Competitive interaction between rice and Sumatran fleabane under large and small plant sizes and populations

Abstract

Background:  The changes in rice production systems, by soybean introduction, have created favorable conditions for Sumatran fleabane [Conyza sumatrensis (Retz.) E.Walker] to increase its potential to establish, spread, and impact crop yields in lowland areas.

Objective:  Therefore, this study to valuate the competitive ability of Sumatran fleabane on flooded and non-flooded rice.

Methods:  Two experiments were conducted in the greenhouse and repeated twice. The experiment was carried out with a completely randomized design, with four replications, testing two plant sizes for Sumatran fleabane (Factor A) (large and small plants), and Sumatran fleabane populations of 0, 35, 71, 141, and 282 plants per m² (Factor B). The experiment was conducted under both flooded and non-flooded conditions for up to 45 days after rice emergence.

Results:  Under non-flooded conditions, there was a reduction in the number of leaves (about 45%), an increase in phyllochron (approximately 75 ºC day/leaf), reduction or absence of tillers, greater decline in shoot and root dry mass, an approximate 50% reduction in Haun scale, and up to 68% reduction in rice height. Under flooded conditions, there was no significant reduction in the number of leaves or in phyllochron. There was less impact on the number of tillers (reduction only at 282 plants m⁻²), smaller reductions in dry mass, and final values with little difference among Sumatran fleabane populations for Haun scale and height.

Conclusions:  Large plants of Sumatran fleabane are highly competitive, especially under non-flooded conditions. Flooding conditions reduces competition effects of Sumatran fleabane, regardless of the size.

Keywords:
Neighborhood Model; Competition; Adaptation

1. Introduction

Conyza spp. are predominantly found in areas where crops such as soybeans, corn, and cotton are grown, causing significant damage if left unmanaged (Oliveira et al., 2021). Characteristics such as ease of dispersal, high seed production, autogamous reproduction, and discontinuous germination give Conyza spp. a high capacity for infestation in various production systems (Bajwa et al., 2016). Additionally, Sumatran fleabane [Conyza sumatrensis (Retz.) E.Walker] has been documented to be resistant to herbicides with five different mechanisms of action (Pinho et al., 2019), leading to challenges in management and increases in control costs. Conyza spp. are more frequent and denser in areas with no-till farming systems (Lazaroto et al., 2008). Thus, it was not considered a problem in irrigated rice areas, where it was present in low densities (Silva et al., 2022). However, an economic, practical, and effective alternative for controlling weedy rice (Oryza sativa f. spontanea) and other weeds followed the introduction of soybeans in these areas (Silva et al., 2022). This practice led to modifications in the rice production system by reducing soil tillage operations and excess water levels, providing a more suitable environment for Sumatran fleabane to thrive.

The distinct production systems in lowland areas influence the density and diversity of species in the area (Ulguim et al., 2018). Therefore, it is likely that species with susceptibility to waterlogged soils, such as Conyza canadensis (Stoecker et al., 1995), may develop in greater desnsities during the soybean cropping cycle in rotation with irrigated rice. This inference is based on environmental changes, primarily resulting from drainage of irrigation water. Furthermore, minimum tillage (MT) during summer or fall is adopted by up to 50% of rice growers in Southern Brazil (Ulguim et al., 2021), where pre-sowing burndown herbicide applications are standard. Under these conditions, the management of Sumatran fleabane becomes particularly challenging due to its resistance to multiple herbicide modes of action (Pinho et al., 2019). Surviving plants in advanced vegetative stages and with considerable height at the time of rice establishment may compete directly with the crop from the onset of its growth cycle.

The cultivation system of directed seeded rice (DSR) may promote greater competition compared to systems using germinated seeds or transplant seedlings because weed emergence coinciding with the crop emergence (Rathika et al., 2020). Studies conducted in India showed that DSR had the highest number of weed species and dry weight of weeds compared to pre-germinated and seedling transplant systems (Jehangir et al., 2021). Therefore, the DSR system may favor the establishment of Sumatran fleabane, since flooding of the crop is recommended from stage V3 for higher yields (Ribas et al., 2021). Thus, it is presumed that the water from rice flooding promotes the physical control of Sumatran fleabane. However, in dryland rice production areas the growth and development of Sumatran fleabane can be favored. Rainfed rice has grown in about 23% of the area and contributes for 10% of cereal production in Brazil (Ministério da Agricultura e Meio Ambiente, 2020). The management practices to mitigate water scarcity may justify the increasing importance of rice production in drylands worldwide (Yu et al., 2024).

