Azelaic acid can efficiently compete for the auxin binding site TIR1, altering auxin polar transport, gravitropic response, and root growth and architecture in Arabidopsisthaliana roots

The present study investigates the phytotoxic potential of azelaic acid (AZA) on Arabidopsis thaliana roots. Effects on root morphology, anatomy, auxin content and transport, gravitropic response and molecular docking were analysed. AZA inhibited root growth, stimulated lateral and adventitious roots, and altered the root apical meristem by reducing meristem cell number, length and width. The treatment also slowed down the roots' gravitropic response, likely due to a reduction in statoliths, starch-rich organelles involved in gravity perception. In addition, auxin content, transport and distribution, together with PIN proteins' expression and localisation were altered after AZA treatment, inducing a reduction in auxin transport and its distribution into the meristematic zone. Computational simulations showed that AZA has a high affinity for the auxin receptor TIR1, competing with auxin for the binding site. The AZA binding with TIR1 could interfere with the normal functioning of the TIR1/AFB complex, disrupting the ubiquitin E3 ligase complex and leading to alterations in the response of the plant, which could perceive AZA as an exogenous auxin. Our results suggest that AZA mode of action could involve the modulation of auxin-related processes in Arabidopsis roots. Understanding such mechanisms could lead to find environmentally friendly alternatives to synthetic herbicides.

A decade of advances in the study of buckwheat for organic farming and agroecology (2013-2023)

During the last decade, research has shown the environment and human health benefits of growing buckwheat (Fagopyrum spp.). This comprehensive review aims to summarize the major advancements made in the study of buckwheat from 2013 to 2023, focusing on its agronomic characteristics, nutritional value, and potential applications in sustainable agriculture. The review examines the diverse applications of buckwheat in organic and agroecological farming systems, and discusses the ability of buckwheat to control weeds through allelopathy, competition, and other sustainable farming methods, such as crop rotation, intercropping and green manure, while improving soil health and biodiversity. The review also explores the nutritional value of buckwheat. It delves into the composition of buckwheat grains, emphasizing their high protein content, and the presence of essential amino acids and valuable micronutrients, which is linked to health benefits such as lowering cholesterol levels, controlling diabetes and acting against different types of cancer, among others. Finally, the review concludes by highlighting the gaps in current knowledge, and proposing future research directions to further optimize buckwheat production in organic or agroecological farming systems. It emphasizes the need for interdisciplinary collaboration, and the integration of traditional knowledge with modern scientific approaches to unlock the full potential of buckwheat as a sustainable crop.

Testing the role of allelochemicals in different wheat cultivars to sustainably manage weeds

BACKGROUND

Selecting wheat varieties with allelopathic potential or high competitiveness against weeds is a sustainable solution for organic farming to eliminate the use of synthetic herbicides. Wheat is one of the most economically important crops. This study focuses on screening the allelopathic or competitive potential of four wheat cultivars, Maurizio, NS 40S, Adesso and Element, on two weeds of interest due to acquired herbicide resistance, Portulaca oleracea and Lolium rigidum, through germination and growth bioassays and the identification and quantification of benzoxazinoids (BXZs) and polyphenols (phenolic acids and flavonoids).

RESULTS

The different cultivars showed different abilities to manage surrounding weeds and different capacity to exude or accumulate specialized metabolites in the presence of those weeds. Furthermore, each cultivar behaved differently depending on the weed present in the medium. The most efficient cultivar to control the tested monocot and dicot weeds was Maurizio, as it effectively controlled germination and growth of L. rigidum and P. oleracea while exuding large amounts of benzoxazinones through the roots, especially the hydroxamic acids 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one and dihydroxy-2H-1,4-benzoxaxin-3(4H)-one. By contrast, NS 40S, Adesso and Element showed the potential to control the growth of just one of the two weeds through allelopathy or competition.

CONCLUSION

This study reveals that Maurizio is the most promising wheat cultivar for sustainable weed control, and that the screening of crop varieties with allelopathic potential, which results in the displacement of synthetic herbicides, is an immediate solution in ecological and sustainable agriculture. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Specialized Metabolites Accumulation Pattern in Buckwheat Is Strongly Influenced by Accession Choice and Co-Existing Weeds

Screening suitable allelopathic crops and crop genotypes that are competitive with weeds can be a sustainable weed control strategy to reduce the massive use of herbicides. In this study, three accessions of common buckwheat Fagopyrum esculentum Moench. (Gema, Kora, and Eva) and one of Tartary buckwheat Fagopyrum tataricum Gaertn. (PI481671) were screened against the germination and growth of the herbicide-resistant weeds Lolium rigidum Gaud. and Portulaca oleracea L. The chemical profile of the four buckwheat accessions was characterised in their shoots, roots, and root exudates in order to know more about their ability to sustainably manage weeds and the relation of this ability with the polyphenol accumulation and exudation from buckwheat plants. Our results show that different buckwheat genotypes may have different capacities to produce and exude several types of specialized metabolites, which lead to a wide range of allelopathic and defence functions in the agroecosystem to sustainably manage the growing weeds in their vicinity. The ability of the different buckwheat accessions to suppress weeds was accession-dependent without differences between species, as the common (Eva, Gema, and Kora) and Tartary (PI481671) accessions did not show any species-dependent pattern in their ability to control the germination and growth of the target weeds. Finally, Gema appeared to be the most promising accession to be evaluated in organic farming due to its capacity to sustainably control target weeds while stimulating the root growth of buckwheat plants.

Application of Indole-Alkaloid Harmaline Induces Physical Damage to Photosystem II Antenna Complexes in Adult Plants of Arabidopsis thaliana (L.) Heynh

Finding herbicides with new and multiple modes of action is a solution to stop the increase in resistant weed species. Harmaline, a natural alkaloid with proven phytotoxic potential, was tested on Arabidopsis adult plants by watering and spraying; watering resulted as the more effective treatment. Harmaline altered several photosynthetic parameters, reducing the efficiency of the light- (Φ_II) and dark-adapted (F_v/F_m) PSII, suggesting physical damages in photosystem II, although dissipation of the energy in excess under the form of heat was not compromised as demonstrated by the significant increase in Φ_NPQ. Metabolomic alterations, such as osmoprotectant accumulation and reduction in sugars' content, also indicate a reduction of photosynthetic efficiency and suggest early senescence and water status alteration induced by harmaline. Data suggest that harmaline might be considered a new phytotoxic molecule interesting for further studies.

Similarities on the mode of action of the terpenoids citral and farnesene in Arabidopsis seedlings involve interactions with DNA binding proteins

The sesquiterpene farnesene and the monoterpene citral are phytotoxic natural compounds characterized by a high similarity in macroscopic effects, suggesting an equal or similar mechanism of action when assayed at IC₅₀ concentration. In the present study, a short-time experiment (24 and 48 h) using an imaging spectrofluorometer allowed us to monitor the in-vivo effects of the two molecules, highlighting that both terpenoids were similarly affecting all PSII parameters, even when the effects of citral were quicker in appearing than those of farnesene. The multivariate, univariate, and pathway analyses, carried out on untargeted-metabolomic data, confirmed a clear separation of the plant metabolome in response to the two treatments, whereas similarity in the affected pathways was observed. The main metabolites affected were amino acids and polyamine, which significantly accumulated in response to both treatments. On the contrary, a reduction in sugar content (i.e. glucose and sucrose) was observed. Finally, the in-silico studies demonstrated a similar mechanism of action for both molecules by interacting with DNA binding proteins, although differences concerning the affinity with the proteins with which they could potentially interact were also highlighted. Despite the similarities in macroscopic effects of these two molecules, the metabolomic and in-silico data suggest that both terpenoids share a similar but not equal mechanism of action and that the similar effects observed on the photosynthetic machinery are more imputable to a side effect of molecules-induced oxidative stress.

Trans-cinnamaldehyde-related overproduction of benzoic acid and oxidative stress on Arabidopsis thaliana

Introduction

Trans-cinnamaldehyde is a specialised metabolite that naturally occurs in plants of the Lauraceae family. This study focused on the phytotoxic effects of this compound on the morphology and metabolism of Arabidopsis thaliana seedlings.

