Walnuts are large deciduous trees in the Juglandaceae family, arguably native to some countries in the Balkan Peninsula and Central Asia [5]. Mature trees are anchored on deep root systems, with big tap roots [6] and heavy, close-grained stems that produce valuable hardwood suitable for furniture. The trunk has large brown/copper glabrous branches with characteristically many-chambered pith. They also have broad canopies of about 18-m wide and 13-m high, and elongated pinnate compound leaves of 5–23 leaflets. Walnuts are commonly propagated vegetatively with grafting and budding, including in vitro culture [7]. Sexually propagated trees take 8–12 years to mature and produce nuts with many variations [8].

Flower differentiation and anthesis start, bearing monoecious flowers [9]. After meeting the chilling requirements that vary from 400 to 1500 h in temperatures between 0 °C and 7 °C [10,11]. This occurs from mid-April for early cultivars to mid-May for late-blooming cultivars [12]. Petal-less male drooping catkins develop on older branches, and clusters of 2–5 female flowers are borne on new terminal shoots or lateral spurs [13,14]. The flowers are heterodichogamous, some cultivars are protogynous, and others are protandrous. Imperfect overlap between pollen shedding and female flower receptivity necessitates the presence of two complementary cultivars to pollinate each other [1].

Walnut fruits consist of a kernel, seed coat, shell, and pericarp, commonly called green husk [15]. The semi-fleshy green pericarp covers a hard corrugated shell enclosing a four-celled edible nut. Walnut husks are composed of the epicarp, mesocarp, and endocarp supplying nutrients for the seed’s growth [16]. The pericarp dehisces allow the seed to fall upon the maturity of nuts. The walnut seed contains two cotyledons enclosed in the shell with a thin lining called the pellicle. Selections of English walnuts are diverse, ranging from round to very elongated shapes, pea-sized to more than 50-mm shell diameter [8].

Physical and phytochemical studies show significant kernel variations across walnut cultivars. Walnut fruits come in different dimensions, shapes, and kernel appearances, and have different content of chemical compounds. The main physical quality features are kernel weight and colour, shell shape and size, kernel ratio, and cracking ratio [17]. Others include sphericity, porosity, volume, bulk and true densities, coefficient of friction, and terminal velocity [18,19].

Walnuts contain numerous phytochemical constituents such as polyphenols, fatty acids, mineral elements, vitamins, digestible proteins, amino acids, carbohydrates, and dietary fibre in their bark, roots, leaves, and fruits [44,45,46,47,48,49].

Polyphenolic compounds are secondary metabolites characterized by phenolic rings synthesized by plants as a defence against stress factors such as radiation, pest, and diseases. Classification of these compounds is based on their chemical structure. They include flavonoids, phenolic acids, stilbenes, lignans, and others [50,51]. A total of 83 phenolic compounds were identified in walnut husks, mainly consisting of naphthoquinones, flavonols, and hydroxycinnamic acids [52]. A related study by Medic et al. [53] identified and quantified 91 individual phenolic compounds. Forty-one of these were in root tissues, and others in petiole, bark, buds, and leaf tissue. They comprised 28 hydroxybenzoic acids, 22 naphthoquinones, 20 flavonols, 11 flavanols, 8 hydroxycinnamic acids, and 2 coumarins.

Kernels are rich in monounsaturated and polyunsaturated fatty acids such as alpha-linolenic acid (ALA), an omega-3 fatty acid, and linoleic acid (LA), an omega-6 fatty acid [68], oleic acid, and small amounts of saturated fatty acids. Studies show that its oil content ranges from 45.6% to 79.4% [5,15,18,45,49,69,70], with unsaturated fatty acids, tocopherols, and phytosterols being the most dominant. The main fatty acids present in walnuts include linoleic, oleic, linolenic, palmitic, and stearic acids [71,72,73,74,75]. Palmitoleic acid, arachidic acid, and eicosenoic acid are present in small amounts [72].

Kernels contain a significant amount of protein ranging from 10.6 to 25.0% [5,18,47,70,76]. Gu et al. [77] reported 40.0–45.0% protein content in J. sigillata kernels. Glutelin is the most abundant protein (72.1%); others (globulin, albumin, and prolamin) are present in smaller amounts [47]. These proteins come with various free amino acids, notably alanine, arginine, and glutamate [78].