Weed competition is one of the main causes of yield loss in irrigated rice cultivation (Fruet et al., 2020; Ribas et al., 2021). This is because some weed species have a superior competitive ability over rice. Studies evaluating the competitive ability of rice with barnyardgrass (Echinochloa spp.) and weedy rice observed reductions in leaf area and dry matter accumulation in the crop due to competition (Agostinetto et al., 2008; Schaedler et al., 2020). Hence, considering the cultivation system in lowland areas, combined with reported cases of herbicide resistance in Sumatran fleabane in Brazil, there is potential for this weed to infest new areas and cause damage to crop yields. Therefore, the objective of this study was to evaluate the competitive ability of Sumatran fleabane populations with rice, considering two plant sizes and two irrigation conditions.

2. Material and Methods

Two experiments were conducted in a greenhouse in Santa Maria, Rio Grande do Sul (RS), Brazil, in two different time periods. The experiments, using neighborhood model (Hasanfard, Chauhan, 2024), were carried out to evaluate the competitive ability of Sumatran fleabane with rice (Figure 1). Each experiment was conducted under distinct irrigation conditions: one experiment was carried out under flooded conditions, and the other experiment was conducted with daily sprinkler irrigation (non-flooded). In both conditions, irrigation from rice sowing onward was performed via a sprinkler system. The flooded condition was established by applying a water layer approximately 4 cm above ground level, starting when the crop was at V3 stage (Counce et al., 2000). Experiments were conducted between February and March 2023 (first period) and repeated between October and November 2023 (second period). The air temperature conditions differed between the periods. In the first period, the air temperature conditions favored the vegetative development of the rice crop (Figure 2). In the second period, temperatures exceeding 25 ºC occurred only in one day. The duration time to emergence-V3 was 12 and 24 days for the first and second period, respectively. However, the accumulated thermal sum (TSa) required for the crop to reach V3 stage was similar in both scenarios, being 190 and 196 ºC day for the first and second period, respectively.

Figure 1
Representative scheme of the treatment arrangement, using a neighborhood model with different Sumatran fleabane (Conyza sumatrensis) populations (0, 1, 2, 4, and 8 per pot or 0, 35, 71, 141 and 282 plants m⁻², respectively) at two plant size (large and small). The single one rice plant was grown per pot. The experiment was conducted under two irrigation conditions: under flooded conditions and with daily sprinkler irrigation (non-flooded)

Figure 2
Daily average air temperature (ºC) during the first and second experimental period in controlled environment, in days after rice (Oryza sativa L.) emergence. The V3 stage (Counce et al., 2000) of rice cultivation indicated in the figures represents the start of flood irrigation

The study was conducted in 3.6L pots (diameter: top 18.9 cm, bottom 15 cm, and height of 19 cm), kept in a completely randomized design, with four replications. The pots were filled with sieved Albaqualfs soil. Fertilization was corrected by applying 10, 40, and 40 kg ha⁻¹ of nitrogen (N), potassium (K₂O), and phosphorus (P₂O₅) at the experiment establishment, plus 45 kg N ha⁻¹ at the V3 stage (Counce et al., 2000). The Sumatran fleabane biotype used for the neighborhood model experiment was collected from a single plant when the seeds were dehiscent, from a commercial lowland field in the municipality of Caçapava do Sul, RS (latitude 30.75º S, longitude −53.55º W).

Seed dormancy was overcome by cold stratification for three days at 8–10ºC. Seeds were germinated in a growth chamber (model EL202/4, manufacturer ELETROlab®) at 25 ºC temperature and 12-hour photoperiod until the plants reached the stage of 2–3 true leaves. In this stage, they were transplanted into pots and grown in a greenhouse until the end of the experiments. Air temperature data (ºC) inside the greenhouse throughout the experiment periods were monitored using a data logger (model AK170, manufacturer AKSO®). The experiments were conducted until 45 days after the emergence (DAE) of rice, covering approximately the critical period for interference prevention (Rathika et al., 2020).

The treatments were arranged in a factorial design where Factor A consisted of two plant sizes of Sumatran fleabane: large plants (the establishment of Sumatran fleabane plants before rice sowing) and small plants (Sumatran fleabane populations established themselves near the rice sowing). At the same time, Factor B consisted of populations of 0, 1, 2, 4, and 8 Sumatran fleabane plants per pot, which represent 0, 35, 71, 141 and 282 plants m⁻², coexisting with a single rice plant. Four seeds of IRGA 424 RI rice cultivar were sown per pot and thinned to one plant per pot 2 DAE.