Material and methods

To evaluate the phytotoxicity of trans-cinnamaldehyde, a dose-response curve was first performed for the root growth process in order to calculate the reference inhibitory concentrations IC50 and IC80 (trans-cinnamaldehyde concentrations inducing a 50% and 80% inhibition, respectively). Subsequently, the structure and ultrastructure of the roots treated with the compound were analysed by light and electron microscopy. Based on these results, the following assays were carried out to in depth study the possible mode of action of the compound: antiauxinic PCIB reversion bioassay, determination of mitochondrial membrane potential, ROS detection, lipid peroxidation content, hormone quantification, in silico studies and gene expression of ALDH enzymes.

Results

Trans-cinnamaldehyde IC50 and IC80 values were as low as 46 and 87 μM, reducing the root growth and inducing the occurrence of adventitious roots. At the ultrastructural level, the compound caused alterations to the mitochondria, which were confirmed by detection of the mitochondrial membrane potential. The morphology observed after the treatment (i.e., appearance of adventitious roots) suggested a possible hormonal mismatch at the auxin level, which was confirmed after PCIB bioassay and hormone quantification by GC-MS. The addition of the compound caused an increase in benzoic, salicylic and indoleacetic acid content, which was related to the increased gene expression of the aldehyde dehydrogenase enzymes that can drive the conversion of trans-cinnamaldehyde to cinnamic acid. Also, an increase of ROS was also observed in treated roots. The enzyme-compound interaction was shown to be stable over time by docking and molecular dynamics assays.

Discussion

The aldehyde dehydrogenases could drive the conversion of trans-cinnamaldehyde to cinnamic acid, increasing the levels of benzoic, salicylic and indoleacetic acids and causing the oxidative stress symptoms observed in the treated seedlings. This would result into growth and development inhibition of the trans-cinnamaldehyde-treated seedlings and ultimately in their programmed-cell-death.

Ecophysiological Responses of Tall Wheatgrass Germplasm to Drought and Salinity

Tall wheatgrass (Thinopyrum ponticum (Podp.) Barkworth and D.R. Dewey) is an important, highly salt-tolerant C3 forage grass. The objective of this work was to learn about the ecophysiological responses of accessions from different environmental origins under drought and salinity conditions, to provide information for selecting superior germplasm under combined stress in tall wheatgrass. Four accessions (P3, P4, P5, P9) were irrigated using combinations of three salinity levels (0, 0.1, 0.3 M NaCl) and three drought levels (100%, 50%, 30% water capacity) over 90 days in a greenhouse. The control treatment showed the highest total biomass, but water-use efficiency (WUE), δ¹³C, proline, N concentration, leaf length, and tiller density were higher under moderate drought or/and salinity stress than under control conditions. In tall wheatgrass, K⁺ functions as an osmoregulator under drought, attenuated by salinity, and Na⁺ and Cl^- function as osmoregulators under salinity and drought, while proline is an osmoprotector under both stresses. P3 and P9, from environments with mild/moderate stress, prioritized reproductive development, with high evapotranspiration and the lowest WUE and δ¹³C values. P4 and P5, from more stressful environments, prioritized vegetative development through tillering, showing the lowest evapotranspiration, the highest δ¹³C values, and different mechanisms for limiting transpiration. The δ¹³C value, leaf biomass, tiller density, and leaf length had high broad-sense heritability (H²), while the Na⁺/K⁺ ratio had medium H². In conclusion, the combined use of the δ¹³C value, Na⁺/K⁺ ratio, and canopy structural variables can help identify accessions that are well-adapted to drought and salinity, also considering the desirable plant characteristics. Tall wheatgrass stress tolerance could be used to expand forage production under a changing climate.

Ultrastructural and hormonal changes related to harmaline-induced treatment in Arabidopsis thaliana (L.) Heynh. root meristem

Harmaline is an indole alkaloid with demonstrated phytotoxicity and recognized pharmacological applications. However, no information is available concerning its mode of action on plant metabolism. Therefore, the present work evaluated bioherbicide mode of action of harmaline on plant metabolism of Arabidopsis thaliana (L.) Heynh. Harmaline induced a strong inhibitory activity on root growth of treated seedlings, reaching IC₅₀ and IC₈₀ values of 14 and 29 μM, respectively. Treated roots were shorter and thicker than control and were characterized by a shorter root meristem size and an increase of root hairs production. Harmaline induced ultrastructural changes such as increment of cell wall thickness, higher density and condensation of mitochondria and vacuolization, appearance of cell wall deposits, increment of Golgi secretory activity and higher percentage of aberrant nuclei. The ethylene inhibitor AgNO₃ reversed high root hair appearance and increment of root thickness, and pTCSn::GFP transgenic line showed fluorescence cytokinin signal in stele zone after harmaline treatment that was absent in control, whereas the auxin signal in the transgenic line DR5 was significantly reduced by the treatment. All these results suggest that the mode of action of harmaline could be involving auxin, ethylene and cytokinin synergic/antagonistic action.

Weed pressure determines the chemical profile of wheat (Triticum aestivum L.) and its allelochemicals potential

BACKGROUND

Common purslane (Portulaca oleracea) and annual ryegrass (Lolium rigidum) are important infesting weeds of field crops. Herbicides are mostly used for weed suppression, while their environmental toxicity and resistance in weeds against them demand considering alternative options, such as the use of allelopathic crops for weed management. Wheat is an important allelopathic crop and present research focused on the identification and quantification of benzoxazinoids (BXZs) and polyphenols (phenolic acids and flavonoids) of the wheat accession 'Ursita' and to screen its allelopathic impact on P. oleracea and Lolium rigidum through equal-compartment-agar (ECA) method.

RESULTS

Weed germination, radicle length, biomass and photosynthetic pigments were altered following co-growth of weeds with Ursita for 10-day. Root exudates from Ursita reduced (29-60%) the seedling growth and photosynthetic pigments of Lolium rigidum depending on co-culture conditions of planting density. Weed pressure caused significant increase in the production of phenolic acids (vanillic, ferulic, syringic and p-coumaric acids) and root exudation of BXZs, in particular benzoxazolin-2-one (BOA), 2-hydroxy-7-methoxy-1,4-benzoxazin-3-one (HMBOA), 2-hydroxy-1,4-benzoxazin3-one (HBOA) and 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) in wheat tissues (shoots, roots) and exudate in root rhizosphere agar medium in response to co-cultivation with Lolium rigidum and P. oleracea, depending on weed/crop density.

CONCLUSION

The work revealed that Ursita is allelopathic in nature and can be used in breeding programs to enhance its allelopathic activity. Meanwhile, there are opportunities to explore allelopathic effect of wheat cultivars to control P. oleracea and Lolium rigidum under field conditions. © 2022 Society of Chemical Industry.

Secondary Metabolites, Ferulic Acid and p-Hydroxybenzoic Acid Induced Toxic Effects on Photosynthetic Process in Rumex acetosa L

The elimination of broadleaf weeds from agricultural fields has become an urgent task in plant and environment protection. Allelopathic control is considered a potential approach because of its exclusive and ecological safety measures. Plant secondary metabolites also called allelochemicals are released from plant leaves, roots, stem, bark, flowers and play significant roles in soil rhizosphere signaling, chemical ecology, and plant defense. The present study was carried out to evaluate the impact of two allelochemicals; ferulic acid (FA) and p-hydroxybenzoic acid (pHBA) on photosynthetic characteristics; F_v/F_m: efficiency of photosystem II photochemistry in the dark-adapted state; ΦPSII: photosynthetic quantum yield; NPQ, non-photochemical quenching; qP, photochemical quenching, and photon energy dissipation (1−qP)/NPQ in Rumex acetosa following 6 days exposure. R. acetosa seedlings were grown in perlite culture, irrigated with Hoagland solution and treated with allelopathic compounds FA and pHBA and were evaluated against the photosynthetic attributes. Both compounds behaved as potent inhibitors of photosynthetic traits such as F_v/F_m, ΦPSII, qP, and NPQ in R. acetosa. Photon energy dissipation (1−qP)/NPQ increased significantly from days 3 to 6. Higher dissipation of absorbed energy indicates the inactivation state of reaction centers and their inability to effectively use the absorbed energy in photosynthesis. These results indicated the potential allelopathic application of FA and pHBA for control of broadleaf weed, Rumex acetosa.

