Introduction

In recent years, Colombia has emerged as a significant producer and exporter of fruits and exotic food products. In 2021, fresh fruit production reached a volume of 12.4 million t, exhibiting an average annual growth of 3.7% since 2017 (Procolombia, 2021). This surge in production within the Colombian fruit sector is attributed to its tropical location, enabling year-round harvesting of fruits. Consequently, Colombia has solidified its position as one of the primary exporting nations of exotic fruits in Latin America (Analdex, 2023). In 2022, approximately 246 million kg of fresh fruits were consumed globally, marking a nearly 7 million kg increases from the previous year. This trend will persist in the upcoming years, which would allow reaching over 303.5 million kg in 2028 (Statista, 2023).

According to Aune et al. (2017), a high consumption of fresh fruits as part of a healthy diet can mitigate the risk of heart disease, cancer, obesity, and diabetes. The authors suggest that a minimum daily intake of 800 g is required to reduce mortality from various causes, offering health and environmental benefits. Moreover, most fruits are low in calories, rich in fiber, contain a high concentration of micronutrients, and are enjoyable for the palate (Guyenet, 2019).

In this context, the term "nutritional density" has been introduced to describe the proportion of nutrients in foods, including vitamins, proteins, fibers, antioxidants, and carbohydrates. This depends on the concentration of nutrients accumulated per unit of mass produced by a crop, and it is directly influenced both by the fertilization practices applied to plants (Barker & Stratton, 2020) as well as by the nutrient availability in the soil, the absorption capacity of the plant, irrigation, and environmental factors (Davis, 2009).

Fertilization in fruit trees has become an indispensable tool for intensive crop production, as the soil lacks the capacity to meet the optimal nutritional requirements demanded by such commercial agricultural operations (Srivastava & Malhotra, 2017). The absence of adequate fertilization leads to nutritional imbalances in plants, resulting in physiological disorders that can impact fruit yield and quality (Kumar & Kumar, 2016).

In this regard, fertilization with calcium (Ca²⁺) positively influences the growth of organs and tissues and stimulates root growth by facilitating the incorporation of materials into cell walls (Prado, 2021). Initially, the Ca²⁺ ion is absorbed by the plant, transported through the xylem with the transpiration flow, and is considered a slightly mobile element within the plant when transported via phloem (Thor, 2019).

Calcium is more abundant in older leaves, and a deficiency of calcium is common in fruits, young leaves, and storage organs (Prado, 2021). In fruits, a deficiency of calcium is attributed to the reduced number of stomata, impeding the transport of calcium through the xylem (Bonomelli et al., 2022). Plant absorption of Ca²⁺ is faster during the initial stages of fruit growth (Casero et al., 2017), as it serves as an enzyme activator and an essential element for cell division, cell elongation, and growth (Prado, 2021).

Applications of calcium (Ca²⁺) in fruits contribute to the maintenance of firmness and cell turgor, reducing the incidence of diseases, and preventing physiological disorders (Jain et al., 2019). Additionally, calcium plays a crucial role in the expression of genes influencing fruit production and quality (Wu et al., 2023), as it stabilizes cell walls and ensures their permeability, protecting them from enzymatic degradation (Zhi et al., 2017). This regulation affects ripening in some fruits and influences respiration rates and ethylene production, thereby extending the quality of fruits during storage (Michailidis et al., 2020).

Among the physiological disorders resulting from an insufficient Ca²⁺ supply or alterations in the balance of Ca with other nutrients (Freitas & Mitcham, 2012), various conditions have been identified, including 'blossom end rot' in tomatoes (Karlsons et al., 2023), 'bitter pit' spots in apples (Griffith & Einhorn, 2023), cork spots in pears (Zhang & Cui, 2023), internal browning in pineapples (Zhang et al., 2022), 'tip burn' in strawberries, 'cracking' in cape gooseberries (Fischer et al., 2021), nectarines (Zhu et al., 2023), grapevines (Yu et al., 2020), and melons (López-Zaplana et al., 2020), as well as the recently identified 'crease' skin wrinkling in citrus (Huai et al., 2022), and these conditions result in significant economic losses (Yu et al., 2020).

