Horticultural Plants and By-Products as Sources of Biological Active Compounds

1. Introduction

Horticultural plants and their by-products from vegetable, herb and fruit cultivation, as well as from food industry operations, warehouses, and retail trade, are abundant in biologically active compounds such as polyphenols, flavonoids, carotenoids, vitamins and minerals [1,2]. They can be a rich source of essential nutrients, antioxidants, anti-infectious and anti-cancer agents, immunomodulators, prebiotics, etc., providing substantial nutritional and medicinal benefits, thus promoting human health and preventing chronic diseases [3,4,5,6,7]. The growth in the processed food market has significantly expanded the fruit and vegetable processing industry, leading to increased waste. Through innovative post-processing techniques, this waste can be converted into high-value active principles and biomaterials, which are used in industries like food, including functional foods and nutraceuticals, pharmaceuticals, cosmetics, and packaging [8,9]. This not only adds value to the waste but also supports the development of circular economy practices [10,11,12,13,14,15,16].

This Special Issue aims to showcase the innovative uses and benefits of biologically active compounds extracted from horticultural plants and their by-products, with a focus on recent advancements in processing methods. Fifteen original articles and one review were published, categorized based on themes such as phytochemical composition and antioxidant activity, extraction techniques, biological properties and health applications, agronomic practices and crop improvement, including the development of novel biocontrol agents. This organization aims to provide a coherent overview of the current research and its implications across various sectors.

2. Overview of Published Articles

2.1. Extraction Techniques for Bioactive Compounds

Nolasco-González et al. presented an optimized ultrasound-assisted extraction (UAE) method to obtain bioactive compounds from Annona muricata leaves, identifying the best conditions as an amplitude of 80%, a pulse cycle of 0.7 s, and a duration of 4.54 min [17]. The extract showed a significantly higher antioxidant capacity compared to traditional methods, such as decoction and infusion. All extracts were non-toxic in the Artemia salina toxicity test. The findings suggest that UAE is an effective method for producing extracts rich in bioactive compounds suitable for therapeutic formulations and nutraceutical products.

The study conducted by Zheng et al. [18] explored the adsorption and desorption characteristics of okicamelliaside (OCS) on five resins to obtain high-purity OCS extracts from Camellia nitidissima leaves for food and pharmaceutical applications. The most effective resin was AB-8, providing adsorption following a pseudo-first-order kinetic model. Optimal conditions included a 30 min adsorption time and 2.5 mg/mL sample concentration, while desorption was best achieved using 60% ethanol at 2.1 times the column volume. This process increased the OCS content from 48.51 mg/g to 290.82 mg/g.

2.2. Phytochemical Composition and Antioxidant Activity

The impact of two drying methods—heat pump drying (HP) and hot air drying (HA)—on the phytochemical composition and antioxidant activity of Lavandula angustifolia flowers was compared [19]. The results showed that the flowers dried using HP retained a significantly higher content of phenols (+66.73%) and anthocyanins (+62.2%), along with greater antioxidant activity (from 60.32% to 284.3% more) compared to the flowers dried using HA. The study suggests that HP drying is a more efficient and sustainable method for producing lavender products with enhanced bioactive compounds and antioxidant properties, which is ideal for the functional food industry.

2.3. Biological Properties and Health Applications

Six traditional Turkish olive cultivars (Gemlik, Domat, Memecik, Ayvalik, Cilli, and Adana Topagi) and the foreign cultivar Manzanilla were tested for fruit skin color, chlorophyll content, fatty acids, antioxidant activity, phenolic compounds, and volatile compounds by Comlekcioglu et al. [20]. The key results include the following: (i) Memecik exhibited the greatest phenolic content, antioxidant activity, ketones, terpenes, and color intensity; (ii) Cilli had the greatest chlorophyll a level, whereas Adana Topagi had higher chlorophyll b and the highest alcohol concentration and (iii) Gemlik contained the largest content of fatty acids, particularly oleic acid. Domat had the most hexanal, whereas Manzanilla had the greatest ester content. Traditional Turkish cultivars were discovered to contain more health-promoting chemicals than Manzanilla, highlighting their potential for breeding programs and industrial applications.