Imaging of Chlorophyll a Fluorescence in Natural Compound-Induced Stress Detection

Imaging of chlorophyll a fluorescence (CFI) represents an easy, precise, fast and non-invasive technique that can be successfully used for discriminating plant response to phytotoxic stress with reproducible results and without damaging the plants. The spatio-temporal analyses of the fluorescence images can give information about damage evolution, secondary effects and plant defense response. In the last years, some studies about plant natural compounds-induced phytotoxicity have introduced imaging techniques to measure fluorescence, although the analysis of the image as a whole is often missed. In this paper we, therefore, evaluated the advantages of monitoring fluorescence images, presenting the physiological interpretation of different possible combinations of the most relevant parameters linked to fluorescence emission and the images obtained.

Allelopathic Potential of Aqueous Extract from Acacia melanoxylon R. Br. on Lactuca sativa

We studied the polyphenol (phenolic compounds and flavonoids) composition and allelopathic effects of Acacia melanoxylon R. Br. aerial foliage aqueous extract (0%, 25%, 50%, 75% and 100%) on the seedling growth and plant biomass of the general biotest species, lettuce (Lactuca sativa). Mean leaf fresh weight, leaf dry weight, root fresh weight and root dry weight were decreased following exposure to Acacia aerial foliage, flowers aqueous extract (AFE) and phyllodes aqueous extract (APE) after 6 days. The reduction in plant dry biomass was more than 50% following treatment with AFE. The decrease in mean root length was approximately 37.7% and 29.20% following treatment with Acacia flowers extract (AFE) at 75% and 100% concentration, respectively. Root dry weight of L. sativa was reduced by both flowers and phyllodes extract. The reduction of protein contents in lettuce leaves following Acacia foliage extract proved that both AFE and APE exhibit polyphenols that causes the toxicity which led to decrease in leaf protein contents. High-Performance Liquid Chromatography (HPLC) was employed to analyze the A. melanoxylon flowers and phyllodes. A total of 13 compounds (accounting for most abundant compounds in flowers and phyllodes) include different flavonoids and phenolic compounds. The phytochemical compounds detected were: Gallic acid, protocatechuic acid, p-hydroxybenzoic acid, p-hydroxybenzaldehyde, vanillic acid, syringic acid, p-coumaric acid, and ferulic acid. The major flavonoid compounds identified include rutin, luteolin, apigenin, and catechin. Allelopathic effects of flower and phyllodes extracts from A. melanoxylon may be due to the presence of above compounds identified by HPLC analysis.

A natural indole alkaloid, norharmane, affects PIN expression patterns and compromises root growth in Arabidopsis thaliana

Norharmane is an indole alkaloid that can be found in several terrestrial plants, as well as in some dinoflagellates and cyanobacteria. The aim of this study was to focus on the way this metabolite impacts the plant metabolism of the model species Arabidopsis thaliana. This metabolite caused increase of secondary and adventitious roots, as well as torsion, toxic effects, and a decrease in root length. Moreover, norharmane altered the cellular arrangement, resulting in unfinished cell walls, decreased auxin content and inhibited PIN proteins activity. All the alterations suggest that norharmane alters polar auxin transport by inhibiting PIN2, PIN3 and PIN7 transport proteins, thus causing a significant inhibitory effect on the growth of A. thaliana seedlings.

Transcriptome and binding data indicate that citral inhibits single strand DNA-binding proteins

The mechanism of phytotoxicity of citral was probed in Arabidopsis thaliana using RNA-Seq and in silico binding analyses. Inhibition of growth by 50% by citral downregulated transcription of 9156 and 5541 genes in roots and shoots, respectively, after 1 h. Only 56 and 62 genes in roots and shoots, respectively, were upregulated. In the shoots, the downregulation increased at 3 h (6239 genes downregulated, vs 66 upregulated). Of all genes affected in roots at 1 h (time of greatest effect), 7.69% of affected genes were for nucleic acid binding functions. Genes for single strand DNA binding proteins (SSBP) WHY1, WHY 2 and WHY3 were strongly downregulated in the shoot up until 12 h after citral exposure. Effects were strong in the root at just 1 h after the treatment and then at 12 and 24 h. Similar effects occurred with the transcription factors MYC-2, ANAC and SCR-SHR, which were also significantly downregulated for the first hour of treatment, and downregulation occurred again after 12 and 24 h treatment. Downregulation of ANAC in the first hour of treatment was significantly (P < 0.0001) decreased more than eight times compared to the control. In silico molecular docking analysis suggests binding of citral isomers to the SSBPs WHY1, WHY2, and WHY3, as well as with other transcription factors such as MYC-2, ANAC and SCR-SHR. Such effects could account for the profound and unusual effects of citral on downregulation of gene transcription.

Genetic evidence for plural introduction pathways of the invasive weed Paterson's curse (Echium plantagineum L.) to southern Australia

Paterson’s curse (Echium plantagineum L. (Boraginaceae)), is an herbaceous annual native to Western Europe and northwest Africa. It has been recorded in Australia since the 1800’s and is now a major weed in pastures and rangelands, but its introduction history is poorly understood. An understanding of its invasion pathway and subsequent genetic structure is critical to the successful introduction of biological control agents and for provision of informed decisions for plant biosecurity efforts. We sampled E. plantagineum in its native (Iberian Peninsula), non-native (UK) and invaded ranges (Australia and South Africa) and analysed three chloroplast gene regions. Considerable genetic diversity was found among E. plantagineum in Australia, suggesting a complex introduction history. Fourteen haplotypes were identified globally, 10 of which were co-present in Australia and South Africa, indicating South Africa as an important source population, likely through contamination of traded goods or livestock. Haplotype 4 was most abundant in Australia (43%), and in historical and contemporary UK populations (80%), but scarce elsewhere (< 17%), suggesting that ornamental and/or other introductions from genetically impoverished UK sources were also important. Collectively, genetic evidence and historical records indicate E. plantagineum in southern Australia exists as an admixture that is likely derived from introduced source populations in both the UK and South Africa.

Transcriptome responses to the natural phytotoxin t-chalcone in Arabidopsis thaliana L

BACKGROUND

New modes of action are needed for herbicides. The flavonoid synthesis intermediate t-chalcone causes apoptosis-like symptoms in roots and bleaching of shoots of Arabidospsis, suggesting a unique mode of action as a phytotoxin.

RESULTS

Using RNA-Seq, transcriptome changes were monitored in Arabidopsis seedlings during the first 24 h of exposure (at 1, 3, 6, 12 and 24 h) to 21 μm t-chalcone (I₅₀ dose), examining effects on roots and shoots separately. Expression of 892 and 1000 genes was affected in roots and shoots, respectively. According to biological classification, many of the affected genes were transcription factors and genes associated with oxidative stress, heat shock proteins, xenobiotic detoxification, ABA and auxin biosynthesis, and primary metabolic processess. These are secondary effects found with most phytotoxins. Potent phytotoxins usually act by inhibiting enzymes of primary metabolism. KEGG pathway analysis of transcriptome results from the first 3 h of t-chalcone exposure indicated several potential primary metabolism target sites for t-chalcone. Of these, p-hydroxyphenylpyruvate dioxygenase (HPPD) and tyrosine amino transferase were consistent with the bleaching effect of the phytotoxin. Supplementation studies with Lemna paucicostata and Arabidiopsis supported HPPD as the target, although in vitro enzyme inhibition was not found.

CONCLUSIONS

t-Chalcone is possibly a protoxin that is converted to a HPPD inhibitor in vivo. © 2019 Society of Chemical Industry.

