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Article

Successional Allelopathic Interactions of Amaranthus palmeri S. Wats. and Cereals

by
Filiz Erbas
Department of Plant Protection, Faculty of Agriculture, Aydin Adnan Menderes University, 09070 Aydin, Türkiye
Sustainability 2025, 17(9), 3871; https://doi.org/10.3390/su17093871
Submission received: 24 February 2025 / Revised: 29 March 2025 / Accepted: 10 April 2025 / Published: 25 April 2025
(This article belongs to the Section Sustainable Agriculture)
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Abstract

:
Plant allelochemicals can affect the germination and growth of other plant species. Petri and pot experiments were conducted to detect the interaction of Amaranthus palmeri with cereals (barley, oat, wheat, and triticale). Aqueous extracts of different tissues of A. palmeri and cereals at several concentrations were used to measure the inhibitory effects on the germination of other plants in the Petri experiments. A. palmeri plants and cereals grown at two different densities were incorporated into a potting mix at two different growing stages to determine the inhibitory effects on the germination and growth of other plants in pot experiments. The relative germination inhibition of A. palmeri was present in the following order: barley > oat > triticale > wheat. The relative germination inhibition of cereals was present in the following order: oat > triticale > barley > wheat. The above-ground parts of the plants were more effective than the roots. The germination of A. palmeri was only affected by wheat, while barley was better at reducing the dry weight in pot experiments. Wheat was found to be the only cereal affected by A. palmeri. Despite the prevailing hypothesis that these plants do not affect each other’s germination and development in nature, it was concluded that using wheat and barley as a cover crop can support A. palmeri management, and delaying wheat planting in the presence of A. palmeri can protect cereals from allelopathic interference.

1. Introduction

According to the International Allelopathy Society, allelopathy is a phenomenon by which plants affect neighboring plants, microflora, and macrofauna by producing allelochemicals that can inhibit or stimulate plant growth [1]. Almost all plant tissues, including leaves, stems, roots, buds, flowers, and seeds, contain chemicals with allelopathic potential [2]. Allelopathy plays an important role in sustainable agriculture and plant biodiversity [3], particularly in relation to the establishment and succession of plant communities [4]. This phenomenon influences soil chemistry and nutrient dynamics, thereby regulating competitive interactions and promoting the growth of desirable plant species. Agricultural systems use many allelopathic methods, including crop rotation, intercropping, cover cropping using living or dead mulch, green manuring, and allelochemical-based bioherbicides.
Wheat, barley, triticale, and oat are cereal grasses with allelopathic effects on other crops and weeds [5,6,7]. In wheat, allelochemicals include phenolic acids, hydroxamic acids, and short-chain fatty acids [8]. Phenolic acids are the main allelochemicals in wheat; they are found in almost all tissues and cultivars [9]. In barley, allelochemicals include alkaloids like gramine and hordenine, phenolic acids, flavonoids, cyanoglucosides, polyamines, and hydroxamic acids. Hydroxamic acids can inhibit the germination, emergence, and growth of many weed species [10]. Triticale shows a high allelochemical weed suppression ability and possesses genes that originate from rye [11]. Some studies have found triticale to be more allelopathic than other cereals [5], while others have found it to be less weed suppressive [12]. However, the allelopathic effects on weeds are very much species-dependent, and the cumulative effects of the benzoxazinoids (BX) compounds and their metabolites may have considerable weed-suppression effects [13]. The amount and ratio of different BXs produced in plant tissue are dependent on the growing conditions and the plant age [14,15]. The allelopathic effects of oats have been proven by previous studies [16,17,18]. In oats, L-tryptophan has been identified as an allelochemical with a strong inhibitory effect on lettuce [19].
Amaranthus palmeri S. Watson (Palmer amaranth) is a dioecious, herbaceous, summer annual weed species native to America [20] and present in Africa, Asia, and Europe [21]. It was recently introduced to Turkiye [22], and with its capability of reducing the yields of summer crops, it has become a threat to the flora in Turkiye. All A. palmeri tissues produce allelochemicals in different concentrations, such as the alcohols 3-methyl-1-butanol and 3-hexen-1-ol; the aldehydes pentanal, 2-methylbutanal, and 3-methylbutanal; the esters ethyl propionate, ethyl butyrate, ethyl isobutyrate, and ethyl 2-methylbutyrate; and the ketones 2-pentanone, 3-pentanone, 3-methyl-2-butanone, 2-heptanone, and 2-nonanone, which can inhibit the germination of certain crops by over 90% [23]. Previous studies have shown that residual A. palmeri tissues effectively inhibit germination and growth of crops and weeds [24,25].
Certain crops have both allelopathic and physical effects on the growth and development of subsequent crops and weeds [26,27]. By cultivating autumn-sown crops, phytotoxins accumulated in the soil can suppress A. palmeri growth in the spring [28]. Furthermore, the by-products of some crops, such as mustards, have been shown to decrease A. palmeri germination [29]. This suggests that A. palmeri and cereals may have reciprocal allelopathic effects on each other’s germination and growth.
The majority of allelochemicals are water soluble [30]. Therefore, studies were conducted to determine allelopathic effects of (a) the aqueous extracts of A. palmeri or cereal plants on the germination of cereal or A. palmeri seeds, respectively, and (b) the incorporation of A. palmeri or cereal plant residues in the soil in which cereal or A. palmeri seeds, respectively, were sown.
In the context of Petri studies, the objective was to ascertain which plant organs were responsible for any allelopathic effects on seed germination. In pot studies, the objective was to determine whether A. palmeri exerted a deleterious effect on sustainable cereal production and whether cereals could be utilized to suppress A. palmeri for the sustainable production of summer crops.

