Chestnut (Castanea spp., Fagaceae family) is an economically important tree in the wood processing industry that occurs in Asia, Europe, Africa, and the Americas ^[1][2][3][1,2,3]. There are four most important and cultivated chestnut varieties: C. molissima (Chinese chestnut) and C. crenata (Japanese chestnut) are distributed in Asia; C. sativa is distributed in continental Europe (European chestnut); and Castanea dentata is distributed in North America (American chestnut) [4]. Chestnuts are a source of nuts and industrial raw materials, including wood, that can be used as firewood, as well as to build timber and barrels for winemaking. In recent years, extracts from chestnut shells (e.g., tannins, polyphenols, and polysaccharides), female flower, and spring buds have been applied in the medical, pharmaceutical, and healthcare fields ^[5][6][7][8][5,6,7,8]. Discarded chestnut shells, inner shells, burs, and leaves have been re-utilized as biomass and catalyst material ^[9][10][9,10]. The important economic and environmental roles of chestnut underlie its high value to ecosystems and agroforestry systems ^{[11][12][13][14]}[11,12,13,14].

Chestnut plantations in Europe and North America are vulnerable to diseases caused by Cryphonectria parasitica and Phytophthora cinnamomi and damage from pests, such as Dryocosmus kuriphilus Yasumatsu (Hymenoptera, Cynipidae) ^{[15][16][17][18][19]}[15,16,17,18,19]. Diseases and pests pose a major threat to the production of chestnut plantations and related ecological systems. Nevertheless, global chestnut production has continually increased over the past 15 years, and this global increase is mainly driven by increases in chestnut production in China, which contributes nearly 90% of the world’s chestnut production [20]. Increases in chestnut production in China have largely been driven by government policies, scientific fertilization and planting technology, the development of new varieties and agricultural tools, and increases in the skills of farmers.

2. Iatrical Benefits and Any Other Functions

Glucan extracted from Castanea mollissima fruit and its selenylation modification derivative have strong antiproliferative effects and could potentially be used as an antitumor drug ^[21][33]. Castanol B (1), a new phenol from a water-soluble extract of shells of Castanea mollissima, suppresses the growth of hepatoma cells and induces cell apoptosis by reducing the levels of TLR4, IKKβ, and NF-κB ^[22][34]. CMP90 is a new water-soluble polysaccharide isolated from Castanea mollissima Blume that inhibits the proliferation of HL-60 cells and the growth of S180 solid tumors [8]. Selenium nanoparticles decorated with 1,6-α-D-glucan (isolated from the fruits of Castanea mollissima Blume) have an anti-proliferative effect on HeLa cells in vitro by inducing apoptosis and S phase arrest ^[23][32]. All extracts from the chestnut shell, inner shell, bur, and leaves exhibit antioxidant activity and inhibit the growth of all Gram-positive and two Gram-negative bacteria ^[24][35]. Ellagic acid and chestanin extracted from the burs of Castanea sativa Miller inhibit the growth of Alternaria alternata and Fusarium solani by inhibiting mycelial growth and spore germination ^[25][36]. It is also reported that chestnut male flower has both excellent antioxidant and antimicrobial properties ^[26][27][37,38]. Portuguese delicacy “pastel de nata” with added extracts of Castanea sativa male flower had higher activity of reducing agents and radical scavengers two days after baking ^[28][39]. The leaf extract of Castanea sativa Mill. reduces the level of malondialdehyde when human sperm samples are treated with H₂O₂ ^[29][40] and lowers serum cholesterol levels in mice ^[30][41]. A new report also showed that 30 g resistant starch had a significant protective role against non-colorectal cancers for patients, and chestnut is rich in resistant starch ^[31][42]. Extracts obtained from chestnut are hypoglycemic and show antioxidant activity, and this mitigates the negative effects of diabetes on the liver ^[32][43]. Bioactive compounds extracted from chestnut shells and bur, such as polyphenols, minerals, lignin, and vitamins, are widely used in the food and medicine industries. Polyphenols extracted from chestnut are considered to be beneficial bioactive compounds in food production and preservation ^[33][34][44,45]. C. mollissima kernel is rich in vitamin C and vitamin E, which are beneficial in humans’ daily diet in reducing the diseases of scurvy and cancer ^[14][35][14,46]. Various macroelements (such as K, P, Ca, and Mg) and microelements (such as Zn, Fe, and Cu) are contained in chestnut kernel, which are beneficial minerals to maintain health ^[36][37][47,48].