Rosmarinic acid induces programmed cell death in Arabidopsis seedlings through reactive oxygen species and mitochondrial dysfunction

Phytotoxic potential of rosmarinic acid (RA), a caffeic acid ester largely found in aromatic species, was evaluated on Arabidopsis through metabolomic and microscopic approaches. In-vitro bioassays pointed out that RA affected root growth and morphology, causing ROS burst, ROS scavengers activity inhibition and consequently, an alteration on cells organization and ultrastructure. In particular, RA-treatment (175 μM) caused strong vacuolization, alteration of mitochondria structure and function and a consistent ROS-induced reduction of their transmembrane potential (ΔΨm). These data suggested a cell energy deficit also confirmed by the metabolomic analysis, which highlighted a strong alteration of both TCA cycle and amino acids metabolism. Moreover, the increase in H2O2 and O2- contents suggested that RA-treated meristems underwent oxidative stress, resulting in apoptotic bodies and necrotic cells. Taken together, these results suggest that RA inhibits two of the main ROS scavengers causing high ROS accumulation, responsible of the alterations on mitochondrial ultrastructure and activity through ΔΨm dissipation, TCA-cycle alteration, cell starvation and consequently cell death on Arabidopsis seedlings. All these effects resulted in a strong inhibition on root growth and development, which convert RA in a promising molecule to be explored for further use in weed management.

Morpho-physiological responses of tall wheatgrass populations to different levels of water stress

Tall wheatgrass [Elymus elongatus subsp. ponticus (Podp.) Melderis] is a perennial forage grass cultivated in dry, saline or alkaline environments. The morpho-physiological characteristics of four populations of tall wheatgrass from different climatic-edaphic origins were evaluated under three conditions of water stress (100%-50%-30% of field capacity). The trial was analyzed with three replicates and two-factor ANOVA in pots within the greenhouse during 35 days. Only dry matter and tiller number showed interaction between populations and water conditions. The most relevant changes in morpho-physiological parameters under strong water stress were reduced dry matter production (48–32% differing among populations), smaller leaf and tiller size (46% and 28%), together with higher water use efficiency (74%), and increased proline and protein contents (144% and 71%), smaller tiller number (30–11% differing among populations) and a slight decrease in leaf water content (3%). The populations differed in growth strategies and morpho-physiological mechanisms to survive water stress, which could be related to their habitat background. The study shows the stability in dry matter production under all levels of water stress, which can be related to the higher tiller number. Due to this plasticity, tall wheatgrass should be studied as a species with great potential to adapt to drought stress.

Carbon (δ13C) and Nitrogen (δ15N) Stable Isotope Composition Provide New Insights into Phenotypic Plasticity in Broad Leaf Weed Rumex acetosa under Allelochemical Stress

Phenolic compounds, hydroquinone and cinnamic acid derivatives have been identified as major allelochemicals with known phytotoxicity from allelopathic plant Acacia melanoxylon R. Br. Several phenolic compounds such as ferulic acid (FA), p-hydroxybenzoic acid (pHBA) and flavonoid (rutin, quercetin) constituents occur in the phyllodes and flowers of A. melanoxylon and have demonstrated inhibitory effects on germination and physiological characteristics of lettuce and perennial grasses. However, to date, little is known about the mechanisms of action of these secondary metabolites in broad-leaved weeds at ecophysiological level. The objective of this study was to determine the response of Rumex acetosa carbon isotope composition and other physiological parameters to the interaction of plant secondary metabolites (PSM) (FA and pHBA) stress and the usefulness of carbon isotope discrimination (Δ¹³C) as indicative of the functional performance of intrinsic water use efficiency (iWUE) at level of plant leaf. R. acetosa plant were grown under greenhouse condition and subjected to PSM stress (0, 0.1, 0.5, 1.0, and 1.5 mM) for six days. Here, we show that FA and pHBA are potent inhibitors of Δ¹³C that varied from 21.0‰ to 22.9‰. Higher pHBA and FA supply enhanced/retard the N_leaf and increased the C_leaf while ratio of intercellular CO₂ concentration from leaf to air (Ci/Ca) was significantly decreased as compared to control. Leaf water content and leaf osmotic potential were decreased following treatment with both PSM. The Ci/Ca decreased rapidly with higher concentration of FA and pHBA. However, iWUE increased at all allelochemical concentrations. At the whole plant level, both PSM showed pronounced growth-inhibitory effects on PBM and C and N concentration, root fresh/dry weight, leaf fresh/dry weight, and root, shoot length of C₃ broad leaf weed R. acetosa. Carbon isotope discrimination (Δ) was correlated with the dry matter to transpiration ratio (transpiration efficiency) in this C₃ species, but its heritability and relationship to R. acetosa growth are less clear. Our FA and pHBA compounds are the potent and selective carbon isotope composition (δ¹³C) inhibitors known to date. These results confirm the phytotoxicity of FA and pHBA on R. acetosa seedlings, the reduction of relative water content and the induction of carbon isotope discrimination (Δ) with lower plant biomass.

Genotypic differences in agro-physiological, biochemical and isotopic responses to salinity stress in quinoa (Chenopodium quinoa Willd.) plants: Prospects for salinity tolerance and yield stability

Quinoa is an important nutritive crop that can play a strategic role in the development of marginal and degraded lands. Genotypic variations in carbon isotope composition (δ¹³C), carbon isotope discrimination (Δ¹³C), ratio of intercellular to atmospheric CO₂ concentration (Ci/Ca), intrinsic water use efficiency (iWUE), seed yield and grain protein contents were analyzed in 6 quinoa cultivars grown in the field under saline conditions (0, 10, 20 dS m^-1). Significant variations occurred in dry biomass, seed yield, plant height, number of branches, number of panicles, panicle weight, harvest index, N and C content. Some genotypes produced yields with values significantly higher than 2.04 t ha^-1 (Q12), with an average increased to 2.58 t ha^-1 (AMES22157). The present study indicates a large variation in Δ¹³C for salinity treatments (3.43‰) and small magnitude of variations among genotypes (0.95‰). Results showed that Δ might be used as an important index for screening, and selection of the salt tolerant quinoa genotypes with high iWUE. Quinoa genotypes differs in foliar ¹³C and ¹⁵N isotope composition, which reflected complex interactions of salinity and plant carbon and nitrogen metabolisms. Grain protein contents were found higher in Q19 and Q31 and lowest in Q26. The study demonstrates that AMES22157 and Q12, were salt tolerant and high yielder while the AMES22157 was more productive. This study provides a reliable measure of morpho-physiological, biochemical and isotopic responses of quinoa cultivars to salinity in hyper arid UAE climate and it may be valuable in the future breeding programs. The development of genotypes having both higher water use efficiency and yield potential would be a very useful contribution for producers in the dry region of Arabian Peninsula.

The Consistency Between Phytotoxic Effects and the Dynamics of Allelochemicals Release from Eucalyptus globulus Leaves Used as Bioherbicide Green Manure

In the worldwide search for new strategies in sustainable weed management, the use of allelopathic plants incorporated into the soil as green manure can help control weeds by releasing allelochemicals into the environment. In previous experiments, Eucalyptus globulus leaves incorporated into the soil as green manure were shown to have a notable potential for weed control. But, 'what was exactly happening at chemical level?' and 'which were the compounds potentially responsible for the phytotoxic effects observed during those greenhouse assays?' In the present study, in-vitro phytotoxicity bioassays and chemical analysis of eucalyptus leaves were carried out in order to explore the relationship between the temporal phytotoxic effects and the dynamics of chemical composition. For that, eucalyptus leaves were removed from soil at different sampling times during 30 days and analyzed for phenolic and volatile organic compounds (VOCs) by HPLC and HS-SPME/GC-MS, respectively. The phytotoxic potential of the aqueous extract and the volatile fraction was tested on the germination and early growth of Lactuca sativa. Eucalyptus leaves incorporated into the soil as green manure showed a continuous release of different phenolic and volatile compounds during a 30-day period of decomposition. Both fractions had phytotoxic effects during the time assayed; however, the target process of phytotoxicity was different: phenolic compounds being the factor causing germination inhibition and VOCs responsible for growth reduction. The dynamics of release of this cocktail of allelochemicals into the soil environment may be the responsible for the phytotoxicity observed in our previous works.

