Postharvest Physiology, Biochemistry and Sustainable Management of Plant Genetic Resources

1. Introduction

Plant genetic resources are the biological basis of food security. Plant genetic resources utilized for food and agriculture consist of diverse seeds and planting materials of traditional varieties and modern cultivars, crop wild relatives, and other wild plant species. These resources are used as food, feed for domestic animals, fibers, textiles, and energy. Their conservation and sustainable use are necessary to ensure crop production and to meet growing environmental challenges and climate change. The erosion of these resources poses a severe threat to the world’s food security in the long term [1]. The postharvest life and quality of a commodity are influenced by the rate at which the many interrelated metabolic reactions of a cell occur. Respiratory metabolism furnishes not only the energy required to drive all these metabolic reactions but also produces the raw material used as substrates by these reactions [2].

Global demand for food is increasing and will likely continue to do so for decades, propelled by a 2.3 billion-person increase in global population and greater per capita incomes expected through the middle of this century. The need to feed a growing population is a constant pressure on crop production, as is coping with an increasingly degraded environment and uncertainties resulting from climate change, the latter necessitating adaptation of farming systems. Both land recovery and more intensive use of existing croplands could contribute to the increased crop production needed to meet food demand, but the environmental impacts and these alternative paths of agricultural expansion are very uncertain at present [1]. The current challenge is to preserve crop biodiversity while, at the same time, fueling expanded production to provide food security for mankind. Predictably, there will always be a divergence between biodiversity protection and human needs, and the task at hand is how best to plot a route between them. With land scarcity, crop production intensification, rather than area expansion, becomes the primary option available. Well-managed ecosystems are essential for ensuring a healthy resource platform necessary to intensify production sustainably from now until 2050 and beyond [1].

The primary purpose of pre-treatment is to prolong the shelf life of vegetables and fruits and minimize losses due to spoilage. Various pre-treatment procedures involve physical, chemical, physiochemical, and biological approaches (Figure 1). Due to the seasonal nature of fruits and vegetables, pre-treatment practices and processing techniques have been aimed at ensuring that safe fruits and vegetables are available for a longer period to accommodate a more varied diet over the year. Pre-treatment techniques led to an increase in palatability, remarkably based on stability during transport, convenience for consumers, and texture. In this way, consumers’ desire for nutritional, natural, and nourishing qualities may also need to be taken into account in the future of processing fruits and vegetables products. Furthermore, the use of sustainable green technologies has begun to be implemented by food technology as part of a transition from conventional pre-treatment techniques [3].

On the other hand, light, in the range of photosynthetically active radiation (PAR), plays several roles in plant life since it is a source of energy for carbohydrate synthesis, acts as a morphogenic signal to regulate several processes, and stimulates a number of light-dependent reactions [4]. In fact, the type and quantity of available radiation influence many physiological, morphogenetic and reproductive processes of plants, also being the main driver for the production of secondary metabolites [4].

Edible coatings and films can maintain the visual appeal, firmness, and moisture content and can extend the storage duration of fruits. This is due to their ability to act as barriers against moisture and gas transmission, prevent lipid oxidation, regulate enzymatic activities, and inhibit microbial spoilage [5]. Edible coatings alter the surrounding environment of fruits by modifying the gas composition inside the fruit, thus slowing down the respiration rate and ethylene generation. This effectively reduces the physiological degradation of fruits [5]. Moreover, it has been shown that edible coatings, including antimicrobial substances, such as organic acids, plant essential oils, and polypeptides, effectively hinder the proliferation of microbes [5]. Edible coatings can decrease enzyme activity, minimize the occurrence of browning reactions, and prevent texture softening [5]. Furthermore, it has the capacity to preserve the inherent volatile flavor and color components [5]. The edible coatings shield food products from microbial contaminants, diminish the effects of deterioration [5], extend the storage duration [5], and minimize lipid oxidation and moisture loss in food products [5]. All these elements were covered by the articles published in this Special Issue, which demonstrate the scope of the efforts made by the scientific community to address the problem of postharvest physiology of fruits. Below is a review of the published studies in the Special Issue.

