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Article

Morphological Diversity and Crop Mimicry Strategies of Weedy Rice Under the Transplanting Cultivation System

by
Yi-Ting Hsu
1,
Yuan-Chun Wang
1,2,
Pei-Rong Du
3,
Charng-Pei Li
3 and
Dong-Hong Wu
1,3,*
1
Department of Agronomy, National Chung Hsing University, Taichung City 402202, Taiwan
2
Agriculture Bureau, Taichung City Government, Taichung City 420018, Taiwan
3
Crop Science Division, Taiwan Agricultural Research Institute, Ministry of Agriculture, Taichung City 413008, Taiwan
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(4), 984; https://doi.org/10.3390/agronomy15040984
Submission received: 30 March 2025 / Revised: 15 April 2025 / Accepted: 17 April 2025 / Published: 19 April 2025
(This article belongs to the Special Issue Weed Biology and Ecology: Importance to Integrated Weed Management)
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Abstract

:
The continued emergence of weedy rice (Oryza sativa L.) in Taiwan poses serious challenges to seed purity and commercial rice cultivation, particularly under transplanting systems. These off-type individuals, often marked by a red pericarp, reduce varietal integrity and complicate seed propagation. This study evaluated the morphological variation among 117 Taiwan weedy rice (TWR) accessions and 55 control cultivars, which include 24 temperate japonica cultivars (TEJ), 24 indica cultivars, and seven U.S. weedy rice (UWR) types. Principal component analysis (PCA) showed that TWR shares vegetative traits with modern cultivars but exhibits grain morphology resembling indica landraces—indicating weak artificial selection pressure on grain traits during nursery propagation. TWR was also found to possess a suite of adaptive weedy traits, including semi-dwarfism, delayed heading, high shattering, and superior seed storability, facilitating its persistence in field conditions. These findings provide critical insights for integrated weed management and cultivar purity strategies, emphasizing the importance of certified seed use, stringent field hygiene, and disruption of weedy rice reproductive cycles.

1. Introduction

Rice (Oryza sativa L.) is one of the three principal global staple crops. According to the Food and Agriculture Organization (FAO), global rice production reached 790 million tons in 2023, with Asia contributing approximately 720 million tons—underscoring the region’s dominance in global rice cultivation. In Taiwan, rice remains the primary staple, cultivated across nearly 230,000 hectares, which represents about one-fourth of the nation’s arable land.
Beginning in 2012, off-type rice plants have been increasingly detected in Taiwan’s rice production areas, including both commercial and certified seed propagation systems. This trend has raised significant concern among breeders and policymakers due to its implications for varietal purity and weed proliferation. These individuals, characterized by yellow hulls and reddish-brown pericarps, are classified as Taiwan weedy rice (TWR) [1,2]. Although morphologically similar to cultivated rice, the red pericarp significantly diminishes seed purity, allowing TWR to become established as an emerging weed species. As with weedy rice (Oryza spp.) in other global regions, TWR belongs to the same species as cultivated rice and exhibits considerable morphological resemblance, making its identification and eradication difficult. A previous study indicated that weedy rice exhibits higher competitiveness due to having higher tillering, panicle number, nutrient uptake, and stress tolerance [3]. Through competition, TWR reduces yield [3,4,5,6] and depresses market value [7,8].
Weedy rice populations globally are known for their pronounced morphological diversity, which enhances their environmental adaptability to conditions such as heat, cold, drought, and biotic stress [3,9]. In the japonica production zones of Italy, for example, weedy red rice can be classified into long-awned, short-awned, and awnless types, reflecting adaptation to a range of agroecological conditions [10,11]. In the United States, compact weedy rice morphotypes with long awns show high germination and seed viability, while looser, awnless types demonstrate superior salt tolerance [12]. In Thailand’s indica-producing regions, the phenotypic traits of weedy red rice have gradually converged with those of cultivated indica varieties, facilitating integration into paddy field management systems [13]. Similar evolutionary patterns have been observed in Europe and the Americas; for instance, Colombian weedy rice exhibits diverse morphological traits, aligning with javanica-like, indica-like, or hybrid backgrounds [14].
In China, studies have reported that weedy rice in provinces such as Northeast China, Jiangsu, and Guangdong possess traits distinct from cultivated varieties, including early flowering, shorter grain-filling periods, earlier maturation, strong seed shattering, and variable dormancy—traits that enhance their competitiveness and seedbank persistence [15,16]. Such features complicate management and facilitate further spread [17,18]. Previous surveys in Taiwan have revealed similar trends. TWR populations demonstrated greater tillering capacity than modern indica and japonica cultivars and significantly higher SHA (13.9–14.3%) compared to 2.9–5.1% in cultivated varieties [19]. TWR also exhibited shorter heading periods, increasing its potential to become established as a persistent weed. Due to its straw-colored hulls and short, awnless grains—traits that closely resemble common japonica cultivars—visual identification of TWR in the field remains challenging.
The red rice with taller height and a longer awn from paddy fields was removed before 1945. However, the modern TWR that has shorter height and is awnless was more similar to cultivated rice; it is hard to distinguish between the cultivar and TWR, which caused farmers’ income loss [1,2]. Due to the significant impact of weedy red rice on Taiwan’s rice production system and the difficulty of control under the transplanting system, a comprehensive investigation into TWR traits is crucial for developing effective management strategies. To address this gap, the present study aims to systematically assess key morphological and physiological traits of TWR, clarify its mimicry and competitive strategies, and provide an empirical foundation for integrated management approaches in Taiwan’s rice agroecosystems.

2. Materials and Methods

2.1. Plant Materials

Based on previous observations, a total of 117 TWR accessions were classified into five phylogenetic clusters, each collected from geographically distant seed fields across Taiwan [1]. As reference populations, 2 groups of cultivated rice, temperate japonica cultivar (TEJ) and indica cultivar (IND), and 1 weedy rice, U.S. weedy rice (UWR), accessions were included, totaling 172 rice accessions for evaluation (Table S1).

2.2. Cultivation and Experimental Design

All rice materials were pre-germinated in a controlled greenhouse environment at the Taiwan Agricultural Research Institute (TARI), Wufeng District, Taichung City. Seedlings at the three-leaf stage were transplanted into field plots for trait evaluation. Standard agronomic practices were followed for fertilization and irrigation, with nutrient applications at 100.8 kg/ha nitrogen (N), 88.4 kg/ha phosphorus pentoxide (P2O5), and 77.6 kg/ha potassium oxide (K2O) (Table S2). In addition, the seed storability and burial experiment design was arranged in a completely randomized design (CRD) and three replicates. Traits investigations were conducted during two cropping seasons in 2018 (2018I and 2018II), and the seed dormancy and soil burial experiments were conducted during the second cropping season in 2019 (2019II) to the first cropping season in 2020 (2020I).