Rice seeds were sown when the Sumatran fleabane plants reached 21 and 26 cm in height (first and second repetition, respectively) for the level of Factor A “large plants”. For the level Factor A “small plants”, the rice seeds were sown when the Sumatran fleabane plants had 4 to 8 true leaves and 1 to 2cm in height, respectively (first and second repetition, respectively). Variations in Sumatran fleabane plant height were closely related to contrasting conditions imposed by different cropping systems. Greater plant height was associated with individuals that were already established in the field at the time of rice seeding, whereas shorter height to plants with emergence closer to rice.

The following variables were evaluated in rice plants three times a week: height (cm), number of leaves (NL) on the main stem, and number of tillers per plant. Height was measured by taking the length from the base of the plant to the apex of the last fully expanded leaf. Leaf and tiller counts were obtained by manually counting each structure on the plants. At 45 DAE, root and shoot biomass were collected and subsequently dried in a forced-air oven at 60 ºC for 72 hours. After drying, the material was weighed. From the leaf count data, the phyllochron (ºC day leaf⁻¹) was calculated using the accumulated thermal sum (TSa, ºC day) (Equation 1). The phyllocron is defined as the time interval between the appearance of successive fully expanded leaves on a plant, typically cereal or grass (Streck et al., 2007).

(1) $\text{TSa}=\sum \text{TSa}=\left(\text{Tavg}-\text{Tb}\right)\times 1\text{ day}$

where TSa is the accumulated thermal sum (ºC day), TSd is the daily thermal sum (ºC day⁻¹), Tavg is the daily average air temperature (ºC), and Tb is the base temperature of the species for the vegetative period, set at 11 ºC (Streck et al., 2007). The phyllochron (ºC day leaf^-1) was calculated from the inverse of the angular coefficient of the linear regression between the NL and TSa (Streck et al., 2007).

Additionally, evaluations were conducted to measure the length of the last expanded leaf (LExpL) and the expanding leaf (ExpL) of the main stem for the calculation of the scale proposed by Haun (1973) (Equation 2):

(2) $\text{HS}=\text{NL}+\left(\text{ExpL}/\text{LExpL}\right)$

where HS (Haun scale) is the number of fully expanded leaves (NL) plus the ratio of the length of the expanding leaf (ExpL) to the length of the last expanded leaf (LExpL), with the condition that if this ratio is less than one, one is added to NL.

Regression analyses were performed using the SigmaPlot v.14.0 software. For the variables height (cm) and Haun scale (HS) of rice, the data were fitted to a three-parameter sigmoidal regression (Equation 3):

(3) $\text{Y}=\text{a}/\left[1+{\text{e}}^{-\left(\text{X}-\text{c}/\text{b}\right)}\right]$

where Y is the analyzed variable, X is the accumulated thermal sum (TSa, ºC day), a is the maximum point of the curve, b is the slope of the curve, and c is the TSa in ºC day that provides 50% of the response. To compare the parameters, the confidence interval (p ≥ 0.95) was checked. Overlaying the confidence interval of the model parameters indicated no significant differences. The HS is an indicative way to determine the vegetative stage, based on leaf number and length. Higher values indicate greater vegetative growth in rice plants, reflecting the conditions to which they were exposed.

The phyllochron variable (ºC day leaf⁻¹) was fitted to a quadratic regression (Equation 4):

(4) $\text{Y}=\text{a}+\left(\text{b}*\text{X}\right)+\left(\text{c}*{\text{X}}^2\right)$

where Y is the response variable of interest, X is the number of Sumatran fleabane plants m⁻², a is the intercept, b is the rate of change at the origin, and c informs the degree of curvature and orientation of the concavity of the parabola.

The variables root and shoot dry mass (weight of the dry roots and shoot), NL, and tillers were fitted to a decreasing exponential regression of two parameters (Equation 5):

(5) $\text{Y}=\text{a}*{\text{e}}^{\left(-\text{b}*\text{X}\right)}$

where Y is the response variable of interest, X is the number of Sumatran fleabane plants m⁻², a is the initial value of the variable, and b is the decay rate of the curve or slope.

