320 mmol·L⁻¹ were used to simulate soil salt stress for the germination test [24,25]. Distilled water was used as a control, and the seeds were cultured in a light incubator with a temperature of 25 °C/35 °C night and day and a light of 12 h: 12 h (D/L). The number of germinated seeds was recorded every day.

4.

Effect of pH on germination.

The seeds of the Cyperaceae weeds were placed in buffers with pH levels of 4, 5, 6, 7, 8 and 9, respectively. The number of germinated seeds was recorded daily, with distilled water as the control. The temperatures were set at 25/35 °C night and day, with 12 h dark and 12 h light.

5.

Effect of water potential on germination.

In accordance with Formula (1),

$$\phi s=-(1.18\times {10}^{-2})\times C-(1.18\times {10}^{-4})\times C^2+(2.67\times {10}^{-4})\times C\times T+(8.39\times {10}^{-7})\times C^2\times T$$

(1)

Here, φs denotes the solution water potential (bar); C represents the concentration of PEG-6000 (g/kg) and T represents the temperature (°C). The quality of PEG6000 required to configure each water potential at 30 °C was calculated, and solutions with water potentials of −0.2, −0.4, −0.6, −0.8 and −1.0 MPa were prepared, respectively [26,27]. Distilled water was used as the control, and 5 mL of the corresponding solution was added to each dish. The dish was placed in a light incubator at a constant temperature of 30 °C, and the other conditions remained unchanged.

The number of germinated seeds of each treatment was recorded daily for 15 days.

2.2.2. Effect of Burial Depth on Seedling Emergence

Seeds of uniform size were selected and disinfected twice with 75% alcohol and then washed with distilled water. After sterilization, two layers of filter paper were spread in a Petri dish with a diameter of 9 cm and wetted with distilled water. The seeds were put into a Petri dish and distilled water was added; then, they were placed in a biochemical incubator (photoperiod (D/L): 12 h/12 h; temperature: 35 °C/25 °C). After the radicle emerged, the pot method was carried out under greenhouse conditions; the seeds were evenly sown in an 8 cm (diameter) × 6 cm (height) pot containing nutrient-mixed soil (sand/matrix = 2:1, produced by Shandong Shangdao Biotechnology Co., Ltd., Jinan, China), and 20 Cyperaceae weed seeds were sown in each pot. Seven treatments of burial depth were set up—0, 0.3, 0.6, 0.9, 1.2, 1.5 and 1.8 cm, respectively—with each treatment replicated four times and each experiment repeated twice. The pots were moved to the greenhouse for cultivation. During the experiment, the soil was kept wet, and the pots absorbed water from the bottom to saturation, and routine management was performed to ensure growth. After 21 days of sowing, the number of seedlings that emerged in each treatment was recorded (when the cotyledon exceeded the soil by 2 mm, the emergence was identified), and the seedling emergence rate was calculated [22].

2.2.3. Herbicidal Activity of Common Herbicides against Cyperaceae Weeds

The pot method was carried out under greenhouse conditions, and the concentration was set according to the recommended dosage of each herbicide registered in the field (the highest recommended dosage (A); the lowest recommended dosage (B); (A + B)/2 is the recommended dosage). The method to germinate the plants was the same as that of Section 2.2.2.

Based on the ‘Guidelines for Pesticide Laboratory Bioassay Tests’ [28,29], in this experiment, three treatments were set for each herbicide—the recommended dose, 1/3 of the recommended dose and 1/9 of the recommended dose, respectively—with each treatment replicated four times. The herbicide information and treatment dose are shown in Table 1; the same amount of 0.1% Tween-80 aqueous solution was sprayed as a blank. The pre-emergence herbicides were applied 24 h after sowing, the post-emergence herbicides were applied at the 3- to 4-leaf stage of the Cyperaceae weeds. An HCL-3000A walking spray tower (Kunshan Hengchuangli Technology Co., Ltd., Suzhou, China) was used to accurately spray the pesticide. The fan-shaped nozzle was applied 50 cm from the top of the weeds, the spray volume was 450 L·hm⁻², and the spray pressure was 0.275 MPa. After spraying, the weeds were placed in a greenhouse for further culture, and the symptoms of herbicide injury were observed regularly. The fresh weight of the aboveground part of the plant was measured 21 days after treatment.

2.3. Data Analysis and Processing

2.3.1. Biological Characteristics of Cyperaceae Weeds

In this experiment, Excel software was used to calculate the data, and Duncan’s new multiple range method in DPS 7.05 software was used to analyze the significance of the difference. The nonlinear fitting and plotting of the seed germination, seedling emergence process and germination performance under salt stress, water potential stress and burial depth were carried out using Sigmaplot 14.0 software. The effects of photoperiod, temperature and pH on seed germination were plotted using Origin 2021.

