Soil microorganisms interacting with residue-derived allelochemicals effects on seed germination

Dynamic response of allelopathic potency of Taxus cuspidata Sieb. et Zucc. mediated by allelochemicals in Ficus carica Linn. root exudates

In a mixed forest, certain plants can release allelochemicals that exert allelopathic effects on neighboring plants, thereby facilitating interspecific coexistence of two species. Previous studies have demonstrated that allelochemicals released from Ficus carica Linn. roots in mixed forest of F. carica and Taxus cuspidata Sieb. et Zucc. has phase characteristics over time, which can improve the soil physicochemical properties, enzyme activity and microbial diversity, thus promoting the growth of T. cuspidata. Based on the irrigation of exogenous allelochemicals, changes in soil fertility (soil physical and chemical properties, soil enzyme activity and soil microelement content) were observed in response to variations in allelochemicals during five phases of irrigation: initial disturbance phase (0-2 d), physiological compensation phase (2-8 d), screening phase (8-16 d), restore phase (16-32 d) and maturity phase (32-64 d), which was consistent with the response of soil microorganisms. The allelopathic response of growth physiological indexes of T. cuspidata, however, exhibited a slight lag behind the soil fertility, with distinct phase characteristics becoming evident on the 4th day following irrigation of allelochemicals. The findings demonstrated that the allelochemicals released by the root of F. carica induced a synergistic effect on soil fertility and microorganisms, thereby facilitating the growth of T. cuspidata. This study provides a comprehensive elucidation of the phased dynamic response-based allelopathic mechanism employed by F. carica to enhance the growth of T. cuspidata, thus establishing a theoretical basis for optimizing forest cultivation through allelopathic pathways.

Seed germination ecology of endangered plant Horsfieldia hainanensis Merr. In China

BACKGROUND

Horsfieldia hainanensis Merr., an indicator species of China's humid tropical rainforests, is endangered due to difficulties with population regeneration. In this study, the biological characteristics and germination adaptability of the seeds were studied for the first time, in order to provide a basis for analyzing the causes of endangerment and strategies for the artificial cultivation of H. hainanensis. The effects of biological characteristics (population, arils, seed coat, seed weight, seed moisture content) and environmental factors (temperature, light, drought, substrate, burial depth) on seed germination and seedling growth of H. hainanensis were studied.

RESULTS AND DISCUSSION

The fruits were found to be capsules containing seeds wrapped in a pericarp and fleshy aril, which provide protection and assist in seed dispersal, but also pose risks to the seeds, as the peel and fleshy aril can become moldy under high temperature and humidity conditions. There were significant differences in fruit morphology and germination characteristics among different populations, and the seed quality of populations in Niandian village, Daxin County, Chongzuo City, Guangxi Zhuang Autonomous Region was better. The arils significantly inhibited seed germination, the germination of large seeds was better, and seedling growth from medium seeds was superior. H. hainanensis seeds were sensitive to dehydration, and intolerant to drought and low temperature, which is typical of recalcitrant seeds. The seeds are suitable for germination on a moist substrate surface with good water retention and breathability at 30-35℃.

The potential for synthesized invasive plant biochar with hydroxyapatite to mitigate allelopathy of Solidago canadensis

Few studies tried to explore the mitigation effect and underlying mechanisms of biochar and their complex for negative allelopathy from invasive plants, which may provide a new way in the invasive plant management. Herein, an invasive plant (Solidago canadensis)-derived biochar (IBC) and its composite with hydroxyapatite (HAP/IBC) were synthesized by high temperature pyrolysis, and characterized by scanning electron microscopy, energy dispersion spectrometer, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Then, both the batch adsorption and pot experiments were conducted to compare the removal effects of kaempferol-3-O-β-D-glucoside (C21 H20 O11 , kaempf), an allelochemical from S. canadensis, on IBC and HAP/IBC, respectively. HAP/IBC showed a stronger affinity for kaempf than IBC due to its higher specific surface area, more functional groups (P-O, P-O-P, PO4 3- ), stronger crystallization [Ca3 (PO4 )2 ]. The maximum kaempf adsorption capacity on HAP/IBC was six times higher than on IBC (10.482 mg/g > 1.709 mg/g) via π-π interactions, functional groups, and metal complexation. The kaempf adsorption process could be fitted best by both pseudo-second-order kinetic and Langmuir isotherm models. Furthermore, HAP/IBC addition into soils could enhance and even recover the germination rate and/or seedling growth of tomato inhibited by negative allelopathy from the invasive S. canadensis. These results indicate that the composite of HAP/IBC could more effectively mitigate the allelopathy from S. canadensis than IBC, which may be a potential efficient approach to control the invasive plant and improve invaded soils.

Exploring the potential of halotolerant bacteria from coastal regions to mitigate salinity stress in wheat: physiological, molecular, and biochemical insights

