**2. Source of potassium fertilizers**

Davy in 1807, but its compounds were already used in processes known from ancient times [2]. From the chemical point of view, K belongs to alkali metal family, being a univalent ion

24 Potassium - Improvement of Quality in Fruits and Vegetables Through Hydroponic Nutrient Management

The K is an essential macronutrient and one of the most important cations in higher plants, constituting about 2–10% of the mass of the dry matter [4]. The K is essential for enzyme activation, protein synthesis, and photosynthesis, as well as modulating osmotic regulation during cell expansion, stomatal movements and tropism [5], and transport of photoassimilates of fabrics sources for fabrics drains [3]. The cytoplasmic concentration of K in plant cells is estimated to be around 100 mM (40–200 mM). This concentration of K appears to be relatively stable, with an optimal concentration of K for enzymatic activity [6]. On the other hand, the concentration of K in the soil is low, being in micromolar range

The absorption of K is performed by means of three groups of membrane proteins, the permeases (KT/HAK/KUP), transporters (Trk/HKT), and by proteins of cation-type antiporter (CPA) [8]. Electrophysiological studies have shown that under K millimolar concentrations, K absorption occurs passively through ion channels and actively through H+−cotransporters when the K concentration is in the micromolar range [9, 10]. When K concentration in the soil solution is below 0.2 mM, high-affinity absorption mechanisms are activated; on the other hand, when K concentrations are above 0.3 mM, mechanisms of low-affinity absorption are

In plants, K is characterized by great mobility, being easily transported to aerial part or even redistributed among various organs of plants. Due to considerable number of functions performed by K in plants, this macronutrient plays an important role in plant growth and devel-

Adequate potassium nutrition is associated with increased fruit production, fruit size, soluble solids increase and ascorbic acid concentration, fruit color improvement, fruit shelf life, and supermarket shelf life [13–16]. Additionally, K is involved in post-harvest quality of vegetables and fruits; it is considered a nutrient associated with quality of products of plant origin due to its important effects on post-harvest attributes such as color, size, acidity, resistance to transportation, handling, storage, nutritional value, and industrial

One of strategies of high-quality food production is the adoption of hydroponic system of cultivation, in which the control of plant nutrition (i.e., concentration of nutrients, pH, and electrical conductivity of nutrient solution) and growing environment (i.e., temperature, luminosity, and humidity) are more effective in obtaining high-quality vegetables compared to field cultivation. Hydroponic vegetable production has increased significantly in recent years worldwide, allowing more efficient use of water and fertilizers, as well as better control of climatic and phytosanitary factors. In addition, hydroponic production increases the quality and productivity of vegetables, resulting in competitiveness and prof-

ray cation of 0.331 nm and hydration energy of 314 mols−1 [3].

of 0.1–1 mM [7].

activated [9, 11, 12].

qualities [17, 18].

itability [19].

opment, as well as food quality.

In hydroponic cultivation, the supply of K is carried out from fertilizers containing this element, which have considerable amounts of other nutrients as accompanying ions (**Table 1**). Thus, the choice of source depends on factors such as availability, commercial value, the requirement of culture by accompanying ion, and saline index. However, it is important that more than one source is available to facilitate the equilibrium of final concentration of K in culture solution, which depends on the requirement of each culture.


**Table 1.** Fertilizers containing potassium that is commonly used in the preparation of nutrient solutions.

### **3. Potassium nutrition in vegetables**

The vegetables are very demanding in K and some species such as tomatoes require it in greater quantity than nitrogen (N) [22]. The factors that contribute to high requirement, in general, are a short cycle and short absorption period associated with high demand for nutrient. An important tool to assist technicians and producers with regard to nutrient quantity and application times is the study of gait of nutrient accumulation [23], which changes according to species. For example, while in tomato the demand for K increases over time, especially in fruiting phase [22], in lettuce plants, the rate of absorption of K decreases during the days of cultivation [24]. Another factor that must be taken into account when choosing the amount of nutrients to provide is where the recommendation information was obtained and should be representative of place where the recommendation will be used. In this context, the manual of recommendation for fertilization of vegetables, [25] clarifies the recommendations that should be followed in the absence of local information.

of K in nutrient solution allowed a higher absorption of Ca and Mg; there is no competition between the nutrients for absorption site. Due to these factors, it is not a concern to describe the concentrations of K used in nutrient solutions as well as adequate levels of K in leaf tissues; the objective of this section on potassium nutrition is to demonstrate some examples of the role that