The germination (emergence) rate calculation formula is as follows:

$$\mathrm{G}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} (\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}) \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\%={\displaystyle \frac{\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{g}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s} \mathrm{o}\mathrm{r} \mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{s}}{\mathrm{total} \mathrm{number} \mathrm{of} \mathrm{seeds} }}\times 100$$

(2)

The nonlinear fitting formula of the seed germination and emergence process is

$$G={\displaystyle \frac{a}{{1+e}^{-\left(\frac{X-t_{50}}{b}\right)}}}$$

(3)

In Formula (3), G is the cumulative germination or emergence rate (%) at time x, a is the maximum germination or emergence rate (%), t₅₀ is the time to reach 50% of the germination or emergence and b is the slope of the equation at t₅₀.

For the mean germination time [(MGT); mean emergence time (MET)] and germination index [(GI); emergence index (EI)],

$$\mathrm{M}\mathrm{G}\mathrm{T}\left(\mathrm{M}\mathrm{E}\mathrm{T}\right)={\displaystyle \frac{\sum (N_i\times D_i)}{\sum N_i}}\times 100$$

(4)

$$\mathrm{G}\mathrm{I}(\mathrm{E}\mathrm{I})=\sum {\displaystyle \frac{N_i}{D_i}}$$

(5)

In Formulas (4) and (5), N_i is the number of germinated seeds or the seedlings at a time of D_i days.

The nonlinear fitting formula of the effect of salt stress and water potential stress on the seed germination rate is

$$G={\displaystyle \frac{G_{max}}{1+{\left(\frac{x}{x_{50}}\right)}^b}}$$

(6)

In the equation, G is the germination rate (%) at NaCl concentration or water potential x, G_max is the maximum germination rate (%), X₅₀ is the NaCl concentration or water potential at which the maximum germination rate is 50% and b is the slope of the equation.

2.3.2. Herbicidal Activity of Commonly Used Herbicides in Paddy Fields against Cyperaceae Weeds

The data of the bioassay results were summarized using Excel software, and the fresh weight inhibition rate was calculated according to Formula (7); the significance of the difference was analyzed using Duncan’s new multiple range method in DPS 7.05 software.

The fresh weight inhibition rate formula is

$$\mathrm{F}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{h} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{i}\mathrm{n}\mathrm{h}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\%={\displaystyle \frac{\mathrm{B}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{k} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l} \mathrm{f}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{h} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}-\mathrm{T}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{f}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{h} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\mathrm{B}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{k} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l} \mathrm{f}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{h} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}}\times 100$$

(7)

Here, the blank control fresh weight denotes the fresh weight of the aboveground plant part of the blank control, and the treatment fresh weight indicates the fresh weight of the aboveground plant part for each treatment.

3. Results

3.1. Effect of Temperature on Germination

The effect of constant temperature on the seed germination of the three Cyperaceae weeds can be seen from the test results (

Table 2. Effect of different temperatures on seed germination of three Cyperaceae weeds.

Treatment		Time to 50% of Seed Germination/d (t50)			Mean Germination Time/d (MGT)			Germination Indexes (GI)
		C. difformis	C. iria	F. littoralis	C. difformis	C. iria	F. littoralis	C. difformis	C. iria	F. littoralis
Constant temperature regimes/°C	15	-	-	-	-	-	-	-	-	-
	20	6.56 ± 0.07 a	6.41 ± 0.05 a	6.68 ± 0.05 a	10.96 ± 0.05 a	10.88 ± 0.04 a	11.04 ± 0.03 a	0.59 ± 0.02 c	0.31 ± 0.03 c	0.25 ± 0.03 c
	25	3.96 ± 0.05 b	4.41 ± 0.05 b	5.35 ± 0.04 b	9.69 ± 0.03 c	9.90 ± 0.04 b	10.38 ± 0.04 b	1.36 ± 0.04 b	1.09 ± 0.05 b	0.58 ± 0.03 b
	30	3.07 ± 0.05 c	3.82 ± 0.05 c	4.24 ± 0.04 c	9.28 ± 0.04 d	9.59 ± 0.03 cd	9.86 ± 0.03 c	1.63 ± 0.02 a	1.45 ± 0.01 a	1.07 ± 0.03 a
	35	2.90 ± 0.05 c	3.56 ± 0.04 c	4.18 ± 0.07 c	9.16 ± 0.04 e	9.49 ± 0.02 d	9.79 ± 0.05 c	1.72 ± 0.03 a	1.51 ± 0.01 a	1.18 ± 0.05 a
	40	4.05 ± 0.01 b	3.85 ± 0.15 c	4.24 ± 0.05 c	9.87 ± 0.04 b	9.66 ± 0.08 c	9.87 ± 0.02 c	0.27 ± 0.03 d	0.36 ± 0.06 c	0.27 ± 0.04 c
Alternating temperature regimes/(°C/°C)	15/25	7.47 ± 0.15 a	6.09 ± 0.07 a	7.80 ± 0.11 a	11.37 ± 0.06 a	10.66 ± 0.02 a	11.58 ± 0.09 a	0.14 ± 0.02 d	0.52 ± 0.08 c	0.07 ± 0.01 d
	20/30	3.88 ± 0.04 b	3.76 ± 0.08 bc	4.87 ± 0.02 b	9.69 ± 0.02 b	9.62 ± 0.04 bc	10.13 ± 0.01 b	1.37 ± 0.03 c	1.33 ± 0.04 b	0.87 ± 0.03 c
	20/35	3.49 ± 0.04 c	3.54 ± 0.09 cd	4.69 ± 0.03 b	9.48 ± 0.02 c	9.51 ± 0.04 cd	10.04 ± 0.02 bc	1.50 ± 0.01 b	1.48 ± 0.03 ab	1.00 ± 0.02 b
	25/35	3.08 ± 0.04 d	3.42 ± 0.11 d	4.46 ± 0.09 c	9.27 ± 0.02 d	9.43 ± 0.05 d	9.91 ± 0.04 cd	1.65 ± 0.01 a	1.54 ± 0.03 a	1.11 ± 0.04 a
	25/40	3.51 ± 0.11 c	3.98 ± 0.08 b	4.42 ± 0.03 c	9.46 ± 0.04 c	9.65 ± 0.04 b	9.88 ± 0.02 d	1.53 ± 0.03 b	1.40 ± 0.04 ab	1.14 ± 0.03 a