Among the treatments widely employed for fruit conservation, foliar applications and dips of Ca(NO₃)₂ and CaCl₂ have been extensively reported. These applications occur during fruit development, at specific intervals, and during the harvest period. The uniform distribution of this solution over the fruits enhances the concentration of Ca in internal tissues, contributing to prolonged shelf life (Dorostkar et al., 2022). However, in some fruits, Ca applications have no effect since sometimes their results are reflected in leaves but not in fruits due to high leaf transpiration (Winkler & Knoche, 2019). While the application of Ca has demonstrated the potential to enhance fruit quality across various crops (Lobos, Retamales, Luengo Escobar et al., 2021), the specific impact of Ca on fruit quality has not been thoroughly studied and summarized. Therefore, the objective of this work was to conduct a comprehensive review that compiles and presents information on the effects of Ca²⁺ on fruit production and quality.

Absorption of Ca by the plants

The movement of calcium (Ca) in the soil towards the roots occurs through mass flow and subsequently it enters the apoplastic space within the apical zone of the root (Doyle et al., 2021). Ca is absorbed as a divalent cation (Ca²⁺) by the cells of the root endodermis (Gulbagca et al., 2020), specifically through channels located in the plasma membrane that can be voltage-dependent or voltage-independent. The first are activated by depolarization or hyperpolarization and are probably encoded by annexins, while the latter are activated by cyclic nucleotides (Lemtiri-Chlieh et al., 2020).

After absorption, Ca is transported with water apoplastically to avoid interference with its role as a secondary messenger. This is because the concentrations of Ca²⁺ in the cytosol must remain in submicromolar ranges (< 0.1 µM) to ensure the ability of cells to generate Ca²⁺ signals (Prado, 2021). Thor (2019) suggests that Ca²⁺ moves mostly via the apoplast from the epidermis of the cortex until it reaches the Caspary band that restricts the apoplastic movement of solutes. Even so, Ca²⁺ can reach the xylem via the apoplast only in regions where the Casparian bands are absent or discontinuous (White & Broadley, 2003). In the selective symplastic pathway, Ca²⁺ enters the cytosol through channel proteins and is pumped to the apoplast of the stele through Ca²⁺-ATPases or the Ca²⁺/H⁺ antiport pathway, ultimately contributing to its upload in the xylem. From there, Ca²⁺ is transported to the shoots where it is unloaded and distributed to the leaf cells through channels activated by cyclic nucleotides (Wang et al., 2017). In this phase, absorption is quite rapid; and Ca²⁺ supplies from the soil are of fundamental importance. Likewise, the Ca²⁺ flow rate is strongly influenced by transpiration (White & Broadley, 2003).

Calcium absorption by the fruits

In the realm of nutrition and calcium physiology, fruits pose challenges in water and nutrient supply due to their low transpiration rates and limited xylem transport networks compared to other plant organs. This limitation hinders the delivery of calcium to fruits (Hocking et al., 2016). Key factors influencing the supply and distribution of calcium in aerial tissues include the mass flow rate of xylem sap (as Ca²⁺ is not highly mobile in the phloem), competition among ions for binding sites in xylem vessel walls, the presence of ionic transporters (including H⁺, influencing pH), membrane channels, and the formation of poorly soluble or insoluble complexes, such as calcium oxalate (Sotiropoulos et al., 2021). Calcium concentration in various cellular compartments can impact water transport through membrane-controlled pathways. An increase in cytosolic calcium concentration may decrease water transport through aquaporins, affecting the relative proportions of apoplastic and symplastic water flow and the magnitude of calcium supply. Symplastic pathways exhibit a lower capacity for long-distance Ca²⁺ movement (Doyle et al., 2021).

In the early stages of fruit development and before the formation of Casparian bands in the root and the suberization of the tissues when the fruit is small, over 50% of the total calcium is contained within it. Therefore, the quantity of calcium fertilization applied to the soil before this stage is of principal importance, as well as the moisture content in the soil, and the presence of many idioblast cells in the root that serve as a Ca sink. Once the Casparian bands are formed, the idioblast cells provide Ca for cell elongation and differentiation (Storey et al., 2003). However, as the fruits expand in later stages, most Ca²⁺ is transported to the leaves, where it cannot be redistributed to the fruits; this results in the dilution of Ca amounts in fruits (Doyle et al., 2021). During this phase, foliar applications of calcium must be implemented through spraying to meet additional nutrient needs and prevent potential physiopathy related to calcium deficiency.