2. Materials and Methods

2.1. Seed Collection and Supply

A. palmeri (AMAPA) seeds collected from the Çukurova Region of Turkiye in 2021 and 2022 were cleaned and stored in paper bags at room temperature until the beginning of the Petri and pot experiments. The Aegean Agricultural Research Institute (Izmir, Turkiye) provided seeds for wheat (cv. Kayra), barley (cv. Imbat), triticale (cv. Ege Yildizi), and oat (cv. Sari).
Plants tested for their allelopathic effects in Petri and pot experiments were grown in fields at the Aydın Adnan Menderes University Research, Application and Production Farm, Turkiye, during the 2020–2021 and 2021–2022 growing seasons for cereals and the 2022 summer for A. palmeri. They were uprooted, washed with tap water containing 1% NaOCl, and dried in a screen house in the shade. Dried plants were separated into leaves, roots, stems, and thyrses (AMAPA) or spikes (cereals) and were stored at 4 °C.

2.2. Petri Experiments

Different tissue types from all species, as well as whole plants, were ground separately to fine powders using a plant grinder at 28,000 RPM for one minute. To extract the water-soluble compounds, present in each tissue, sterile distilled water was added to 200 or 100 g of tissue powder, depending on tissue type, to achieve a final volume of 1000 mL to make 20 or 10% (w/v) stock solutions, respectively. The concentration of the stock solution was determined according to the maximum amount of water that the tissue powder could absorb. Solutions were continuously mixed in an orbital shaker at 100 RPM for 24 h. Extracts were filtered through two layers of sterile cheesecloth, then centrifuged at 3000 RPM for 10 min to precipitate any remaining powder. The purified stock solutions were diluted with sterile distilled water as needed in order to obtain desired working concentrations (Table 1).
Seed surfaces were sterilized with 5% NaOCl for 5 min, then rinsed three times with sterile distilled water for 5 min.
Due to the diminutive size of AMAPA seeds, 50 seeds per Petri dish were utilized, whereas 20 seeds per Petri dish were used for larger cereal seeds. Seeds were placed on filter paper in a 9 cm Petri dish and 5 mL of either sterile distilled water or aqueous extract (Table 1) was added. Petri dishes were sealed with parafilm and placed in an MIT-600 growth chamber set to 50% humidity at 25 °C and at 10,000 lux for 16 h daily for AMAPA germination and set to 50% humidity at 15 °C and at 10,000 lux for 12 h daily for cereal germination. Germination was checked for at 1, 3, 5, 7, 10, 14, 21, and 28 days. Seeds with radicles of ≥ 2 mm in length were counted as germinated and were removed from the Petri dish. Each seed species × aqueous extract species × aqueous extract concentration was tested twice, using four Petri dishes per test, in a completely randomized design.

2.3. Pot Experiments

2.3.1. The Effects of Cereal Residues on Germination and Development of AMAPA

The effects of wheat, barley, triticale, and oat plant residues on AMAPA germination and development were determined by growing AMAPA in pots in which a cereal species had previously been grown to an early or late growth stage at low or high densities and had then been incorporated into the soil. Firstly, ninety-six 10 L pots (28 cm diameter; 25 cm tall) were filled with a mixture of 50% soil, 25% peat, and 25% perlite and were fertilized with ~100 kg ha−1 (0.615 g) of ammonium sulfate and ~200 kg ha−1 (1.23 g) of 15-15-15 NPK. Either 18 or 35 seeds (~250 or 500 seeds m−2, respectively) were sown per pot. Pots were placed in an open area and plant growth continued until the early (tillering) or late (harvest time) growth stage was reached. At that point, the cereal plants were uprooted, cut into parts, and incorporated back into the soil of the same pots. Three weeks later, 50 AMAPA seeds were sown in each of these pots, as well as in four pots containing fertilized soil/peat/perlite without any cereal residues. After the first two AMAPA seeds germinated, any other germinating seeds were counted and removed from the pots. The two AMAPA plants in each pot were irrigated as needed until they reached the flowering stage, at which point plant height was measured and plants were harvested to obtain dry weights. This experiment was performed during the 2020–2021 and 2021–2022 growing seasons, using four replications of each cereal species × cereal seeding rate × cereal growth stage, using a completely randomized design. Monthly mean temperatures were 17.5 and 17.2 °C during the first and second experiments, respectively. Applications that were made during the pot experiments with cereal residues are listed in Table 2.