Menga et al. [21] characterized 54 snap bean lines grown under organic farming for traits such as pod color, shape, width, protein, and sugar content. After cooking, firmness and color parameters were assessed. Fourteen lines with varying firmness and colors were further analyzed for phenolic content and antioxidant activity pre- and post-cooking. Cooking generally reduced antioxidant activity by 39%; however, purple line SBP053 showed minimal loss (3.1%). The study suggests these traits can guide breeding programs to develop snap beans suited for specific markets and consumer preferences.

Kittibunchakul et al. [22] evaluated the nutritional content, antioxidant activities, and health-related properties of sacha inchi (Plukenetia volubilis L.) oil extraction by-products—specifically the husk and shell—to enhance their potential as food sources. The husk contained higher protein, carbohydrates, and dietary fiber, while the shell had more fat and energy. The shell also had 1.6 times more total phenolic content, resulting in 1.8–2.7 times higher antioxidant and 1.2 times greater anti-glycation activity than the husk. Different phenolic compounds in each by-product led to different enzymatic activity inhibitory activities. The findings suggest that these by-products hold promise as valuable food ingredients, warranting further research on their health benefits, toxicity, and bioavailability.

The study conducted by Zahnit et al. [23] investigated the antioxidant, pharmacological, and mineral properties of Artemisia campestris L., a plant used in traditional medicine. Various extracts from its aerial parts were tested for phenolic content, antioxidant capacity, and mineral composition using ICP-OES and LC-MS/MS. The extracts exhibited strong antioxidant activity and enzymatic activity inhibition, particularly against α-amylase, acetylcholinesterase, and butyrylcholinesterase. They also showed high UV absorption, indicating photoprotective potential. Eleven polyphenols and essential minerals like calcium, iron, magnesium, and zinc were identified. The findings suggest A. campestris has significant potential for use in pharmaceutical formulations and health applications due to its rich bioactive and mineral content.

The study of Norkum et al. [24] presents the physiological characteristics, genetic variation, and chemical composition of Gymnema inodorum (Chiang Da), a vegetable from northern Thailand known for its anti-diabetic properties. Two commercial lines (COM1 and COM2) and eight local accessions (BAC1-8) cultivated in the same study plot were analyzed. Morphological and genetic data have revealed clear distinctions between commercial lines and local accessions, with commercial strains exhibiting closely related leaf structures. The results showed that genetic differentiation was related to morphological and nutritional properties but not chemical constituents. The accessions were divided into two groups: those with extensive edible parts and those with a higher content of phytochemicals. The findings could support the selection and crossing of Chiang Da strains with desired traits for the functional foods industry.

The review of Petruskevicius et al. [25] presents the anthocyanin-rich fruit species, known for their medicinal properties and potential health benefits, especially in treating fever, neurodegenerative disorders, and ageing-related diseases. Anthocyanins are also known for their antimicrobial and antiviral properties (including activity against SARS-CoV-2). The review also discusses various extraction methods that effectively preserve these polyphenolic compounds and addresses industrial application challenges.

2.4. Agronomic Practices and Crop Improvement

The application of biostimulants and successive harvesting are sustainable techniques that can enhance crop yield and quality while simultaneously reducing inputs. A greenhouse experiment tested four different biostimulants (based on enzymatic hydrolysate from Fabaceae species, one containing betaine, alginic acid, and caidrin, and another from alfalfa extract, algae, and molasses rich in low-molecular-weight amino acids) on wild arugula, showing that successive harvests increased production by 41% compared to the initial harvest, while the biostimulants increased production by 38% compared to untreated plants. Biostimulants have also reduced nitrate content by 24% and improved the mineral content in leaves. These practices show promise for cultivating leafy vegetables by increasing yield and quality, making them suitable strategies for sustainable agricultural production [26].

Papa et al. [27] used atomic absorption spectrometry to explore the effects of various microbial and non-microbial biostimulants on the concentration of essential macro- and micronutrients in San Marzano and Datterino tomatoes, as well as in spinach. All the tested biostimulants increased the nutrient content in crops. Notably, the application of Trichoderma harzianum T22 on Pixel tomatoes significantly increased the levels of iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), and selenium (Se). In San Marzano tomatoes, a biostimulant based on tropical plant extracts increased the concentrations of Fe, Mn, Zn, Cu, and Se. Similarly, a protein hydrolysate derived from plants significantly increased the daily nutrient intake from spinach. The findings suggest that biostimulants can serve as ecological tools for the biofortification of both fruits and leafy vegetables, enhancing their nutritional value for human health.