3. Industrial Production and Technology

3.1. Chestnut Products

Instead of burning waste chestnut shell, many studies have subjected them to biorefinery processes. The use of 5% (w/v) activated charcoal can yield 70.3% (w/v) phenolic compounds, and the phenol radical scavenging activity is higher in the first activated charcoal eluate than in crude chestnut shell hydrolyzate [9]. Castanea mollissima shell can be used in various reactions (e.g., propane dehydrogenation), and it shows high catalytic performance and is low in cost ^[38][49]. In vitro digestion has shown that the total phenol content and antioxidant activity are higher in the outer/inner skin of digested chestnut than in undigested chestnut, and this could be used as a source of raw materials for antioxidant-rich active substances [5].

3.2. Improvements in Industrial Technologies

High amounts of tannins are present in chestnut wood and bark, and these tannins are widely used in the wood, culinary, and medicinal industries; they are also used as a fuel, as an additive in animal fodder, and for tanning leather ^[6][26][6,37]. Experiments of the hydrothermal hydrolysis of sweet chestnut tannins have revealed that 250 °C for 30 min are the optimal conditions for maximizing the content of total tannins and phenols [6]. Optimized choline chloride-based deep eutectic solvents can be used to extract and recover ellagic acid from waste chestnut shells [7]. The content of flavonoids and tannins from male chestnut flowers is maximized when the following optimal conditions of ultrasonic-assisted extraction are used: 24 ± 3 min, 259 ± 16 W, and 51 ± 7% ethanol ^[39][50].

4. Food Science

4.1. Beneficial Extractions from Chestnut

The glycemic index is negatively correlated with the relative crystallinity, which indicates that the starch crystalline structure has a retardant effect on digestibility ^[40][51]. Various factors, such as the drying temperature and the order of long-range, short-range, and other molecular compounds in chestnut, affect the digestibility of starch ^[41][52]. Animals fed with ethylenediaminetetraacetic acid, a tannin-rich extract from chestnut, promote pig fattening by increasing the body weight, average daily weight gain, and the feed-to-gain ratio of pigs ^[42][53]. The addition of chestnut to the diet increases the intramuscular fat content of the biceps femoris muscle of pigs compared with commercial feed; a diet with chestnut also alters the content of some volatile compounds ^[43][54]. The organic acid composition is affected by hot air convective drying treatment, and the effect depends on the chestnut variety ^[44][55]. Soy protein isolate films containing chestnut bur extract might provide an effective packaging and preservation method given that antioxidant activity is increased in films with chestnut bur extract ^[45][56]. A blend of chestnut and wheat flour or rice flour can be used to generate a high-quality paste ^[46][57]. According to different authoresearchs, C. mollissima bur and inner shell are remarkable sources of bioactive ingredients. C. mollissima bur is rich in phenolic acids, flavonoids, and total tannin ^[47][48][58,59]. C. mollissima inner shell contains multiple polyphenolic components, which have a beneficial health care function to the human body ^[49][60]. Chestnut shells are optimized for alkaline delignification to co-produce ligin and bio-ethanol ^[50][61]. Flavonoids procyanidin B3, quercetin-3-O-glycoside, and steroidal sapogenins extracted from C. mollissima shells are natural resource pigments, which are natural additives in production in the food industry ^[51][62].

4.2. Postharvest Biology and Technology

The postharvest quality of chestnut has been a major focus of research in recent years. Chestnuts treated by high-pressure processing (400, 500, and 600 MPa for 5 min at 20 °C) have prolonged storage times, fewer molds and insect larvae, and nuts with higher quality ^[52][63]. Chestnuts coated with chitosan have lower abundances of microorganisms under refrigeration (0 °C, 90% HR) after 6 months compared with control chestnuts ^[53][64]. Chitosan nanoparticles loaded with thymol coating reduce the decline in the soluble sugar and starch content of chestnut and inhibit the growth of mold and yeast ^[54][65].

