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

The study, published in Plant Cell Reports, focused on Nicotiana tabacum, the common tobacco plant long used as a laboratory model. Researchers led by Zoltán Katona and Éva Hideg exposed only the fourth true leaf of each plant to a broadband UV source filtered to remove wavelengths below 280 nanometers, delivering a biologically effective UV-B dose of 6.8 kilojoules per square meter over two days. The leaf directly above it, the fifth, remained completely shaded from UV. When the team later analyzed both leaves, they found that the unexposed systemic leaf had undergone biochemical changes that closely mirrored those in the treated leaf.

Two classes of molecules took center stage. The first were class III peroxidases, a large family of enzymes that plants deploy to manage reactive oxygen species and to reinforce cell walls. Using native polyacrylamide gel electrophoresis, the researchers separated seven distinct peroxidase isoforms from leaf extracts, labeled A through G according to their apparent molecular weights. In unexposed control plants, the dominant activities sat in the 40 to 75 kilodalton range, particularly isoforms C and D. After UV treatment, the pattern shifted: activities of isoforms D, E, F and G rose, while band C diminished, a reorganization that likely reflects either the selective activation of different peroxidase genes or altered post-translational glycosylation of the same gene products.

The remarkable finding was that the systemic fifth leaf, which had never been irradiated, displayed essentially the same peroxidase rearrangement as the directly exposed fourth leaf. This is the first demonstration that UV radiation can systemically reprogram the isoperoxidase profile of a plant, extending earlier work by the same group showing that low-dose UV raises antioxidant capacity and photosynthetic performance in leaves above the treatment zone. The team had previously implicated hydrogen peroxide as a mobile mediator of that systemic antioxidant effect, and the new results suggest the same signal cascade reaches deep into the plant’s enzymatic defense machinery.

Enzymes, however, are only half the story. Peroxidases need substrates to work on, and the second arm of the study examined the phenolic compounds that serve as both peroxidase substrates and direct antioxidants. Using high-performance liquid chromatography with diode array detection, the researchers profiled leaf extracts from four groups: directly UV-exposed leaves, systemic leaves, and the corresponding leaves of negative controls that received no UV at all and positive controls in which whole plants were irradiated at a fourfold higher dose for four days.

The chromatographic analysis revealed that chlorogenic acids dominated the phenolic pool, accounting for 80 to 97 percent of total extractable phenolics. These included 5-O-caffeoylquinic acid, the classic chlorogenic acid, along with its crypto- and neo-chlorogenic acid isomers. Even the modest, single-leaf UV dose increased total phenolic content by roughly 35 to 50 percent, and critically, the systemic leaf showed an increase of the same magnitude. Under the higher whole-plant dose, the effect was larger still, with chlorogenic acid itself showing the most pronounced rise.

Flavonoids told an even more dramatic story. Although they made up only 3 to 5 percent of phenolics in control leaves, they proved far more responsive to UV. Total flavonoid content tripled in the directly exposed leaves under the low-dose treatment and rose similarly in the systemic leaves. Under the four-times-higher whole-plant regimen, flavonoids surged approximately thirtyfold. The dominant flavonol was quercetin-3-O-rutinoside, accompanied by smaller amounts of quercetin-3-O-glucoside and kaempferol-3-O-rutinoside. The shift toward quercetin derivatives is biochemically meaningful: quercetins carry two hydroxyl groups on their B ring, making them substantially better antioxidants than the monohydroxylated kaempferols, and they absorb UV radiation effectively, shielding the leaf’s photosynthetic apparatus from below.

This quercetin bias is consistent with a well-established mechanism. UV exposure selectively activates the enzyme flavonoid 3′-hydroxylase, which redirects flux within the flavonoid pathway toward dihydroxylated compounds. Similar shifts have been documented in petunia, Arabidopsis and other species, and the same enzyme is known to respond to other stresses, including salinity, nutrient depletion and temperature extremes. That raises an important interpretive point for the Hungarian team: the systemic response may not be a UV-specific preparation but rather a general preemptive stress response, priming the whole plant against a broad range of challenges rather than narrowly fortifying against future ultraviolet exposure.

How the signal travels from the irradiated leaf to its unexposed neighbor remains an open question. The researchers consider local upregulation of phenolic biosynthesis in the systemic leaf, triggered by a mobile signal, more likely than physical transport of the phenolics themselves, since there is little evidence that colorless flavonols or phenolic acids are moved between tissues the way anthocyanins are shuttled into vacuoles. Hydrogen peroxide is the leading candidate messenger, supported by the team’s earlier work and by independent studies showing that hydrogen peroxide treatment stimulates phenylpropanoid biosynthesis genes in lettuce and differentially regulates peroxidase proteins in rice roots. But hormones, nitric oxide and calcium waves may also participate, and pinpointing the source of the systemic hydrogen peroxide, whether chloroplasts, peroxisomes or the apoplast, is a priority for future work.

The practical implications could be significant, particularly for controlled-environment agriculture. Because even a quarter of the acclimation dose produced measurable systemic benefits without harming photosynthesis, targeted low-dose UV treatment of a fraction of the plant canopy might be enough to elevate antioxidant and nutritionally valuable secondary metabolites across an entire crop. That would reduce energy costs and treatment time in vertical farms and greenhouses while boosting the resilience and quality of produce. Beyond agriculture, the study reinforces a broader biological message: plants do not operate as collections of autonomous leaves but as integrated networks, capable of coordinated, whole-organism responses that prepare tissues never touched by a stressor to withstand it. In the case of ultraviolet light, what happens to one leaf clearly does not stay on one leaf.

Subject of Research: Systemic UV-induced peroxidase and phenolic antioxidant responses in Nicotiana tabacum leaves

Article Title: Systemic induction of peroxidase and phenolic responses to UV radiation in Nicotiana tabacum

Article References: Katona, Z., Czégény, G., Csepregi, K., & Hideg, É. (2026). Systemic induction of peroxidase and phenolic responses to UV radiation in Nicotiana tabacum. Plant Cell Reports, 45(10), Article 292. https://doi.org/10.1007/s00299-026-03985-5

Image Credits: AI Generated

DOI: 10.1007/s00299-026-03985-5

Keywords: ultraviolet radiation, systemic response, Nicotiana tabacum, peroxidase isozymes, phenolic profiles, chlorogenic acid, flavonoids, quercetin, antioxidant defense, hydrogen peroxide signaling, plant stress, controlled-environment agriculture