The study conducted by Perisoara et al. [28] investigated the development of a biocontrol phytostimulant using Tagetes erecta flower extract combined with rhizobacteria to combat phytopathogenic fungi and enhance crop yield. The hydroalcoholic extract was characterized by its phenolic, flavonoid, and sugar content using various analytical techniques. It exhibited antioxidant and phytostimulatory properties in radish and cucumber plants and promoted the growth of Bacillus rhizobacteria. The extract reduced mycelium growth of pathogens like Fusarium graminearum and Monilinia laxa, and a synergistic antifungal effect was observed when combined with rhizobacteria. The findings suggest that T. erecta extract, along with rhizobacteria, can be effective biocontrol agents and plant growth promoters, with potential applications in sustainable agriculture. Further research is needed to optimize field application and formulation stability.

The investigation into the antifungal potential of aqueous extracts from pomegranate peel (PP), avocado peel (AP), and avocado pit (AP), by-products isolated through microwave-assisted extraction as biocontrol agents against pathogenic fungi for plants, was accomplished [29]. The PP extract significantly inhibited the growth of the Penicillium expansum mycelium and was the most effective against Rhizoctonia solani in vitro. The AP extract reduced the growth of Aspergillus niger by 10.21% after seven days. The combined extracts AP and AS (50:50 ratio) initially suppressed the growth of Botrytis cinerea for 3 days, but the effects diminished by the 7th day. The growth of Fusarium oxysporum was slightly reduced (by 6%) by equal volumes of all extracts. In vivo tests have confirmed that the PP extract effectively suppressed the damage caused by R. solani in tomato plants. The study generally supports using PP and AP extracts as natural alternatives to synthetic fungicides for controlling fungal pathogens in crops.

According to Teixeira et al. [30], the ethanolic extract from garlic peel (GPE) effectively inhibited the growth of Colletotrichum acutatum in apples, indicating its potential as a natural alternative to synthetic fungicides for managing fruit diseases. The extract seems to target the integrity of the plasma membrane by affecting the biosynthesis of ergosterol and the structure of the cell wall. Its complex mixture of antifungal compounds suggests multiple cellular targets, which may reduce the likelihood of resistance development, while also being safe for the environment.

The effects of various irrigation rates (0%, 50%, 100%, and 150% of reference evapotranspiration, ET0) on soil microbiological activity, plant physiological responses, fruit production, and chemical composition in a calafate orchard (Berberis microphylla G. Forst.) were investigated by Betancur et al. [31]. Irrigation at 50% ET0 significantly increased soil enzyme activity (urease, dehydrogenase, and acid phosphatase), stomatal conductance, and chlorophyll index, resulting in a 60% increase in fruit output above other treatments. Despite the higher yield, the 50% ET0 irrigation had comparable amounts of total anthocyanins and antioxidant capability to the 100% ET0 treatment. In contrast, the 0% and 150% ET0 treatments had higher stress levels, which enhanced anthocyanin concentration and antioxidant capacity. Overall, irrigating at 50% ET0 improved water efficiency while increasing fruit output and preserving quality in calafate orchards.

The aim of the work carried out by Valšíková-Frey et al. [32] was to compare the impact of brown mulch film versus bare soil on the quality and yield of six pepper crops for two years. Mulching significantly increased production for all varieties, with ‘Lungy’ showing the highest yield, while peppers grown without mulch had higher levels of vitamin C, antioxidant capacity, polyphenols, and soluble solids. The “Yolo Wonder” cultivar had the highest polyphenol and antioxidant concentrations, while “Semaroh” had the highest content of vitamin C and soluble solids, but the lowest yield with mulch. The findings highlight the trade-offs between yield and nutritional quality depending on the cultivation methods.

3. Summary and Future Outlook

In conclusion, these studies underscore the significant, yet underexplored, potential of horticultural products and by-products as essential resources for health and nutrition, while also promoting sustainable agricultural practices. Future research should adopt a multidisciplinary approach, focusing on improved extraction technologies, bioavailability assessments, industrial applications, environmental impacts, and consumer awareness [33,34]. By exploring these dimensions, researchers can unlock the full benefits of horticultural goods, leading to innovative solutions that enhance human health and foster a sustainable food system.