1. Introduction
Rice is one of the most significant irreplaceable staples foods in many countries of the world and its availability is connected to food security [
1]. Rice is the seed of the monocot plant
Oryza sativa (Asian rice) and
Oryza glaberrima, which belong to the grass family, Poaceae, and has twenty wild species and two cultivated
Oryza sativa (Asian rice) and
Oryza glaberrima (African rice) [
2]. The growing global population has emphasized increasing food production, particularly rice, which is a staple diet for the majority of the world’s population [
3]. Weeds, however, have a tremendous impact on agricultural production and weed losses have exceeded all other agricultural damage instigated by insects, nematodes, disease, rodents, etc [
4]. Bangladesh, as well as Malaysia’s self-sufficiency policy, has centered on rice, the country’s primary staple food [
5]. Therefore, improving the rice yield is always the priority for rice breeding [
6].
Weeds are unwanted plants that do not give farmers economic output that is otherwise difficult for farmers to control [
7]. It also prerequisites to be prudently managed to limit the effect of weeds on crop yields [
8]. This is particularly concerning because barnyard grass is one of the top 15 weed species capable of herbicide resistance [
9], with cases reported in 23 countries, primarily in rice but also grown extensively in agricultural fields [
10]. It is a prolific species with high tillering potential [
11], apart from that, their biological and ecological similarities to rice make this species one of the world’s most common concerns [
12].
Allelopathy is a chemical technique that enables a plant to compete for a limited number of resources [
13]. According to Zhijie Zhag [
14], allelopathy (chemical interactions between plants) has been shown to affect individual performance, community organisation, and plant invasions. Several compounds have been identified as potential rice allelochemicals, including phenolic acids, fatty acids, phenylalkanoic acids, hydroxaicacids, terpenes, indoles, the diterpenoid momilactones, etc [
15]. Cell division and proliferation, cellular morphology, cell surface permeability, antioxidant activity, plant growth regulatory system, photosynthesis, water and nutrient uptake, and other physiological and biochemical processes in plants have shown to be affected by allelochemicals [
16]. Allelopathic substances impede plant development at all phases, from seed germination to maturity, including seed sprouting, seedling development, dry matter, and biochemical components, according to several studies [
17]. Allelopathy is one of the numerous plant interactions regulated by a variety of elements including nutrition, light, temperature, humidity, and others [
18]. Rice’s allelopathy has been thoroughly examined, and a wide variety of rice types have been proven to hamper the development of various plant species during co-cultivation [
19]. When looking at the behaviour of target crops, it has been observed that rice root and shoot growth are the most sensitive to the stress imposed by allelochemicals [
20]. Extracts from the jungle rice shoot have shown a greater inhibitory effect on root length and seedling dry weight, according to Sitthinoi [
21]. Soleymani and shahrajabian [
22] found that sesame extract density and plant organs had a significant impact on percent germination, coleoptile weight, radical, and coleoptile length. Kadioglu et al. [
23] found that aqueous extracts of different weeds can inhibit the germination process, length of root and shoot, and crops’ dry weight, and Chung et al. [
24] stated that water-soluble phytochemicals in rice can prevent the germination of
E. cruss-galli.
Allelochemicals produced by rice cultivars likely impacted the nitrogen intake of adjacent plants, affecting the growth of shoot length of the tested plants [
25]. Allelochemicals generated by one crop species can affect the growth, productivity, and quality of crops belonging to the same crop family [
21]. Allelopathy is a trait of herbs and grasses, as well as the main crops, which are normally part of the Poaceae family (Grass family). This promises to be a more effective means of protecting the area in the future [
26].
Research on the allelopathic capacity of several rice cultivars against spinach (
Spinacia Oleracea) was published by [
27]. The allelopathic efficacy of some rice accessions versus barnyard grass was also examined by [
28,
29]. Jafari et al. [
30] investigated the allelopathic potential of rice (
Oryza sativa L.) varieties (
Echinochloa crus-galli) on barnyard grass. Although allelopathy was found in some rice accessions from Bangladesh and Malaysia against various test plants, still, it is unknown which accessions are the best against barnyard grass. There are no published articles that identify the best allelopathic accession between Bangladesh and Malaysia at this time. Resistant weeds are becoming more common, so biological control with allelopathic rice is important. Herbicide varieties are a useful option for weed control in rice fields. This demands a bigger population for germplasm screening, resulting in very confusing research. If chemical weed control is truly required, allelopathic rice may minimise the herbicidal dosage. There is a surge of interest in exploiting plant allelopathy to improve weed management approaches in agriculture over and above to develop new weed control strategies to overcome the limitations of synthetic herbicides. All of these constraints have been alleviated to a considerable extent by using plant breeding. Increasing rice yield, avoiding the use of chemical pesticides, is alternatively a critical challenge in agricultural production. Therefore, precise weed management is required to ensure global food security. This research would support in lowering dependency on chemical herbicides, ecologically sound as well as resolving food security together with boosting the national economy. Consequently, the goal of this study was to screen and identify the most potentially allelopathic rice varieties along with quantifying the percentage of barnyard grass growth inhibition caused by allelopathic effects of rice accessions.