3. Results and Discussion

For both experimental periods, a three-parameter sigmoidal non-linear regression model adjustment was made for Haun scale and height (cm) (Figures 3 and 4, Tables 1 and 2). An exception occurred in the population with 282 Sumatran fleabane plants m⁻², with large plants, under flooded conditions, in the second period (Figure 4, Table 2). Generally, greater competition was observed in those with large plants of Sumatran fleabane in non-flooded conditions. This is attributed to lower values of parameter a (Equation 3) for HS and plant height, in which the confidence interval for the parameter did not overlap with population 0 in most comparisons (Table 1 and 2). Additionally, plant interference was more significant under non-flooded conditions (Figures 3 and 4). Under flooded condition, no differences were observed in parameter a for any comparison, regardless of Sumatran fleabane size, based on confidence intervals (Table 1 and 2). For large plant size of Sumatran fleabane and densities under non-flooded conditions, there was a reduction in the a parameter value for Haun scale and height when compared to the control in the first period (Table 1). Under the of large Sumatran fleabane plants condition, the Haun scale was reduced by approximately 50%, and the final height (cm) decreased by 60% compared to the control (Figure 3A and C). Under non-flooded conditions, normal growth of Sumatran fleabane plants continued, and competition with the crop persisted throughout the entire experimental period. Under flooding conditions, there was a negative impact on Sumatran fleabane growth and development, affecting its competitive ability.

Figure 3
Haun Scale and height (cm) of rice (Oryza sativa L.) for the first experimental period, according to the accumulated thermal sum (TSa, ºC day), competing with different populations of Sumatran fleabane (Conyza sumatrensis) m⁻², with two plant sizes (large and small), and exposed to non-flooded condition (A and C) and flooded condition (B and D)

Figure 4
Haun Scale and height (cm) of rice (Oryza sativa L.) for the second experimental period, according to the accumulated thermal sum (TSa, ºC day), competing with different populations of Sumatran fleabane (Conyza sumatrensis) m⁻², with two plant sizes (large and small), and exposed to non-flooded condition (A and C) and flooded condition (B and D)

Table 1
Parameters of the sigmoidal equation for the Haun Scale (HS) and height (cm) of rice (Oryza sativa L.) for the first experimental period, according to the accumulated thermal sum (TSa, ºC day), competing with different populations of Sumatran fleabane (Conyza sumatrensis) (plants m⁻²), with two plant sizes (large and small), and exposed to non-flooded and flooded conditions. Santa Maria, RS, 2023