Data are mean ± SE. Letters identify significant differences between the means of the three species under the same treatment conditions (p < 0.05). - means that there is no seed germination under this treatment.

, Figure 1a). The germination rate of the three weeds was more than 80% at 35 °C, but the germination time (t₅₀ and MGT) of F. littoralis was significantly longer than that of C. difformis and C. iria. When the temperature dropped to 25 °C, the difference between the three Cyperaceae weeds began to increase. The germination rate of C. difformis was 98%, that of C. iria was 84.67% and that of F. littoralis was only 53.33%. The t₅₀ and MGT of C. difformis are the shortest and those of F. littoralis are the longest. When the temperature was reduced to a constant temperature of 20 °C, the germination rate of the three decreased, but the germination rate of C. difformis was greater than 60% and significantly higher than that of C. iria and F. littoralis; the germination time of the three was C. iria > C. difformis > F. littoralis. When the temperature was reduced to a constant temperature of 15 °C, the germination rate of C. iria was less than 5%, while C. difformis and F. littoralis did not germinate. When the temperature rose to 40 °C, the germination rate of the three decreased significantly and there was no significant difference in the average germination time.

The effect of alternating temperature on the seed germination of the three Cyperaceae weeds can be seen from the test results (Table 2, Figure 1b). At 20 °C/35 °C, 25 °C/35 °C and 25 °C/40 °C, the germination rate of the three Cyperaceae weeds was the highest. The germination rate of C. difformis and C. iria was close to 100%, and the germination rate of F. littoralis was 82%~87.33%. And the t₅₀ and MGT of the three weed seeds were F. littoralis > C. iria > C. difformis. When the temperature dropped to 20 °C/30 °C, the germination rate of the three Cyperaceae weeds decreased, but the germination rate of C. difformis and C. iria was still above 93.33%. When the temperature decreased to 15 °C/25 °C, the germination rate of the three decreased significantly, but the germination rate of C. iria was significantly higher than that of C. difformis and F. littoralis, indicating that the germination ability of C. iria was stronger than that of the other two weeds at low temperatures. The t₅₀ and MGT of C. iria were the shortest at the lower two alternating temperatures. The calculation results of the germination index also reflected a similar trend. At the lowest temperature, the germination index of C. iria was significantly higher than that of C. difformis and F. littoralis.

3.2. Effect of Photoperiod on Seed Germination

The experimental results of the effects of different photoperiods on the germination of the three Cyperaceae weed seeds (Figure 2) showed that under the treatment of 24 h dark/0 h light, none of the three weeds germinated. Under the treatment of 6 h dark/18 h light and 0 h dark/24 h light, the germination rates of the three weeds were between 99.33% and 100%, indicating that the light time had a significant effect on the germination of the three Cyperaceae weed seeds. However, under the treatment of 6 h dark/18 h light, the differences between the species began to increase. Among them, the germination rate of F. littoralis only reached 88% under the 12 h dark/12 h light treatment, which was significantly lower than that of C. difformis and C. iria, which fully indicated that F. littoralis was more sensitive to dark stress.

3.3. Effect of Salt Stress on Seed Germination

The experimental results of the effects of different salinity levels on the seed germination of the three Cyperaceae weeds (Figure 3) showed that with an increase in NaCl concentration, the germination rate of the three weed seeds showed a decreasing trend. At 160 mmol·L⁻¹, C. difformis and F. littoralis did not germinate, and C. iria did not germinate at 320 mmol·L⁻¹. The germination rates of C. difformis, C. iria and F. littoralis under different NaCl concentrations were nonlinearly fitted; the concentrations that inhibited the maximum germination rate by 50% were 32.16, 19.10 and 40.27 mmol·L⁻¹, respectively. The results of the t₅₀ and MGT of the three weeds showed that an increase in NaCl concentration significantly prolonged the germination time of the three weeds. However, there was no significant difference in the average germination time between the three Cyperaceae weeds (

Table 3. Effect of different salinity levels on seed germination of three Cyperaceae weeds.