2.3. Traits Investigation

2.3.1. Seedling Morphological Traits

Seedling vigor was assessed using eight filled seeds per variety and three biological replicates. Seeds were sterilized, rinsed, and soaked in tap water for 15 h at 30 °C in the dark before being set in a germination pouch (13 × 15 cm2; Mega international, Minneapolis, MN, USA). Germination was conducted in darkness at 30 °C and 75% relative humidity for five days. A camera was positioned 60 cm above the samples after 5 days. Seedling shoot length (SL) and mesocotyl length (ML) were quantified from standardized digital images. ImageJ (Fiji, version 2.3.0; https://imagej.net/Fiji, accessed on 16 April 2025) software [20] was used for measurements after cropping and preprocessing, and the mesocotyl was measured from the seed base to the coleoptile node using the segmented line tool [21]. Root surface area (RA) was evaluated using GiARoots software (version 0.6.0; https://mybiosoftware.com/tag/gia-roots, accessed on 16 April 2025) [22], applying empirically adjusted RGB threshold values across image batches.

2.3.2. Field Agronomic Traits

The agronomic traits and yield components were investigated during cultivation and at maturity, which followed the methods and standards established by the International Rice Research Institute (IRRI) [23]. The recorded traits included heading date (HD), plant height (PH), pericarp color (SC), hull color (HC), spike number (SN), number of total grains (GNT), fertility rate (FER), seed shattering rate (SHA), and seed dormancy. Each variety (or line) was planted in three-row plots, with 30 plants per plot, with a planting density of 30 cm × 15 cm.

Growth-Related Traits

PH was measured in 2018, with measurements taken at different growth stages in 2018I and 2018II, included the seedling stage, tillering stage, boosting stage, heading stage, grain-filling stage, and maturing stage. Four plants were sampled as replicates for each variety or accession to calculate the average growth rate (Equation (1)).
(plant heightlast day − plant heightfirst day)/(daylast day − plant heightfirst day)
The leaf area index (LAI) was measured with a plant canopy analyzer (LAI-2200; LI-COR Bioscience, Lincoln, NE, USA), which collected four measurement values between two plants, along with one blank (Blank) reading [24]. The raw data were then processed using FV2200 software (version 2.0.3) to calculate the LAI value. This experiment was conducted in 2018, with collecting times at 53 days after transplanting in 2018I and 39 days after transplanting in 2018II, corresponding to the late tillering-to-panicle initiation stage. HD was recorded as the number of days from transplanting to the emergence of the panicle from the flag leaf sheath for each variety or accession.

Yield Components

SHA was measured by selecting the strongest panicle from 4 plants of each variety or accession during the third week after heading [25]. SHA was calculated as GNT per panicle/the total number of filled grains, which was conducted during 2018I. Yield components, including SN, GNT, and FER, were investigated for 4 plants of each variety or accession after harvest in both cropping seasons of 2018.

Grain Appearance

The rice grains of each variety or accession were scanned using a scanner (MRS-600U2ADF; MICROTEK, Hsinchu, Taiwan). The scanned images of the rice grains were analyzed with the “SmartGrain” software (version 1.2; https://www.naro.affrc.go.jp/archive/nias/qtl/SmartGrain/, accessed on 16 April 2025) [26] to obtain data on the grain length (GL) and grain width (GW). The ratio of grain length to grain width (GLW) was calculated as follows: grain length/grain width. SC and HC were categorized based on the IRRI color classification [23].

2.4. Seed Dormancy and Soil Seedbank Persistence

2.4.1. Preharvest Sprouting

The dormancy of the panicle (preharvest sprouting rate, PHS), conducted in 2019II, was assessed using the ex vitro test with a roll-paper method for the germination rate experiment. Rice panicles were collected at 40 and 54 days after heading as testing materials (PHS40 and PHS54). Three panicles were sampled per accession, each from an individual plant, constituting biological replicates. For each variety or accession, three panicles were sampled for replication. The panicles in roll-paper were then placed in a dark chamber at 30 °C for germination. After 7 days, the germination rate was calculated based on the proportion of germinated seeds among the filled seeds. Germinated seeds were identified when the embryo root or embryo axis was visibly protruding from the seed coat. When the germination rate exceeded 80%, the seeds were considered to have lost dormancy [27].

2.4.2. Seed Storability

The harvested seeds were placed in a 40 °C oven for 3 days until they showed a moisture content of approximately 13%, and then the rice seeds were stored in a room environment. Germination rate investigations were conducted on seeds stored for 3, 8, and 12 months (RT3M, RT8M, RT12M). For each batch, three replications were performed, with 100 filled rice seeds selected for each replication. These seeds were placed in sterile Petri dishes lined with moisture filter paper and then placed in a dark growth chamber at 30 °C for germination. The seeds were removed after 7 days, and the germination rate was calculated as the proportion of germinated seeds among the filled seeds. Germinated seeds were identified when the embryo root or embryo axis was visibly protruding from the seed coat. The seeds stored for 3 and 8 months were harvested in 2018II, while those stored for 12 months were harvested in 2019I.

2.4.3. Soil Burial Experiments

In the burial experiments, 100 seeds from each variety were placed in a nylon mesh bag (6 × 9 cm, 125 mesh), and each variety or accession was tested in two replicates. Considering that seed germination ability is significantly suppressed at burial depths of 10–15 cm [28], two burial depths of 5 cm and 10 cm were selected on a flooded field in 2019II and 2020I. Additionally, seeds were buried at 5 cm and 10 cm depths in upland fields in 2020I. The burial period for 2019II was from 21 August to 5 October 2019. The average monthly temperatures for August, September, and October were 27.8 °C, 28 °C, and 24.5 °C, respectively, with accumulated rainfall of 346 mm, 32.5 mm, and 0.5 mm, respectively, and accumulated rainy days of 19, 3, and 0 days, respectively. The burial period for 2020 I was from 30 March to 13 May 2020. The average monthly temperatures for March, April, and May were 20.8 °C, 24.2 °C, and 24.9 °C, respectively, with accumulated rainfall of 143.5 mm, 45 mm, and 463.5 mm, respectively, and accumulated rainy days of 12, 4, and 20 days, respectively. The investigated method referred to [29], where seeds were excavated after 45 days of burial and the number of decayed seeds and germinated seeds counted.
Complete intact seeds were used for germination testing. After 7 days, the number of germinated seeds was counted, whereas the remaining seeds were tested for viability with tetrazolium staining: a 1% tetrazolium solution was applied after vertically cutting the seed embryos, and the seeds were inspected after 3 h in a 30 °C chamber [17]. The rice seeds used in the experiment were harvested in 2019I and stored for 2 months and 9 months before the burial experiments in 2019II and 2020I, respectively. The burial period for 2019II was from 21 August to 5 October 2019. The burial period for 2020I was from 30 March to 13 May 2020.

2.5. Statistical Analysis

Statistical analyses were conducted using R statistical software (R version 4.1.3). Analysis of variance (ANOVA) was performed using the “agricolae” package [30] to evaluate variations among populations. Since traits such as HC and brown rice color are categorical, they were excluded from the variance analysis. For variables that reached a significant level, Fisher’s protected least significant difference test (LSD test) was applied to determine significant differences among populations. The “ggplot2” package [31] was used to draw dual-axis plots, line charts, box plots, and violin plots. Principal component analysis (PCA) and biplot were determined using the “FactoMineR” package [32] and the “factoextra” package [33]. Additionally, eigenvalues and factor loadings of each principal component were calculated to assess the correlation between traits and principal components.