2.3.2. The Effects of AMAPA Residues on Germination and Development of Cereals

One or two AMAPA grown in pots (28 cm in diameter; 25 cm in height; 10 L volume) were uprooted, cut into pieces, and equally incorporated into the pots at the vegetative (30 and 60 g 10 L−1 pot) or generative stage (125 and 250 g 10 L−1 pot). AMAPA plants were then uprooted, cut into parts, and incorporated back into the soil of the same pots. Three weeks later, 35 cereal seeds were sown in each of these pots, as well as four pots containing soil/peat/perlite without any AMAPA residues, and 0.615 g (~100 kg ha−1) of ammonium sulfate and 1.23 g (~200 kg ha−1) of 15-15-15 NPK was applied. After the first four plants germinated, any other germinating seeds were counted and removed from the pots. Plants were irrigated as needed until harvest, when plant height, above-ground weight, ear weight, thousand-grain weight, and yield of cereals were determined. This experiment was performed during the 2021 and 2022 growing seasons, using four replications of AMAPA density × AMAPA growth stage × cereal species, implemented with a completely randomized design. Monthly mean temperatures were 16.5 and 16.6 °C during the first and second experiments, respectively. Applications that were made during the pot experiments with AMAPA residues are listed in Table 3.

2.4. Statistical Analysis

Firstly, the absolute values obtained from the experiments were converted into relative values according to their control to perform statistical analyses. The following formula was used to obtain relative values except for mean germination time values.
R   % = V a l u e   o f   t h e   v a r i a b l e V a l u e   o f   t h e   m e a n   o f   c o n t r o l × 100
Mean germination time (MGT) was calculated using the following formula [31], where ni = number of seeds germinated on i. day; di = number of i. days from the beginning of the test; and N = total number of seeds germinated at the termination of the experiment. As the Petri studies were conducted for a period of 28 days, the calculation of the mean germination time (MGT) in the Petri dishes where no germination was observed would be rendered undefined if the result were to be divided by zero. Therefore, in order to incorporate the obtained results in statistical analyses, the average germination time in these Petri dishes was accepted as 29.
M G T = ∑ ( n i × d i ) / N
The following formula was used to calculate the delay in mean germination time (MGT) compared to the control of the treatments in Petri dishes.
D e l a y   i n   M G T = M G T   o f   t h e   v a r i a b l e − M G T   o f   t h e   m e a n   o f   c o n t r o l  
To comply with the assumptions of analysis of variance, extreme values were removed, and the LN or LN (100 + x) transformation of the relative values was used as needed—for example, when relative values were negative. Note that figures show non-transformed relative values.
For Petri experiments, the main and interaction effects of replicate, cereal species and treatment were checked by treating the cereal species and treatment as fixed factors and the replicate as a random factor. For pot experiments examining AMAPA performance in the presence of cereal residues, cereal species, seeding rate, and growth stage were treated as fixed factors and the replicate as a random factor. For pot experiments examining cereal performance in the presence of AMAPA residues, cereal species and treatment were treated as fixed factors and the replicate as a random factor.
General linear model/univariate procedure was performed to determine the effects of factors and Duncan’s multiple range test at p ≤ 0.05 was used to separate means when the analysis of variance was significant. IBM SPSS Statistics 21 (IBM Corp., Armonk, NY, USA) was used for all analyses.

3. Results

3.1. Petri Experiments

3.1.1. Effects of Cereal’s Aqueous Extracts on Relative Germination and Delay in MGT of AMAPA

Since the main and interaction effects of cereals and treatments were significant for relative germination and delay in the MGT of AMAPA (Table S1), results for each cereal are given separately in Figure 1 and Figure 2.
AMAPA germination % and mean germination time were affected differently depending on cereal species, tissue type, and aqueous extract concentration. Most cereal extracts decreased AMAPA germination relative to the control. Exceptions were extracts from wheat stem at 5%, and wheat root at 5 and 10%, which increased AMAPA germination, and triticale and oat root at 5%, which had no significant effects. Extracts from all barley tissues at all concentrations decreased AMAPA germination. The most effective tissue for suppressing AMAPA germination at 10% extract concentration depended on the species: spikes and whole plant (barley); leaves (wheat and oat); and spikes (triticale). In general, extracts from above-ground tissues had greater effects than root extracts. For each tissue and extract concentration, the extracts from barley had the greatest inhibitory effects (Figure 1).
Changes in MGT of AMAPA also varied according to cereal species and tissue. The MGT of AMAPA in control dishes was determined as 1.60, 1.72, 1.53, and 1.58 days in experiments conducted using barley, wheat, triticale, and oat extracts, respectively. AMAPA germination was delayed the most by the 20% spike extract from each cereal. Similarly to germination %, the most effective tissue for delaying AMAPA germination at 10% extract concentration depended on the species: spikes and whole plant (barley); leaves (wheat and oat); and leaves and spikes (triticale). For each tissue and extract concentration, extracts from barley delayed germination the most. It was observed that the 5% root extracts of cereals other than barley accelerated germination slightly compared to the control.