8. Discussion
Germination is a consequent enhancement of plant metabolic activity [
39]. The yield and quality of any crop plant depend on germination. In the sandwich method, the germination of barnyard grass is low when treated with allelopathic rice accessions, and these actions might have been due to the effect of the allelochemicals released from the rice variety which affected the germination as well as the seedling growth of barnyard grass. Significant allelopathic effects of rice accessions were also observed in the sandwich method on the percentage of germination [
28]. Kumbhar and Patel [
40] unveiled that the phenolic compounds commonly released from weeds, such as alkaloid, terpenoid, flavonoid, tannin, saponin, influence the germination and establishment of crop stands by disrupting various essential processes such as respiration and enzyme activity. In addition, Kabir et al. [
27] showed that due to high phytotoxic effects, the spinach germination percentage decreased in consequence of the rice variety WITA12 and BRRIdhan 44. Ma et al. [
41], and other researchers, reported the inhibition of target test seed germination by allelopathic effects of rice accessions. A delay in seed germination was reported by Kruse et al. [
42]. Allelochemicals (released from allelopathic rice species) are comparable to plant hormones as it causes delayed germination [
43].
The outcome of the relay seeding showed that the allelochemicals released from different rice root exudates impede the seed germination of the tested plant. Different sorts of responses were seen during the relay seeding process when several allelochemicals were produced from the rice seeds spouting, influencing the germination of spinach seeds [
27]. All weed species had the greatest germination rates in control treatments compared to all other treatments, suggesting rice accessions had a suppressive impact or allelopathic capacity on weed species germination [
44].
For the mean germination time (sandwich), it was observed that some varieties offered low values. The low mean germination time may be due to the effect of p-coumaric acid present in the allelopathic accessions. The rice plants released different secondary metabolites which subsequently inhibited or stimulated the growth and development of the test plant. The total germination of all weed species was likewise pointedly (
p ≤ 0.01) affected by the ordered aqueous extracts of different rice cultivars [
44]. Rice released some allelochemicals, among them, p-coumaric acid has reported being active against barnyard grass at high concentrations [
45]. The mean germination time (relay seeding method) findings of this study matched those of Jabeen and Ahmed [
46], who studied the effects of
Asphodelus tenuifolius and
Fumaria indica on maize seeds and reported that the species were responsible for inhibiting germination. Rice has a comparable inhibitory effect on weed seed germination [
47,
48].
In the sandwich method, the negative effect of rice exudates caused stunted roots with pruned root tips. Similar findings for
Ipomoea, Cymbopogon, Hyptis, and Alternanthera were also reported in wheat [
49]. Ismail et al. [
32] reported that several rice species, particularly Manik and Makmur, reduced
E. crusgalli’s root length by more than 75% because of the allelopathic impact of rice species. This was consistent with the observations reported by Navarez and Olofsdotter [
34]. Momilactones and phenolic compounds are the most common allelochemicals found in rice, according to Jabran [
50]. Momilactone A and B, diterpenes originally isolated from rice husks, have been identified as major allelochemicals in rice [
51]. Smashed seeds (used this method) contain rice husk that may have played a role in the reduction in germination and growth of roots and shoots.
On the other hand, in relay seeding, it might be due to the phytotoxic effect of the plant. Phenolics present in rice plants play a role in rice allelopathy against weeds [
52]. Berendji et al. [
53] found that root exudates of allelopathic rice accessions showed the highest inhibitory activity in some traits of barnyard grass seedlings. Furthermore, Kabir et al. [
27] discovered that the rice cultivar WITA-12’s allelopathic effect was reduced due to the root length of spinach, i.e., about 60%. In another study, Ismail et al. [
32] found that the rice variety Manik and Makmuer could reduce the root length of
E. crus-galli by more than 80% and 75%, respectively. Khang et al. [
54] reported that most of the vanillin and vanillic acid treatments had strong inhibitory effects on lettuce and radish seedling development, as well as high stimulatory activity on rice root elongation.
According to Karim et al. [
25], the allelopathic effect (sandwich method) of several rice cultivars developed by BRRI, Bangladesh, caused the greatest shoot length drop (37.18%) in lettuce. Kabir et al. [
27] and Karim et al. [
25] also found a similar response of rice varieties on the shoot growth of weeds. All of the effects of rice varieties on the shoot length of the test plant give the idea that rice accessions had distinguishing genotypic traits that are linked to allelopathy. The rice cultivars had no stimulating impact on this investigation.
The allelopathic effect (relay method) of diverse rice cultivars developed by BRRI, Bangladesh, resulted in the largest loss of shoot length in lettuce (37.18%) [
25]. Allelochemicals can alter the concentrations of plant growth regulators or promote phytohormone imbalances, limiting the growth and development of target plants [
28]. Additionally, the concentration of momilactone B in plants and root exudates is greatly raised when rice plants are cultivated near barnyard grasses [
55].
