Allelopathy is the phenomenon by which one plant releases biochemicals, through root exudates, decaying residues or leachates, that influence the growth and development of neighboring plants. In continuous cropping systems, where a single species is cultivated on the same land year after year, these compounds can accumulate to phytotoxic levels, a condition agronomists call soil sickness. The research team, led by Nazish Akhtar and corresponding author Mo Shadab, set out to test whether tobacco’s root chemistry could be a driver of this syndrome. They prepared three distinct treatments from tobacco plants: an aqueous extract of the roots, actual rhizosphere soil collected from around living tobacco roots, and soil amended with ground tobacco root powder or residue. Each treatment was applied to chickpea and pea seeds and seedlings under controlled bioassay conditions, allowing the researchers to separate the effects of soluble exuded chemicals from those of the intact soil environment.

The results were striking and consistent across both crops. Germination rates fell significantly as the concentration of the root aqueous extract increased, and seedlings that did emerge were stunted, with shorter shoots and reduced root elongation. Because root growth is typically the first casualty of allelochemical exposure, the team paid particular attention to the architecture of the seedlings’ underground organs, where direct contact with the toxic compounds occurs. The photosynthetic machinery also took a hit: levels of chlorophyll and carotenoid pigments declined in treated seedlings, which means the plants had less capacity to capture light energy and convert it into chemical fuel. Two enzymes that sit at the heart of plant carbon and nitrogen metabolism, carbonic anhydrase and nitrate reductase, were similarly suppressed, indicating that the allelochemicals interfered not just with structure but with core biochemical pathways.

Perhaps the most revealing data came from the stress markers. Proline, an amino acid that plants accumulate as an osmoprotectant under duress, rose sharply in the exposed seedlings, as did malondialdehyde, a breakdown product of membrane lipids that serves as a classic fingerprint of oxidative damage. At the same time, the activities of three antioxidant enzymes, superoxide dismutase, catalase and peroxidase, were significantly elevated. Together, these shifts tell a coherent mechanistic story: the tobacco-derived compounds trigger the overproduction of reactive oxygen species inside the recipient plants’ cells, and the seedlings respond by ramping up their enzymatic antioxidant defenses. The fact that the defense response was activated at all shows the plants recognized the assault, but the concurrent lipid peroxidation shows the defenses were not fully able to contain the damage.

Crucially, the phytotoxicity was not confined to laboratory extracts. When the researchers grew the legumes in genuine tobacco rhizosphere soil, growth and physiological parameters declined in much the same pattern. This is a key finding, because aqueous extracts can sometimes exaggerate effects that would not materialize in a real field. The persistence of inhibitory activity in whole soil suggests that tobacco’s allelochemicals are chemically stable enough, or are replenished steadily enough by root turnover and residue decomposition, to remain active in the growing medium. For farmers who rotate tobacco with legumes or who practice continuous monoculture, the implication is that the soil itself becomes a reservoir of plant-suppressing chemistry long after the tobacco crop has been harvested.

To probe the damage at the cellular level, the team turned to the onion (Allium cepa) root meristem, a standard and sensitive cytogenetic model whose rapidly dividing cells make chromosomal disturbances easy to detect. Exposure to the tobacco root extract and rhizosphere soil produced a spectrum of chromosomal abnormalities in the dividing cells, indicating that the allelochemical mixture is not merely growth-inhibiting but genotoxic. Chromosomal aberrations in root meristems are a well-established warning sign in environmental mutagenesis screening, and their appearance here suggests that some of the tobacco-derived compounds can penetrate meristematic tissue and disrupt the machinery of mitosis, whether by interfering with spindle formation, DNA replication or chromosome segregation.

The microscopic damage extended to the surface of the plants as well. Using scanning electron microscopy, the researchers documented alterations in stomatal morphology in the treated seedlings. Stomata are the adjustable pores through which plants exchange carbon dioxide and water vapor with the atmosphere, and changes in their structure can compromise gas exchange, transpiration control and ultimately photosynthetic efficiency. The combination of reduced pigment content, suppressed carbon-fixation enzymes and distorted stomata paints a picture of seedlings struggling on multiple fronts simultaneously, from the molecular biochemistry inside their cells to the anatomical features that regulate their interaction with the environment.

What, chemically speaking, is doing all this damage? To answer that question, the team subjected the tobacco root extract to gas chromatography–mass spectrometry profiling, a technique that separates volatile and semi-volatile compounds and identifies them by their mass fragmentation patterns. The analysis identified 25 compounds, among them nicotine-related alkaloids and phytosterols. Nicotine is, of course, the signature alkaloid of tobacco, synthesized in the roots and transported throughout the plant, and previous studies have shown that nicotine released into soils can be taken up by subsequent crops and can affect soil microorganisms. The authors are careful to note that these compounds are already known constituents of tobacco; the novelty of the study lies in connecting this chemical inventory with the full cascade of physiological and cytological responses in recipient plants, providing a mechanistic bridge between what tobacco roots release and what neighboring crops suffer.