Casero et al. (2017) report that in apple (Malus domestica Borkh.) cv. 'Golden Smoothee' calcium is absorbed by fruits during the initial stages of development, accumulating until 80 d after full bloom (DAFB). The absorption peaks at 38 DAFB and may be antagonistic with nutrients such as potassium (K⁺), ammonium (NH₄⁺), and magnesium (Mg²⁺) at the early stage of fruit development. Similarly, Bonomelli et al. (2022) find significant differences in calcium distribution in orange (Citrus sinensis) fruits cv. 'Fukumoto' with applications at various stages of development (30 DAFB, 44 DAFB, 66 DAFB, and 99 DAFB). The application at 30 DAFB resulted in the accumulation of 61% of calcium in the peel and 39% of calcium in the fruit pulp. This suggests that calcium mobility to the fruit pulp varies with phenological stages and fruit morphology and is more substantial at the early stages of development.

Doyle et al. (2021) note an inverse correlation between fruit growth rate and calcium content in fruits. In blueberry fruits, calcium content increases during early fruit development, with higher levels observed in peel and seed tissues compared to a fruit pulp (Doyle et al., 2021). Calcium deficiency symptoms may manifest a few weeks after anthesis, primarily in the distal zone of the fruits.

Functions of calcium in the fruits

Adequate supply and transport of calcium are crucial for fruit development, as only a small portion of the absorbed calcium is transported to the fruits. Its accumulation relies on xylem flow due to the low mobility of calcium in the phloem (Gao et al., 2019). Calcium accumulation in the fruits primarily occurs during the initial stages of fruit development, where calcium absorption is linked to cell division and metabolism, particularly during the initial fruit expansion. In the later stages of fruit growth, calcium absorption predominantly influences cell-to-cell adhesion (Hocking et al., 2016).

Interactions between Ca and the cell wall during fruit development

Calcium combines with pectic acid in the cell wall to form calcium pectate, constituting the structural skeleton of the cell wall. This structure prevents the disintegration of the gel layer in the cell wall, enhancing the structural strength of the cell wall (Zhang & Wang, 2019). A sufficient calcium supply inhibits the entry of hydrolases and reduces changes in the pectic composition of the cell wall, thereby maintaining fruit firmness. Additionally, being a structural component of the cell membrane, calcium forms bridges connecting phospholipids and proteins within the plasma membrane. This interaction affects phase transition and fluidity, preserving membrane integrity (Xu et al., 2022).

The cell wall comprises various polysaccharides forming a network of cellulose microfibrils linked to a matrix of pectins through α-1,4-glycosidic bonds and hemicelluloses (Hocking et al., 2016). Additionally, it contains minor structural components such as proteins, lignins, and phenolic compounds (Polko & Kieber, 2019). Pectins stand out as the primary polysaccharide of the cell wall in most fruits, and calcium-pectin bonds significantly influence their physical, structural, and resistance properties (Cui et al., 2021).

The prevalence of either ionic or ester bonds among pectins plays a crucial role in the physical properties of fruit cell walls, impacting the solubility of pectins (Hocking et al., 2016). Homogalacturonan, the primary pectic polysaccharide, constitutes 60% of all pectins in the cell wall (Gawkowska et al., 2018), interconnected with Ca²⁺ ions to form pectates, collectively known as the egg-box (Huang et al., 2023).

During cell growth and expansion, acidification of the cell wall takes place, displacing calcium bound to pectin through the protonation of carboxyl groups (Hocking et al., 2016). This phenomenon is triggered by changes in pectin concentration, leading to the release of apoplastic Ca²⁺ [Ca2+]. These changes induce secretion and modification of pectins, crucial for fruit growth (Gao et al., 2019). According to Hocking et al. (2016), apoplast pH is influenced by xylem sap pH under conditions of ample water supply, potentially resulting in significant alterations in cell wall dynamics and composition.