Other biochemicals present in walnut kernels include carbohydrates (5.0–24.0%); minerals (phosphorus, potassium, and traces of sulphur); micro-elements (Ca, Mg, Fe, Cu, Zn, and I); vitamins: E, C, B1, B2, and A [5]; dietary fibre [49].

The green husk is a fleshy outer layer enclosing the shell of a nut [79]. Upon nut ripening, the husk gradually darkens. As a by-product, it is considered waste upon harvesting [13]. Sebahattin et al. [80] report that the average fresh husks-biomass to total walnut-biomass ratio is 57.15%. Husks are therefore produced in large amounts, and poor disposal lead to environmental pollution. However, they are a natural source of bioactive compounds, which can be used for diverse purposes.

Similar to other parts of the tree, numerous bioactive polyphenolic compounds associated with antioxidant and antimicrobial properties have been identified in green husks [81]. These compounds, as secondary metabolites, cover a large group of heterogeneous bioactive compounds. Their synthesis is regulated by different enzymes in different metabolic pathways, and they have several roles in fruit growth processes. The amount of these compounds depends on the genotype, environmental and climatic conditions as well as geographical conditions. Furthermore, it depends on the development stage of the fruit. It is necessary to follow up on these changes and consider the date of extraction when the amount reaches its maximum concentration [82]. The open green husks have higher antioxidant capacity and polyphenol content than the closed ones [83].

The basis of walnut husk utilization could be the high antioxidant activity of these phenolic compounds. The isolated and identified phenolic compounds are divided into different groups according to their chemical structure. Ellagic acid and tannic acid belong to hydrolyzable tannins, both present in walnut husks. Naphthoquinones, naphthoquinone glycosides, and naphthalenes are also isolated in green husks and widely studied. Juglone, the most important phenolic compound in the husk, is a naphthoquinone. Derivatives of juglone are also present. Other compound groups are α-tetralones, α-tetralones glycosides, and α-tetralones dimers. The most significant hydroxybenzoic acids in husk are gallic acid, protocatechuic acid, syringic acid, vanillic acid, salicylic acid, 3,4-dihydroxybenzoic acid and 2,3-dihydroxybenzoic acid, and benzoic acid. Caffeic acid, ferulic acid, chlorogenic acid, p-coumaric acid, sinapic acid, chlorogenic acid, and trans-ferulic acid are also characterized in green husks, which make the group of hydroxycinnamic acids. Flavonoids are represented in the husk by (+)-catechin, (−)-epicatechin, myricetin, quercetin, sudachitin, cirsilineol, and 5,6,4′-trihydroxy-7,3′-dimethoxy-flavone, apigenin, eriodictyol, kaempferol, rutin. Other secondary metabolites, such as diarylheptanoids, are isolated and identified as well, e.g., rhoiptelol, juglanin A, juglanin B, and juglanin C [15].

The choice of extraction solvent is a critical consideration because of its chemical variability and complexity. Many research groups have investigated the effects of different extraction solvents and how the development stage affects the content of phenolics as well. A negative correlation was established between growth and the concentration of the phenolic compounds [82]. According to the investigation by Zhang [84], extraction by methanol, ethanol, and acetone shows the highest content of phenolic compounds and antioxidant activity compared to ethyl-acetate and water. Hexane yielded the least number of phenolic compounds. Fernandez-Agulló et al. [81] investigated the extraction yield of methanol, ethanol, methanol/water 50/50, and ethanol/water 50/50. It was established that ethanol/water 50/50 resulted in higher bioactivity and that the antioxidant properties depend on the concentration of the extraction solvent. In a related study, Hama et al. [85] used methanol (80%) for phenolic extraction with chloroform, ethyl acetate, and n-butanol. Ethyl acetate yielded the highest amount of phenolics, followed by chloroform and n-butanol, respectively.

A comparative study of antioxidant activity and individual phenolic compounds in green husk extracts using three different solvents (70% ethanol, 40% ethanol, and 40% ethanol/sugar 50/50) was performed by Cosmolescu and co-workers [86]. In the case of 70% ethanol, it was found that the concentrations of gallic, vanillic, chlorogenic, caffeic, syringic, salicylic, ellagic acids, juglone, catechin, epicatechin, myricetin, and quercetin were the highest. In the case of 40% ethanol, ferulic acid and rutin were more, compared to the other compounds mentioned above. Forty per cent ethanol and sugar (the traditional way of walnut liqueur production) resulted in the highest amount of rutin. Jakopic and co-workers [87] examined the effect of different ethanol concentrations on total phenolic concentration and the number of individual compounds. Generally, they found that increasing ethanol concentration resulted in increased phenolic concentrations. However, in some cases (gallic, chlorogenic, vanillic, and syringic acid, (+)-catechin, and juglone), they achieved better extraction yield with 40% ethanol. In another study, methanol gave better extraction for juglone, (+)-catechin, gallic, protocatechuic, and chlorogenic acids, whereas ethanol resulted in higher amounts of ellagic and sinapic acids [88].