5. Soil, Fertilizer, and Endophytic Fungi

5.1. Soil and Fertilizer Conditions

The optimal soil conditions for chestnut are deep, soft, and original volcanic soil rich in phosphorus, potassium, and organic matter ^[55][66]. The content of most macronutrients (e.g., total nitrogen, available phosphorus, potassium, and calcium) in chestnut leaves and orchards is low, which indicates that chestnut trees could benefit from fertilization ^[56][67]. Nine- and ten-year-old chestnut trees are largest when they are fertilized with suitable amounts of N, P, and K fertilizer ^[57][68]. The vertical distribution of plant nutrients under a balanced fertilization regime (N₂:P₁:K₂) is affected by soil type (e.g., loam, clay loam, and sandy loam) ^[58][69]. Different levels of N and P application increase the content of some elements, such as N and Cu, but decrease the content of other elements (e.g., Fe and Mn under N application, and K, Ca, and Mg under P application) in the leaves of chestnut ^[59][70]. The N concentration in chestnut leaves and the average nut yield are higher under lime application plus a compound NPK fertilizer than lime plus phosphorus fertilizer and unfertilized control orchards ^[60][71]. Potassium silicate application increases the accumulation of phytoliths in the leaf, cell wall, and xylem and enhances the tolerance of plants to high temperature by increasing the efficiency of photosystem II and the content of photosynthetic pigments ^[61][72]. Regardless of the sampling period, the concentration of boron in leaves is the factor most strongly correlated with chestnut productivity, and leaves are the most useful tissue for the early diagnosis of boron deficiency during the bloom period ^[62][73]. The conversion of native forest to Chinese chestnut plantations increases the total nitrogen stock but decreases carbon storage in soil following intensive management ^[63][74].

5.2. Endophytic Fungi of Chestnut Trees

Trees can obtain their nutrients through mycorrhizae and nutrient recycling. The fitness and productivity of chestnuts are affected by different types of fungi, and ectomycorrhizal fungi (EMF) affect both the roots and above-ground parts of plants ^[64][65][66][75,76,77]. The symbionts of EMF and chestnut contribute to the strength of the roots, including their ability to deeply penetrate the soil, which enhances the growth of chestnut trees. Various planting methods have been used to increase the growth and survival of EMF to promote EMF root colonization and the restoration of chestnuts ^[67][68][78,79]. The EMF species Scleroderma spp., Laccaria spp., and Cenococcum geophilum can disperse and colonize new habitats rapidly, including young chestnut trees ^[69][80], and Amanita, Boletus, Cantharellus, Cortinarius, Lactarius, Russula, and Tricholoma are dominant in mature trees ^[70][81]. Chestnut blight induces decreases in the rate of photosynthesis, EMF colonization, and species diversity, and blight does not have an effect on the parent trees or neighboring five-month-old seedlings, which are more tolerant of reductions in the content of photosynthate ^[71][82]. The success of EMF colonization differs among chestnut tree species infected with P. cinnamomi and healthy ones, and EMF have more extrametrical hyphae in healthy trees than in infected trees ^[72][83]. The decrease in mycorrhizal root tip density and ECM species richness might explain the defoliation caused by Swiss needle cast disease ^[73][84]. Thus, EMF can be used to enhance the survival of chestnut when chestnut trees are reintroduced to regions vulnerable to blight outbreaks.

6. Ecological Environment of Chestnut Orchards

The ecological features of chestnuts are manifold and include soil conditions, temperature, and altitude; natural and anthropogenic factors are also important, as well as whether management is traditional or intensive ^{[2][55][74][75][76]}[2,66,100,101,102]. Compared with infested chestnut seeds, non-infested and mast seeds are cached more by rodents, which promotes seed germination and forest regeneration ^[77][103]. C. sativa is an invasive alien species on the Canary Islands that many have suggested should not be completely eradicated given that they are reservoirs of lichen biodiversity [11]. Study of the American chestnut in the southwestern portion of its historical range has revealed that a suitable habitat is present at higher elevations and in regions with higher forest canopy cover ^[78][104]. Claudia Mattioni et al. (2017) identified three main gene pools of sweet chestnut and a significant genetic barrier among populations, and this provided new insights into the biogeographic history, geographic locations of different gene pools, and priority areas possessing high genetic diversity in Europe ^[79][105]. A 30-year study has shown that the spread of chestnut in northeastern Turkey is mostly driven by temperature and total precipitation ^[80][106]. The phenology, morphometric parameters, ripening time, and average yield of different genotypes of sweet chestnut can vary among locations ^[81][107]. Compared with other co-occurring tree species, sweet chestnut has a lower survival probability because of its low shade tolerance, poor competitiveness, preference for dry conditions, and summer temperatures [12]. Latitudinal variation among populations is associated with variation in phenology and the xerothermic index; the phenology of chestnut trees in central and southern Mediterranean populations was earlier than that of the northern populations, and drought plays a key role in this pattern ^[82][108].