3.1.2. Effects of AMAPA Aqueous Extracts on Relative Germination and Delay in MGT of Cereals

The main and interaction effects of cereals and treatments were also significant for the relative germination and delays in the MGT of cereals (Table S2). Therefore, results for each cereal were given separately in Figure 3 and Figure 4.
All AMAPA extracts at all concentrations significantly suppressed germination of each cereal species. Comparing the effects of extracts of the same concentration (10%), different AMAPA tissues had greater effects on different cereal species: leaves, thyrses, and whole plant (barley); thyrses and whole plant (wheat); leaves (triticale); and all plant organs (oats). While the relative germination of wheat, barley, and triticale decreased by an average of 48.5%, 65.6%, and 72.2%, respectively, this rate was determined to be 97.1% in oats (Figure 3).
The MGTs of cereals in control dishes were 3.69, 4.05, 6.11, and 5.60 days in barley, wheat, triticale, and oat, respectively. Barley and wheat germination was most delayed by treatment with 10% whole plant AMAPA extract. Triticale germination was most delayed by treatment with 10% leaf or thyrse extract, while germination was accelerated by treatment with 5% extracts from all tissues (except thyrse), as well as 10% extracts from roots. Oat germination was delayed by at least 10 days by treatment with all AMAPA extracts, and more concentrated extracts were associated with a longer delay in germination. Overall, AMAPA extracts had the greatest effect on the MGT of oat, followed by barley, then wheat, and then triticale (Figure 4).

3.2. Pot Experiments

3.2.1. The Effects of Cereal Residues on AMAPA Germination and Growth

Reductions in AMAPA germination in the presence of cereal residues were not significant when only the main effects of growth stage, seeding density, and cereal type were considered. However, significant interaction effects were identified (Table S3). AMAPA germination was significantly reduced when seeds were planted in soil with residues of wheat grown to an early (tillering) growth stage at low (18 plants per pot) or high (35 plants per pot) densities or to a late (harvest) growth stage at high densities (Figure 5). AMAPA germination was significantly increased when planted in soil with residues of barley grown to an early growth stage at low densities; however, it was not significantly affected by soil combined with oat or triticale residues.
AMAPA plants were significantly taller when grown in soil with late-growth-stage barley or triticale residues. AMAPA plant height was not affected by the presence of wheat or oat residues (Figure 6).
AMAPA dry weight significantly decreased when grown in soil with barley planted at low density (Figure 7). However, AMAPA dry weight significantly increased when grown in soil with late-growth-stage barley planted at high density. AMAPA dry weight was not affected by wheat, oat, or triticale residues.

3.2.2. The Effects of A. palmeri Residues on Cereal Germination and Growth

An analysis conducted to ascertain the effects of A. palmeri density (1 or 2 plant per pot) and growth period (vegetative or generative) on the relative development of cereals revealed that the main effects of the factors were statistically non-significant (Tables S4 and S5). However, given the particular importance of the interaction effects of repeat x cereals, the results are given separately for each cereal in the figures.
Neither cereal seed germination nor cereal plant height were significantly affected by the presence of AMAPA residues in the soil (Figure 8 and Figure 9).
The above-ground weight of wheat significantly decreased when soil contained residues of one AMAPA plant grown to an early (vegetative) growth stage. The above-ground weight of barley significantly increased when soil contained residues of two AMAPA plants grown to a late (generative) growth stage. The above-ground weight of oat decreased when soil contained residues of two AMAPA plants, and the above-ground weight of triticale increased with all AMAPA residue treatments. However, these differences were not statistically significant (Figure 10).
The ear weights of oat, barley, and triticale significantly increased with all AMAPA residue treatments, while the ear weights of wheat were not significantly different (Figure 11).
Yield of barley, triticale, and oat significantly increased when soil contained residues of two (for barley and triticale) or one (for oat) late-growth-stage AMAPA plants. Wheat yield significantly decreased when soil contained residues of one early-growth stage AMAPA plant. No other AMAPA residue treatments significantly affected cereal yield (Figure 12).
The thousand-grain weight of barley was significantly greater when grown in soil containing two early-growth-stage AMAPA plants. No other AMAPA residue treatment significantly affected the thousand-grain weight of cereals (Figure 13).