Barekat et al. [89] established that phenolic compounds in walnut husks could have significant antioxidant power ranging from 256.5 to 746.8 score g⁻¹ dry weight (DW) by using the PAOT (total antioxidant power) method. This method uses two specific electrodes to measure the changes in the electrochemical potential in the reaction medium. These changes are in correlation with the antioxidant properties, and results are expressed in PAOT score (total antioxidant power) per gram of dw walnut husk. The identified phenolic compounds include tannins, flavonoids, stilbenes, lignans, quinones, diarylheptanoids, and phenolic acids [44,51,65,90].

Ghasemi et al. [95] observed variances in phenolic and flavonoid content in husks with changes in altitude and temperature. A positive correlation between total phenolic and flavonoid content with altitude was observed; however, the correlation with the temperature was negative [95]. Ghesami’s team attributed the significant differences in phenolic and flavonoid content to the differences in geographical and climatic conditions where samples were obtained.

Mikulic-Petkovsek et al. [98] observed a decline in total phenolic content concerning the physiological stage of the nuts. However, it was noted that the biosynthesis of phenolic compounds in healthy plants may be triggered by pathological infections. Xanthomonas arboricola pv. juglandis infected tissues of the husk exhibited higher content of hydroxycinnamic acids, gallic acid, quercetin, and catechin than uninfected tissues. Similar observations were made by Sheng et al. [44]. The total phenolic and flavonoid content in husks declined with increased maturity to subsequent ripening of the walnut fruit. The differences in phenolic content reported by the research groups [44,98] may be related to the sample origin, the genotype, the ripening status of the samples, the applied solvent, and measurement methods.

Despite being underutilized, green husks remain an important agricultural by-product and a natural source of bioactive compounds. Like other parts of the walnut tree, the green husk has been used in traditional medicine to relieve pain and treatment of cancer, diabetes, microbial infections and skin and heart diseases [15,79]. The walnut green husk is a source of glucans and pectins, which are useful in human pathology treatment and induce defence mechanisms to respond to wounding [15]. The husks contain juglone, which is a natural dye used in the textile and wood industry, as well as tanning leather [83,99]. Allelopathic compounds in husks serve as pesticides and herbicides [100,101]. It is also important to note that green husks are used in making wine, traditional liqueur, and jam [102,103], whereas green nuts are used in pharmaceuticals and cosmetics [81].

The numerous bioactive compounds present in walnut husks are associated with both positive and negative effects on abiotic and biotic factors such as soil, water, microorganisms, and insects, including herbivores. They play a significant role in human, animal, and plant health because these compounds have antioxidant and antiradical properties. They exhibit anticarcinogenic, antimutagenic, anti-inflammatory, cardioprotective, astringent, antiseptic, anthelmintic, and antiaging effects that are beneficial to human and animal health [45,104,105,106,107,108].

According to the literature, these compounds have antimicrobial and antifungal properties that prevent and control plant diseases [109]. They inhibit the initiation and progress of X. arboricola pv. juglandis, the walnut blight disease-causing bacteria [3] and pathogenic fungi such as Botrytis cinerea, Alternaria alternata, Fusarium culmorum, Rhizoctonia solani, and Phytophthora cactorum [110]. Both 4-coumaric acid and p-hydroxybenzoic acid, at minimum concentrations of 1 gL⁻¹ and 2 gL⁻¹, respectively, inhibit the growth of Colletotrichum gloeosporioides fungi responsible for walnut anthracnose disease [109]. Maleita et al. [111] established that at 50 ppm, 1,4-Naphthoquinone can cause 42% mortality in the juvenile root-knot nematode (Meloidogyne hispanica), thus a potential alternative to synthetic nematicides. Walnut and tea intercrops improved the availability of soil micronutrients such as potassium, nitrogen, phosphorus, and organic matter, including enhanced microbial diversity [112].