During fruit growth, pectins undergo de-esterification, catalyzed by pectin methylesterases (PMEs), releasing carboxylic residues linked to calcium (Hocking et al., 2016). The level of PME activity and the availability of Ca²⁺ within the apoplast significantly impact the strength of the cell wall, its expansion, as well as the disassembly and remodeling of pectin (Khan et al., 2019). In the ripening phase, calcium inhibits polygalacturonase (PG) activity, and a reduction in calcium concentration favors increased PG activity (Hocking et al., 2016). PG exhibits its maximum expression at 100 d after anthesis (DAA) in grapes, indicating a programmed role in cell wall disassembly at maturity (Khan et al., 2019). At the initial stages of fruit growth, low activity of PG and pectate lyases (PL) is regulated by specific isoforms of these enzymes (Hocking et al., 2016). However, as the fruit matures, extensive depolymerization of pectins occurs, resulting in shorter subunits that are more soluble, indicating a decrease in the xyloglucan and cellulose networks in the cell wall. This is attributed to the action of PG and PL in tomato fruits. In this context, Nie et al. (2022) tested targeted mutations of the SIPG gene and found that it was possible to delay the loss of firmness and water in tomato fruits.

The strength of the bonds forming Ca pectates is dependent on pH, with apoplastic values between 6 and 7 enhancing these bonds. Furthermore, the formation and dissolution of pectate gels by Ca bonds depend on the level of de-esterification, specifically the concentration of free carboxyl groups and free calcium ions (Frempong et al., 2022). As the dissolution and hydration of pectic gels occur, the breaking of calcium bonds with pectins increases, resulting in a reduction in cell wall rigidity evident in small deformations. This process occurs at a pH of 3, where the resistance of the links is minimal (Lara-Espinoza et al., 2018).

According to Gao et al. (2019), an adequate calcium concentration in the fruits delays and decreases the climacteric peak and ethylene production. This is largely dependent on the phosphorylation of calcium-dependent proteins, inhibiting the activity of 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) and ethylene-forming enzyme (EFE), subsequently reducing the contents of 1-aminocyclopropane-1-carboxylic acid (ACC).

Moreover, calcium treatments also diminish microbial growth and susceptibility to pathogens, delaying fruit ripening and extending postharvest life (Gao et al., 2019). However, an excessive calcium supply can elevate cytosolic calcium concentration, potentially causing membrane damage and increase respiratory intensity.

Calcium exerts an inhibitory effect on enzymes and coenzymes during fruit ripening and softening (Huai et al., 2022). It significantly suppresses the expression of genes that encode cell wall-degrading enzymes such as cellulase, pectinesterase, polygalacturonase, β-galactosidase, and pectate lyase. However, when pectin degradation occurs, there is an increase in free Ca²⁺, resulting in the loss of cell-to-cell polymerization and fruit softening (Gao et al., 2019). Additionally, in the cell membrane, calcium inhibits lipid peroxidation initiated by the activation of the enzyme lipoxygenase. Calcium also enhances the activity of enzymes neutralizing reactive oxygen species, including superoxide dismutase, peroxidase, catalase, and the enzymes of ascorbate-glutathione cycle (Xu et al., 2022).

According to Saure (2005), the concentration of calcium varies according to the different growth rates of each part of the fruit. Calcium in the fruit can be found as water-soluble Ca or exchangeable Ca; the latter increases during fruit development until reaching 84% of the total Ca that indicates that as the fruit matures, Ca is found in more available forms. Likewise, Ca can be redistributed, since, in ripe fruits, the concentration of Ca is higher in the peel and lower in the pulp section just below the peel.

As a secondary messenger calcium elevates the level of phosphorylation in fruits, inducing the activation of MAKP (mitogen-activated protein kinases) cascades. These cascades control protein stability and regulate the expression of genes that inhibit ethylene synthesis, ultimately prolonging the post-harvest life of the fruits (Yu et al., 2018).

Effect of calcium on the physical properties of the fruits

The physical transformations within fruits play a pivotal role in determining their postharvest handling. These changes primarily manifest in the cell walls of the fruits and are associated with the modification and continuous solubilization of pectins. Generally, controlled depolymerization of the cell wall structure occurs, leading to the loss of cell adhesion, which results in the associated reduction of fruit firmness and the leakage of internal juices during the ripening process. Consequently, this leads to mass loss and a decline in overall quality (Fig. 1).

