**6. Potassium affecting plant growth and yield**

In relation to calcium, it has been demonstrated that increased salinity may induce its deficiency [20]. The reduction in Ca2+ absorption may lead to loss of plasma membrane integrity,

fer to aerial part, in order to maintain a positive relationship between those nutrients and Na<sup>+</sup>

The high salinity of some fertilizers, mainly of KCl, compromises the growth and distribution of the roots, as well as the absorption of water and nutrients [23]. Potassium chloride is the main source of K for agriculture, followed by potassium sulfate used on a smaller scale. Potassium sulfate has a lower salinity effect than potassium chloride, which makes it more

Plants undergo changes in their metabolism when maintained under adverse environmental conditions. Plant tissues are endowed with different response systems to control the production of free radicals. Due to their specific compartmentalization in the cells, the enzymes and organic compounds formed in situations of environmental stress can be determined. In saline conditions, there is a reduction in the availability of water to the plants; as water tends to move from point larger to the smaller the osmotic potential (of the salinized nutritious solution toward the plant), there will be greater energy expenditure for its absorption. The greater or lesser effort to overcome the osmotic potential difference varies according to vegetable species for adaptation to different salinity conditions [25]. In addition, this factor may influence the photosynthetic process, since the content of chlorophyll in the plants will be affected [26]. The high saline concentration in the solution can cause nutritional imbalance, toxicity of some ions, and interference in the hormonal balance, which are able to decrease the plasticity of the

The role of calcium in vegetable adaptation to saline stress is complex and not well defined. Saline stresses were observed in the positive effects of this nutrient. The effects of K and Mg are little studied because they have a beneficial effect on the plant to increase the tolerance of

Applications of high and continuous doses of KCl may also raise the chloride ion content in the plant, leading to a chlorosis and necrosis of the leaves, as well as a drop in production. Chlorine does not enter into the constitution of organic compounds, being necessary for the

When applied externally, Ca+2 decreases saline stress by means of an unknown function that

cytoplasm at 100–200 mol m−3 by active transport, and NaCl promotes a rapid increase in its concentration in the cytoplasm, probably acting as a signal of general stress. Although there is no confirmation that this increase is a salinity tolerance effect, the higher concentrations of Ca+2 in the cytoplasm may be transient. Results suggest that this increase, as a function of exposure to NaCl, may be reduced by the increase in Ca-ATPase activity [29]. The eggplant presents resistance to salinity induced by potassium sources, being considered a plant that

absorption sites, which can reduce the Na<sup>+</sup>

absorption low-affinity component. Calcium is usually maintained in the

influx

[21]. Salinity-

transfer rates and only slight reduction in Ca2+ trans-

with consequent loss of the absorption capacity of some ions, especially K+

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

cell, causing reduction in the permeability of the cytoplasmic membrane.

photolysis of water, during photosynthesis and electron transport [28].

selectivity and inhibits K+

can be used in conditions of high osmotic potential [24].

tolerant varieties tend to have higher K+

suitable for the preparation of nutrient solutions [24].

vegetables to salinity in the nutrient solution [27].

and Cl<sup>−</sup>

preserves K+

mediated by the K+

/Na<sup>+</sup>

ions [22].

Salinization is a problem that invariably occurs in protected environments, due to the accumulation of salts present in fertilizers. This problem tends to aggravate over time with greater or lesser speed, according to the practices adopted. The effects of salinity on fruit and leaf vegetables are intense, causing flowers to fall, alteration of the fruits color, flowers abortion, and burn on leaf margins [30] (**Figure 4**).

**Figure 4.** Images of the effects of salinity on eggplant.

**Figure 5.** Root volume of eggplant (*Solanum melongena* L.), cultivar Embu, as a function of potassium doses and sources.