Condition	Weed size	Sumatran fleabane population	Haun Scale
			a*		b*		c*
Non-flooded	Large	0	10.67	(10.3-11.0)1	140.73	(127.6-153.9)	270.02	(255.0-285.0)
		35	8.39	(7.8-9.0)	198.83	(171.3-226.3)	297.76	(261.9-333.6)
		71	5.88	(5.5-6.3)	187.63	(155.8-219.5)	250.30	(215.8-284.8)
		141	5.02	(4.6-5.4)	194.29	(153.5-235.1)	203.10	(166.8-239.4)
		282	4.64	(4.2-5.1)	214.52	(165.7-263.3)	236.16	(185.0-287.3)
	Small	35	10.28	(9.8-10.7)	155.03	(137.7-172.4)	264.28	(244.9-283.7)
		71	10.13	(9.8-10.4)	138.48	(126.1-150.9)	249.75	(236.5-263.0)
		141	9.76	(9.4-10.1)	144.14	(128.3-160.0)	256.74	(239.4-274.1)
		282	9.31	(8.8-9.8)	150.51	(128.1-172.9)	242.53	(219.3-256.7)
Flooded		0	10.72	(10.3-11.2)	153.99	(138.6-169.4)	280.72	(262.6-298.9)
	Large	35	10.74	(9.8-11.7)	181.37	(156.2-206.5)	372.61	(331.7-413.5)
		71	11.34	(10.8-11.9)	173.51	(161.2-185.9)	405.67	(383.8-427.5)
		141	10.93	(9.6-12.3)	188.85	(158.1-219.6)	415.01	(357.7-472.3)
		282	12.44	(10.2-14.6)	217.24	(181.2-253.3)	494.73	(407.5-582.0)
	Small	35	10.72	(10.3-11.2)	157.13	(141.2-173.0)	270.27	(252.1-288.4)
		71	10.95	(10.5-11.4)	156.21	(142.2-170.2)	277.73	(261.3-294.1)
		141	11.08	(10.7-11.5)	147.22	(133.3-161.1)	275.84	(259.7-292.0)
		282	11.11	(10.8-11.4)	144.83	(133.6-156.1)	270.12	(257.3-282.9)
Rice height (cm)
Non-flooded		0	64.3	(61.7-66.9)	111.74	(97.5-126.0)	266.93	(250.4-283.5)
	Large	35	37.99	(34.6-41.3)	146.42	(112.9-179.9)	274.6	(235.9-313.3)
		71	23.3	(22.5-24.1)	110.5	(98.6-122.4)	278.05	(263.9-292.2)
		141	20.75	(18.9-22.6)	154.68	(113.2-196.2)	219.71	(108.4-259.1)
		282	20.12	(18.6-21.7)	169.84	(133.7-206.0)	231.19	(195.4-267.0)
	Small	35	69.34	(64.9-73.7)	133.9	(113.0-154.8)	308.98	(282.5-335.5)
		71	67.13	(63.5-70.7)	125.69	(105.9-145.5)	269.04	(246.3-291.8)
		141	58.71	(55.6-61.8)	118.54	(98.5-138.6)	255.15	(232.7-277.6)
		282	58.91	(54.9-62.9)	132.56	(106.0-159.1)	255.44	(226.3-284.6)
Flooded		0	68.74	(65.1-72.4)	128.82	(110.4-147.3)	289.39	(267.2-311.6)
	Large	35	74.59	(66.2-82.9)	145.82	(118.8-172.9)	407.94	(362.8-453.1)
		71	71.6	(62.3-80.9)	138.37	(109.5-167.3)	429.96	(380.0-480.0)
		141	73.17	(63.4-82.9)	155.28	(127.8-182.8)	453.41	(399.8-507.0)
		282	83.27	(67.4-99.2)	187.8	(157.2-218.4)	537.62	(456.2-619.0)
	Small	35	71.01	(68.4-73.6)	109.79	(96.9-122.7)	268.39	(253.3-283.5)
		71	74.62	(70.7-78.6)	123.04	(105.1-141.0)	293.14	(271.3-315.0)
		141	70.2	(66.3-74.1)	122.96	(103.2-142.7)	278.98	(255.7-302.3)
		282	68.01	(63.5-72.6)	131.64	(107.0-156.3)	273.65	(254.2-302.1)

• ^*
  All parameters had a p-value of < 0.0001. ¹Confidence interval (p ≥ 0.95).

Table 2
Parameters of the sigmoidal equation for the Haun Scale (HS) and height (cm) of rice (Oryza sativa L.) for the second experimental period, according to the accumulated thermal sum (TSa, ºC day), competing with populations of Sumatran fleabane (Conyza sumatrensis) (plants m⁻²), with two plant sizes (large and small), and exposed to non-flooded and flooded conditions. Santa Maria, RS, 2023