2. Overview of Published Articles

In the literature, postharvest applications of n-butanol have been claimed to increase greasiness in apple skins by altering wax composition via effects on gene expression [6]. In the first paper included in the Special Issue, Charoenphun et al. (Contribution 1) examined the exogenous application of aqueous n-butanol at various concentrations (0.2–0.6%) in controlling pericarp browning and suppressing different oxidoreductase enzymes in the pericarp under prolonged ambient storage conditions (8 days). The results showed that the increased storage significantly impacted the color characteristics of fruits. Membrane permeability loss (MPL), malondialdehyde (MDA), and reactive oxygen species (ROS) were found to increase. The browning-related enzymes (PAL and PPO), membrane-degrading enzymes (PLD and LOX), and antioxidant enzymes (SOD, CAT, and GPX) continuously increased in all pericarp samples throughout the storage. The authors concluded that the positive effect of n-butanol application was dose-dependent; higher concentrations (0.4–0.6%) performed well in protecting the fruit from deterioration.

Physical environmental factors, such as light, temperature, humidity, gas composition, and mechanical force, contribute to the regulation of the ripening of fruits and vegetables. Light is an important environmental factor affecting the development and postharvest ripening of fruits and vegetables, and different spectra have different effects [7]. Deng [8] showed that LED blue light irradiation accelerates color change, enhances chlorophyll degradation and carotenoid synthesis, increases the expression of genes related to ethylene-induced ripening, and increases the ethylene sensitivity of citrus fruits. However, the mechanism by which light promotes postharvest ripening and senescence in fruits has not been clearly established. Zhou et al. (Contribution 2) evaluated the correlation between the regulation of postharvest ripening and energy metabolism in banana fruits under red light irradiation, providing theoretical groundwork to guide research on the regulation of ripening by LED red light. The red light-irradiated bananas had higher total color differences and higher rates of chlorophyll degradation and carotenoid synthesis than those of the ethephon-treated group during the storage period. Red light irradiation promoted banana fruit ripening and senescence mainly by promoting carotenoid synthesis; capturing absorbed light energy; accelerating energy metabolism; effectively enhancing the activities of the respiratory and energy metabolism-related enzymes H⁺ adenosine triphosphatase, Ca²⁺ adenosine triphosphatase, succinate dehydrogenase, cytochrome C oxidase, and malic enzyme; and promoting organic acid degradation. They concluded that LED red light can be used as a new physical ripening technology for bananas, with a similar effect to that of traditional ethephon treatment.

The application of natural-based coating materials, including waxes and resins, on fresh fruit can be used by low-income countries where natural resources are available, which is recognized as an eco-friendly approach. These materials are becoming very popular in citrus postharvest, as they reduce losses by the differential permeability of CO₂, O_2, and water vapor, decreasing the metabolic rate and water losses [9,10].

The citrus-processing industry generates large amounts of by-products every year, including peel (flavedo and albedo), pulp (juice sacs), rag (segment wall and central core), and seed residues [11] that account for around half of the total fruit weight [12]. Citrus by-products may become waste and a possible source of environmental pollution if not adequately processed [13]. These by-products are potent natural sources of bioactive compounds, such as phenolics and antioxidants, that are conducive to human health. Within this context, better practices on citrus by-product application are essential to determine their potential as healthy beneficial products, such as outstanding low-cost antioxidant sources [11,12,13]. Carvalho et al. (Contribution 3) investigated the use of carnauba wax/wood resin-based coating and cold storage on postharvest life of Valencia Late and Natal IAC sweet oranges, as well as the physicochemical quality and antioxidant capacity of its by-products. Valencia Late and Natal IAC fruits had proper quality in both years, satisfying the requirements of the fresh market and processing industry. The flavedo and albedo sections displayed the highest concentration of bioactive compounds, such as phenolics, flavonoids, and antioxidant activity. The coating treatment associated with cold storage was efficient in terms of the preservation of fruit color and retardation of weight loss for up to 60 days for both varieties. The sensory profile and quality of the carnauba wax/wood resin-treated fruits were preserved all over the cold storage period, while uncoated fruits ranked low for most of the sensory attributes. Together, Valencia Late and Natal IAC fruits contain a high level of healthy beneficial compounds, which may be exploited as a natural source of low-cost antioxidants. Further, the carnauba wax/wood resin coating associated with cold storage effectively reduces weight loss and color progression in sweet orange fruits, in addition to preserving overall physicochemical and sensory quality.