The elevation of K content in the solution can induce nutritional imbalance for the plants, due to antagonism, competitive inhibition, and noncompetitive inhibition among nutrients, in addition to synergism, which can cause a differentiated dynamics between cations in the leaves and roots of plants. However, little is known about the interactions between cations

regardless of the source used, the electrical conductivity increases linearly (**Figure 7**). However, it is observed that the values of electrical conductivity are significantly higher with

The electrical conductivity ranges between 3.82 and 1.33, with a mean of 2.49 dS m−1 when a

SO4

conductivity was evidenced during the experimental period, and this reduction was more pronounced during 60 days after transplantation because of the onset of flowering and fruit-

0.89 dS m−1 and the average of 2.16 dS m−1, while the range was between 3.30 and 0.28 dS m−1

was as high as 6.27 and as low as 1.30 having an average of 3.78 dS m−1 (**Figure 8C**). When

was applied, electrical conductivity values obtained were between 4.27 and 1.03 dS m−1

O 250 kg ha−1 for KCl fertilizer was applied, whereas values were between 4.24 and

O 500-kg ha−1 KCl dose, the electrical conductivity ranges between 3.46 and

treatments remained within the ranges from 7.12 to 1.82 and from 3.36 to

as a source of potassium fertilization generates a direct form of competition

the use of KCl, indicating an increase in nutritious solution salinity [34].

SO4

1.25 dS m−1 (**Figure 8D**), with a mean of 4.47 and 2.11 dS m−1, respectively [35].

O induced by different sources. When the K<sup>2</sup>

O doses are increased,

57

(**Figure 8A**). A decreasing trend of electrical

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O 750 kg ha−1 especially for KCl, and the range

O 1000-kg ha−1 electrical conductivity

O affect production, and excess K induces

as potassium source (**Figure 8B**). A greater fluctuation

caused by excess doses of K2

0.86 dS m−1 and averaged 2.55 dS m−1 for K2

of electrical conductivity was observed after K2

with a mean of 2.65 dS m−1. Subsequently, at a dose of K2

with Mg2+ in the roots of eggplants, high doses of K2

**Figure 7.** Electrical conductivity (EC) in function to sources and potassium doses.

with a mean of 1.79 dS m−1 for K2

SO4

SO4

dose of K2

K2 SO4

ing. In case of K2

in KCl and K2

The use of K2

**Figure 6.** Roots of plants of eggplant (*Solanum melongena* L.), grow crops "Embu," according to doses and potassium sources.

Comparatively higher root volume was found when potassium chloride was used as the source of potassium fertilization than potassium sulfate (**Figures 5** and **6**). Considering the use of K2 SO4 , it is observed that the root volume increases with increasing doses, up to an estimated maximum value of 645 kg ha−1 K2 O; from here, there is a decrease, indicating a stressing effect on the plant. On the other hand, with KCl as source there is no definite trend of increase or decrease in the root volume, values found being stable and higher than those found with the K2 SO4 source [30].

#### **7. Cation dynamics in leaves and fruits of vegetables**

Many problems have been observed related to excessive fertilization, leading the nutritious solution to an accumulation of salts. Although the water used in irrigation in the protected crop is of good quality, using the fertigation technique increases the risk of salinization [31].

In the process of nutrient absorption, the cationic interactions at the adsorption sites and the concentration of nutrient ions in the solution are important aspects in plant nutrition and crop production. The rate of absorption of a nutrient by the plant depends on the cations dissolved in the solution in dynamic equilibrium with the cations of the nutritious solution exchange complex [32]. The absorption of a nutrient is also affected by the nature of the complementary cations, that is to say, there is influence of an ion adsorbed in the release of another ion to the solution, besides the relations that involve the cations [33].

The elevation of K content in the solution can induce nutritional imbalance for the plants, due to antagonism, competitive inhibition, and noncompetitive inhibition among nutrients, in addition to synergism, which can cause a differentiated dynamics between cations in the leaves and roots of plants. However, little is known about the interactions between cations caused by excess doses of K2 O induced by different sources. When the K<sup>2</sup> O doses are increased, regardless of the source used, the electrical conductivity increases linearly (**Figure 7**). However, it is observed that the values of electrical conductivity are significantly higher with the use of KCl, indicating an increase in nutritious solution salinity [34].