Condition	Weed size	Sumatran fleabane population	Haun Scale
			a*		b*		c*
Non-flooded		0	9.78	(8.3-11.3)1	110.47	(87.5-133.5)	232.49	(192.3-272.7)
	Large	35	11.56	(1.5-21.6)	214.38	(123.0-305.7)	420.25	(70.6-769.9)
		71	4.23	(4.0-4.5)	55.58	(38.6-72.5)	97.58	(85.1-110.1)
		141	6.55	(1.8-8.3)	155.08	(94.0-216.1)	197.37	(104.6-290.2)
		282	6.24	(5.3-7.1)	131.74	(99.6-163.9)	181.71	(139.5-223.9)
	Small	35	8.97	(8.0-9.9)	124.51	(105.0-144.0)	212.13	(181.3-243.0)
		71	7.97	(7.3-8.6)	110.28	(94.2-126.3)	190.63	(169.4-211.9)
		141	7.60	(6.9-8.3)	120.03	(103.4-136.6)	204.65	(179.9-229.4)
		282	7.19	(6.1-8.3)	129.26	(100.3-158.2)	205.93	(161.6-250.3)
Flooded		0	9.79	(9.1-10.5)	112.17	(101.5-122.8)	226.93	(208.9-244.9)
	Large	35	16.06	(8.4-23.7)	176.16	(136.7-215.6)	409.33	(251.5-567.2)
		71	14.00	(6.2-21.8)	186.92	(130.7-243.1)	385.36	(185.2-585.6)
		141	20.48ns	(-6.5-47.5)	222.34	(140.0-304.7)	556.61	(70.5-1042.7)
		282	16.98ns	(11.4-22.6)	231.59	(189.5-273.7)	480.42ns	(343.0-617.9)
	Small	35	10.35	(9.2-11.5)	122.46	(107.8-137.1)	257.31	(227.1-287.5)
		71	10.43	(9.2-11.6)	123.60	(107.8-139.4)	258.40	(225.7-291.1)
		141	13.04	(7.5-18.6)	160.79	(118.7-202.9)	353.45	(216.0-490.9)
		282	10.38	(7.3-13.4)	133.79	(61.1-171.4)	276.72	(189.9-363.5)
Rice height (cm)
Non-flooded		0	47.82	(33.7-61.9)	95.13	(63.2-127.1)	269.03	(201.8-336.3)
	Large	35	18.16	(11.3-25.0)	120.86	(40.1-201.6)	183.94	(79.1-288.7)
		71	12.51	(11.7-13.4)	78.79	(58.3-99.2)	114.91	(100.5-129.3)
		141	14.57	(13.1-16.0)	66.13	(43.8-88.4)	143.42	(123.2-163.7)
		282	16.87	(13.4-20.3)	96.64	(61.9-131.4)	204.81	(155.7-253.9)
	Small	35	35.22	(30.6-39.9)	87.38	(68.4-106.3)	224.56	(195.0-254.2)
		71	32.25	(28.8-35.7)	77.76	(59.9-95.6)	198.77	(175.8-221.7)
		141	28.95	(26.1-31.8)	73.27	(57.0-89.5)	196.99	(176.5-217.5)
		282	20.99	(18.8-23.2)	73.92	(55.0-92.8)	181.17	(159.5-202.8)
Flooded	Large	0	46.96	(41.4-52.5)	79.40	(62.5-95.3)	229.48	(204.6-254.4)
		35	52.23	(33.7-70.8)	111.16	(82.8-139.6)	325.04	(240.7-409.3)
		71	46.86	(29.0-64.7)	101.97	(72.6-131.3)	320.75	(236.1-405.4)
		141	44.71	(24.0-65.4)	118.85	(85.3-152.4)	348.18	(235.6-460.7)
		282	No adjustment.
	Small	35	43.09	(36.6-49.6)	80.12	(63.4-96.9)	256.69	(255.6-287.7)
		71	48.18	(36.9-59.5)	93.70	(69.3-118.1)	273.47	(220.6-326.3)
		141	48.38	(40.4-56.4)	91.24	(75.8-106.7)	286.61	(250.9-322.3)
		282	46.81	(34.8-58.8)	100.74	(80.6-120.9)	317.06	(260.2-373.9)

• ^*
  The parameters that presented a significant p-value obtained values equal to or less than 0.0228 for a, 0.0054 for b and 0.0236 for c. ns: not significant. ¹Confidence interval (p≥0.95)

Under flooding conditions, it was observed that the Haun scale and height values were similar to the control at the end of the experiment, at both plant size of Sumatran fleabane, based on the a parameter of the model (Equation 3) (Figure 3B and D, Table 1). However, during the early stages of the experiment, generally up to 548 ºC day, a reduction in Haun scale and rice height was observed in those treatments with large plants of Sumatran fleabane, which was noticeable when compared to the control. In contrast, this behavior was not observed in treatments with small Sumatran fleabane plants. This is because small plants of Sumatran fleabane remained submerged under a 4cm water layer in the early stages of development, in flooded conditions (Figure 3B and D).

Early exposure of Sumatran fleabane plants to flooding led to their death 13 days from the onset of flooding (data not shown), with a TSa of approximately 389 ºC day (Figure 3B and D). Such results justify the values of parameter b or slope in Equation 3 being similar to those from control and with an overlapping confidence interval for the parameter, for Haun scale and height in the first experimental period. At the same time, Sumatran fleabane plants that sprouted prior to crop emergence died 18 days after the water layer was applied, when TSa was close to 480 ºC days (data not shown) (Figure 3B and D). Under flooded conditions, the rice crops showed a trend towards stabilization in growth and development with or without the Sumatran fleabane presence, suggesting low interference of the weed species with rice crops.