NaCl Concentration/(mmol·L−1)	Time to 50% of Seed Germination/d (t50)			Mean Germination Time/d (MGT)			Germination Indexes (GI)
	C. difformis	C. iria	F. littoralis	C. difformis	C. iria	F. littoralis	C. difformis	C. iria	F. littoralis
0	3.04 ± 0.04 c	3.04 ± 0.10 c	3.22 ± 0.14 c	9.29 ± 0.02 c	9.30 ± 0.06 d	9.377 ± 0.05 c	1.60 ± 0.04 a	1.55 ± 0.08 a	1.12 ± 0.07 a
10	3.82 ± 0.20 b	3.55 ± 0.09 c	3.45 ± 0.07 c	9.68 ± 0.09 b	9.59 ± 0.04 cd	9.49 ± 0.02 bc	0.96 ± 0.07 b	0.89 ± 0.05 b	0.85 ± 0.01 b
20	4.18 ± 0.15 ab	3.40 ± 0.18 c	3.69 ± 0.12 bc	9.79 ± 0.04 ab	9.52 ± 0.06 d	9.587 ± 0.03 bc	0.85 ± 0.05 b	0.61 ± 0.06 c	0.68 ± 0.03 c
40	4.47 ± 0.18 a	4.18 ± 0.16 b	4.14 ± 0.09 ab	9.97 ± 0.10 a	9.83 ± 0.02 bc	9.757 ± 0.03 ab	0.63 ± 0.09 c	0.47 ± 0.05 cd	0.56 ± 0.02 d
80	4.56 ± 0.30 a	4.53 ± 0.10 b	4.38 ± 0.34 a	10.03 ± 0.10 a	9.92 ± 0.04 b	9.913 ± 0.15 a	0.21 ± 0.08 d	0.41 ± 0.05 d	0.21 ± 0.03 e
160	-	6.35 ± 0.31 a	-	-	10.80 ± 0.14 a	-	-	0.08 ± 0.02 e	-
320	-	-	-	-	-	-	-	-	-

Data are mean ± SE. Letters identify significant differences between the means of the three species under the same treatment conditions (p < 0.05). - means that there is no seed germination under this treatment.

). Although the NaCl concentration inhibiting 50% germination of C. iria was significantly lower than that of C. difformis and F. littoralis, the germination index of C. iria was significantly higher than that of C. difformis and F. littoralis at 160 mmol·L⁻¹ and 320 mmol·L⁻¹ NaCl concentrations, which fully demonstrated that C. iria was more able to tolerate higher salt during germination. In addition, according to the germination index, with each increase in salinity, there was a significant decrease in germination for all species (Table 3).

3.4. Effect of pH on Germination

Under the condition of pH 4~9, the germination rate of each treatment of C. difformis was between 94% and 100%, the germination rate of each treatment of C. iria was between 96% and 100% and the germination rate of each treatment of F. littoralis was between 81.33% and 84%. There was no significant difference between each treatment and the water control (Figure 4).

3.5. Effect of Water Potential on Germination

The experimental results of the effects of different water potentials on the germination of the three Cyperaceae weed seeds (Figure 5) showed that with a continuous decrease in water potential, the germination rate of the three weed seeds decreased and the three weeds could not germinate at −1.0 MPa. When the water potential was reduced to −0.8 MPa, the germination rate of C. iria was only 9.33%, while neither C. difformis nor F. littoralis could germinate. The germination rates of C. difformis, C. iria and F. littoralis under different water potentials were nonlinearly fitted; the water potentials that inhibited the maximum germination rate by 50% were −0.56, −0.59 and −0.47 MPa, respectively, indicating that the tolerance of C. iria to water potential reduction was slightly higher than that of C. difformis and that F. littoralis was the most sensitive to water potential stress. The t₅₀, MGT and GI of the three showed a decreasing trend (

Table 4. Effect of water potentials on seed germination of three Cyperaceae weeds.

Osmotic Potential/MPa	Time to 50% of Seed Germination/d (t50)			Mean Germination Time/d (MGT)			Germination Indexes (GI)
	C. difformis	C. iria	F. littoralis	C. difformis	C. iria	F. littoralis	C. difformis	C. iria	F. littoralis
0	3.04 ± 0.04 c	3.03 ± 0.08 e	3.67 ± 0.03 c	9.27 ± 0.03 b	9.30 ± 0.03 d	9.56 ± 0.01 d	1.63 ± 0.02 a	1.62 ± 0.02 a	1.28 ± 0.05 a
−0.2	2.99 ± 0.01 c	3.31 ± 0.07 d	3.85 ± 0.01 b	9.22 ± 0.01 b	9.40 ± 0.03 d	9.65 ± 0.01 c	1.58 ± 0.02 a	1.55 ± 0.03 a	1.14 ± 0.03 b
−0.4	3.30 ± 0.05 b	4.02 ± 0.07 c	4.01 ± 0.01 b	9.39 ± 0.02 b	9.69 ± 0.03 c	9.75 ± 0.01 b	1.41 ± 0.02 b	1.27 ± 0.02 b	0.86 ± 0.04 c
−0.6	6.39 ± 0.14 a	6.50 ± 0.05 b	6.27 ± 0.10 a	10.90 ± 0.07 a	10.96 ± 0.02 a	10.85 ± 0.02 a	0.28 ± 0.04 c	0.41 ± 0.02 c	0.09 ± 0.01 d
−0.8	-	6.04 ± 0.05 a	-	-	10.76 ± 0.04 b	-	-	0.09 ± 0.02 d	-
−1.0	-	-	-	-	-	-	-	-	-

Data are mean ± SE. Letters identify significant differences between the means of the three species under the same treatment conditions (p < 0.05). - means that there is no seed germination under this treatment.