Potassium Nutrition in Fruits and Vegetables and Food Safety through Hydroponic System

http://dx.doi.org/10.5772/intechopen.71742

27

It is possible to manage potassium nutrition in hydroponic systems considering the cultivation of plants with different objectives, since the maintenance of high K/Na ratio in the cytosol is of vital importance for the functioning of plant cells [33, 34], for example, the production of K-poor edible plants for groups of people with chronic kidney disease who have difficulty excreting K. In an experiment with strawberry plants grown under a hydroponic system and

mation, [35] obtained strawberry fruits with low K contents when the concentration of K in the nutrient solution corresponded to 1/32 of the control treatment. For this concentration of K, there was no reduction in yield and fruit quality. However, in a study with melon plants under hydroponic conditions [36] did not obtain similar results, since the melon plants absorbed and stored considerable amounts of K before the application of the restriction treatments of K in the nutrient solution. In this study, a significant redistribution of K of the veg-

In another study, there was the equivalent substitution of K for Na in sugar beet plants cultivated in a hydroponic system [37]. In this study, the equivalent substitution of K for Na did not promote growth reduction, but only significantly reduced the calcium contents in the shoot and root. Plants that support the substitution of K for Na without damage to the growth and ionic homeostasis are called natrophilic, being included in this classification sugar beet [37].

The management of K in hydroponic system can improve the growth of plants cultivated under conditions of saline stress. In a study with five tomato genotypes, [38] observed that 2 mM of K supplementation mitigated the effect of saline stress by promoting greater leaf, root, and fruit yield. However, it should be considered that the response to K addition was

In order to evaluate induced changes in the proteomic level by the substitution of K for Na or even K deficiency in sugar beet plants, [39] observed that a wide range of physiological pro-

tion, glycolysis, and tricarboxylic acid cycle. Stimulation to the photosynthetic process was observed when there was K deficiency; however, due to the presence of Na, the cellular respiration process was affected. This study evidenced that Na is able to repair some damage due to K deficiency, but it did not replace K as an essential element to the growth of plants.

The attention to adequate potassium nutrition must occur from the acquisition of seeds because nutritional status of mother plant affects not only the final yield of crop but also the

genotypic, since the Pearson cultivar presented the best response to the addition of K.

cesses were impaired by K deficiency, such as light reactions of photosynthesis, CO<sup>2</sup>

**4. Potassium nutrition and quality of seeds and seedlings**

in the phases comprised between anthesis and fruit for-

assimila-

K has on the plants, ranging from the seed to the final quality of the product.

decreasing concentrations of KNO3

etative structures was observed for the melon fruits.

In relation to evaluation of nutritional status of crops, the main ways to do this is through visual analysis of deficiency or excess symptoms and nutrient content of plant tissue (leaf analysis). The visual analysis has the advantage of low cost, for dispensing laboratory analysis. However, when the plant expresses the symptoms of deficiency in the histological plane, much of productivity is compromised, which is an undesirable situation for producers. Leaf analysis has the advantage of detecting symptoms of deficiency or excess of nutrient that are not being demonstrated in the histological plane, by comparison with pre-established reference values.

The same care described about the use of recommendations should also be taken in relation to the use of reference values. For example, Trani et al. [26] indicate that for sugar beet, the critical K content ranges from 20 to 40 g kg−1 for field crops. However, if there is a change in cultivation system with an increase in green fertilization [27] and hydroponic cultivation [28], the critical levels of K increased to 70 and 84 g kg−1, respectively.

A relevant nutritional aspect related to plant nutrition is a relationship between nutrients, because of its dependence on the chemical nature of nutrient. It also can affect absorption through root system, thus absorption rate of one ion can be affected by another, which are competing for the same membrane carrier [29]. This fact will depend on its concentration on nutrient solution, the permeability of nutrient to membrane, and its mechanism of absorption [30]. Thus, increasing the concentration of particular nutrient in nutrient solution may interfere with the plant's absorption of other nutrients. In this context, the important relationship between K, Ca, and Mg is framed and these three nutrients are found in expressive concentrations in plant tissues.

The K competes with Ca and Mg for the same membrane carrier and increase in K concentration in nutrient solution implies reduction in Mg uptake. Similarly, increasing K concentration may reduce Ca uptake, because K is preferentially transported in plant compared to Ca [31]. The Ca competes with Mg, which makes the absorption reduced and this is due to the high energy of hydration and the larger size of ionic ray of Mg2+ ion, when compared with the Ca2+ ion. Due to competition, it is possible to observe Mg deficiency in plants, which means high doses of potassium and calcium fertilizers [3]. According to Forster and Mengel [32], the reduced concentration of K in nutrient solution allowed a higher absorption of Ca and Mg; there is no competition between the nutrients for absorption site. Due to these factors, it is not a concern to describe the concentrations of K used in nutrient solutions as well as adequate levels of K in leaf tissues; the objective of this section on potassium nutrition is to demonstrate some examples of the role that K has on the plants, ranging from the seed to the final quality of the product.

**3. Potassium nutrition in vegetables**

that should be followed in the absence of local information.

the critical levels of K increased to 70 and 84 g kg−1, respectively.