For the second experimental period, temperature conditions promoted slower growth and development of rice; however, weeds had more favorable conditions, given its ability to tolerate milder temperatures. This occurs because Conyza spp. emergence peaks in autumn-winter, and in spring its development accelerates, leading to flowering induction (Soares et al., 2017). This fact favored the competitive ability of Sumatran fleabane over the rice plants in the neighborhood model study. Additionally, another factor that contributed to variations in the results is that the Sumatran fleabane plants had more days to develop before flooding initiation in the pots. Therefore, small plants of Sumatran fleabane plants attained greater height at the time of water layer application, allowing them not to be submerged, unlike the first experimental period. Consequently, there was a greater impact on rice development under non-flooded conditions (Figure 4a and C) compared to the control, evidenced by the difference in a values (Equation 3) for approximately half of the comparisons with the treatments (Table 2). Consequently, it was observed that during this experimental period, less than 10% of experimental units presented dead Sumatran fleabane plants (data not shown). This fact allowed the crop and weed to continue competing until the end of the experiment, even under flooded conditions.

There was no influence on the variable Haun scale and rice height under flooded conditions (Figure 4B), and the values of a (Equation 3) were similar among treatments (Table 2). Under non-flooded conditions, there was an average reduction of 68% in height of rice plants, based on the a parameter values (Equation 3), compared to the control treatment where there were large plants of Sumatran fleabane (Table 2). Yet, comparing the a values (Equation 3) of different Sumatran fleabane populations and for the controls, under flooded conditions, the reduction was only 2% (Table 2). This result corroborates what was observed in the first experimental period, where the presence of the water layer harmed the competitive ability of Sumatran fleabane and favored the rice crop.

In the first period, the model was exclusively adjusted for large plants of Sumatran fleabane under non-flooded conditions, regarding the phyllochron variables and NL (Figure 5). For the second period, models were adjusted, except for the NL variable for small plants of Sumatran fleabane, under flooded conditions (Figure 6). Under non-flooded conditions, with small plants of Sumatran fleabane, there was no significant difference in phyllochron among different Sumatrann fleabane populations due to overlapping confidence intervals. On the other hand, with large plants of Sumatran fleabane, the phyllochron variable had higher values (Figures 5A and 6A). As for flooded conditions, in the first period, there was no significant difference in the phyllochron variable, regardless of the plant size of Sumatran fleabane (Figure 5B).

Figure 5
Phyllochron (ºC day leaf⁻¹), number of leaves, and tillers of rice (Oryza sativa L.) competing with different populations of Sumatran fleabane (Conyza sumatrensis) m⁻², with two plant sizes (large and small) under non-flooded condition (A, C, and E) and flooded condition (B, D, and F), at 45 days after crop emergence of the first experimental period. The bars indicate the confidence interval

Figure 6
Phyllochron (ºC day leaf⁻¹), number of leaves, and tillers of rice (Oryza sativa L.) competing with different populations of Sumatran fleabane (Conyza sumatrensis) m⁻², with two plant sizes (large and small) under non-flooded condition (A, C, and E) and flooded condition (B, D, and F), at 45 days after crop emergence of the second experimental period. The bars indicate the confidence interval

The phyllochron values for the controls (without Sumatran fleabane presence) ranged from 73.3 to 75.1 and 48.4 to 50.2 ºC day leaf⁻¹, for the first and second experimental periods, respectively. Consistent with these results, phyllochron values for rice in RS range from 42.6 to 77.7 ºC day leaf⁻¹, depending on the cultivar and sowing season (Streck et al., 2007). Thus, it can be observed that there was a delay in crop development in competition with Sumatran fleabane, especially when large plants of Sumatran fleabane, under non-flooded conditions in both periods (Figures 5A and 6A). This is evidenced by the higher value of the curve decay coefficient (parameter b, Equation 4) (Tables 1 and 2), which represents a sharper decline in rice leaf emergence as the Sumatran fleabane population increases, resulting in increased phyllochron values.

In general, the higher Sumatran fleabane densities, the higher phyllochron value observed in rice plants, which resulted in a lower final NL (Figures 5 and 6). These findings reflected the negative effects of weed competition on rice development. Under non- flooded conditions with large plants of Sumatran fleabane, the decay coefficient of the curve (parameter b, Equation 4) was 0.0033 and 0.0028 for the first (Figure 5C) and second (Figure 6C) experimental periods, respectively. The NL serves as the primary means for capturing solar radiation, converting it into assimilates — essentially energy for the plant (Taiz et al., 2017). In experiments evaluating the competitive ability of rice with barnyardgrass (Agostinetto et al., 2008) and jointvetch (Aeschynomene denticulata Rudd) plants (Galon et al., 2015), there was an average reduction of 45% and 26% in the crop leaf area, respectively, in the lowest evaluated proportions. Consequently, the reduction in photosynthetically active areas delays the growth and development of rice, decreasing its competitiveness.