3. Results

3.1. Seedling Vigor and Vegetative Growth Characteristics

In this study, a total of 172 rice varieties (lines) were investigated in the TWR and three control groups (UWR, TEJ, IND) for their traits, followed by seedling growth vigor, existing morphology in the vegetative period, appearance traits of grains in the reproductive period after heading, and factors contributing to weed accumulation after seed maturity.
SL, RA, and ML were investigated during the seedling growth period. The SL of TWR was significantly higher than TEJ. TWR had the highest RA, significantly higher than UWR and TEJ. On the other hand, in the result of ML, UWR had the longest at 15.1 ± 4.9 mm, whereas TWR had the shortest at 2.4 ± 2.4 mm, among all the groups (Table 1). LAI, HD, and PH were investigated during 2018. In 2018I, the mean LAI of TWR was similar to IND but significantly higher than TEJ and UWR. In 2018II, TWR recorded the highest LAI (2.1 ± 0.7), significantly exceeding values observed in both the TEJ and UWR populations. The average HD of TWR in 2018I was 74.0 ± 3.3 days, similar to IND but significantly later than TEJ and UWR. In 2018II, TWR (64.7 ± 3.7 days) was also close to IND; UWR had the latest HD, whereas TEJ had the earliest (64.7 ± 3.7 days).
Based on the above results, the SL and RA of TWR and UWR were superior to those of IND and TEJ. These traits enhance the ability of weedy rice to compete with cultivated rice for nutrients and sunlight [34], thereby improving its survival rate during the seedling stage in the field. Additionally, the ML of UWR was more obvious [35] than that of TWR, suggesting that ML may not significantly affect TWR’s adaptation to the transplantation system. This finding implies that different cultivation systems and environmental conditions could drive weedy rice to evolve distinct adaptive traits. During the vegetative stage, TWR exhibited a larger LAI, which enhances light capture and contributes to its photosynthetic advantage. While most weedy rice varieties typically showed early-maturing characteristics, TWR displayed a late-maturing trait (Table 1). It was speculated that when cultivated rice grains reach maturity in the field, TWR may still be in an immature, green state (Figure S1). This delayed maturity could make TWR more difficult to identify and eliminate during harvest, as it blended with cultivated rice.
During the two cropping seasons of 2018, changes in the PH and growth rate were investigated throughout the rice growth period. In 2018I, PH was measured at 28, 42, 56, 70, and 84 days after transplantation, with the 28th day corresponding to the tillering stage. The PH of TWR and TEJ was the lowest during the seedling stage and grain-filling stage, but with no significant difference between them, whereas UWR had the greatest PH, followed by IND. However, UWR reached full maturity and was harvested 84 days after heading; therefore, no PH data were available for this stage. Regarding the growth rate of PH, TWR was similar to TEJ throughout the growth period, except during heading to the grain-filling stage, where it was higher. UWR showed the highest growth rate during the seedling-to-tillering stage and the booting-to-heading stage (Figure 1A). In 2018II, PH was measured at 28, 49, 56, 63, 70, and 77 days after transplanting. The PH of TWR was also similar to that of TEJ, except during the tillering-to-boosting stage. The results of the PH of UWR and IND were similar in 2018I. For the growth rate of PH, TWR was similar to TEJ before the boosting stage; then, the subsequent growth rate was higher than TEJ. Therefore, the PH of TWR was gradually similar to TEJ after the heading stage. The PH growth rate of IND was similar to TEJ from the tillering stage to the spikelet differentiation stage (from day 0 to 28 and from day 29 to 49), which gradually increased after that, and the growth rate of UWR was higher than other groups during the cultivation period (Figure 1B).
The results showed that TWR had a similar PH to TEJ only during the tillering and grain-filling stages. This trait may help TWR remain concealed in the field during rice cultivation. In contrast, UWR exhibited the greatest PH throughout the entire cultivation period, which may provide an advantage in competing for sunlight and nutrients.

3.2. Reproductive Traits and Grain Morphology

For SN of the yield components, TWR and IND had the highest SN in 2018I, at almost 18 spikes, followed by UWR, and TEJ was the lowest. In 2018II, UWR had the highest SN, followed by TWR; and, furthermore, the GNT of TWR was the highest in 2018. TWR had the highest FER in 2018I (86 ± 9%), but showed the lowest in 2018II (69 ± 11%); and TEJ and IND were greater than 80% in 2018, while UWR was greater than 70%. In addition, UWR and TWR had the highest SHA, while they were also significantly higher than TEJ in 2018 (Table 2).
The results of SN, GNT, and SHA were significantly better than TEJ and IND, with TWR having the highest GNT. TWR could produce a large number of seeds after completing its life cycle, which mixed into the grains of cultivated rice in the plant harvest or spread to other fields through shared equipment, and the trait of high SHA also raised the risk of the weedy rice seeds entering the soil seed bank.
In the results of HC, TWR had mainly straw-colored, such as TEJ and IND, with only 2 of the 117 accessions lines presenting purple furrows on straw; but UWR had more diverse HC, with one straw-colored, two brown furrows on straw, one brown furrow on straw, and three black furrows on straw (Figure 2A). Otherwise, 106 (91%) of the 117 TWR accessions had a red pericarp; another 9% had a purple, variable-purple, light-brown, or white pericarp; while UWR also had predominantly a red pericarp; and only one accession had brown color. In contrast, all the SC of TEJ presented as light brown, whereas IND mainly showed light brown, with a few white (Figure 2B). Those grain appearances indicated most TWR accessions were straw-colored for HC and red for SC (Figure 2A,B).
It was observed that the GL, GW, and GLW of TWR differed significantly from the other three rice groups (Fisher’s LSD at the 0.05 probability level). TWR had the shortest GL, while UWR had the longest, followed by IND and TEJ (Figure 2C). TEJ showed the widest GW, followed by TWR and IND, whereas UWR showed the lowest. Regarding GLW, UWR had the highest value, while TWR had an intermediate value, falling between TEJ and IND. These results indicated that the grain color of TWR was similar to TEJ and IND. However, the grain shape of TWR aligned with the short-grain type, which resembled TEJ. In contrast, UWR exhibited more diverse grain colors and a long grain shape.