Salinity stress, a significant global abiotic stress, is caused by various factors such as irrigation with saline water, fertilizer overuse, and drought conditions, resulting in reduced agricultural production and sustainability. In this study, we investigated the use of halotolerant bacteria from coastal regions characterized by high salinity as a solution to address the major environmental challenge of salinity stress. To identify effective microbial strains, we isolated and characterized 81 halophilic bacteria from various sources, such as plants, rhizosphere, algae, lichen, sea sediments, and sea water. We screened these bacterial strains for their plant growth-promoting activities, such as indole acetic acid (IAA), phosphate solubilization, and siderophore production. Similarly, the evaluation of bacterial isolates through bioassay revealed that approximately 22% of the endophytic isolates and 14% of rhizospheric isolates exhibited a favorable influence on seed germination and seedling growth. Among the tested isolates, GREB3, GRRB3, and SPSB2 displayed a significant improvement in all growth parameters compared to the control. As a result, these three isolates were utilized to evaluate their efficacy in alleviating the negative impacts of salt stress (150 mM, 300 mM, and seawater (SW)) on the growth of wheat plants. The result showed that shoot length significantly increased in plants inoculated with bacterial isolates up to 15% (GREB3), 16% (GRRB3), and 24% (SPSB2), respectively, compared to the control. The SPSB2 strain was particularly effective in promoting plant growth and alleviating salt stress. All the isolates exhibited a more promotory effect on root length than shoot length. Under salt stress conditions, the GRRB3 strain significantly impacted root length, leading to a boost of up to 6%, 5%, and 3.8% at 150 mM, 300 mM, and seawater stress levels, respectively. The bacterial isolates also positively impacted the plant's secondary metabolites and antioxidant enzymes. The study also identified the WDREB2 gene as highly upregulated under salt stress, whereas DREB6 was downregulated. These findings demonstrate the potential of beneficial microbes as a sustainable approach to mitigate salinity stress in agriculture.

Effects of 13C isotope-labeled allelochemicals on the growth of the invasive plant Alternanthera philoxeroides

The secondary metabolites of indigenous plants have significant allelopathic inhibitory effects on the growth and development of invasive alien plants. Methyl palmitate (MP) and methyl linolenate (ML) were used as exogenous allelopathic substances. The research investigated the differences of inhibitory effects of MP and ML on the growth of seedlings of Alternanthera philoxeroides, and calculated their morphological characteristics, biomass, physiological indicators and the response index (RI). The synthetical allelopathic index (SE) of 1 mmol/L MP was the smallest (- 0.26) and the allelopathic inhibition was the strongest; therefore, it was selected as a 13C-labeled allelochemical. The distribution of 1 mmol/L MP in different parts of A. philoxeroides and the correlation between the biomass ratios of roots, stems and leaves and the 13C content were studied by 13C stable isotope tracing experiments. Atom percent excess (APE) between roots, stems and leaves of A. philoxeroides treated with 1 mmol/L MP were significantly different in terms of magnitude, with leaves (0.17%) > roots (0.12%) > stems (0.07%). The root, stem and leaf biomass ratios of invasive weeds had great significant positive correlation with 13C content (p < 0.01, R2 between 0.96 and 0.99). This current research provides a new idea and method for the control of A. philoxeroides, but large-scale popularization remains to be studied.

Insights into the impacts of autotoxic allelochemicals from rhizosphere of Atractylodes lancea on soil microenvironments

Atractylodes lancea suffers from continuous cropping obstacles that have become a major constraint in its cultivation, but there is still little information on the autotoxic allelochemicals and their interaction with soil microorganisms. In this study, we firstly identified the autotoxic allelochemicals from rhizosphere of A. lancea and determined their autotoxicity. Third-year continuous A. lancea cropping soils, i.e., rhizospheric soil and bulk soil, compared with control soil and one-year natural fallow soil were used to determine soil biochemical properties and microbial community. Eight allelochemicals from A. lancea roots were detected and exhibited significant autotoxicity effects on seed germination and seedling growth of A. lancea with the highest content of dibutyl phthalate in rhizospheric soil and lowest IC₅₀ value of 2,4-di-tert-butylphenol inhibiting seed germination. The contents of soil nutrients and organic matter, pH value, and enzyme activity were altered between different soils, and the parameters of fallow soil were close to those of the unplanted soil. The PCoA analysis indicated that the community composition of both bacteria and fungi were differed significantly among the soil samples. Continuous cropping decreased OTUs numbers of bacterial and fungal communities, and natural fallow restored them. The relative abundance of Proteobacteria, Planctomycetes, and Actinobacteria decreased, and that of Acidobacteria and Ascomycota increased after three years cultivation. The LEfSe analysis identified 115 and 49 biomarkers for bacterial and fungal communities, respectively. The results suggested that natural fallow restored the structure of soil microbial community. Overall, our results revealed that autotoxic allelochemicals caused the variations of soil microenvironments and resulted in replantation problem of A. lancea, and natural fallow alleviated the soil deterioration by remodeling the rhizospheric microbial community and restoring soil biochemical properties. These findings provide important insights and clues for solving the continuous cropping problems and guiding the management of sustainable farmland.

Changes in the Soil Fungal Community Mediated by a Peganum harmala Allelochemical

Plants can release phytotoxic allelochemicals into the environment, not only to suppress other plants’ growth, but also alter community structure of soil microbiota, however, the mechanism are often complicated. We designed a consecutive cultivation procedure to evaluate the allelopathic effect of harmaline, the major active allelochemical produced by the desert plant Peganum harmala, on soil microorganisms. Harmaline was added to the soil at 20 μg/g, and after five generations of cultivation, the Chao1, Pielou, Shannon and Simpon indexes changed significantly. In particular, the relative abundances of the dominant fungi, Alternaria sp. and Fusarium sp., declined drastically by 84.90 and 91.90%, respectively. Further in vitro bioassays confirmed that harmaline indeed suppressed growth of 6 Alternaria and Fusarium strains isolated from P. harmala rhizosphere soil. We thus suspect that P. harmala might produce harmaline as an effective carry-on pesticide to defend against general pathogens such as Alternaria sp. and Fusarium sp. and favor itself. Our consecutive cultivation procedure has successfully magnified the core signals from the chaotic data, implying that it can be applied to measure the effects of other allelochemicals on soil microbiota.