). Although the germination time of C. difformis and C. iria was shorter than that of C. iria under some conditions, the germination index of C. iria was significantly higher than that of C. difformis and C. iria under lower water potential conditions (−0.6 and −0.8 MPa), which fully indicated that C. iria could tolerate a lower water potential during germination.

3.6. Effect of Burial Depth on Emergence

The experimental results of the effects of different burial depths on the emergence of three Cyperaceae weeds (Figure 6) showed that with an increase in burial depth, the emergence rates of the three weeds showed decreasing trends, among which the decreasing trend of C. difformis was the most obvious. Under the condition of a soil burial depth of 1.2 cm, the seeds of C. difformis and F. littoralis were not observed to emerge. A nonlinear fitting of the emergence rate of the three could be obtained. The burial depths resulting in a 50% inhibition of the emergence of C. difformis, C. iria and F. littoralis were 0.27, 1.06 and 0.42 cm, respectively, indicating that the emergence ability of C. iria was significantly higher than that of C. difformis and F. littoralis. It indicated a strong correlation between the vertical distribution of Cyperaceae seeds in the soil and their ability to successfully emerge, with the soil surface being the optimal depth for their germination and emergence.

3.7. Herbicidal Activity of Commonly Used Herbicides in Paddy Fields against Cyperaceae Weeds

From

Table 5. Fresh weight inhibition rate of pre-emergence herbicides on three Cyperaceae weeds in paddy field.

Herbicide Name	Dosage/(g hm−2)	C. difformis		C. iria		F. littoralis
		Fresh Weight/g	Fresh Weight Control Efficacy/%	Fresh Weight/g	Fresh Weight Control Efficacy/%	Fresh Weight/g	Fresh Weight Control Efficacy/%
Pretilachlor	47.25	0.02	78.38 ± 3.31 b	0.31	100 ± 0.00 a	0.09	62.48 ± 1.42 c
	141.75	0.00	100 ± 0.00 a	0.25	100 ± 0.00 a	0.04	84.58 ± 1.06 d
	425.25	0.00	100 ± 0.00 a	0.10	100 ± 0.00 a	0.02	92.91 ± 1.21 b
Butachlor	120	0.24	100 ± 0.00 a	0.00	100 ± 0.00 a	0.04	84.72 ± 1.30 a
	360	0.00	100 ± 0.00 a	0.00	100 ± 0.00 a	0.03	87.99 ± 0.99 c
	1080	0.00	100 ± 0.00 a	0.00	100 ± 0.00 a	0.00	100 ± 0.00 a
Pendimethalin	96.25	0.00	78.97 ± 4.29 b	0.00	43.99 ± 1.77 c	0.04	84.45 ± 1.08 a
	288.75	0.00	92.91 ± 1.35 c	0.00	61.50 ± 2.57 c	0.03	88.27 ± 0.72 c
	866.25	0.00	100 ± 0.00 a	0.00	83.46 ± 3.73 cd	0.00	100 ± 0.00 a
Thiobencarb	206.25	0.24	81.16 ± 3.30 b	0.63	63.35 ± 5.08 b	0.06	77.22 ± 2.48 b
	618.75	0.08	100 ± 0.00 a	0.43	78.93 ± 1.74 b	0.02	93.04 ± 0.85 b
	1856.25	0.00	100 ± 0.00 a	0.19	88.65 ± 0.72 bc	0.00	100 ± 0.00 a
Prometryne	58.33	0.21	67.28 ± 2.94 c	0.41	34.27 ± 7.39 c	0.06	73.67 ± 0.55 b
	175	0.00	100 ± 0.00 a	0.24	74.11 ± 1.19 b	0.06	76.81 ± 0.99 e
	525	0.00	100 ± 0.00 a	0.13	79.45 ± 1.35 d	0.03	87.99 ± 0.99 c
Anilofos	30	0.37	85.71 ± 3.25 b	0.74	35.16 ± 8.72 c	0.13	45.57 ± 3.54 d
	90	0.00	96.75 ± 1.63 b	0.29	76.19 ± 4.52 b	0.09	65.35 ± 1.57 f
	270	0.00	100 ± 0.00 a	0.23	86.94 ± 3.43 bc	0.02	90.31 ± 2.25 bc
Oxyfluorfen	6	0.16	98.61 ± 0.49 a	0.73	72.26 ± 0.96 b	0.03	87.31 ± 2.95 a
	18	0.04	100 ± 0.00 a	0.27	78.07 ± 1.62 b	0.00	100 ± 0.00 a
	54	0.00	100 ± 0.00 a	0.15	90.92 ± 0.54 b	0.00	100 ± 0.00 a
Oxadiazon	46.67	0.82	22.23 ± 2.49 e	1.02	9.42 ± 3.54 e	0.21	13.96 ± 5.02 f
	140.00	0.36	74.27 ± 3.98 d	0.50	55.36 ± 2.79 d	0.11	55.01 ± 5.02 e
	420.00	0.24	72.52 ± 3.28 b	0.16	85.75 ± 2.02 cd	0.07	70.78 ± 3.03 d
Oxadiargyl	8.00	0.61	40.35 ± 3.58 d	0.43	61.47 ± 2.77 c	0.22	10.01 ± 1.41 f
	24.00	0.25	84.21 ± 3.72 c	0.22	80.81 ± 1.57 c	0.03	89.56 ± 3.29 b
	72.00	0.00	100 ± 0.00 a	0.11	89.97 ± 1.57 bc	0.01	94.43 ± 0.81 ab
Bensulfuron-methyl	3.75	1.06	7.02 ± 1.47 f	0.22	80.66 ± 2.46 b	0.20	16.97 ± 1.90 f
	11.25	0.94	22.23 ± 2.62 e	0.12	88.95 ± 1.06 b	0.13	48.74 ± 5.30 e
	33.75	0.84	30.41 ± 3.09 c	0.07	94.18 ± 1.54 ab	0.08	68.92 ± 3.73 d
Control	-	1.13	-	1.12	-	0.24	-