3.3. Dormancy and Soil Seedbank Accumulation Potential

This experiment aimed to determine the changes in seed status of the TWR and the three control groups after the seeds entered the soil seed bank. The results from two paddy trails of the 2018 and 2019 burial experiment indicated that approximately 98% and more than 95% of the seeds from TWR, TEJ, and IND decayed during the burial period. In 2018II, less than 1% of the undecayed seeds remained viable after being buried in the paddy field for 45 days at a depth of 5 cm, while the percentage of decayed seeds for UWR was 90.94 ± 5.80%, with only 1% of the undecayed seeds still viable. Furthermore, after being buried at a depth of 10 cm in the paddy field for 45 days, more than 95% of the TWR, TEJ, and IND seeds had decayed, and less than 2% of the undecayed rice seeds remained viable. In contrast, only about 30% of the UWR seeds had decayed, while 48.77% of the undecayed rice seeds retained their vigor (Table 3).
In the second burial experiment conducted during 2019I, most seeds of TWR, TEJ, and IND had decayed (>95%). The survival rate of non-decayed seeds was extremely low after 45 days of burial at depths of 5 cm and 10 cm in the paddy field. In contrast, UWR exhibited a relatively lower decay rate, with 90.59 ± 8.03% and 60.14 ± 28.57% decay at the 5 cm and 10 cm depths, respectively. A small proportion of UWR seeds germinated during the burial process; among the non-decaying rice seeds, only a small number of seeds buried under 10 cm were still active. The proportion of seeds that entered mild and deep dormancy were 0.35 ± 0.93% and 1.74 ± 3.63%, respectively, which was presumed to be affected by the seed quality, resulting in a decline in the proportion of the rice seeds that survived in the second rice burial test in the paddy fields (Table 3). The results indicated that the seeds entered a state of deep dormancy during the burial period to avoid an unfavorable environment.
The results of the rainfed-field burial test showed that most rice seeds germinated during the burial process. Among them, TWR had the highest proportion of germinated seeds and the lowest proportion of decayed seeds at burial depths of 5 cm and 10 cm, while a very small proportion (<1%) of the undecayed seeds remained viable. In contrast, nearly all UWR seeds either germinated or decayed (>99%) during the burial period. Meanwhile, a higher percentage of TEJ seeds remained viable of seeds entering a mild dormancy stage at burial depths of 5 cm and 10 cm (Table 3).
From the results of the burial test, it was observed that TWR exhibited no significant dormancy traits, with most seeds decaying in the continuously flooded environment, and only a tiny percentage entering dormancy, whereas most of the rice seeds germinated when the environment was suitable in the rainfed field. On the other hand, UWR exhibited stronger dormancy traits, with a higher percentage of seeds entering dormancy under unfavorable environmental conditions.
The evaluation of germination at different stages of maturity could help determine the innate dormancy traits of different rice variety groups. In this study, the germination rate ranged from 0% to 89% on the 40th day after heading, with TWR, IND, and TEJ exhibiting higher rates, while UWR had the lowest rate. By the 54th day after heading, the germination rate of all groups increased, ranging from 0% to 94%. TWR and IND had the highest rates, both exceeding 60%, followed by TEJ, and, in contrast, UWR remained the lowest at 8 ± 10% (Figure 3A).
The results of preharvest sprouting showed that TWR had a weaker innate dormancy trait, with a sprouting rate exceeding 60% on day 54 after heading. As a result, TWR seeds exhibited strong germination vigor. However, earlier harvesting of TWR in the field was necessary, as self-grown seedlings germinated later than transplanted seedlings. Furthermore, UWR had stronger innate dormancy traits, which were advantageous for its adaptation to the local agricultural environment.
The harvested seeds were stored at room temperature, and their germination rate was investigated at 3, 8, and 12 months after harvest to evaluate the storability of each group. After 3 months of storage, the average germination rate for all groups exceeded 95%, with no significant differences. After 8 months, TWR seeds had the highest germination rate, followed by UWR and IND, while TEJ had the lowest rate. Among the groups, TWR showed the smallest decline in germination rate (14%) after 8 months of storage, followed by UWR (29%) and IND (30%). After 12 months of storage at room temperature, the average germination rate of weedy rice remained above 60%. TWR showed a 22% decline, while UWR and IND each decreased by only 7%. However, TEJ had the lowest germination rate at 9 ± 14% (Figure 3B).
Overall, TWR maintained a high germination rate for up to 8 months of storage, but its decline became more pronounced after 12 months of storage, though the average germination rate still remained above 60%, whereas UWR had a greater reduction in germination rate by 8 months of storage, with greater variability within the group; but the decline slowed thereafter, maintaining a germination rate above 60% after 12 months of storage.

3.4. Morphological Variation and Competitive Traits

To select representative traits as variables for principal component analysis, a correlation matrix analysis was conducted for the above 27 traits (Table S3). The results of the correlation matrix showed that traits from both cropping seasons in 2018 were significantly and moderately-to-highly correlated with each other. Therefore, the data from 2018II, which had higher data completeness, were used for subsequent principal component analysis. The tillering stage is an important period for establishing the field competitiveness of weedy rice. Therefore, correlation analysis of PH was conducted between the tillering stage and final stage. The results showed a significant and moderate-to-high positive correlation at both stages, and PH was selected as the variable for subsequent analysis.
Regarding yield-related traits, SN and GNT in 2018II showed a highly positive correlation. Variables with higher cos2 values (indicating better representation in the component space) and strong factor loadings on PC1 and PC2 were retained for the final PCA model. GNT, which had a higher cos2 value between the first two principal components, was selected as a variable (Figure S2C). Additionally, FER had a lower cos2 value; therefore, it was not included as a target trait.
In the burial test, most of the rice seeds had decayed and were therefore excluded from the subsequent principal component analysis. Ultimately, 13 target traits were selected as the original variables for the analysis, categorized into five groups based on their attributes: grain appearance (GL, GW, GLW); rice growth period (final plant height [PH77d], LAI, HD); seedbank accumulation factors (RT8M and PHS54); seedling growth (SL, RA, ML); and yield-related traits (SHA and GNT).
PCA reduced multiple variables into a few principal components. The results showed that the eigenvalues of the first two principal components were 1.89 and 1.70, with variance proportions of 27% and 22%, respectively, together explaining 50% of the total variance (Table S4). In the first principal component, RA, RT8M, SL, PHS54, and GNT had high factor loadings of 0.44, 0.42, 0.41, 0.35, and 0.33, respectively, primarily representing soil seedbank accumulation factors and seedling trait groups.
PC1 was primarily defined by seedling vigor, grain number, and seed storability, suggesting a competitive advantage in early growth and seedbank persistence. PC2 reflected traits associated with plant morphology, such as HC, ML, and PH—critical for visual mimicry or emergence from depth. In addition, the above five variables had higher cos2 values and contribution values for PC1, which respectively indicated that these variables had a better quality of representation of the variable on the component and higher contribution values for PC1 (Figure S2). In the second principal component, GW, HC, GL, ML, and PH77d had larger loadings, which were 0.46, −0.44, −0.39, −0.39, and −0.33, respectively, mainly belonging to the rice appearance trait group. The above traits also had higher cos2 values and contribution values for PC2 (Figure S2).
The loci of the first two principal component vectors were projected onto a 2-dimensional distribution map, and the degree of dispersion of each cluster was circled. On the PC1 component axis, the positively correlated variables of RA, RT8M, SL, PHS54, and GNT were classified in the right hemisphere (Figure S2B). TWR was mainly distributed in this area, indicating that TWR had a more advantageous performance in the above traits, while TEJ and UWR were farther away, performing weaker in seedling growth, seed storability, and preharvest sprouting. Among them, UWR was farther away from the variable of PHS54, indicating that UWR had a stronger dormancy trait.
On the PC2 component axis, the positively correlated variables of HC, GL, ML, and PH77d were classified in the upper hemisphere, where UWR was mainly distributed. The results indicated that UWR had the traits of darker hull color, long grains, long mesocotyls, and tall plants. The negatively correlated variable GW was classified in the lower hemisphere (Figure S2B), where TEJ was mainly distributed. It indicated that TEJ was more prominent in grain width. In addition, more individuals in TWR were close to the lower hemisphere, indicating that most TWR had the traits of short grains, short plants, and straw-colored hulls (Figure 4).
The results of PCA indicated that TWR was primarily characterized by strong seedling growth potential (SL and RA), larger LAI, a higher GNT, and greater seed storability. In contrast, UWR exhibited stronger dormancy (PHS54), prominent mesocotyl traits, and tall plant traits. Regarding plant morphology and grain appearance, TWR featured short plants, short grains, and straw-colored hulls, resembling TEJ. Under the transplantation system, a similar appearance helped reduce the likelihood of TWR being eliminated. On the other hand, the diverse HC and tall plant traits of UWR suggest that in the direct-seeding system, weedy rice morphology is less influenced by human selection, while its taller stature enhances its competitive ability in capturing sunlight.

