Allelopathic Efficiency of Plant Extracts to Control Cyanobacteria in Hydroponic Culture

Cyanobacteria rapidly form harmful algal blooms (HABs) that cause serious nutritional imbalances in crop production via hydroponics. Allelopathic extracts from plants can be applied as a solution for ecologically sustainable control of algal blooms. In this study, the effects of 11 aqueous extracts of 10 allelopathic plants in controlling Microcystis aeruginosa were evaluated. Among the extracts, walnut husk and rose leaf extracts exhibited high inhibitory levels for efficient control of algae. High inhibitory levels were achieved owing to large amounts of water-soluble tannins, especially tannic acid. The effective extracts were applied to a hydroponic system cultivated on leafy perilla vegetables. Although the severe doses (IC90) did not guarantee complete algal control due to partial algal regrowth, walnut husk and rose leaf extracts only exerted strong persistent effects on algae control. Persistent algae inhibition contributed to the increase in perilla growth and leaf quality. Rose leaf was potentially a more useful resource for controlling algae in a hydroponic system because the application of rose leaf extract efficiently controlled the algae and was less toxic to perilla growth. In contrast, the treatment of walnut husk extract also controlled algae but inhibited perilla growth with pale green leaves.

Juglone Inactivates Pseudomonas aeruginosa through Cell Membrane Damage, Biofilm Blockage, and Inhibition of Gene Expression

Due to the strong drug resistance of Pseudomonas aeruginosa (P. aeruginosa), the inhibition effects of conventional disinfectants and antibiotics are not obvious. Juglone extracted from discarded walnut husk, as a kind of plant-derived antimicrobial agent, has the advantages of naturalness, high efficiency, and low residue, with a potential role in the inhibition of P. aeruginosa. This study elucidated the inhibitory effect of juglone on the growth of plankton and the formation of P. aeruginosa biofilm. The results showed that juglone (35 μg/mL) had an irreversible inhibitory effect on P. aeruginosa colony formation (about 107 CFU/mL). The integrity and permeability of the cell membrane were effectively destroyed, accompanied by disorder of the membrane permeability, mass leakage of the cytoplasm, and ATP consumption. Further studies manifested that juglone could induce the abnormal accumulation of ROS in cells and block the formation of the cell membrane. In addition, RT-qPCR showed that juglone could effectively block the expression of five virulence genes and two genes involved in the production of extracellular polymers, thereby reducing the toxicity and infection of P. aeruginosa and preventing the production of extracellular polymers. This study can provide support for the innovation of antibacterial technology toward P. aeruginosa in food.

Phenolic Response to Walnut Anthracnose (Ophiognomonia leptostyla) Infection in Different Parts of Juglans regia Husks, Using HPLC-MS/MS

This study compares the individual phenolic response of husk tissues of Juglans regia L., infected to different degrees of severity with walnut anthracnose, which is one of the most serious and widespread walnut diseases worldwide. A comparison among three differently susceptible cultivars, ‘Franquette’, ‘Milotai 10’ (‘M10’), and ‘Milotai intenziv’ (‘M10-37’), is made. In our methodology, high performance liquid chromatography coupled with mass spectrometry is used to identify and quantify the compounds. Our results show that flavanols, flavonols, and naphthoquinones account for more than 95% of the phenolic compounds identified in the walnut husk. The higher total analyzed phenolic content in tissues is more affected by walnut anthracnose confirmed that phenolics play a major role in the plant’s response against pathogens. A difference between cultivars is observed, since French cultivar ‘Franquette’ responds differently to walnut anthracnose infection than Hungarian cultivars ‘M10’ and ‘M10-37’. Naphthoquinones and flavanols have a very similar response to walnut anthracnose infection. The resistance of cultivars may be due to the reaction time of the plant and the speed with which it recognizes the pathogen and responds quickly to the infection by containing it while it has not yet spread. Flavonols may be the most important phenolic compounds in disease control, since they respond more rapidly to infection than flavanols and naphthoquinones. They also play an inhibitory role in the early stages of viral and bacterial infections.

The Analysis of Phenolic Compounds in Walnut Husk and Pellicle by UPLC-Q-Orbitrap HRMS and HPLC

Husk and pellicle as the agri-food waste in the walnut-product industry are in soaring demand because of their rich polyphenol content. This study investigated the differential compounds related to walnut polyphenol between husk and pellicle during fruit development stage. By using ultra-high performance liquid chromatography-quadrupole-orbitrap (UHPLC-Q-Orbitrap), a total of 110 bioactive components, including hydrolysable tannins, flavonoids, phenolic acids and quinones, were tentatively identified, 33 of which were different between husk and pellicle. The trend of dynamic content of 16 polyphenols was clarified during walnut development stage by high-performance liquid chromatography (HPLC). This is the first time to comprehensive identification of phenolic compounds in walnut husk and pellicle, and our results indicated that the pellicle is a rich resource of polyphenols. The dynamic trend of some polyphenols was consistent with total phenols. The comprehensive characterization of walnut polyphenol and quantification of main phenolic compounds will be beneficial for understanding the potential application value of walnut and for exploiting its metabolism pathway.

Preliminary results about the efficacy of abamectin trunk injection against the walnut husk fly (Rhagoletis completa)

The walnut husk fly (Rhagoletis completa) is one of the main pests affecting common walnut in both Europe and America. This work examines the effects of abamectin on the development of walnut husk fly larvae by injecting the product Vertimec 1.8 EC (Syngenta) into the trunks of walnut trees in Hungary. In the case of properly injected trees, the infection rate was negligible and the abamectin content in the husk samples ranged between 1.54 and 3.00 ng/g; controls show a very high (> 90%) infestation rate and an abamectin content under the detection limit. Abamectin residue measured in walnut kernel did not exceed the maximum residue limit; moreover, the active ingredient content was below the detection limit (0.0003 mg/kg) in all the collected kernel samples. Our results confirm that trunk injection is a viable method for walnut pest control.