The electrical conductivity ranges between 3.82 and 1.33, with a mean of 2.49 dS m−1 when a dose of K2 O 250 kg ha−1 for KCl fertilizer was applied, whereas values were between 4.24 and 0.86 dS m−1 and averaged 2.55 dS m−1 for K2 SO4 (**Figure 8A**). A decreasing trend of electrical conductivity was evidenced during the experimental period, and this reduction was more pronounced during 60 days after transplantation because of the onset of flowering and fruiting. In case of K2 O 500-kg ha−1 KCl dose, the electrical conductivity ranges between 3.46 and 0.89 dS m−1 and the average of 2.16 dS m−1, while the range was between 3.30 and 0.28 dS m−1 with a mean of 1.79 dS m−1 for K2 SO4 as potassium source (**Figure 8B**). A greater fluctuation of electrical conductivity was observed after K2 O 750 kg ha−1 especially for KCl, and the range was as high as 6.27 and as low as 1.30 having an average of 3.78 dS m−1 (**Figure 8C**). When K2 SO4 was applied, electrical conductivity values obtained were between 4.27 and 1.03 dS m−1 with a mean of 2.65 dS m−1. Subsequently, at a dose of K2 O 1000-kg ha−1 electrical conductivity in KCl and K2 SO4 treatments remained within the ranges from 7.12 to 1.82 and from 3.36 to 1.25 dS m−1 (**Figure 8D**), with a mean of 4.47 and 2.11 dS m−1, respectively [35].

The use of K2 SO4 as a source of potassium fertilization generates a direct form of competition with Mg2+ in the roots of eggplants, high doses of K2 O affect production, and excess K induces

**Figure 7.** Electrical conductivity (EC) in function to sources and potassium doses.

Comparatively higher root volume was found when potassium chloride was used as the source of potassium fertilization than potassium sulfate (**Figures 5** and **6**). Considering the

**Figure 6.** Roots of plants of eggplant (*Solanum melongena* L.), grow crops "Embu," according to doses and potassium sources.

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

stressing effect on the plant. On the other hand, with KCl as source there is no definite trend of increase or decrease in the root volume, values found being stable and higher than those

Many problems have been observed related to excessive fertilization, leading the nutritious solution to an accumulation of salts. Although the water used in irrigation in the protected crop is of good quality, using the fertigation technique increases the risk of salinization [31]. In the process of nutrient absorption, the cationic interactions at the adsorption sites and the concentration of nutrient ions in the solution are important aspects in plant nutrition and crop production. The rate of absorption of a nutrient by the plant depends on the cations dissolved in the solution in dynamic equilibrium with the cations of the nutritious solution exchange complex [32]. The absorption of a nutrient is also affected by the nature of the complementary cations, that is to say, there is influence of an ion adsorbed in the release of another ion to the

, it is observed that the root volume increases with increasing doses, up to an

O; from here, there is a decrease, indicating a

use of K2

SO4

found with the K2

estimated maximum value of 645 kg ha−1 K2

source [30].

solution, besides the relations that involve the cations [33].

**7. Cation dynamics in leaves and fruits of vegetables**

SO4

**Figure 8.** Electrical conductivity (EC) corrected for function and the sources and doses 250 (A), 500 (B), 750 (C) and 1000 (D) kg K<sup>2</sup> O (KCl and K2 SO4 ) in relation to the days after transplantation (DAT).

are disputed at the same site of the carrier in the membrane. No competitive inhibition happens when binding occurs at different sites of the carrier. In the first case, the effect of the inhibitor can be annulled by increasing the concentration of the inhibited element, which does not occur at the second case. An example of competitive inhibition is observed

**Ion present Second ion Effect of the second on the first**

<sup>+</sup> Not competitive inhibition

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, NH<sup>4</sup> Not competitive inhibition