3.5. Crop Mimicry Features of Weedy Rice Between Taiwan and U.S.

Based on 13 traits, the functions of weedy rice in cultivation areas can be categorized into three groups: competitive ability, crop mimicry, and seedbank density (Table 4). By comparing weedy rice with the major cultivated rice varieties in the region, we clarified its invasion and adaptation strategies across different agricultural systems. Rice cultivation in Taiwan is mainly based on the transplantation system, and the area of TEJ accounts for about 90% of the total rice cultivation area. Therefore, TEJ was used as the control for TWR. For competitiveness, TWR excels in SL, RA, LAI, SN, and GNT. The self-grown seedlings of weedy rice in the field germinated late, but they could develop rapidly in the seedling stage and increase the tiller number, competing with transplanted rice for resources. After completing their life cycle, they could produce a large number of seeds, which was conducive to seed amplification and propagation.
In addition, TWR exhibited crop mimicry of PH, HD, and grain appearance (GW, GL, HC). Taiwan practices intensive agriculture, where more human resources are dedicated to rice management. As a result, off-type plants in the field are easily eliminated, which accelerated the selection of traits that resemble cultivated rice. Generally, weedy rice shares a similar appearance with cultivated rice and typically exhibits early maturity. However, TWR is more late-maturing, which helped it avoid being recognized and eliminated after the harvest of cultivated rice, as the seed coat of TWR did not turn red upon full maturation.
TWR did not exhibit obvious dormancy traits but was characterized by strong seed storability (RT8M), a high SHA, and large GNT. Taiwan, located in a subtropical region, does not require overwintering conditions, making seed dormancy a less crucial survival trait for weedy rice. Instead, producing a large quantity of seeds, high shattering, and demonstrating exceptional seed longevity were more advantageous for accumulating population in the soil seedbank. Additionally, rice seeds could spread to other fields through centralized seedling nurseries and shared equipment, remaining dormant until favorable conditions allow them to germinate as self-grown seedlings. These competitive traits, combined with crop mimicry, enable TWR to successfully complete its reproductive cycle and sustain its population into the next generation.
On the other hand, U.S. rice production primarily adopts a direct-seeding system, with indica cultivars as the dominant cultivated rice. As a result, indica cultivars were used as the control. UWR showed prominent traits of ML and PH but did not show a significant advantage in seedling growth, SN, or GNT. ML is particularly beneficial for weedy rice seeds buried deep in the soil, allowing them to emerge and develop into self-grown seedlings.
Additionally, the difference in germination time between self-grown seedlings of UWR and direct-seeded rice is smaller, and the tall trait during the growing period may make UWR more competitive with cultivated rice for resources. Regarding crop simulation traits, UWR exhibited a late HD and slender grain shape (GL, GW). In the field, weedy rice may experience pollen mixing with neighboring cultivated rice, causing the appearance of grains and HD to converge with locally cultivated varieties over long periods of natural evolution. Furthermore, U.S. direct seeding is part of an extensive system involving lower human labor input per unit area, resulting in a reduced frequency of artificial selection and a more diverse range of rice HC.

4. Discussion

4.1. Competitive Advantage Traits Under the Transplant System

The competitive advantage traits of TWR, particularly seedling vigor, are superior to those of TEJ, enabling it to compete with three-leaf-old transplanted seedlings (Table 1). It was also reported that weedy rice showed a higher competitive advantage than cultivated rice [34,36,37] due to its higher environment adaptation, such as higher photosynthesis use efficiency [38,39].
Its strong competitiveness is due to its rapid seed germination, rapid growth at the seedling stage, stronger root system, efficient nutrient utilization (such as N), and higher biomass potential (by increasing PH or producing a large number of tillers) [3,40,41], such that the results of SN and GNT were significantly better than TEJ (Table 2).
Mesocotyl is an embryonic structure that plays an important role in pushing the shoot tip across the soil surface during germination, whose length is affected by light, temperature, and water, and, as a result, is responsive to sowing depth, water content, and soil salinity [42]. In a direct-seeding system, a longer ML promotes early germination [41], which aims to compete with direct-seedling cultivars. However, if the ML of TWR was too long in the transplantation system, it might be removed during seedling cultivation because the height of the planting container is approximately 3.5 cm, leading TWR to have a shorter ML. Therefore, a shorter ML could help TWR mimic TEJ and adapt to the transplantation system.
As it is known, weedy rice has a shorter growth period, matures earlier, and shatters more easily than cultivated rice [16,43,44]. The greater canopy height of mature weedy rice plants allows for higher photosynthetic energy capture, while seedlings emerging from the soil seedbank typically grow between rows, enhancing their ability to establish canopy dominance. The results indicated that TWR had the similar PH to TEJ and late-maturing characteristic (Figure 1 and Figure S1, and Table 1); we surmise that could make TWR more difficult to identify and eliminate during harvest, letting it blend easily with cultivated rice in Taiwan’s transplanting system.

4.2. Crop Mimicry and Visual Camouflage of TWR

There was research accounting that weedy rice morphology was polymorphism and phenotypic plasticity [45,46]. The data show the PH of TEJ and TWR were the semi-dwarf type (Table 1, Figure 1 and Figure 2), which indicated the indica background of TWR also carried the semi-dwarf sd1 allele [1]. Generally, weedy rice such as UWR with higher height could have a greater ability for light interception [39], whereas shorter weedy rice like TWR could tolerate shade and low light [38]. These studies showed that the deviation from the usual growth habit, such as UWR, may have a higher survival advantage in direct seeding systems in large field areas due to higher PH. The morphological diversity of TWR is likely shaped by hybridization with local cultivars, as well as standing variation within red rice gene pools, contributing to its polymorphic appearance and adaptability [38,47]. However, in Taiwan, farmers use the transplanting system for rice production, and moreover small arable area, making it easy to manually remove plants with differences in PH and HD, resulting in TWR having morphological features that are gradually similar to the cultivars. Furthermore, the color of TWR grains was straw-colored, and their appearance was similar to TEJ (Figure 2 and Figure 4), without awns or brown-colored grains, which increases the difficulty of manual inspection. However, the color of brown rice is red, and its high amylose content affected its commercial value and farmer income [48]. Knowledge of the weedy rice dispersal mechanisms that were used for contaminated rice seeds is one of the most important dispersal mechanisms for weedy rice [49], indicating TWR were used too.