The underlined number indicates the recommended dose of the pesticide in the field. Data are mean ± SE. Letters identify significant differences between the means of the three species under the same treatment conditions (p < 0.05). - means no relevant data.

, it can be seen that among the ten pre-emergence herbicides selected in the experiment, pretilachlor, butachlor, pendimethalin, prometryne, thiobencarb, anilofos and oxadiargyl completely inhibited the germination and emergence of C. difformis at the recommended dose in the field and the fresh weight inhibition rate was 100%. The fresh weight inhibition rates of pretilachlor, butachlor, oxyfluorfen and bensulfuron-methyl at the recommended doses in the field were all above 90.92%. Pendimethalin, thiobencarb, anilofos, oxadiazon and oxadiargyl could also effectively control C. iria; the fresh weight inhibition rate was between 83.46% and 89.97%. F. littoralis was sensitive to butachlor, pendimethalin, thiobencarb and oxyfluorfen and completely inhibited the germination and emergence of F. littoralis at the recommended dose in the field. Pretilachlor and oxadiargyl could also effectively control F. littoralis; the fresh weight inhibition rate was 92.91%~94.43%. In addition, the control effect of oxadiazon on the three Cyperaceae weeds was not good. At the recommended dose, the fresh weight inhibition rate was between 70.78% and 82.75%. Bensulfuron-methyl was almost ineffective on C. difformis, and the fresh weight inhibition rate was only 30.41% at the recommended dose.

The results of the post-emergence herbicide screening test are shown in

Table 6. Fresh weight inhibition rate of post-emergence herbicides on three Cyperaceae weeds in paddy field.