For dry matter (sandwich), according to Karim et al. [
29], allelopathic rice accession resulted in poor root growth of the test plant due to the allelochemicals produced by the rice plant. In other findings, the allelopathic variety, WITA12, produced the highest drop of dry matter and that was in Kataribhog (79%) [
27]. The inhibition rate based on the dry weight of the seedling was higher than that of the emergence and seedling length of barnyard grass [
56]. The allelopathic effects of root exudates significantly inhibited the height, number of tillers, and total dry weight of barnyard grass plants [
57].
In relay, it might be due to the effect of stunted root length and shoot length. Poor seed germination, effects on the root, shoot development, reduction in dry weight, and the impediment on coleoptiles elongation are the most widely reported morphological effects [
29]. In several studies, allelopathic substances have been proven to delay plant development at all stages viz. from seed to maturity, including seed germination, seedling development, leaf area, dry matter, and biochemical components [
58]. The greatest inhibitory impact on barnyard grass seed germination, seedling length, and dry weight was found in a bioassay in view of nine recognized allelochemicals and their combinations [
59]. Another research study disclosed that allelochemicals resulted in a considerable reduction in root and shoot length, fresh and dry weights, and total chlorophyll and protein content [
60].
In sandwich, most of the allelopathic rice accessions displayed outstanding germination inhibition in barnyard grass seed in the sandwich method. The accessions were sorted according to API i.e., BR17 > BR23 > BR26 > Line (16-3-38-9) > Putra1 > BRRIdhan39 > BRRIdhan75 > BRRIdhan46 > Line (L50-38-8)> MRQ74 > Putra2 > Saitta > BRRIdhan69 > BRRIdhan82 > BRRIdhan45 > MRIA1 > BRRIdhan48 (
Table 3). Again, the percent reduction of API was highest in BR17 (40.62%), next to BR23 (34.14%) and BR26 (31.07%), and lowest in BRRIdhan 48 (5.81%) (
Table 3). It might be due to the phytotoxic effect of all traits. A good number of researchers used this parameter in the detection of allelopathy [
29]. According to API, BR17 has placed at the top, followed by BR 23, BR26, and Line (16-3-38-9). Salam et al. [
61] suggested that BR17 is the most allelopathic among 102 Bangladeshi rice cultivars. S M R Karim et al. [
25] stated that the allelopathic potential of most of the selected rice varieties is good, which resulted in 30 to 35% API, which was supported by Ismail et al. [
32]. Mazid et al. [
28] observed that rice accession MR73 caused more than 40% API other than some traditional and modern rice accessions also presented > 30% API. Siyar et al. [
62] reported that allelochemicals are often associated with cellular damage and abnormalities in the photosystem. So, this is one of the reasons for inhibition.
In the case of the relay seeding method, when many plant species were grown along with rice cultivars in the field and/or laboratory, it was illustrated that the rice cultivars inhibited the growth of the plants [
63]. This approach exposed a significant barnyard grass inhibition in all the chosen rice accessions. The changes in germination rate, mean germination time, root length, shoot length, and dry matter amount are all substantially linked with the API.
Weeds raise rice production costs by prompting the use of pesticides to manage them, as well as reducing rice grain quality, lowering net revenue [
64]. Molecular genetics (QTL) and biotechnology, as well as traditional breeding procedures, might be employed to generate the selected elite cultivars. Breeding allelopathic rice types would make a significant contribution to long-term rice production [
4]. Chemical fertilizers are highly used in modern agriculture, particularly in wheat production [
65]. It is critical to address both the economic and ecological benefits of rice field weed management for agricultural development [
12]. Due to the deleterious effects of commercial herbicides, it is essential to investigate many alternative weed management strategies [
21] and allelopathy promises to be one of them [
66]. Allelopathy is an eco-friendly weed management tool, which is practiced to combat the impacts of environmental pollution [
67]. Kato-Noguchi and Ino evaluated allelopathic potentiality of 60 conventional and 42 high yielding cultivars, where they assessed against common weeds such as cress, lettuce,
Echinochloa cruss-gralli, and
Echinochloa colonum, with Bangladeshi modern rice variety BR17, and displayed the highest allelopathic potentiality across all bioassays [
68]. In another research, Kato-Noguchi et al. [
69] found a similar finding when they analyzed the allelopathic activity of 102 contemporary and traditional varieties and disclosed that BR17 (modern variety) is the most allelopathic among all. However, donor plants produce allelochemicals that impact receiver plants, and these plants respond to the donor plants by modifying gene expression [
67]. In recent years, weed management programs have tended to emphasize nonchemical weed management, i.e., safety measures, or are “environmental or eco-friendly” in general [
4]. Rice that possesses allelopathic potential has been reported from all around the world [
50].