4.3. Population Dispersal Mechanisms of TWR

The factors of the seed dormancy [50], seed shattering [51], and seed longevity [17] of weedy rice would help their population dispersal. From the results of the burial test and preharvest sprouting, it was observed that TWR exhibited no significant seed dormancy; especially, most seeds were decaying in the continuously flooded environment (Table 3 and Figure 3). However, there are two cropping seasons in Taiwan; we surmised that TWR does not need deep dormancy to pass the fallow season. On the other hand, UWR exhibited stronger dormancy traits, with a higher percentage of seeds entering dormancy under unfavorable environmental conditions (Table 3 and Figure 3). In fact, a large variation in the seed dormancy of weedy rice has been reported, for example, no dormancy [18] and a few days to several years after its harvest [50,52]. This variation in seed dormancy in weedy rice is attributed to genetic and environmental factors, like temperature, moisture, and storage conditions [51,53].
The SHA of TWR was determined by the seed shattering with external force, which was higher than TEJ (Table 2). Previous studies on seed shattering of weedy rice had demonstrated that it is related with population dispersal [46,51]. Shared use of combine harvesters is very common among smallholder farms in Taiwan. Moreover, the harvest period in Taiwan was delayed from the south-to-north area, allowing for long-distance spread. The mechanical harvesting of rice could increase the TWR seeds in the cultivar rice grains or in the soil, and indirectly reduce the dependence on high shattering by natural factors. In addition, the straw-colored rice panicles (Figure 2 and Figure S2) also increase the possibility of seed mixing, and the commercial practice of seedling propagation further promote the spread of weedy rice through the transfer of seedlings from southern to northern regions.

4.4. Seedbank Accumulation Dynamics in Subtropical Environments

In contrast, weedy rice entered winter immediately after harvest, and the large amount of shattering helped to increase the seed bank in the soil [16,17,43,54]. In subtropical regions, two cropping seasons could be grown in a year, and the winter period is shorter compared to temperate regions; when accumulating the soil seedbank density, TWR relied less on seed dormancy. The soil is immediately tilled after harvest, particularly, which would prompt the accumulation of weedy rice seeds in the soil seedbank due to shattering during mechanical harvesting. This leads to an increase in natural seedlings from the soil seed bank in the second cropping season.
During the winter in Taiwan, fields were left fallow with no irrigation, or in some areas, only a rainfed pattern is maintained during the second cropping season, without any weed management. This allowed weedy rice in the soil seedbank to lack sufficient moisture but maintain highly advantageous seed longevity, waiting for the return of moisture [55]. Furthermore, the winter fallow period increases the chances for maintaining the density of the soil seedbank.

5. Conclusions

This study examined the life cycle dynamics of TWR from germination through plant maturation and seed shattering. Key early-stage traits—including RA and SL, as well as SN and LAI, during the seedling and vegetative growth stages—consistently demonstrated that TWR possesses significantly greater early vigor than cultivated rice. This elevated vigor facilitates rapid population establishment and confers a competitive advantage under natural field conditions. During the vegetative and reproductive stages, the HD and PH of TWR exhibited phenotypic similarity to cultivated rice. This convergence suggests the use of crop mimicry as a survival strategy, potentially enabling TWR to escape early detection and removal. At the maturation stage, TWR demonstrated high seed shattering capacity, indicative of an evolved adaptive mechanism that enhances seed dispersal and contributes to the expansion of the weedy rice population. Furthermore, TWR seeds exhibited prolonged viability within the soil seedbank, ensuring long-term persistence in paddy ecosystems. Its relatively weak dormancy facilitates rapid post-harvest germination, allowing immediate establishment in subsequent cropping cycles and thereby complicating management practices. The combination of high seedling vigor, delayed heading, crop mimicry, and extended seed viability presents substantial challenges for visual detection and conventional control methods. The persistent replenishment of the soil seedbank, driven by high shattering rates and seed longevity, underscores the necessity for an integrated management strategy. We recommend an integrated management strategy that includes the use of certified seeds, selective herbicide application in the seedling stage, timely removal of weedy rice individuals by the heading stage, and post-harvest flooding to induce the germination of shattered seeds before the plowing approach. These practices can help reduce seedbank replenishment and off-type proliferation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15040984/s1, Table S1: List of materials used in the study; Table S2: Summary of methodological approaches evaluated in this study; Table S3: Correlation coefficient matrix of 27 target traits derived from Taiwan weedy rice and 3 rice populations; Table S4: The principal analysis of Taiwan weedy rice population and 3 check rice populations was conducted on the 13 traits; Figure S1: Morphological characteristics of Taiwan weedy rice and 3 rice populations; Figure S2: The principal components analysis among Taiwan weedy rice and 3 populations.

Author Contributions

Conceptualization, D.-H.W. and Y.-T.H.; methodology, C.-P.L., Y.-C.W., P.-R.D. and D.-H.W.; software, Y.-C.W. and P.-R.D.; formal analysis, C.-P.L. and P.-R.D.; writing—original draft preparation, D.-H.W. and Y.-T.H.; writing—review and editing, D.-H.W.; visualization, P.-R.D. and D.-H.W.; supervision, D.-H.W.; funding acquisition, D.-H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially supported by the Ministry of Science and Technology, Taiwan, R.O.C. (Grant Nos. 108-2628-B-055-001, 109-2628-B-055-001, 110-2628-B-055-001, and 111-2313-B-055-001-MY3).

Data Availability Statement

Data is contained within the article or Supplementary Material.

Acknowledgments

The authors express their sincere gratitude to Ming-Hsin Lai and Hsing-Mu Yen, as well as the technical staff of the Rice Laboratory at the Taiwan Agricultural Research Institute (TARI), for their invaluable assistance in paddy field management and phenotypic data collection. We also thank Hsin-Yu Ma (M.Sc. candidate, Department of Agronomy, National Taiwan University) for her contributions to seedling trait measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
FAOFood and Agriculture Organization
FER fertility rate
GLgrain length
GLW ratio of grain length to grain width
GNTnumber of total grains
GWgrain width
HChull color
HDheading date
INDindica cultivar
LAIleaf area index
ML mesocotyl length
PCA principal component analysis
PHplant height
PHSpreharvest sprouting rate
RA root surface area
SCpericarp color
SHA seed shattering rate
SLseedling shoot length
SNspike number
TARITaiwan Agricultural Research Institute
TEJtemperate japonica cultivar
TWRTaiwan weedy rice
UWRU.S. weedy rice