4. Discussion

The present study investigated the allelopathic effects of various cereals on A. palmeri, and vice versa. Effects on germination % and mean germination time (MGT) were tested by applying aqueous extracts to seeds in Petri dishes. Effects on growth parameters were tested by growing plants in soil containing residues of the potentially allelopathic species.
To this end, a series of Petri experiments were conducted which showed that aqueous extracts from different cereal tissues (leaves, roots, spikes, stems, whole plant) significantly lowered A. palmeri seed germination % and increased mean germination time (MGT). Of the four cereals tested (wheat, oat, barley, and triticale), barley extracts had the greatest effect, and extracts from above-ground cereal tissues were more effective than root extracts. Unlike in this study, Ben-Hammouda et al. [32] found barley leaf extracts to be more effective than root or stem extracts. However, Ben-Hammouda et al. did not test barley spike extracts; the present study found that the most effective part of barley after spikes was the whole plant. The results reported here are in agreement with the findings of Chon and Kim [33], who investigated the allelopathic effects of four distinct cereals on Echinochloa crus-galli and found that barley and wheat exhibited a higher degree of success in inhibiting germination. However, no study has yet been conducted to ascertain the most efficacious extracts. Previous studies used various methods to extract phenolic compounds from barley [34]. Hordenine and gramine released from barley damage white mustard radicles [35], while N-methyltyramine reduces lettuce root length [36]. Since in this study, A. palmeri germination was most inhibited by aqueous extracts from barley spikes, it can be hypothesized that chlorogenic acid, an allelochemical generally found in seeds [10], may be the phenolic compound responsible for this effect. As was evidenced by Petri studies, barley was also found to be more efficacious than other cereals in terms of delaying the MGT of A. palmeri; it was assumed that this would provide a competitive advantage to the subsequent crop competing with A. palmeri. However, this could not be tested in the pot studies, which did not involve spike tissue.
In the pot experiments, different cereals, different plant densities (18 and 35 plants per pot), and two plant growth stages (end of tillering = early and harvest time = late) were evaluated. It was observed that wheat was more effective than other cereals in decreasing A. palmeri seed germination. These results align with the results of Steinsiek et al. [37] and Alghamdi et al. [38], who studied the allelopathic effects of wheat straw and found it effective at inhibiting the seed germination of certain weed species. It has been reported in many other studies that wheat has allelopathic effects on weed species [8,39,40] and other plants [9,41,42]. Norsworthy et al. [43] showed >81% control of A. palmeri by using wheat as a cover crop in glyphosate-resistant cotton. Wheat extracts also reduce germination by 86% and biomass accumulation by 78–82% in the congener Amaranthus retroflexus [8,44]. In another study, soil-incorporated wheat and triticale residues reduced A. retroflexus root elongation [45]. Wheat straw mulch suppressed the growth of Amaranthus spp. [46]. Similarly to these studies, the effect of aqueous wheat extracts and the incorporation of wheat into the soil on the germination of A. palmeri was observed in our study.
However, triticale did not decrease the germination and growth parameters of A. palmeri. It is known that allelochemicals obtained from rye inhibit the germination and seedling growth of A. palmeri [47], and rye cultivars reduce the establishment of this weed [48]. Since triticale is a hybrid of wheat and rye and demonstrated a higher allelopathic effect than wheat in some studies [49], it was expected to have a greater effect than wheat. While 20% aqueous extracts of triticale spikes were significantly effective in reducing A. palmeri germination in Petri experiments, in the soil, the allelopathic chemicals probably degraded to non-effective metabolites and/or concentrations. The absence of benzoxazinoids [50] could be another reason for the ineffectiveness of triticale on A. palmeri. Spikes and leaves in both triticale and wheat were the most effective treatments for inhibiting A. palmeri germination in Petri experiments, although their practical use in the field is doubtful, since growers would prefer to harvest spikes rather than incorporate them in the soil. Even so, incorporating wheat residues into the soil could be recommended to decrease A. palmeri seed germination.
Oat also had a strong effect on A. palmeri germination in Petri experiments that did not translate to the pot experiments. The aqueous extracts of above-ground tissues were especially effective at inhibiting A. palmeri germination. Carraro-Lemes et al. [19] found that increasing the concentration of Avena spp. was able to inhibit lettuce germination completely. Wang et al. [51] reported that low concentrations of oat stubble have a positive effect on cucumber seed germination, while higher concentrations have an inhibitory effect. Chovancova et al. [17] determined that 10% aqueous oat extract suppresses maize germination by over 15% and delays maize mean germination time by about 2 days. Oat cultivar residues also have cover crop characteristics and are able to suppress Chenopodium album and Capsella bursa-pastoris preceding pea growth [16]. This can also be observed in a dry weight decrease in A. palmeri, especially when oats were added at the early stage and with denser usage, albeit without statistical evidence, as shown in this study.