Unravelling the bioherbicide potential of Eucalyptus globulus Labill: Biochemistry and effects of its aqueous extract

In the worldwide search for new strategies in sustainable weed management, the use of plant species able to produce and release phytotoxic compounds into the environment could be an effective alternative to synthetic herbicides. Eucalyptus globulus Labill. is known to be a source of biologically active compounds responsible for its phytotoxic and allelopathic properties. Our previous results demonstrated the bioherbicide potential of eucalyptus leaves incorporated into the soil as a green manure, probably through the release of phytotoxins into the soil solution. Thus, the aims of this study were to understand the phytotoxicity of the eucalyptus leaves aqueous extract applied in pre- and post-emergence, and to identify and quantify its potentially phytotoxic water-soluble compounds. The effects were tested on the germination and early growth of the model target species Lactuca sativa and Agrostis stolonifera, and on physiological parameters of L. sativa adult plants after watering or spraying application. Dose-response curves and ED50 and ED80 values for eucalyptus aqueous extracts revealed pre-emergence inhibitory effects on both target species, effects being comparable to the herbicide metolachlor. While spraying treatment reduced the aerial and root biomass and increased the dry weight/fresh weight ratio of lettuce adult plants, watering application reduced protein contents and chlorophyll concentrations with respect to control, reflecting different modes of action depending on the site of phytotoxin entry. Via HPLC analyses, a total of 8 phenolic compounds (chlorogenic, two ρ-coumaric derivatives, ellagic, hyperoside, rutin, quercitrin, and kaempferol 3-O-glucoside) and other 5 low weight organic acids (citric, malic, shikimic, succinic and fumaric acids) were obtained from aqueous extract, the latter being identified for the first time in E. globulus. Despite some phytotoxic effects were found on lettuce adult plants, the use of eucalyptus aqueous extract would be discarded in post-emergence, whereas it was promising as a pre-emergence bioherbicide.

Plasma membrane depolarization precedes photosynthesis damage and long-term leaf bleaching in (E)-chalcone-treated Arabidopsis shoots

The plant phenolic compound (E)-chalcone has been previously found to induce noticeable seedling size reduction and progressive de-greening (bleaching) in shoots of Arabidopsis thaliana seedlings. In this work, we demonstrate that this progressive de-greening occurring on Arabidopsis shoots after (E)-chalcone treatment, is directly linked to early plasma membrane depolarization and dramatic effects on chloroplasts structure and function. Later effects in chalcone-treated seedlings included ROS accumulation, pigment degradation, reduced photosynthetic activity, bleaching, and eventually cell death. De-greening and pigment degradation induced by (E)-chalcone were partially reversed when NaCl was added together with chalcone, which could be related to restoration of altered pH gradients. All these results suggest that rapid alteration of plasma membrane potential after chalcone treatment is a major component of the mode of action of (E)-chalcone on Arabidopsis metabolism.

Auxin-like effects of the natural coumarin scopoletin on Arabidopsis cell structure and morphology

The mode of action and phytotoxic potential of scopoletin, a natural compound belonging to the group of coumarins, has been evaluated in detail. Analysis conducted by light and electron transmission microscopy showed strong cell and tissue abnormalities on treated roots, such as cell wall malformations, multi-nucleated cells, abnormal nuclei and tissue disorganization. Scopoletin compromised root development by inducing wrong microtubule assembling, mitochondrial membrane depolarization and ultimate cell death, in a way similar to auxin herbicides. The structural similarities of the natural compound scopoletin and the auxin herbicide 2,4-D, as well as the ability of scopoletin to fit into the auxin-binding site TIR1, were analyzed, suggesting that the phytotoxic activity of scopoletin matches with that exhibited by auxinic herbicides.

Soil Cd, Cr, Cu, Ni, Pb and Zn sorption and retention models using SVM: Variable selection and competitive model

The aim of this study was to model the sorption and retention of Cd, Cu, Ni, Pb and Zn in soils. To that extent, the sorption and retention of these metals were studied and the soil characterization was performed separately. Multiple stepwise regression was used to produce multivariate models with linear techniques and with support vector machines, all of which included 15 explanatory variables characterizing soils. When the R-squared values are represented, two different groups are noticed. Cr, Cu and Pb sorption and retention show a higher R-squared; the most explanatory variables being humified organic matter, Al oxides and, in some cases, cation-exchange capacity (CEC). The other group of metals (Cd, Ni and Zn) shows a lower R-squared, and clays are the most explanatory variables, including a percentage of vermiculite and slime. In some cases, quartz, plagioclase or hematite percentages also show some explanatory capacity. Support Vector Machine (SVM) regression shows that the different models are not as regular as in multiple regression in terms of number of variables, the regression for nickel adsorption being the one with the highest number of variables in its optimal model. On the other hand, there are cases where the most explanatory variables are the same for two metals, as it happens with Cd and Cr adsorption. A similar adsorption mechanism is thus postulated. These patterns of the introduction of variables in the model allow us to create explainability sequences. Those which are the most similar to the selectivity sequences obtained by Covelo (2005) are Mn oxides in multiple regression and change capacity in SVM. Among all the variables, the only one that is explanatory for all the metals after applying the maximum parsimony principle is the percentage of sand in the retention process. In the competitive model arising from the aforementioned sequences, the most intense competitiveness for the adsorption and retention of different metals appears between Cr and Cd, Cu and Zn in multiple regression; and between Cr and Cd in SVM regression.

Evaluation of photosynthetic performance and carbon isotope discrimination in perennial ryegrass (Lolium perenne L.) under allelochemicals stress

Ferulic (FA) and p-hydroxybenzoic acid (pHBA) are commonly found as phenolic compounds (PHC) in many forage and cereal crops. Although the effects of these PHC on seedling growth are relatively explored, not many information is available regarding the phytotoxicity on ecophysiological processes of perennial ryegrass adult plants. The experiment was conducted with the aim to evaluate the phytotoxic potential of PHC on the seedling growth, leaf water relation, chlorophyll fluorescence attributes and carbon isotope discrimination adult plants of perennial ryegrass (Lolium perenne L.). The results clearly indicated that PHC behaved as potent inhibitors of chlorophyll fluorescence yield (F_v/F_m) in leaves of L. perenne and plants showed poor tolerance against allelochemicals stress. Quantum yield (ΦPSII), chlorophyll fluorescence quenching (qP) and non-photochemical quenching (NPQ) were decreased following exposure to FA and pHBA. The portion of absorbed photon energy that was thermally dissipated (D) in L. perenne was decreased. Exposure of the L. perenne seedlings to FA and pHBA stress led to a decrease in fresh/dry weight, relative water content and leaf osmotic potential. Carbon isotope composition ratio (δ¹³C) was significantly less negative than the control following treatment with FA or pHBA. The results suggested that PHC uptake was a key step for the effectiveness of these secondary metabolites and their phytotoxicity on L. perenne adult plants was mainly due to the alteration of leaf water status accompanied by photosystem II damage. Acquisition of such knowledge may ultimately provide a better understanding about the mode of action of the tested compounds.

Terpenoid trans-caryophyllene inhibits weed germination and induces plant water status alteration and oxidative damage in adult Arabidopsis

trans-Caryophyllene (TC) is a sesquiterpene commonly found as volatile component in many different aromatic plants. Although the phytotoxic effects of trans-caryophyllene on seedling growth are relatively explored, not many information is available regarding the phytotoxicity of this sesquiterpenes on weed germination and on adult plants. The phytotoxic potential of TC was assayed in vitro on weed germination and seedling growth to validate its phytotoxic potential on weed species. Moreover, it was assayed on the metabolism of Arabidopsis thaliana adult plants, through two different application ways, spraying and watering, in order to establish the primary affected organ and to deal with the unknown mobility of the compound. The results clearly indicated that TC inhibited both seed germination and root growth, as demonstrated by comparison of the ED50 values. Moreover, although trans-caryophyllene-sprayed adult Arabidopsis plants did not show any effect, trans-caryophyllene-watered plants became strongly affected. The results suggested that root uptake was a key step for the effectiveness of this natural compound and its phytotoxicity on adult plants was mainly due to the alteration of plant water status accompanied by oxidative damage.