The broader significance of the work lies in its implications for agricultural practice. Soil sickness under continuous monoculture is a costly and poorly understood problem worldwide, often attributed vaguely to nutrient depletion, pathogen buildup or autotoxicity. By demonstrating that tobacco root-derived allelochemicals persist in rhizosphere soil at levels sufficient to impair germination, growth, photosynthesis and genome integrity of following crops, the study strengthens the case that root-mediated allelopathy deserves a place among the primary suspects. The findings also feed into a growing interest in allelopathy as a source of bioherbicides and as a factor to consider in crop rotation design, since the same chemistry that suppresses weeds can suppress desirable crops as well. Understanding which compounds are responsible, and at what concentrations they accumulate, could eventually help breeders select tobacco varieties with reduced allelopathic output or help agronomists design rotations and soil amendments that break down the phytotoxic residues.

The research, conducted at the Allelopathy and Plant Taxonomy Laboratory of Aligarh Muslim University’s Department of Botany and supported by a fellowship from the Council of Scientific and Industrial Research in New Delhi, adds tobacco to a lengthening list of crop and weed species whose root chemistry reshapes the plant communities around them. For the chickpea and pea farmers who share fields with tobacco, the message is sobering: the most damaging legacy of a tobacco crop may not be visible at harvest time at all, but hidden in the soil, waiting in the rhizosphere for the next seed to be planted.

Subject of Research: Root-mediated allelopathic effects of tobacco on legume crops and onion cytogenetics

Article Title: Physiological and biochemical responses of Cicer arietinum and Pisum sativum and Cytogenetic Responses in Allium cepa exposed to root-derived extract and rhizosphere soil of Nicotiana tabacum

Article References: Physiological and biochemical responses of Cicer arietinum and Pisum sativum and Cytogenetic Responses in Allium cepa exposed to root-derived extract and rhizosphere soil of Nicotiana tabacum. (n.d.). https://doi.org/10.1007/s44473-026-00225-2

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00225-2

Keywords: allelopathy, Nicotiana tabacum, Cicer arietinum, Pisum sativum, Allium cepa, rhizosphere soil, allelochemicals, oxidative stress, chromosomal abnormalities, GC-MS analysis, soil sickness, plant physiology

Rice is a cornerstone of global food security, feeding more than half of the world’s population and serving as a staple crop across Asia and Africa. Yet the crop’s productivity is consistently undermined by weeds, which compete fiercely for light, water and nutrients during the critical early phase of establishment. The research team, led by Anthony Oluwadamilola Ogungbemi of Olusegun Agagu University of Science and Technology in Okitipupa, Ondo State, approached this problem through the lens of allelopathy, the chemical interaction between plants mediated by secondary metabolites that can either inhibit or promote the growth of neighbouring organisms. Understanding these interactions, the team argued, could point toward weed-management strategies that reduce reliance on synthetic herbicides.

Both study species are chemically rich. Vernonia amygdalina and Chromolaena odorata contain an array of bioactive compounds, including sesquiterpene lactones, flavonoids and phenolics, many of which have documented effects on cell division, hormone signalling and oxidative stress in plants. Previous studies had produced a confusingly mixed picture: C. odorata extracts have been reported to suppress seedling growth in soybean and millet yet to leave rice seedlings untouched in some trials, while V. amygdalina extracts have inhibited growth in rice, cassava and wheat but enhanced growth in maize, lettuce and cucumber. The Nigerian team hypothesised that both extracts would act in a concentration-dependent manner, with C. odorata proving the more consistently inhibitory of the two.

To test this, the researchers collected fresh leaves of both species from the wild near the university’s botanical garden, with specimens verified by the institution’s herbarium and assigned voucher numbers. Three hundred grams of each plant material were washed, sliced, ground in a ceramic mortar and filtered through cheesecloth and Whatman number one filter paper to produce what the team describes as fresh crude aqueous extracts. These full-strength preparations, designated 100 percent weight per volume, were then diluted with distilled water to generate 50 percent and 25 percent treatments, alongside water-only controls. Because allelochemicals can degrade rapidly, the filtrates were freshly prepared every three days throughout the experimental period.