Firmness

Firmness is a critical quality parameter in fruit marketing (Huang et al., 2023). Calcium enhances fruit firmness by strengthening the cell wall structure, reinforcing cell adhesion, and forming Ca pectate when combined with pectic acid (Zhang & Wang, 2019). Additionally, exogenous calcium application causes the accumulation of homogalacturonans and an increased number of pectin networks (Huang et al., 2023).

Calcium applications are typically done pre-harvest alongside fertilization, while in other instances, they are directly administered to fruits during the final stages of development or post-harvest. These applications have demonstrated positive outcomes in enhancing or sustaining fruit firmness over an extended period. However, according to Sena et al. (2024), foliar Ca fertilizations prove to be more effective than soil applications in augmenting Ca levels within the fruits. In this context, Khakpour et al. (2022) observe that the soil application of 150 g of Ca(NO₃)₂ per apple tree results in fruits exhibiting increased firmness compared to the control group. Conversely, Álvarez-Herrera et al. (2022) found no significant disparities in fruit firmness when employing fertilizations ranging from 0 to 100 kg ha^-1 of Ca.

Figure 1.: Effect of calcium on the physical and chemical properties of fruits. TTA - total titratable acidity, TSS - total soluble solids.

Liu et al. (2023) further notes that CaCl₂ application reduces the amount of water-soluble pectins, indicating prolonged firmness in persimmon (Diospyros kaki) fruits. Ali et al. (2021) applied CaCl₂ through pre-harvest sprays (1%, 2%, and 3%) and significantly increased peach fruit firmness by 28%, 13%, and 24%, compared to the control. Overall, calcium applications have successfully maintained fruit firmness for longer durations in various fruits such as jujube (Ziziphus mauritiana Lamk.) (Jain et al., 2019), papaya, melon, guava (Thakur et al., 2019), apple (Xu et al., 2022), plum (Sinha et al., 2019), grapevine (Shi et al., 2023), blueberry (Lobos, Retamales, Luengo Escobar et al., 2021), lemon (Frempong et al., 2022), and kiwi (Sotiropoulos et al., 2021). Also, Melo et al. (2022) find that two weekly applications of 0.3 g L^-1 of CaCl₂ increase the calcium content and firmness in tomato fruits. During postharvest, Liu et al. (2023) immersed freshly harvested pear fruits in 5 g L^-1 of Ca lactate for 10 min, resulting in fruits with greater firmness at 8 DAH (28.8 N) compared to the control (27.3 N). A similar effect was observed in loquat (Eriobotrya japonica Lindl), where Ca lactate increased fruit firmness from 4.1 to 4.6 N at 30 DAH.

Effect of Ca on the chemical properties of the fruits

Among the chemical changes during fruit development are depolymerization and modifications in the side chains of pectin, degradation of xyloglucan, increasing activity of non-catalytic proteins (arabinogalactan proteins, AGP), and processes related to ripening such as the accumulation of solutes and acid degradation, some of which are detailed below.

Ca in physiological fruit disorders

Calcium deficiencies usually appear in fruits at the initial stages, and although sometimes they are not noticeable to the naked eye, they can cause disorders that affect the development and quality of fruits and, consequently, the postharvest behavior. In this regard, low levels of Ca²⁺ in fruits cause bitter spots, blossom end rot, cracking, and watery fruits, among other physiological disorders (Gao et al., 2019). On the other hand, if high concentrations of calcium occur, it can cause cellular toxicity, cell walls that are too rigid, and abnormalities in the growth and development of the fruits (Hocking et al., 2016).

Conclusions

Foliar applications of calcium have a more significant impact on fruits when carried out in the initial stages of development. Calcium positively influences fruit mass and reduces fresh mass loss during storage. The softening of fruits is delayed due to calcium inhibitory action on enzymes degrading the cell wall, maintaining firmness for an extended period, and favoring fruit storage. Calcium applications maintain high total acidity values while generating low total soluble solids (TSS) values, delaying TSS peaks during postharvest. Calcium decreases ethylene production, delays the climacteric peak, and reduces its intensity, along with delaying color changes in fruits. Calcium also reduces the appearance of physiological disorders related to deficiencies, playing a crucial role in preventing disorders that affect fruit quality. It's noteworthy that the deficiency of this nutrient causes specific symptoms in each species.