The present study demonstrated that the incorporation of harvest-stage, densely planted oats, barley, and triticale into soil had a slight effect on A. palmeri germination, although this was not statistically significant. The incorporation of tillering-stage barley into the soil promoted A. palmeri germination. A series of pot experiments demonstrated that the application of cereal products does not reduce A. palmeri height. In fact, the incorporation of harvest-stage barley and triticale into the soil has been observed to enhance A. palmeri height through the provision of essential nutrients. Schulz and Wieland [52] demonstrated that benzoxazolinones were detoxified in numerous plants, including Amaranthus albus. This finding potentially explains the observed lack of effect of wheat extracts, which was effective on the germination but not the height, and dry weight of A. palmeri in the present study. However, the opposite was observed in the case of barley and early- and late-stage soil mixing applications. These applications proved ineffective in impeding A. palmeri germination, resulting in a decline in dry weight compared to the control group. This phenomenon is hypothesized to be attributable to barley’s incorporation into the soil during both stages, which manifested an allelopathic effect at both concentrations. However, after this initial stage, barley’s impact transitioned from an allelopathic state to an organic element effect at higher concentrations. It can also be concluded from this that, since the spikes were not incorporated into the soil in the pot studies, the different alkaloids, phenolic acids, flavonoids, cyanoglucosides, polyamines, and hydroxamic acids were effective in barley at the early stage [10].
When we look at the Petri experiments where the effect of A. palmeri on cereal germination was examined, it was found that leaf, thyrse, and whole plant parts were more effective in reducing cereal germination % and delaying mean germination time, and the most affected cereal was oat. Residues of A. palmeri were found to be effective in reducing germination and growth rate of corn, soybean, tomato, and A. palmeri, and its residues at 160,000 ppm and 80,000 ppm significantly reduced soybean leaf area by 97% and 94%, respectively [24]. Overall, an increase in A. palmeri residue in the soil reduced soybean growth and development [53]. Since we do not have any data on the allelochemicals in A. palmeri, other than the studies conducted by Connick in the 1980s, and no analysis was conducted to determine the allelochemicals in this study, we cannot discuss the effective allelochemicals here. However, we can say that aqueous extracts such as leaves, thyrses, and whole plants that were found to be very effective in Petri studies did not give the same results in terms of inhibiting germination in pot studies, and when soil microbiota is involved, germination is not prevented by allelochemicals secreted from A. palmeri. Although mixing A. palmeri into the soil at different growth stages and densities did not have any effect on the cereals in terms of germination and plant height, wheat was statistically affected in terms of above-ground weight and yield at the lower density and vegetative stage incorporation of A. palmeri. This phenomenon is believed to be attributable to the allelopathic effect of A. palmeri, which subsequently transitions into a nutrient element effect. A. palmeri was found to have a generally beneficial effect on other cereals as a nutrient. Statistical analyses revealed significant increases in various growth parameters, with the exception of germination and plant height.
Although there are studies on the allelopathic effect of A. palmeri, allelochemical metabolism in the soil remains to be determined. It is recommended that further studies be conducted to determine these metabolites in order to clarify why and how the allelopathic effect of this weed in soil decreases. One of the reasons why the allelopathic effect is detected in Petri dishes with extracts but not in pot studies may be that allelochemicals are not sufficiently released into the environment by the plants. Another reason may be that extracts were made from dried plants, but fresh residues were mixed into the pots.
Despite the fact that it does not provide a complete solution to the issue of A. palmeri, we believe that it would be beneficial to grow cereals like wheat and barley as a cover crop and mix them into the soil before planting summer crops. The accumulation of biomass by these cereals may exert a more pronounced suppressive effect on A. palmeri germination and development at the field level. As shown by Sheekofa et al. [54], summer plants such as cotton can be negatively influenced by allelochemicals excreted by cereal cover crops. Therefore, the termination time of cereals that affect A. palmeri but not summer crops should also be studied. It has been suggested [55] that large seeds and deeply planted summer crops could reduce allelopathic effects of cereals on growth of the subsequent crop.
Despite the strong interactions observed between cereals and A. palmeri in Petri studies, the most effective concentrations of aqueous extract identified in these studies are not readily available in nature. The 5% concentration, which was largely ineffective and provided a higher concentration of allelochemicals than the highest planting density used in pot studies. Consequently, as the interval between the growing periods of A. palmeri and cereals increases and environmental factors become influential, it appears improbable for them to be able to exert a deleterious effect on each other’s germination and development.
However, it is imperative to consider that allelopathic effects may vary between varieties. Bertholdsson [56] demonstrated that selection and breeding in some cereals could reduce allelopathic activity. Since allelochemicals can be degraded by physical, chemical, and microbial activities [57], it is recommended that more allelopathic cereals are cultivated and that research is conducted in fields to see whether the effects are meaningful to growers.