The germination bioassay used Jamila rice, also known as variety FARO 52 or WITA 4, a high-yielding, iron-toxicity-tolerant cultivar obtained from farmers in Kontagora whose seed originally came from the National Cereals Research Institute in Badeggi. Seeds were surface-sterilised with a diluted sodium hypochlorite solution, rinsed thoroughly, and placed five at a time into sterilised Petri dishes lined with filter paper. The experiment followed a completely randomised design with five replicates per treatment, maintained under a 12-hour light-dark cycle at laboratory temperatures of roughly 30 degrees Celsius by day and 26 degrees at night. Radicle and plumule lengths were measured every three days for 21 days, and germination was scored whenever a radicle extended at least one millimetre beyond the seed coat.

The results revealed two strikingly different patterns. Chromolaena odorata produced a strictly linear, concentration-dependent suppression of seedling growth: the more concentrated the extract, the shorter the radicles and plumules, following a clear declining gradient from control through 25, 50 and 100 percent treatments. By 21 days after planting, the full-strength extract had reduced radicle length by 34 percent and plumule length by 12 percent relative to the control, differences that reached statistical significance. The researchers attribute this inhibition to flavonoids, phenolics and sesquiterpene lactones known to occur in the weed’s leaves, compounds capable of disrupting auxin and gibberellin signalling pathways and of inducing oxidative stress that damages cell membranes during root and shoot elongation.

Vernonia amygdalina, by contrast, displayed what ecologists call a biphasic hormetic response, a phenomenon in which a substance is stimulatory at low doses but toxic at high doses. At a dilute 25 percent concentration, the bitter leaf extract actually stimulated rice seedling growth, increasing radicle elongation by 18 percent and plumule growth by 5 percent compared with untreated controls at 21 days after planting. At full strength, however, the same extract proved phytotoxic, inhibiting radicle growth by 31 percent and plumule growth by 12 percent during the earliest days of development. In other words, the very same plant that poisons rice seedlings at one dose nourishes their growth at a gentler one, a duality that mirrors earlier reports of enhancement in maize, lettuce and cucumber at low concentrations.

Perhaps the most intriguing temporal finding is that these allelopathic effects are largely transient. Inhibitory stress from both extracts was most pronounced during early ontogeny, roughly between two and eleven days after planting, when seedlings are most vulnerable to chemical interference. By day 14, and definitively by the end of the 21-day bioassay, the statistical differences among treatments had largely dissolved, with no significant growth differences remaining. This suggests that the allelochemicals either lose potency over time through degradation, or that rice seedlings recover metabolically once past their most sensitive developmental window. For agronomists, this transient window defines precisely when a botanical herbicide would need to act, and when a bio-stimulant would need to be applied to catch the growth-promoting phase.

The study’s authors are careful to acknowledge its limitations. The bioassay was conducted in Petri dishes on filter paper, an environment that cannot reproduce the complexities of field soil, where microbial communities, adsorption to soil particles and leaching all alter the bioavailability of allelochemicals. Moreover, the team used crude aqueous extracts, so the specific compounds responsible for stimulation or inhibition were not isolated or identified. The observation window was also limited to early seedling growth, leaving effects on later vegetative development, grain yield and grain quality unknown. The researchers call for future work to characterise the active molecules chemically, to validate the effects under real field conditions, and to apply repeated-measures analyses that can tease apart treatment and time interactions.

Even with those caveats, the implications are tantalising for sustainable agriculture in rice-growing regions of West Africa and beyond. A low-dose Vernonia amygdalina preparation could plausibly be developed into a bio-stimulant that gives rice seedlings a head start, while concentrated extracts of either species, and of Chromolaena odorata in particular, could serve as natural herbicides that knock down weeds without synthetic chemistry. The critical caveat is dosage: crossing the threshold between stimulation and inhibition risks suppressing the very crop a farmer intends to protect, a phenomenon the authors describe as autotoxic or non-target crop suppression. As the global search intensifies for crop-protection tools that are effective, affordable and environmentally benign, this study suggests that two of the tropics’ most abundant and troublesome plants may be sitting, quite literally, on the fence between problem and solution.

Subject of Research: Allelopathic, concentration-dependent effects of Vernonia amygdalina and Chromolaena odorata leaf extracts on rice germination and early seedling growth

Article Title: Allelochemics impacts of leaf extracts from Vernonia amygdalina and Chromolaena odorata on germination of rice

Article References: Ogungbemi, A. O., Oyebanji, R. O., & Osewole, A. (2026). Allelochemics impacts of leaf extracts from Vernonia amygdalina and Chromolaena odorata on germination of rice. Discover Plants, 3(1), Article 411. https://doi.org/10.1007/s44372-026-00872-x

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00872-x

Keywords: allelopathy, allelochemicals, Vernonia amygdalina, Chromolaena odorata, Oryza sativa, rice germination, hormesis, seedling growth, bioherbicide, bio-stimulant, plant biochemistry, sustainable agriculture