Loss of Gravitropism in Farnesene-Treated Arabidopsis Is Due to Microtubule Malformations Related to Hormonal and ROS Unbalance

Mode of action of farnesene, a volatile sesquiterpene commonly found in the essential oils of several plants, was deeply studied on the model species Arabidopsis thaliana. The effects of farnesene on the Arabidopsis root morphology were evaluated by different microscopic techniques. As well, microtubules immunolabeling, phytohormone measurements and ROS staining helped us to elucidate the single or multi-modes of action of this sesquiterpene on plant metabolism. Farnesene-treated roots showed a strong growth inhibition and marked modifications on morphology, important tissue alterations, cellular damages and anisotropic growth. Left-handed growth of farnesene-treated roots, reverted by taxol (a known microtubule stabilizer), was related to microtubule condensation and disorganization. As well, the inhibition of primary root growth, lateral root number, lateral root length, and both root hairs length and density could be explained by the strong increment in ethylene production and auxin content detected in farnesene-treated seedlings. Microtubule alteration and hormonal unbalance appear as important components in the mode of action of farnesene and confirm the strong phytotoxic potential of this sesquiterpene.

The plant secondary metabolite citral alters water status and prevents seed formation in Arabidopsis thaliana

Based on previous results, which showed that the secondary metabolite citral causes disturbances to plant water status, the present study is focused on demonstrating and detailing these effects on the water-related parameters of Arabidopsis thaliana adult plants, and their impact on plant fitness. Clear evidence of effects on water status and fitness were observed: plants treated with 1200 and 2400 μm citral showed decreased RWC, reduced Ψs , increased Ψw and reduced stomatal opening, even 7 days after the beginning of the experiment. Plant protection signals, such as leaf rolling or increased anthocyanin content, were also detected in these plants. In contrast, 14 days after beginning the treatment, treated plants showed signs of citral-related damage. Moreover, the reproductive success of treated plants was critically compromised, with prematurely withered flowers and no silique or seed development. This effect of citral on fitness of adult plants suggests a promising application of this natural compound in weed management by reducing the weed seed bank in the soil.

Characterization of xanthophyll pigments, photosynthetic performance, photon energy dissipation, reactive oxygen species generation and carbon isotope discrimination during artemisinin-induced stress in Arabidopsis thaliana

Artemisinin, a potent antimalarial drug, is phytotoxic to many crops and weeds. The effects of artemisinin on stress markers, including fluorescence parameters, photosystem II photochemistry, photon energy dissipation, lipid peroxidation, reactive oxygen species generation and carbon isotope discrimination in Arabidopsis thaliana were studied. Arabidopsis ecotype Columbia (Col-0) seedlings were grown in perlite and watered with 50% Hoagland nutrient solution. Adult plants of Arabidopsis were treated with artemisinin at 0, 40, 80, 160 μM for one week. Artemisinin, in the range 40-160 μM, decreased the fresh biomass, chl a, b and leaf mineral contents. Photosynthetic efficiency, yield and electron transport rate in Arabidopsis were also reduced following exposure to 80 and 160 μM artemisinin. The ΦNPQ and NPQ were less than control. Artemisinin treatment caused an increase in root oxidizability and lipid peroxidation (MDA contents) of Arabidopsis. Calcium and nitrogen contents decreased after 80 and 160 μM artemisinin treatment compared to control. δ13C values were less negative following treatment with artemisinin as compared to the control. Artemisinin also decreased leaf protein contents in Arabidopsis. Taken together, these data suggest that artemisinin inhibits many physiological and biochemical processes in Arabidopsis.

Higher peroxidase activity, leaf nutrient contents and carbon isotope composition changes in Arabidopsis thaliana are related to rutin stress

Rutin, a plant secondary metabolite that is used in cosmetics and food additive and has known medicinal properties, protects plants from UV-B radiation and diseases. Rutin has been suggested to have potential in weed management, but its mode of action at physiological level is unknown. Here, we report the biochemical, physiological and oxidative response of Arabidopsis thaliana to rutin at micromolar concentrations. It was found that fresh weight; leaf mineral contents (nitrogen, sodium, potassium, copper and aluminum) were decreased following 1 week exposure to rutin. Arabidopsis roots generate significant amounts of reactive oxygen species after rutin treatment, consequently increasing membrane lipid peroxidation, decreasing leaf Ca(2+), Mg(2+), Zn(2+), Fe(2+) contents and losing root viability. Carbon isotope composition in A. thaliana leaves was less negative after rutin application than the control. Carbon isotope discrimination values were decreased following rutin treatment, with the highest reduction compared to the control at 750μM rutin. Rutin also inhibited the ratio of CO2 from leaf to air (ci/ca) at all concentrations. Total protein contents in A. thaliana leaves were decreased following rutin treatment. It was concluded carbon isotope discrimination coincided with protein degradation, increase lipid peroxidation and a decrease in ci/ca values may be the primary action site of rutin. The present results suggest that rutin possesses allelopathic potential and could be used as a candidate to develop environment friendly natural herbicide.

Comparative analysis of allelopathic effects produced by four forestry species during decomposition process in their soils in Galicia (NW Spain)

The development of toxicity produced by vegetable litter of four forest species (Quercus robur L.,Pinus radiata D.Don.,Eucalyptus globulus Labill, andAcacia melanoxylon R.Br.) was studied during the decomposition process in each of the soils where the species were found. The toxicity of the extracts was measured by the effects produced on germination and growth ofLactuca saliva L. var. Great Lakes seeds. The phenolic composition of the leaves of the four species was also studied using high-performance liquid chromatographic analysis (HPLC). It was verified that toxicity was clearly reflected in the first stages of leaf decomposition inE. globulus andA. melanoxylon, due to phytotoxic compounds liberated by their litter. At the end of half a year of decomposition, inhibition due to the vegetable material was not observed, but the soils associated with these two species appeared to be responsible for the toxic effects. On the other hand, the phenolic profiles are quite different among the four species, and greater complexity in the two toxic species (E. globulus andA. melanoxylon) was observed.

Individual and joint activity of terpenoids, isolated from Calamintha nepeta extract, on Arabidopsis thaliana

Four terpenoids, camphor, pulegone, trans-caryophyllene and farnesene, previously found in Calamintha nepeta (L.) Savi methanolic extract and essential oils were assayed on germination and root growth of Arabidopsis thaliana (L.) Heynh. None of the terpenes, singularly or in combination, was able to inhibit the germination process. Farnesene and trans-caryophyllene caused a strong inhibitory effect on root growth, and pulegone, at the highest concentrations, reduced lateral root formation. Although the mixture of camphor-trans-caryophyllene with or without farnesene did not cause any effect on root growth, the addition of pulegone induced a marked synergistic activity. Moreover, the addition, at low concentration, of farnesene to pulegone-camphor-trans-caryophyllene mixture further increased the inhibitory effect on root elongation. These results suggested that the inhibitory effects caused by C. nepeta methanolic extract may depend on the combined action of different molecules.

Citral induces auxin and ethylene-mediated malformations and arrests cell division in Arabidopsis thaliana roots

Citral is a linear monoterpene which is present, as a volatile component, in the essential oil of several different aromatic plants. Previous studies have demonstrated the ability of citral to alter the mitotic microtubules of plant cells, especially at low concentrations. The changes to the microtubules may be due to the compound acting directly on the treated root and coleoptile cells or to indirect action through certain phytohormones. This study, performed in Arabidopsis thaliana, analysed the short-term effects of citral on the auxin content and mitotic cells, and the long-term effects of these alterations on root development and ethylene levels. The results of this study show that citral alters auxin content and cell division and has a strong long-term disorganising effect on cell ultra-structure in A. thaliana seedlings. Its effects on cell division, the thickening of the cell wall, the reduction in intercellular communication, and the absence of root hairs confirm that citral is a strong phytotoxic compound, which has persistent effects on root development.