5. Conclusions

Despite the observation of substantial effects in Petri dish experiments and some effects in pot experiments, it is important to recognize that such observations do not necessarily reflect the complexity and variability of natural systems. As the temporal distance between the growing seasons of A. palmeri and cereals increases and environmental factors become influential, it appears improbable that they will exert deleterious effects on each other’s germination and development. However, it can be concluded that including wheat into crop rotation as a cover crop would be beneficial in suppressing A. palmeri germination, while growing barley as a cover crop would suppress its biomass. When cultivating wheat in areas where A. palmeri was previously present, it can be hypothesized that it would be advantageous to delay planting and await increased rainfall, which would in turn stimulate the microbial activity in the soil and the metabolic neutralization of allelochemicals.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17093871/s1, Table S1: Effects of cereal aqueous extracts on relative germination and delay in MGT of AMAPA in Petri experiments; Table S2: Effects of AMAPA aqueous extracts on relative germination and delay in MGT of cereals in Petri experiments; Table S3: Effects of cereal density and stage on relative germination, height, and dry weight of AMAPA in pot experiments; Table S4: Effects of AMAPA’s stage and density on the relative germination, height, and above-ground weight of cereals in pot experiments; Table S5: Effects of AMAPA’s stage and density on the relative spike weight, yield, and thousand grain weight of cereals in pot experiments.

Funding

This research was funded by the Aydin Adnan Menderes University Scientific Research Project Coordination Unit/Türkiye (No: ADU-ZRF-21005).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The author declares no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AMAPAAmaranthus palmeri
LNNatural logarithm
MGTMean germination time
NaOClSodium hypochlorite
NPKNitrogen, phosphorus, potassium
RPMRound per minute
w/vWeight/volume