The vegetables are very demanding in K and some species such as tomatoes require it in greater quantity than nitrogen (N) [22]. The factors that contribute to high requirement, in general, are a short cycle and short absorption period associated with high demand for nutrient. An important tool to assist technicians and producers with regard to nutrient quantity and application times is the study of gait of nutrient accumulation [23], which changes according to species. For example, while in tomato the demand for K increases over time, especially in fruiting phase [22], in lettuce plants, the rate of absorption of K decreases during the days of cultivation [24]. Another factor that must be taken into account when choosing the amount of nutrients to provide is where the recommendation information was obtained and should be representative of place where the recommendation will be used. In this context, the manual of recommendation for fertilization of vegetables, [25] clarifies the recommendations

26 Potassium - Improvement of Quality in Fruits and Vegetables Through Hydroponic Nutrient Management

In relation to evaluation of nutritional status of crops, the main ways to do this is through visual analysis of deficiency or excess symptoms and nutrient content of plant tissue (leaf analysis). The visual analysis has the advantage of low cost, for dispensing laboratory analysis. However, when the plant expresses the symptoms of deficiency in the histological plane, much of productivity is compromised, which is an undesirable situation for producers. Leaf analysis has the advantage of detecting symptoms of deficiency or excess of nutrient that are not being demonstrated in the histological plane, by comparison with pre-established reference values. The same care described about the use of recommendations should also be taken in relation to the use of reference values. For example, Trani et al. [26] indicate that for sugar beet, the critical K content ranges from 20 to 40 g kg−1 for field crops. However, if there is a change in cultivation system with an increase in green fertilization [27] and hydroponic cultivation [28],

A relevant nutritional aspect related to plant nutrition is a relationship between nutrients, because of its dependence on the chemical nature of nutrient. It also can affect absorption through root system, thus absorption rate of one ion can be affected by another, which are competing for the same membrane carrier [29]. This fact will depend on its concentration on nutrient solution, the permeability of nutrient to membrane, and its mechanism of absorption [30]. Thus, increasing the concentration of particular nutrient in nutrient solution may interfere with the plant's absorption of other nutrients. In this context, the important relationship between K, Ca, and Mg is framed and these three nutrients are found in expressive concentrations in plant tissues.

The K competes with Ca and Mg for the same membrane carrier and increase in K concentration in nutrient solution implies reduction in Mg uptake. Similarly, increasing K concentration may reduce Ca uptake, because K is preferentially transported in plant compared to Ca [31]. The Ca competes with Mg, which makes the absorption reduced and this is due to the high energy of hydration and the larger size of ionic ray of Mg2+ ion, when compared with the Ca2+ ion. Due to competition, it is possible to observe Mg deficiency in plants, which means high doses of potassium and calcium fertilizers [3]. According to Forster and Mengel [32], the reduced concentration It is possible to manage potassium nutrition in hydroponic systems considering the cultivation of plants with different objectives, since the maintenance of high K/Na ratio in the cytosol is of vital importance for the functioning of plant cells [33, 34], for example, the production of K-poor edible plants for groups of people with chronic kidney disease who have difficulty excreting K. In an experiment with strawberry plants grown under a hydroponic system and decreasing concentrations of KNO3 in the phases comprised between anthesis and fruit formation, [35] obtained strawberry fruits with low K contents when the concentration of K in the nutrient solution corresponded to 1/32 of the control treatment. For this concentration of K, there was no reduction in yield and fruit quality. However, in a study with melon plants under hydroponic conditions [36] did not obtain similar results, since the melon plants absorbed and stored considerable amounts of K before the application of the restriction treatments of K in the nutrient solution. In this study, a significant redistribution of K of the vegetative structures was observed for the melon fruits.

In another study, there was the equivalent substitution of K for Na in sugar beet plants cultivated in a hydroponic system [37]. In this study, the equivalent substitution of K for Na did not promote growth reduction, but only significantly reduced the calcium contents in the shoot and root. Plants that support the substitution of K for Na without damage to the growth and ionic homeostasis are called natrophilic, being included in this classification sugar beet [37].

The management of K in hydroponic system can improve the growth of plants cultivated under conditions of saline stress. In a study with five tomato genotypes, [38] observed that 2 mM of K supplementation mitigated the effect of saline stress by promoting greater leaf, root, and fruit yield. However, it should be considered that the response to K addition was genotypic, since the Pearson cultivar presented the best response to the addition of K.

In order to evaluate induced changes in the proteomic level by the substitution of K for Na or even K deficiency in sugar beet plants, [39] observed that a wide range of physiological processes were impaired by K deficiency, such as light reactions of photosynthesis, CO<sup>2</sup> assimilation, glycolysis, and tricarboxylic acid cycle. Stimulation to the photosynthetic process was observed when there was K deficiency; however, due to the presence of Na, the cellular respiration process was affected. This study evidenced that Na is able to repair some damage due to K deficiency, but it did not replace K as an essential element to the growth of plants.