The role of peroxidases on the mode of action of chalcone in Arabidopsis roots

Chalcone is a secondary metabolite belonging to the group of flavonoids. It has shown strong phytotoxic activity on Arabidopsis roots, as inductor of programmed cell death, and inhibitor of root growth and root hair formation. Peroxidases are particularly abundant in root meristems and are involved in the formation and interconversion of reactive oxygen species (ROS), which play a critical role on root and root hair development. Therefore, we report here the role of peroxidases in Arabidopsis root development during chalcone treatment. A strong inhibition of peroxidase activity was detected in the apical root meristems after chalcone treatment, which reflects the important role of these enzymes on the mode of action of this secondary metabolite.

The natural compound trans-chalcone induces programmed cell death in Arabidopsis thaliana roots

Chalcone (1,3-diphenyl-2-propen-1-one) is an aromatic ketone precursor of important molecules in plants such as flavonoids or anthocyanins. Its phytotoxicity has been demonstrated on different plant species, but to date little is known about the mechanisms of action of this secondary metabolite at plant cellular level. Detailed analysis by light and transmission electron microscopy (TEM) was conducted to examine the root meristems' ultrastructure of control and chalcone-treated Arabidopsis seedlings. Mitochondrial dysfunction was analysed by measuring mitochondrial membrane potential with JC-1 fluorochrome. Finally, acridine orange/ethidium bromide staining was used for the detection of programmed cell death. Microscopy revealed tissue alterations, inhibition of root hair formation and important changes after 7 and 14 d at the chalcone IC(50) value. Chalcone-treated cells showed signs of programmed cell death such as mitochondrial condensation, disruption of organelles and chromatin fragmentation. Acridine orange/ethidium bromide staining confirmed the programmed cell death, which could be induced by the reduction of mitochondrial transmembrane potential (ΔΨ(m)) that was detected after chalcone treatment. These results confirm the phytotoxic activity of chalcone on Arabidopsis seedlings, the alteration of mitochondrial membrane potential and the induction of programmed cell death.

A chlorophyll fluorescence analysis of photosynthetic efficiency, quantum yield and photon energy dissipation in PSII antennae of Lactuca sativa L. leaves exposed to cinnamic acid

This study investigated the effects of cinnamic acid (CA) on growth, biochemical and physiological responses of Lactuca sativa L. CA (0.1, 0.5, 1.0 and 1.5 mM) treatments decreased plant height, root length, leaf and root fresh weight, but it did not affect the leaf water status. CA treatment (1.5 mM) significantly reduced F(v), F(m), photochemical efficiency of PSII (F(v)/F(m)) and quantum yield of PSII (ΦPSII) photochemistry in L. sativa. The photochemical fluorescence quenching (qP) and non-photochemical quenching (NPQ) were reduced after treatment with 1.5 mM CA. Fraction of photon energy absorbed by PS II antennae trapped by "open" PS II reaction centers (P) was reduced by CA (1.5 mM) while, portion of absorbed photon energy thermally dissipated (D) and photon energy absorbed by PSII antennae and trapped by "closed" PSII reaction centers (E) was increased. Carbon isotope composition ratios (δ(13)C) was less negative (-27.10) in CA (1.5 mM) treated plants as compared to control (-27.61). Carbon isotope discrimination (Δ(13)C) and ratio of intercellular CO(2) concentration (ci/ca) from leaf to air were also less in CA treated plants. CA (1.5 mM) also decreased the leaf protein contents of L. sativa as compared to control.

Benzoxazolin-2(3H)-one (BOA) induced changes in leaf water relations, photosynthesis and carbon isotope discrimination in Lactuca sativa

The effects are reported here of Benzoxazolin-2(3H)-one (BOA), an allelopathic compound, on plant water relations, growth, components of chlorophyll fluorescence, and carbon isotope discrimination in lettuce (Lactuca sativa L.). Lettuce seedlings were grown in 1:1 Hoagland solution in perlite culture medium in environmentally controlled glasshouse. After 30 days, BOA was applied at concentration of 0.1, 0.5, 1.0 and 1.5 mM and distilled water (control). BOA, in the range (0.1-1.5 mM), decreased the shoot length, root length, leaf and root fresh weight. Within this concentration range, BOA significantly reduced relative water content while leaf osmotic potential remained unaltered. Stress response of lettuce was evaluated on the basis of six days of treatment with 1.5 mM BOA by analyzing several chlorophyll fluorescence parameters determined under dark-adapted and steady state conditions. There was no change in initial fluorescence (F₀) in response to BOA treatment while maximum chlorophyll fluorescence (F(m)) was significantly reduced. BOA treatment significantly reduced variable fluorescence (F(v)) on first, second, third, fourth, fifth and sixth day. Quantum efficiency of open PSII reaction centers (F(v)/F(m)) in the dark-adapted state was significantly reduced in response to BOA treatment. Quantum yield of photosystem II (ΦPSII) electron transport was significantly reduced because of decrease in the efficiency of excitation energy trapping of PSII reaction centers. Maximum fluorescence in light-adapted leaves (F'(m)) was significantly decreased but there was no change in initial fluorescence in light-adapted state (F'₀) in response to 1.5 mM BOA treatment. BOA application significantly reduced photochemical fluorescence quenching (qP) indicating that the balance between excitation rate and electron transfer rate has changed leading to a more reduced state of PSII reaction centers. Non photochemical quenching (NPQ) was also significantly reduced by BOA treatment on third, fourth and fifth day. BOA had dominant effect on C isotope ratios (δ¹³C) that was significantly less negative (-26.93) at 1.0 mM concentration as compared to control (-27.61). Carbon isotope discrimination (Δ¹³C) values were significantly less (19.45) as compared to control (20.17) at 1.0 mM. BOA also affect ratio of intercellular to air CO₂ concentration (ci/ca) that was significantly less (0.66) as compared to control (0.69) when treated with 1.0 mM BOA. Protein content of lettuce leaf tissue decreased under BOA treatment at 1.5 mM concentration as compared to control.

Allelochemical stress inhibits growth, leaf water relations, PSII photochemistry, non-photochemical fluorescence quenching, and heat energy dissipation in three C3 perennial species

In this study, the effect of two allelochemicals, benzoxazolin-2(3H)-one (BOA) and cinnamic acid (CA), on different physiological and morphological characteristics of 1-month-old C(3) plant species (Dactylis glomerata, Lolium perenne, and Rumex acetosa) was analysed. BOA inhibited the shoot length of D. glomerata, L. perenne, and R. acetosa by 49%, 19%, and 19% of the control. The root length of D. glomerata, L. perenne, and R. acetosa growing in the presence of 1.5 mM BOA and CA was decreased compared with the control. Both allelochemicals (BOA, CA) inhibited leaf osmotic potential (LOP) in L. perenne and D. glomerata. In L. perenne, F(v)/F(m) decreased after treatment with BOA (1.5 mM) while CA (1.5 mM) also significantly reduced F(v)/F(m) in L. perenne. Both allelochemicals decreased ΦPSII in D. glomerata and L. perenne within 24 h of treatment, while in R. acetosa, ΦPSII levels decreased by 72 h following treatment with BOA and CA. There was a decrease in qP and NPQ on the first, fourth, fifth, and sixth days after treatment with BOA in D. glomerata, while both allelochemicals reduced the qP level in R. acetosa. There was a gradual decrease in the fraction of light absorbed by PSII allocated to PSII photochemistry (P) in R. acetosa treated with BOA and CA. The P values in D. glomerata were reduced by both allelochemicals and the portion of absorbed photon energy that was thermally dissipated (D) in D. glomerata and L. perenne was decreased by BOA and CA. Photon energy absorbed by PSII antennae and trapped by 'closed' PSII reaction centres (E) was decreased after CA exposure in D. glomerata. BOA and CA (1.5 mM concentration) decreased the leaf protein contents in all three perennial species. This study provides new understanding of the physiological and biochemical mechanisms of action of BOA and CA in one perennial dicotyledon and two perennial grasses. The acquisition of such knowledge may ultimately provide a rational and scientific basis for the design of safe and effective herbicides.

Early senescence induced by 2-3H-benzoxazolinone (BOA) in Arabidopsis thaliana

Measurements of chlorophyll a fluorescence, nutrient and trace elements, total protein content and malonyldialdehyde in leaves of Arabidopsis thaliana between 1 and 192 h after treatment with 0, 1 or 3 mM 2-3H-benzoxazolinone (BOA), together with imaging of chlorophyll a fluorescence and of the distributions of hydrogen peroxide and superoxide anion, suggested that the primary phytotoxic action of BOA is the induction of premature senescence, and that oxidative stress is a secondary effect that sets in a day or two later.