For the number of tillers, it was possible to fit the exponential model, except in the flooded condition with small plants of Sumatran fleabane, in the first period (Figure 5F), with a R² above 0.80. The greatest impact on the reduction of tiller numbers was observed in the non-flooded condition with large plants of Sumatran fleabane. In this situation, starting from populations of 71 and 35 Sumatran fleabane plants m⁻², for the first and second period, respectively, rice plants did not produce any tillers (Figure 5E and 6E). In fact, the rice tillering was higher in flooded conditions. Competition of rice with jointvetch showed an average reduction of 38% in the number of tillers, regardless of the proportions of rice plants and its competitor (Galon et al., 2015). In the flooded condition, this impact was reduced, with tiller production observed at populations of 282 and 71 Sumatran fleabane plants m⁻², for the first and second periods, respectively (Figure 5F and 6F). This reduction can be observed by the coefficient b of the models, with a greater impact in the second period, with values of 0.7433 and 0.0249 for the non-flooded and flooded conditions, respectively. The tiller emergence rate varies according to the cultivar, sowing density, and environmental conditions; however, it number has a direct influence on the main productivity component of the crop, which is the number of panicles per area (Li et al., 2021). Moreover, tillering capacity is an important characteristic that confers competitiveness to the crop (Dass et al., 2017).

For the shoot dry mass variable, it was possible to fit the exponential model, except for the condition with flooding and large plants of Sumatran fleabane, in the first period (Figure 7B), with a R² above 0.95. When analyzing large plants of Sumatran fleabane, the values of coefficient b are similar for both periods, being 0.0734 and 0.0782 for the first and second periods, respectively (Figure 7A and 8A). For root dry mass, the model was adjusted, with R² values equal to or greater than 0.97 (Figure 7 and 8). The exception to this was in small plants of Sumatran fleabane, in the first period, both in non-flooded and flooded conditions (Figure 7C and D). Overall, the greatest reduction in roots and shoot dry mass occurred under non-flooded conditions with large plants of Sumatran fleabane. In competition studies, the negative impact of competition on roots mass is generally greater compared to the shoot part, probably due to competition for nutrients, besides the fact that weeds have a greater competitive capacity than cultivated plants (Kiær et al., 2013).

Figure 7
Dry mass (g) of shoot and roots of rice (Oryza sativa L.) competing with different populations of Sumatran fleabane (Conyza sumatrensis) m⁻², with two plant sizes (large and small) under non-flooded condition (A and C) and flooded condition (B and D), at 45 days after crop emergence of the first experimental period. The bars indicate the confidence interval

Figure 8
Dry mass (g) of shoot and roots of rice (Oryza sativa L.) competing with different populations of Sumatran fleabane (Conyza sumatrensis) m⁻², with two plant sizes (large and small) under non-flooded condition (A and C) and flooded condition (B and D), at 45 days after crop emergence of the second experimental period. The bars indicate the confidence interval

Overall, based on the results of both experimental periods, the lower competitive ability of rice with Sumatran fleabane occurred when large plants were already established, especially under non-flooded conditions. In this situation, there was a pronounced reduction in the NL, tillers, height, root, and shoot dry mass of rice. However, when the water was applied (V3 stage of rice), the competitive ability of Sumatran fleabane decreased due to plant mortality.

4. Conclusions

Under non-flooded conditions, large Sumatran fleabane plants are more competitive than rice plants. Small plants of Sumatran fleabane do not significantly affect the growth and development of rice, regardless of the irrigation condition.

Under flooded conditions, the competition ability of Sumatran fleabane is reduced due to its mortality, mitigating the impact this species has on rice crops. Even though higher densities of Sumatran fleabane increase its competition ability, they are less detrimental under flooding conditions.

Acknowledgements

We would like to thank the National Council for Scientific and Technological Development (CNPq) for the research grants awarded to several of the authors, both undergraduate and graduate, during this work.

• Funding
  We would like to thank Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS, grant no. 09/2023) (Proc. 24/2551-0001181-1) for their financial support; and in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes), Brasil - Finance code 001.

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Edited by

• Editor in Chief:
  Carol Ann Mallory-Smith

• Associate Editor:
  Silvia Fogliatto

Publication Dates

• Publication in this collection
  15 Sept 2025
• Date of issue
  2025

History

• Received
  16 Mar 2025
• Accepted
  07 July 2025