Herbicide Name	Dosage/(g hm−2)	C. difformis		C. iria		F. littoralis
		Fresh Weight/g	Fresh Weight Control Efficacy/%	Fresh Weight/g	Fresh Weight Control Efficacy/%	Fresh Weight/g	Fresh Weight Control Efficacy/%
MCPA-sodium	58.33	1.43	38.34 ± 2.65 g	1.80	17.75 ± 3.59 d	0.32	65.19 ± 3.75 b
	175.00	0.46	80.02 ± 1.72 cd	1.49	31.69 ± 0.69 ef	0.15	83.58 ± 0.27 bc
	525.00	0.28	88.09 ± 1.51 c	0.86	60.57 ± 3.27 e	0.12	86.80 ± 0.50 b
Quintrione	83.33	1.32	42.82 ± 2.80 fg	1.78	18.40 ± 1.30 d	0.61	34.99 ± 5.68 de
	250.00	0.89	61.60 ± 2.41 ef	1.64	24.82 ± 1.78 g	0.48	48.87 ± 1.82 f
	750.00	0.56	75.86 ± 1.70 e	1.31	39.89 ± 1.10 g	0.37	60.18 ± 3.13 ef
Pyrazosulfuron-ethyl	2.50	1.22	47.40 ± 2.30 ef	1.45	33.38 ± 3.33 c	0.65	29.77 ± 5.73 ef
	7.50	0.94	59.46 ± 2.06 f	1.19	45.57 ± 2.48 d	0.41	56.24 ± 3.16 e
	22.50	0.55	76.24 ± 1.54 e	0.85	61.07 ± 3.44 e	0.18	80.54 ± 2.94 c
Bensulfuron-methyl	3.75	1.20	48.05 ± 4.29 ef	1.84	15.53 ± 3.83 d	0.69	25.44 ± 3.10 ef
	11.25	0.96	58.69 ± 1.12 f	1.21	44.43 ± 4.84 d	0.31	66.65 ± 2.04 d
	33.75	0.69	70.14 ± 2.79 f	0.57	73.72 ± 1.57 d	0.19	79.36 ± 1.75 cd
Oxaziclomefone	5.00	1.25	46.15 ± 2.63 efg	1.79	17.78 ± 3.47 d	0.30	67.51 ± 3.29 b
	15.00	0.87	62.61 ± 2.49 ef	1.60	26.73 ± 1.99 fg	0.18	80.29 ± 1.49 c
	45.00	0.55	76.25 ± 1.29 e	1.10	49.66 ± 3.12 f	0.13	86.15 ± 0.38 b
Halosulfuron-methyl	4.19	0.95	59.10 ± 2.94 d	1.19	45.63 ± 5.77 b	0.07	92.09 ± 0.28 a
	12.56	0.55	76.31 ± 2.13 d	0.63	71.06 ± 2.10 b	0.05	94.88 ± 0.28 a
	37.69	0.36	84.62 ± 2.07 c	0.39	82.07 ± 1.79 c	0.00	100 ± 0.00 a
Florpyrauxifen-benzyl	3.00	0.33	85.65 ± 0.40 b	0.85	60.90 ± 0.62 a	0.45	51.70 ± 0.36 c
	9.00	0.15	93.55 ± 0.58 b	0.37	82.89 ± 1.17 a	0.33	64.31 ± 0.53 d
	27.00	0.05	98.04 ± 0.30 a	0.25	88.75 ± 0.38 b	0.13	85.68 ± 0.56 b
Bispyribac-sodium	2.92	1.09	53.05 ± 3.29 de	1.69	22.69 ± 0.29 d	0.64	30.95 ± 0.60 e
	8.75	0.86	62.90 ± 1.16 ef	1.67	23.51 ± 0.32 g	0.55	41.33 ± 0.72 g
	26.25	0.38	83.70 ± 0.65 cd	1.54	29.42 ± 1.09 h	0.34	63.74 ± 0.72 e
Bentazone	140.00	0.05	98.04 ± 0.25 a	1.67	23.28 ± 0.33 d	0.35	62.96 ± 1.27 b
	420.00	0.02	99.02 ± 0.12 a	0.88	59.50 ± 1.54 c	0.14	84.80 ± 0.41 b
	1260.00	0.00	100 ± 0.00 a	0.00	100 ± 0.00 a	0.03	97.20 ± 0.27 a
Penoxsulam	2.50	0.61	73.67 ± 1.68 c	1.87	14.44 ± 1.94 d	0.54	41.90 ± 0.82 d
	7.50	0.39	83.23 ± 0.96 c	1.36	37.65 ± 0.95 e	0.35	62.34 ± 1.09 d
	22.50	0.16	93.03 ± 0.94 b	1.07	50.79 ± 0.88 f	0.23	75.57 ± 0.91 d
Benzobicyclon	20.83	1.16	49.88 ± 3.57 ef	1.36	37.65 ± 2.47 bc	0.74	20.83 ± 1.73 f
	62.50	0.78	66.47 ± 0.70 e	1.09	50.27 ± 1.06 d	0.46	50.66 ± 1.22 f
	187.50	0.47	79.61 ± 0.44 de	0.78	64.17 ± 0.19 e	0.40	56.89 ± 1.04 f
Control	-	2.31	-	2.18	-	0.93	-

The underlined number indicates the recommended dose of the pesticide in the field. Data are mean ± SE. Letters identify significant differences between the means of the three species under the same treatment conditions (p < 0.05). - means no relevant data.

. When spraying at the 3- to 4-leaf stage of the weeds, bentazone had the best control effect on the three Cyperaceae weeds at the recommended dose; the fresh weight inhibition rate was above 97.20%. The inhibition rates of florpyrauxifen-benzyl and halosulfuron-methyl on the fresh weight of the three weeds were also more than 82.07%. MCPA-sodium, bispyribac-sodium and penoxsulam could also effectively control C. difformis at the recommended dose in the field, and the fresh weight inhibition rate was above 83.70%. MCPA-sodium, bensulfuron-methyl and oxaziclomefone could effectively control F. littoralis at the recommended dose in the field, and the fresh weight inhibition rate was above 80.54%.

4. Discussion

The results showed that the three Cyperaceae weed seeds could germinate at a constant temperature of 25 °C~35 °C and alternating temperatures of 20 °C/30 °C~25 °C/40 °C, which was consistent with the ambient temperature during the rice-planting season [30]. This may be one of the main reasons why these weeds are widely distributed in various regions of China. Light is an important factor affecting seed germination. This study showed that the three Cyperaceae weed seeds were light-requiring seeds, and the seeds of the three Cyperaceae weeds could not germinate under dark conditions; this is similar to most other paddy field weeds, such as Oryza sativa L. [31]. In addition, seed light sensitivity determines whether seeds can germinate when buried in soil. The closer to the soil surface they are, the higher the germination rate and seedling emergence rate. Due to the small size of the seeds of C. difformis and F. littoralis, the energy they provide on their own may not be enough to support the embryo in breaking through the soil layer. The oxygen content and light intensity decrease with the soil depth. When seeds were buried at a depth of 1.2 cm, no seedling emergence was observed. The seeds of C. iria were larger than those of C. difformis and F. littoralis. When the burial depth reached 1.06 cm, the inhibition rate of the emergence of C. iria reached 50%. When the burial depth was 1.8 cm, the emergence rate was less than 10%. In addition, other Cyperaceae weeds also showed this feature, such as Kyllinga polyphylla Kunth, which did not emerge at all when the buried depth was 0.5 cm [32]. Thus, early sowing practices such as tillage and straw mulching can reduce the emergence of these three Cyperaceae weeds. Studies have shown that after the rice is sown, deep-water layer management can also effectively inhibit the weeds’ emergence [33].