Reduced photosynthetic activity is directly correlated with 2-(3H)-benzoxazolinone accumulation in lettuce leaves

2-(3H)-benzoxazolinone (BOA) is a secondary plant metabolite previously found to inhibit plant growth and development. The phytotoxic activity of BOA has been extensively demonstrated over the last years. However, the relation of BOA phytotoxicity with BOA accumulation in plant leaves has not been thoroughly investigated. In this work, BOA phytotoxicity on photosynthesis (PhiPSII and Pn) of lettuce (Lactuca sativa L. cv. Great Lakes) was studied, and these results were correlated with BOA quantities in the leaves. BOA-treated plants showed reduced photosynthesis rate 6 h after the beginning of the treatment, and the efficiency of photosystem II started to be affected 10 h after treatment. These results were correlated with an increasing concentration of BOA in leaves that starts 6 h after treatment and shows a maximum at 96 h.

Classification and regression trees (CARTs) for modelling the sorption and retention of heavy metals by soil

The sorption and retention of mixtures of heavy metals by soil is a complex process that depends on both soil properties and competition between metals for sorption sites. In this study, the sorption and retention of mixtures of Cd, Cr, Pb, Cu, Zn and Ni by a representative sample of soils from Galicia (N.W. Spain) was reproduced considerably more precisely by binary decision-tree regression models constructed using the CART algorithm than by linear regression models. Of the six metals competing for sorption sites in these experiments, Pb, Cu and Cr were sorbed and retained to a greater extent than Cd, Ni and Zn. Non-linear tree regression models constructed with CART fitted the data better than linear models, especially for Cd, Ni and Zn; and with both kinds of model the data for Pb, Cu and Cr were fitted better than those for Cd, Ni and Zn (the difference being much more marked for linear models), suggesting that the influence of soil properties on the sorption and retention of the latter three metals was limited by the preferential binding of the former three.

Degradation of fuel oil in salt marsh soils affected by the Prestige oil spill

We assessed natural degradation of fuel oil in three marshes from Galicia (Spain) affected by the Prestige oil spill (Baldaio, Barizo, and Muxía). Soil samples collected from polluted and unpolluted areas on four different dates were used to determine total petroleum hydrocarbon content and fuel-oil components. Natural degradation was monitored by analysing changes in the proportion of saturated hydrocarbons, aromatics, asphaltenes and resins in the soils, and also by evaluating the degree of depletion of saturated hydrocarbons on each sampling date. We additionally assessed the phytoremediation potential of Lolium perenne, L., Convolvulus arvensis L. and Raphanus raphanistrum L. All marsh soils exhibited natural degradation of saturated and aromatic hydrocarbons to between 85 and 95% in most cases. In contrast, asphaltenes and resins were degraded to a lesser extent (viz. 64-76% in Barizo 1, Muxía and Traba; 39-44% in Baldaio; and only 12% in Barizo 2, where flooding by the river continues to introduce balls of fuel oil into the soil). Monitoring analyses revealed natural degradation to be dependent on the thickness of the pollutant layer. Field plots sown with L. perenne L. exhibited no significant differences in fuel-oil degradation from untreated plots.

2-3H-Benzoxazolinone (BOA) induces loss of salt tolerance in salt-adapted plants

In order to test the stress hypothesis of allelopathy of Reigosa et al. (1999, 2002), the combined action of a well-established allelochemical compound (2-3H-benzoxazolinone, BOA) and a common abiotic stress (salt stress) were investigated in lettuce (Lactuca sativa L.). In a previous study (Baerson et al. 2005), we demonstrated that the primary effects of BOA are related to the expression of genes involved in detoxification and stress responses, which might serve to simultaneously alleviate biotic and abiotic stresses. Through analysis of the same physiological and biochemical parameters previously studied for BOA alone (Sánchez-Moreiras & Reigosa 2005), we observed specific effects of salt stress alone, as well as for the two stresses together (BOA and salt). This paper demonstrates that plants showing tolerance to salt stress (reduced stomatal density, increased proline content, higher K(+) concentration, etc.) become salt sensitive (markedly low Psiw values, high putrescine content, increased lipid peroxidation, etc.) when simultaneously treated with the allelochemical BOA. We also report additional information on the mechanisms of action of BOA, and general stress responses in this plant species.

The natural compound benzoxazolin-2(3H)-one selectively retards cell cycle in lettuce root meristems

Benzoxazolin-2(3H)-one (BOA) is a natural plant product that is phytotoxic to target plant species, inhibiting germination and growth and causing oxidative damage. We investigated its effects on the root meristems of seedlings of lettuce (Lactuca sativa) by means of light and transmission electron microscopy, flow cytometry, and conventional determination of mitotic index. Flow cytometry analyses and mitotic index showed a retard of cell cycle in BOA-treated meristems with selective activity at G2/M checkpoint.

Phytotoxic effects of 21 plant secondary metabolites on Arabidopsis thaliana germination and root growth

This study investigated potential phytotoxic effects on germination and root growth of 21 plant secondary metabolites (sinapinic, syringic, vanillic, ferulic, p-coumaric, chlorogenic, gallic, gentisic, protocatechuic, p-hydroxybenzoic, and trans-cinnamic acids, and eucalyptol, quercetin, vanillin, syringaldehyde, rutin, 2-benzoxazolinone, protocatechualdehyde, tyrosol, juglone, and L-mimosine) in the plant model Arabidopsis thaliana. Eleven of the 21 molecules showed significant inhibitory effects on germination, and 17 inhibited root growth. Inhibitory effects on root growth were more evident when nutrients were not added. We present dose response curves for germination effects and IC50 values for each compound, along with possible explanations of the observed inhibitory actions in terms of molecular structure.

Whole plant response of lettuce after root exposure to BOA (2(3H)-benzoxazolinone)

The goal of our work was to expand the knowledge about plant stress response to the allelochemical 2(3H)-benzoxazolinone (BOA). We focused on physiological processes that are affected by this secondary metabolite. Physiological and biochemical characteristics of plants exposed to BOA help us to better understand its mode of action and open the gate to the use of allelochemicals as "natural" herbicides. Measurements on photosynthesis, fluorescence, water relations, antioxidant enzymes (superoxide dismutase, peroxidase), ATPases, and lipid peroxidation indicated that a phytotoxic effect follows BOA exposition. This effect was intense enough to interfere with plant growth and development and to produce "induced senescence." Based on this, we propose a multifaceted mode of action for BOA with effects at different levels and in different parts of the plant.

Detoxification and transcriptome response in Arabidopsis seedlings exposed to the allelochemical benzoxazolin-2(3H)-one

Benzoxazolin-2(3H)-one (BOA) is an allelochemical most commonly associated with monocot species, formed from the O-glucoside of 2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one by a two-step degradation process. The capacity of Arabidopsis to detoxify exogenously supplied BOA was analyzed by quantification of the major known metabolites BOA-6-OH, BOA-6-O-glucoside, and glucoside carbamate, revealing that detoxification occurs predominantly through O-glucosylation of the intermediate BOA-6-OH, most likely requiring the sequential action of as-yet-unidentified cytochrome P450 and UDP-glucosyltransferase activities. Transcriptional profiling experiments were also performed with Arabidopsis seedlings exposed to BOA concentrations, representing I(50) and I(80) levels based on root elongation inhibition assays. One of the largest functional categories observed for BOA-responsive genes corresponded to protein families known to participate in cell rescue and defense, with the majority of these genes potentially associated with chemical detoxification pathways. Further experiments using a subset of these genes revealed that many are also transcriptionally induced by a variety of structurally diverse xenobiotic compounds, suggesting they comprise components of a coordinately regulated, broad specificity xenobiotic defense response. The data significantly expand upon previous studies examining plant transcriptional responses to allelochemicals and other environmental toxins and provide novel insights into xenobiotic detoxification mechanisms in plants.