To extend the shelf life of fresh fruit after harvest, cold storage is the first issue, whether in combination with the controlled atmosphere or not. During storage, postharvest losses occur due to mechanical damage, diseases, and physiological disorders [7]. Important structural and metabolic components of plant/fruit cells include fatty acids and lipids. Changes in the lipid composition of the membrane frequently have detrimental effects on the cell’s capacity to adapt to high temperatures and other stressful situations, which can result in a variety of physiological storage diseases in fruit [14]. Uarrota (Contribution 4) investigated the changes in polar metabolites, phenolic compounds, and fatty acids in the skin of Hass avocados stored under two distinct conditions. Fruits were mostly correlated with palmitoleic, palmitic, and oleic acids. Phenolic content increased at the beginning of storage and decreased at the end of storage for one orchard and contrarily for others, indicating that the result was dependent on the orchard and storage condition. The differences in fatty acids, polar metabolites, and phenolics were dependent on orchard and storage conditions.

The worldwide population is increasing; therefore, to meet growing food demand, sustainable agricultural production is needed. The use of chemical pesticides has been the best way to control pests in recent decades. However, the use of these chemicals has caused several health problems in humans since these chemicals enter the food chain and eventually the human body, some of them being carcinogens. Concerns about human health and environmental problems prioritize safer and more sustainable crop production [15]. In this sense, Ortiz and Sansinenea (Contribution 5) reviewed the application of Bacillus genes and Bacillus formulations to crops and their safety for human health to provide a more comprehensive understanding of this topic. The authors comment that transgenic plant technology can be used to address global food scarcity, particularly in developing countries. Genetically modified organisms are a controversial topic that needs to be considered more carefully.

Wild edible fruits are an important alternative to agriculture worldwide that suffers from genetic erosion due to severe genetic diversity reduction and domestication hindrance. Underutilized Prunus spinosa fruits are increasingly being considered as genetic resources and are marginally used by small farmers, constituting a real safety valve for the sustainability of the processing plum value chain. Ilhan (Contributor 6) studied fruits of eight wild-grown blackthorn (Prunus spinosa) genotypes that were sampled and subjected to sensory, morphological, biochemical, and antioxidant characterization. Aroma, taste, and juiciness were used as the criteria for sensory analysis, and a trained panel of ten experts established and evaluated the sensory characteristics of the fruits of the blackthorn. The results indicated significant differences among genotypes for most of the traits. The data showed that the analyzed blackthorns had bigger fruits, indicative of their suitability for fresh and dried consumption; had higher juiciness, indicating their suitability for processing; and had higher human health-promoting compounds (higher total phenolic content and antioxidant capacity), making them suitable for future use as functional foods and as promising sources of natural antioxidants.

Sweet cherry may develop surface pitting during prolonged cold storage, and susceptibility among varieties is related to metabolites in response to cold and mechanical damage. Low-temperature storage is necessary to preserve the quality attributes of various fruits; however, fruits might develop symptoms of chilling injury, leading to oxidative damage and lipid peroxidation. It has been observed that sweet cherries show a higher incidence of pitting damage after prolonged storage at 1 °C. To understand fruit physiological problems, several researchers have evaluated different treatments to control disorders, such as CaCl₂, gibberellic acid, hydrogen sulfide, and UV-C light exposure [16,17]. Hernandez et al. (Contributor 7) evaluated the metabolic changes in sweet cherry fruits subjected to melatonin treatment and induced surface pitting. Melatonin treatment attenuated the severity of pitting damage during cold storage. In addition, melatonin application appeared to modulate metabolic responses due to the regulation of metabolic pathways related to abiotic stress. Upregulation of different secondary metabolites was observed after 16 h of melatonin treatment and cold storage. Moreover, some metabolites of the sphingolipid and sulfur metabolism were upregulated after 10 days.

3. Conclusions

The scope of this Special Issue is broad and includes the physiology, biochemistry, and effects of various biotic and abiotic stressors in different horticultural crops and on the sustainable management of plant genetic resources. Many published manuscripts presented advances in the pre-treatment effect of the shelf-life of fruits. The physiology and biochemistry of the bioactive compounds are also addressed in the manuscripts, including abiotic stress and some sustainable management approaches to genetic resources. Despite this, the effect of biotic stressors on the postharvest physiology of fruits was not reported and needs a deep understanding for future works.