<sup>+</sup> Competitive inhibition

2− Competitive inhibition

2− Competitive inhibition

<sup>−</sup> Not competitive inhibition

<sup>+</sup> Competitive inhibition

2− Synergism

<sup>−</sup> Competitive inhibition

2− Not competitive inhibition

Mg2+, Ca2+ K+ Competitive inhibition

K+ Ca2+ (high concentration) Competitive inhibition

2− Cl<sup>−</sup> Competitive inhibition

Zn2+ Mg2+ Competitive inhibition Zn2+ Ca2+ Competitive inhibition

K+ Ca2+ (low concentration) Synergism

Synergism occurs when the presence of one element enhances the absorption of another, for example, Ca2+ in low concentrations increases the absorption of cations and anions (Viets effect), due to its role in maintaining the functional integrity of membranes, which has a consequence in the practice of fertilization; another example is Mg2+ which increases the absorp-

The black bottom or rot apical of the tomato (**Figure 9**) is a very common anomaly in fruits. It can cause high losses, above 50% of the fruits produced, especially in the lower parts. It is characterized by black spots, hard and dry in the apical extremity, and well visible from the formation of the fruits. The main cause is the lack of calcium in the tissue, caused by the competitive inhibition between K, Ca, and Mg, which causes Ca deficiency. This anomaly occurs very frequently in tomato culture in hydroponic system, because of the accelerated growth of plant, due to the environment conditions and the fact that calcium is still in the plant's

phloem. This problem is aggravated when water deficiency occurs.

between Ca, Mg, and K [36].

H2 PO4

K+

H2 BO3

SO4

SO4

MoO4

MoO4

<sup>−</sup> Al3

<sup>−</sup> NO<sup>3</sup>

2− SeO4

2− SO4

Zn2+ H2

Fe2+ Mn2

Zn2+ H2

<sup>−</sup> H2

Cu2+ MoO4

−

BO3

PO4

PO4

**Table 5.** Examples of interionic effects [36].

, Ca2+ Al3

tion of phosphorus [36].

competitive inhibition between cations; however, the use of K2 SO4 is less harmful, when used in excess, than that of KCl [34].

The elements are absorbed by the plants at different speeds, generally following the decreasing order as follows:


The accompanying ion, as a consequence of this, also influences at the absorption of its pair, so, for example, the maximum absorption of NH<sup>4</sup> + will occur when it is accompanied by NO<sup>3</sup> − , the speed will be minimal if accompanied by H2 PO4 − . **Table 5** presents examples of interionic effects.

The inhibition consists in the reduction of the mineral absorption due to the presence of another one, being considered competitive inhibition when the element and the inhibitor


**Table 5.** Examples of interionic effects [36].

competitive inhibition between cations; however, the use of K2

2− > H2

PO4

> Mg2+ > Ca2+

The elements are absorbed by the plants at different speeds, generally following the decreas-

**Figure 8.** Electrical conductivity (EC) corrected for function and the sources and doses 250 (A), 500 (B), 750 (C) and 1000

) in relation to the days after transplantation (DAT).

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

The accompanying ion, as a consequence of this, also influences at the absorption of its pair,

The inhibition consists in the reduction of the mineral absorption due to the presence of another one, being considered competitive inhibition when the element and the inhibitor

+

PO4 −

in excess, than that of KCl [34].

SO4

O (KCl and K2

− > Cl<sup>−</sup>

+ > K+ > SO4

> Na<sup>+</sup>

so, for example, the maximum absorption of NH<sup>4</sup>

the speed will be minimal if accompanied by H2

ing order as follows:

**1.** Anions—NO<sup>3</sup>

**2.** Cations—NH<sup>4</sup>

effects.

(D) kg K<sup>2</sup>

SO4

will occur when it is accompanied by NO<sup>3</sup>

. **Table 5** presents examples of interionic

is less harmful, when used

− , are disputed at the same site of the carrier in the membrane. No competitive inhibition happens when binding occurs at different sites of the carrier. In the first case, the effect of the inhibitor can be annulled by increasing the concentration of the inhibited element, which does not occur at the second case. An example of competitive inhibition is observed between Ca, Mg, and K [36].

Synergism occurs when the presence of one element enhances the absorption of another, for example, Ca2+ in low concentrations increases the absorption of cations and anions (Viets effect), due to its role in maintaining the functional integrity of membranes, which has a consequence in the practice of fertilization; another example is Mg2+ which increases the absorption of phosphorus [36].