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Figure 1. Effects of aqueous extracts from cereal tissues on relative germination of AMAPA in Petri experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05).
Figure 1. Effects of aqueous extracts from cereal tissues on relative germination of AMAPA in Petri experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05).
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Figure 2. Effects of aqueous extracts from cereal tissues on delay in MGT of AMAPA in Petri experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05).
Figure 2. Effects of aqueous extracts from cereal tissues on delay in MGT of AMAPA in Petri experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05).
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Figure 3. Effects of aqueous extracts from AMAPA tissues on relative germination of cereals in Petri experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05).
Figure 3. Effects of aqueous extracts from AMAPA tissues on relative germination of cereals in Petri experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05).
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Figure 4. Effects of aqueous extracts from AMAPA tissues on delay in MGT of cereals in Petri experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05).
Figure 4. Effects of aqueous extracts from AMAPA tissues on delay in MGT of cereals in Petri experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05).
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Figure 5. Effects of cereals on relative germination of AMAPA (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations: 18 = 18 plant per pot; 35 = 35 plant per pot; Early = tillering stage; Late = harvest stage.
Figure 5. Effects of cereals on relative germination of AMAPA (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations: 18 = 18 plant per pot; 35 = 35 plant per pot; Early = tillering stage; Late = harvest stage.
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Figure 6. Effects of cereals on relative height (cm) of A. palmeri (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan test (p < 0.05). Abbreviations: 18 = 18 plant per pot; 35 = 35 plant per pot; Early = tillering stage; Late = harvest stage.
Figure 6. Effects of cereals on relative height (cm) of A. palmeri (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan test (p < 0.05). Abbreviations: 18 = 18 plant per pot; 35 = 35 plant per pot; Early = tillering stage; Late = harvest stage.
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Figure 7. Effects of cereals on dry weight (g) of A. palmeri (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations: 18 = 18 plant per pot; 35 = 35 plant per pot; Early = tillering stage; Late = harvest stage.
Figure 7. Effects of cereals on dry weight (g) of A. palmeri (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations: 18 = 18 plant per pot; 35 = 35 plant per pot; Early = tillering stage; Late = harvest stage.
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Figure 8. Effects of AMAPA on relative germination of cereals (%) in pot experiments. Error bars indicate standard errors. Abbreviations: Veg = vegetative; Gen = generative; 1 = one AMAPA per pot; 2 = two AMAPA per pot.
Figure 8. Effects of AMAPA on relative germination of cereals (%) in pot experiments. Error bars indicate standard errors. Abbreviations: Veg = vegetative; Gen = generative; 1 = one AMAPA per pot; 2 = two AMAPA per pot.
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Figure 9. Effects of AMAPA on relative height (cm) of cereals (%) in pot experiments. Error bars indicate standard errors. Abbreviations: Veg = vegetative; Gen = generative; 1 = one AMAPA per pot; 2 = two AMAPA per pot.
Figure 9. Effects of AMAPA on relative height (cm) of cereals (%) in pot experiments. Error bars indicate standard errors. Abbreviations: Veg = vegetative; Gen = generative; 1 = one AMAPA per pot; 2 = two AMAPA per pot.
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Figure 10. Effects of A. palmeri on relative above-ground weight (g) of cereals (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations: Veg = vegetative; Gen = generative; 1 = one A. palmeri per pot; 2 = two A. palmeri per pot.
Figure 10. Effects of A. palmeri on relative above-ground weight (g) of cereals (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations: Veg = vegetative; Gen = generative; 1 = one A. palmeri per pot; 2 = two A. palmeri per pot.
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Figure 11. Effects of A. palmeri on relative ear weights (g) of cereals (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations.: Veg = Vegetative, Gen = Generative, 1 = One A. palmeri per pot, 2 = Two A. palmeri per pot.
Figure 11. Effects of A. palmeri on relative ear weights (g) of cereals (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations.: Veg = Vegetative, Gen = Generative, 1 = One A. palmeri per pot, 2 = Two A. palmeri per pot.
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Figure 12. Effects of A. palmeri on relative yield (g) of cereals (%) in pot experiments. Error bars indicate standard error. Small letters indicate differences between subjects according to Duncan test (p < 0.05). Abbreviations: Veg = vegetative; Gen = generative; 1 = one A. palmeri per pot; 2 = two A. palmeri per pot.
Figure 12. Effects of A. palmeri on relative yield (g) of cereals (%) in pot experiments. Error bars indicate standard error. Small letters indicate differences between subjects according to Duncan test (p < 0.05). Abbreviations: Veg = vegetative; Gen = generative; 1 = one A. palmeri per pot; 2 = two A. palmeri per pot.
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Figure 13. Effects of A. palmeri on the relative thousand-grain weight (g) of cereals (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations: Veg = vegetative; Gen = generative; 1 = one A. palmeri per pot; 2 = two A. palmeri per pot.
Figure 13. Effects of A. palmeri on the relative thousand-grain weight (g) of cereals (%) in pot experiments. Error bars indicate standard errors. Small letters indicate differences between subjects according to Duncan’s test (p < 0.05). Abbreviations: Veg = vegetative; Gen = generative; 1 = one A. palmeri per pot; 2 = two A. palmeri per pot.
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Table 1. The concentrations of aqueous extracts used in the experiments.
Table 1. The concentrations of aqueous extracts used in the experiments.
AMAPA Extracts and ConcentrationsCereal Extracts and Concentrations
Leaf/10 and 5%Leaf/10 and 5%
Root/10 and 5%Root/20, 10 and 5%
Thyrse/10 and 5%Spike/20, 10 and 5%
Stem/10 and 5%Stem/10 and 5%
Whole Plant/10 and 5%Whole Plant/20, 10 and 5%
Table 2. Applications during the experiments used to determine the effects of cereals on the germination and growth of AMAPA.
Table 2. Applications during the experiments used to determine the effects of cereals on the germination and growth of AMAPA.
ApplicationsFirst
Experiment		Second
Experiment
Cereal seeds were sown16 November 20202 November 2021
Cereals were incorporated into the potting mix at the end of the tillering stage (early stage)22 March 20217 March 2022
AMAPA seeds were sown in the early-stage pots14 April 202128 March 2022
Germinated AMAPA seeds were counted
(Early stage)		24 May 2021
7 June 2021		28 April 2022
21 June 2022
Cereals were incorporated into the potting mix at harvest time (late stage)24 May 20212 June 2022
AMAPA seeds were sown in the late-stage pots14 June 202114 June 2022
Germinated AMAPA seeds were counted
(late stage)		28 June 2021
12 July 2021		21 June 2022
5 July 2022
Height and dry weight of AMAPA were
measured (early stage)		13 July 2021
16 July 2021		27 July 2022
30 July 2022
Height and dry weights of AMAPA were
measured (late stage)		12 September 2021 15 September 202131 August 2022
3 September 2022
Table 3. Applications during the experiments used to determine the effects of AMAPA on the germination and growth of cereals.
Table 3. Applications during the experiments used to determine the effects of AMAPA on the germination and growth of cereals.
ApplicationsFirst ExperimentSecond Experiment
AMAPA seeds were sown 17 August 202110 August 2022
AMAPA were incorporated into the potting mix at the vegetative stage 22 September 202113 August 2022
AMAPA were incorporated into the potting mix at the generative stage7 October 202110 October 2022
Cereal seeds were sown 1 November 20211 November 2022
Germinated cereals were counted 8 November 2021
29 November 2021		8 November 2022
25 November 2022
Height, total spike weight, total grain weight per plant, and thousand-grain weight of cereals were measured and calculated8 June 20229 June 2023
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Erbas, F. Successional Allelopathic Interactions of Amaranthus palmeri S. Wats. and Cereals. Sustainability 2025, 17, 3871. https://doi.org/10.3390/su17093871

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Erbas F. Successional Allelopathic Interactions of Amaranthus palmeri S. Wats. and Cereals. Sustainability. 2025; 17(9):3871. https://doi.org/10.3390/su17093871

Chicago/Turabian Style

Erbas, Filiz. 2025. "Successional Allelopathic Interactions of Amaranthus palmeri S. Wats. and Cereals" Sustainability 17, no. 9: 3871. https://doi.org/10.3390/su17093871

APA Style

Erbas, F. (2025). Successional Allelopathic Interactions of Amaranthus palmeri S. Wats. and Cereals. Sustainability, 17(9), 3871. https://doi.org/10.3390/su17093871

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