C. difformis, C. iria and F. littoralis are strongly competitive in paddy fields and have a certain adaptability and tolerance to environmental stress. They can make full use of their own growth advantages to seize the living space and cause a large reduction in rice production. Regarding the establishment of weed populations in the field, their germination and emergence are closely related to external environmental conditions, in addition to their own genetic factors. Water potential, salinity and pH are all key factors restricting their germination. This study showed that the germination rate of the three Cyperaceae weed seeds was the highest when the water potential was 0 MPa. As the paddy field was often in a state of water accumulation for a long time, the water content of the soil reached saturation and the water potential was almost 0 MPa; so, the wet environment of the paddy field was more suitable for the occurrence of such weeds. When the water potential was -0.8 MPa, the germination rates of C. difformis, F. littoralis and C. iria were 0%, 0% and 9.33%, respectively, which may explain why C. iria is more prevalent in dry land compared to the other two species. A large number of studies have confirmed that most of the seeds of malignant weeds have a strong adaptability to soil pH [22]; C. difformis, C. iria and F. littoralis also conform to this characteristic. They can germinate under conditions of pH 4~9; therefore, the pH of the soil cannot limit the germination of the seeds of the three Cyperaceae weeds, as the pH of most of the soil in China is 5~8, which is suitable for the germination of the three Cyperaceae weeds. And the three Cyperaceae weeds show a certain tolerance under different salt concentrations. C. difformis and F. littoralis can germinate in the range of 0~80 mmol·L⁻¹, and C. iria can germinate in the range of 0~160 mmol·L⁻¹. Therefore, soil salinization also has difficulty in limiting the spread of these three Cyperaceae weeds. Although the germination and emergence ability of C. difformis and F. littoralis were weaker than those of C. iria, they could fully meet the environmental conditions required for their germination and emergence in actual production; that is, all three weeds could adapt to various environments in the field. Therefore, there is a risk that these weeds will continue to spread in China’s rice-growing areas. Among them, the germination and seedling emergence ability of F. littoralis is weaker than that of C. difformis and C. iria, so its distribution in the field will be limited, which explains the difference in the degree of harm of the three weeds to a certain extent. In order to avoid further damaging by C. difformis, C. iria and F. littoralis, it is necessary to strengthen the monitoring of the spread trend of the three weeds and to further clarify their damage characteristics; this is of great significance for the formulation of scientific prevention and control decisions, for timely prevention and control before emergence, the improvement of prevention and control effects and a reduction in yield losses [34].

The use of chemical herbicides is a more economical and efficient method for weed control in paddy fields [35]. The results of this study showed that the use of pretilachlor, butachlor, oxyfluorfen and other herbicides for pre-emergence control could effectively inhibit the germination and emergence of the three Cyperaceae weeds. The post-emergence herbicides, halosulfuron-methyl, florpyrauxifen-benzyl and bentazone have good control effects on the three Cyperaceae weeds. It is worth mentioning that the half-life of these herbicides in soil is between a few days and a few weeks and that they are low-residual herbicides. The activity of the different herbicides against the three Cyperaceae weeds was different. In fields with a high incidence of F. littoralis, MCPA-sodium, pyrazosulfuron-ethyl and oxaziclomefone could be used for control. Shi Xianluo et al. showed that oxaziclomefone had a poor control effect on C. difformis and that penoxsulam could effectively control C. difformis [36]. This study also showed that oxaziclomefone had low activity against C. difformis and that penoxsulam had high activity against C. difformis. In general, the effects of the pre-emergence herbicides were better than those of the post-emergence herbicides. Therefore, using pre-emergence herbicides in paddy fields is a more economical and efficient method. At this stage, a method of co-ordinated treatment should be adopted for the prevention and control of Cyperaceae weeds; that is, while controlling Cyperaceae weeds, it is necessary to take into account other major weed species in the paddy fields, select the appropriate dosage and use time and alternate the use of herbicides with different modes of action.

5. Conclusions

In summary, the three Cyperaceae weeds are able to germinate under a wide range of temperature, salinity and pH conditions. Their ability to germinate and emerge from the top soil layers enables them to adapt well to paddy field habitats, posing a risk of spread. This study demonstrated that both pre-emergence herbicides, including pretilachlor, butachlor, and oxyfluorfen, and post-emergence herbicides, such as halosulfuron-methyl, florpyrauxifen-benzyl, and bentazone, exhibited a high efficacy against these weeds at the recommended doses under greenhouse conditions. The further validation of these findings through field experiments is warranted to confirm their efficacy in natural settings. According to the test results, appropriate herbicides can be selected according to local conditions to achieve efficient control of such weeds.