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Agronomy 15 00984 g001
Figure 1. The dual y-axis showing plant height growth curve and growth rate of Taiwan weedy rice and three rice populations during (A) first and (B) second cropping season in 2018. The graph of line and bar on the dual y-axis plot are plant growth curve and daily increased height during different periods, respectively. The left y-axis is for lines and the right y-axis is for bars. IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. The same letter(s) within the same period indicates that the means are not significantly different at the 5% level as determined by Fisher’s protected LSD test.
Figure 1. The dual y-axis showing plant height growth curve and growth rate of Taiwan weedy rice and three rice populations during (A) first and (B) second cropping season in 2018. The graph of line and bar on the dual y-axis plot are plant growth curve and daily increased height during different periods, respectively. The left y-axis is for lines and the right y-axis is for bars. IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. The same letter(s) within the same period indicates that the means are not significantly different at the 5% level as determined by Fisher’s protected LSD test.
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Figure 2. Grain appearance index of Taiwan weedy rice accessions compared with cultivated rice and other weedy-type rice populations. (A) The frequency of hull color scale and grain images for four populations. (B) The frequency of the pericarp color scale and brown rice images for four populations. (C) Three grain traits for four populations. IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. GW, grain width; GL, grain length; GLW, grain length-to-width ratio. The same letter(s) within the same trait indicates that the means are not significantly different at the 5% level as determined by Fisher’s protected LSD test.
Figure 2. Grain appearance index of Taiwan weedy rice accessions compared with cultivated rice and other weedy-type rice populations. (A) The frequency of hull color scale and grain images for four populations. (B) The frequency of the pericarp color scale and brown rice images for four populations. (C) Three grain traits for four populations. IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. GW, grain width; GL, grain length; GLW, grain length-to-width ratio. The same letter(s) within the same trait indicates that the means are not significantly different at the 5% level as determined by Fisher’s protected LSD test.
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Figure 3. The box plots showing (A) variation of seed preharvest sprouting at 40th and 54th day after rice heading and (B) variation of seed storability after 3, 8, and12 months storage among Taiwan weedy rice and three populations. IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. The same letter(s) within the same time indicates that the means are not significantly different at the 5% level as determined by Fisher’s protected LSD test.
Figure 3. The box plots showing (A) variation of seed preharvest sprouting at 40th and 54th day after rice heading and (B) variation of seed storability after 3, 8, and12 months storage among Taiwan weedy rice and three populations. IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. The same letter(s) within the same time indicates that the means are not significantly different at the 5% level as determined by Fisher’s protected LSD test.
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Figure 4. Principal components analysis biplot for the 13 variables showing the survival characteristic among Taiwan weedy rice and 3 populations. Thirteen variables were divided into 5 clusters based on the type of each trait, including “Grain appearance” (HC, GL, GW), “Growth period” (PH77d, LAI, HD), “Seedbank” (RT8M, PHS54), “Seedling” (SL, RA, ML), and “Yield” (SHA, GNT). The vectors of the variable were colored according to the clusters The first and second principal coordinates account for 29.4% and 21.4% of total variation, respectively. IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. HC, hull color; GL, grain length; GW, grain width; PH77d, plant height 77 days after transplanting; LAI, leaf area index; HD, heading date; RT8M, seed germination percentage after 8 months storage (seed storability); PHS54, germination percentage 54 days after rice heading (seed preharvest sprouting); SL, seeding shoot length; RA, root surface area; ML, Mesocotyl length; SHA, seed shattering rate; GNT, number of total grains.
Figure 4. Principal components analysis biplot for the 13 variables showing the survival characteristic among Taiwan weedy rice and 3 populations. Thirteen variables were divided into 5 clusters based on the type of each trait, including “Grain appearance” (HC, GL, GW), “Growth period” (PH77d, LAI, HD), “Seedbank” (RT8M, PHS54), “Seedling” (SL, RA, ML), and “Yield” (SHA, GNT). The vectors of the variable were colored according to the clusters The first and second principal coordinates account for 29.4% and 21.4% of total variation, respectively. IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. HC, hull color; GL, grain length; GW, grain width; PH77d, plant height 77 days after transplanting; LAI, leaf area index; HD, heading date; RT8M, seed germination percentage after 8 months storage (seed storability); PHS54, germination percentage 54 days after rice heading (seed preharvest sprouting); SL, seeding shoot length; RA, root surface area; ML, Mesocotyl length; SHA, seed shattering rate; GNT, number of total grains.
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Table 1. Five morphological traits in the seedling stage and vegetative stage of two weedy rice and two rice cultivar populations.
Table 1. Five morphological traits in the seedling stage and vegetative stage of two weedy rice and two rice cultivar populations.
Pop †Num.Seedling StageVegetative StageReproductive Stage
SL ‡ (mm)RA (mm2)ML (mm)LAI (m2m−2)HD (Days)
1st Season2nd Season1st Season2nd Season
IND2449.5 ± 14.5a §110.4 ± 20.6ab5.8 ± 5b1.8 ± 0.5b1.9 ± 0.7a74.9 ± 6.6a64.5 ± 7.2a
TEJ2427.3 ± 8.2b68.8 ± 17.6c4.4 ± 1.9b1.3 ± 0.4c1.4 ± 0.6b66.0 ± 6.5b56.7 ± 10.1b
TWR11751.1 ± 10.7a115.9 ± 21.7a2.4 ± 2.4c2.0 ± 0.5a2.1 ± 0.7a74.0 ± 3.3a64.7 ± 3.7a
UWR754.1 ± 9.9a93.9 ± 26.1b15.1 ± 4.9a0.6 ± 0.3d1.3 ± 0.7b65.3 ± 5.9b66.7 ± 7.7a
† IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. ‡ SL: seeding shoot length; RA, root surface area; ML, mesocotyl length; LAI, leaf area index; HD, heading date. § Values are expressed as the mean and standard deviation of each population, and means within a column followed by the same letter(s) are not significantly different at the 5% level by Fisher’s protected LSD test.
Table 2. Four traits related to propagation and dissemination of two weedy rice and two rice cultivar populations.
Table 2. Four traits related to propagation and dissemination of two weedy rice and two rice cultivar populations.
Pop †Num.SN ‡GNTFertility (%)Seed Shattering Rate (%)
1st Season2nd Season1st Season2nd Season1st Season2nd Season1st Season2nd Season
IND2418.8 ± 7.0a §16.1 ± 3.7c8029.9 ± 2821.8a6349.6 ± 1221.2b82 ± 15ab80 ± 11a1.3 ± 0.8b8.2 ± 6.6a
TEJ2414.1 ± 3.3b12.1 ± 1.7d6334.9 ± 2503.1b4436.2 ± 1063c85 ± 11a86 ± 9a0.5 ± 0.8b2.8 ± 4.1b
TWR11718.8 ± 4.6a19.5 ± 5.3b9013.4 ± 2394.3a7917.0 ± 2090.1a86 ± 9a69 ± 11b3.0 ± 2.4a11.9 ± 9.9a
UWR715.8 ± NA ¶ab24.3 ± 5.3a7643.0 ± NA ¶ 4893.1 ± 2211.8bc74 ± 16b79 ± 5a3.9 ± 3.1a14.4 ± 9.7a
† IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. ‡ SN, spike number; GNT, number of total grains. § Values are expressed as the mean and standard deviation of each population, and means within a column followed by the same letter(s) are not significantly different at the 5% level by Fisher’s protected LSD test. ¶ Due to severe lodging and high seed shattering, sample availability was limited.
Table 3. The proportions of five seed states among weedy rice and two cultivar populations after burial trials at different soil burial depths and field conditions.
Table 3. The proportions of five seed states among weedy rice and two cultivar populations after burial trials at different soil burial depths and field conditions.
TrialDepth
(cm)		Pop †Num.During Burial TrailIntact Seed
Decayed Seed (%)Germinated Seed (%)Germinated Seed (%)Non-Viable Seed (%)Viable Seed (%)
Paddy trial 15IND2498.99 ± 1.82a ‡NA 0.00 ± 0.00 0.98 ± 1.8b0.03 ± 0.15b
TEJ2498.71 ± 2.91aNA 0.00 ± 0.00 1.08 ± 2.17b0.22 ± 0.84ab
TWR11797.98 ± 5.06aNA 0.00 ± 0.00 1.69 ± 4.18b0.33 ± 1.6ab
UWR790.94 ± 5.80bNA 0.00 ± 0.00 7.82 ± 4.95a1.24 ± 1.77a
10IND2496.51 ± 7.13aNA 0.00 ± 0.00 1.74 ± 2.96b1.75 ± 4.48b
TEJ2496.87 ± 11.03aNA 0.00 ± 0.00 0.67 ± 1.37b2.46 ± 10.04b
TWR11795.49 ± 7.27aNA 0.00 ± 0.00 2.43 ± 4.42b2.08 ± 4.25b
UWR729.75 ± 27.46bNA 0.00 ± 0.00 21.48 ± 11.72a48.77 ± 25.38a
Paddy trial 25IND2497.22 ± 3.78a0.52 ± 2.34b0.00 ± 0.00 2.24 ± 2.98ab0.02 ± 0.1a
TEJ2496.16 ± 5.19a0.00 ± 0.00b0.00 ± 0.00 3.84 ± 5.19b0.00 ± 0.00a
TWR11797.16 ± 4.23a0.86 ± 2.06b0.00 ± 0.00 1.96 ± 3.72ab0.03 ± 0.17a
UWR790.59 ± 8.03b5.41 ± 6.9a0.00 ± 0.00 4.00 ± 6.32a0.00 ± 0.00a
10IND2498.55 ± 1.46a1.2 ± 1.28ab0.00 ± 0.00b0.23 ± 1.02b0.02 ± 0.11b
TEJ2499.13 ± 1.00a0.85 ± 1.01b0.00 ± 0.00b0.02 ± 0.1b0.00 ± 0.00b
TWR11797.79 ± 3.67a1.88 ± 3.63ab0.00 ± 0.00b0.31 ± 0.78b0.02 ± 0.12b
UWR760.14 ± 28.57b3.73 ± 4.97a0.35 ± 0.93a34.03 ± 26.87a1.74 ± 3.63a
Upland trial5IND2413.05 ± 9.78b85.87 ± 12.77a0.52 ± 1.71ab0.54 ± 1.67a0.02 ± 0.1a
TEJ2422.84 ± 14.41a75.47 ± 17.19b1.05 ± 2.36a0.56 ± 1.02a0.08 ± 0.4a
TWR1178.87 ± 8.92b90.69 ± 9.76a0.14 ± 0.88b0.27 ± 0.83a0.02 ± 0.15a
UWR710.26 ± 4.91b89.60 ± 4.89a0.00 ± 0.00b0.14 ± 0.24a0.00 ± 0.00a
10IND2424.51 ± 12.87a74.00 ± 14.87b1.06 ± 2.22b0.37 ± 1.49a0.06 ± 0.31a
TEJ2426.86 ± 17.86a69.66 ± 21.27b3.21 ± 5.53a0.21 ± 0.65ab0.06 ± 0.3a
TWR11714.87 ± 10.54b84.68 ± 10.77a0.35 ± 1.24b0.03 ± 0.16b0.07 ± 0.57a
UWR726.44 ± 12.96a73.06 ± 13.19b0.21 ± 0.39b0.29 ± 0.4ab0.00 ± 0.00a
† IND, indica cultivar; TEJ, temperate japonica cultivar; TWR, Taiwan weedy rice; UWR, U.S. weedy rice. ‡ Values are expressed as the mean and standard deviation of each population, and means within a column within the same depth followed by the same letter(s) are not significantly different at the 5% level by Fisher’s protected LSD test.
Table 4. The adaptive strategies of Taiwan weedy rice and U.S. weedy rice in agricultural environments.
Table 4. The adaptive strategies of Taiwan weedy rice and U.S. weedy rice in agricultural environments.
Trait §Taiwan Weedy Rice †U.S. Weedy Rice ‡
Competitive AbilityCrop
Mimicry		Seedbank DensityCompetitive
Ability		Crop
Mimicry		Seedbank
Density
Seedling vigor (SL)Positive Same
Seedling vigor (RA)Positive Same
Seedling vigor (ML)Negative Positive
Growth (HD) Positive
(pericarp color)		Negative PositivePositive
(early shattering)
Growth (LAI)Positive Negative
Growth (PH in 77d)SamePositive (dwarf) Positive (tall)Negative
Yield (SN)Positive Same
Yield (GNT)Positive PositiveSame
Yield (SHA) Positive Positive
Grain (GL) Positive (similar) Positive (similar)
Grain (GW) Positive (similar) Positive (similar)
Grain (HC) Positive Negative
Dormancy (PHS54) Negative Positive
Longevity (RT8M) Positive Same
† Taiwan weedy rice: positive and negative indicate comparison with temperate japonica cultivar. ‡ U.S. weedy rice: positive and negative indicate comparison with indica cultivars. § HC, hull color; GL, grain length; GW, grain width; PH77d, plant height 77 days after transplanting; LAI, leaf area index; HD, heading date; RT8M, seed germination percentage after 8 months storage (seed storability); PHS54, germination percentage 54 days after rice heading (seed preharvest sprouting); SL, seeding shoot length; RA, root surface area; ML, Mesocotyl length; SHA, seed shattering rate; GNT, number of total grains.
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Hsu, Y.-T.; Wang, Y.-C.; Du, P.-R.; Li, C.-P.; Wu, D.-H. Morphological Diversity and Crop Mimicry Strategies of Weedy Rice Under the Transplanting Cultivation System. Agronomy 2025, 15, 984. https://doi.org/10.3390/agronomy15040984

AMA Style

Hsu Y-T, Wang Y-C, Du P-R, Li C-P, Wu D-H. Morphological Diversity and Crop Mimicry Strategies of Weedy Rice Under the Transplanting Cultivation System. Agronomy. 2025; 15(4):984. https://doi.org/10.3390/agronomy15040984

Chicago/Turabian Style

Hsu, Yi-Ting, Yuan-Chun Wang, Pei-Rong Du, Charng-Pei Li, and Dong-Hong Wu. 2025. "Morphological Diversity and Crop Mimicry Strategies of Weedy Rice Under the Transplanting Cultivation System" Agronomy 15, no. 4: 984. https://doi.org/10.3390/agronomy15040984

APA Style

Hsu, Y.-T., Wang, Y.-C., Du, P.-R., Li, C.-P., & Wu, D.-H. (2025). Morphological Diversity and Crop Mimicry Strategies of Weedy Rice Under the Transplanting Cultivation System. Agronomy, 15(4), 984. https://doi.org/10.3390/agronomy15040984

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