The black bottom or rot apical of the tomato (**Figure 9**) is a very common anomaly in fruits. It can cause high losses, above 50% of the fruits produced, especially in the lower parts. It is characterized by black spots, hard and dry in the apical extremity, and well visible from the formation of the fruits. The main cause is the lack of calcium in the tissue, caused by the competitive inhibition between K, Ca, and Mg, which causes Ca deficiency. This anomaly occurs very frequently in tomato culture in hydroponic system, because of the accelerated growth of plant, due to the environment conditions and the fact that calcium is still in the plant's phloem. This problem is aggravated when water deficiency occurs.

**Figure 9.** Physiological anomaly called black bottom or apical rot.

#### **8. Changes in leaf proline protein induced by potassium**

The proline concentration was significantly modified independently of potassium source, and higher level in this parameter occurred under potassium rate of K2 O 1000 kg ha−1 [35], as shown in **Figure 10A**. As for the protein content, with the increase in K2 O concentration there was a reduction in the content (**Figure 10B**).

Under normal conditions, proline is produced using glutamate and arginine while glutamate is the main pathway in stress conditions [37]. When plant experiences stress such as inadequate situations of mineral, salt, and water, proline protects the cell against denaturation processes, because this organic compound is highly soluble in water. It is accumulated in the cytoplasm of cells present in leaves, stems, and roots. Abiotic stresses like salt stress to *Oryza sativa* plants showed several biochemical consequences at different proline levels [38]. Significant changes in *Glycine max* plants under water deficit as an abiotic stress [39] were also found.

In tomato culture, the accumulation of proline was detected within the first 24 h of the beginning of the treatment with excess fertilizers, observing its osmoregulatory activity. Halophytic or glycophytic plants adapt to high saline concentrations by lowering the osmotic potential of their tissues, with increased solutes absorption (Na and Cl ions). However, in less tolerant species, the growth is inhibited in function to the toxic effect of the accumulation of solutes [41].

**Figure 10.** Concentration of proline (A) and soluble protein (B) in the gram of fresh matter mass (MMF) in function to

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Plants have a high requirement for K for mainly maintaining a high K content in the cytoplasm, mainly to ensure enzyme activity [42]. A high concentration of K in cytosol and chloroplast stroma is also required to maintain anion neutralization and an appropriate pH level for cell functioning [21]. It can also participate in the control of stomatal opening and closing which is essential for photosynthesis. Despite the great importance of K, excess of it can reduce the osmotic potential of the solution, making the nutritious solution saline, resulting in a modified nutritious solution in which the growth of most species is prejudiced by the presence of

**9. Effect of potassium sources on the antioxidant activity**

the sources and doses of potassium.

Some authors affirm that proline has functions linked to processes of adaptation to water deficit; however, others point to proline as an indicator of stress. Although there is no clear evidence of proline accumulation in tolerant species, its accumulation in species sensitive to water deficit has been observed, and this mechanism seems to be part of the protection against this type of stress [40].

The synthesis of proline has special importance in plants, because it is closely related to the water potential of the tissues. Plants in conditions of water stress or saline have high levels of proline compared to plants under normal conditions. This phenomenon seems to be related to the mechanism of protection against lack of water, because proline helps reduce the water potential of tissues and thus retain water. It is not by chance that the solubility of proline is much superior (162 g in 100 mL) than that of the other protein amino acids (in the range of <1–25 g in 100 mL). Although the two proline synthesis pathways are equally important under normal conditions, the evidence is more favorable to direct glutamate pathway (without acetylation) in water stress conditions [39].

**Figure 10.** Concentration of proline (A) and soluble protein (B) in the gram of fresh matter mass (MMF) in function to the sources and doses of potassium.

In tomato culture, the accumulation of proline was detected within the first 24 h of the beginning of the treatment with excess fertilizers, observing its osmoregulatory activity. Halophytic or glycophytic plants adapt to high saline concentrations by lowering the osmotic potential of their tissues, with increased solutes absorption (Na and Cl ions). However, in less tolerant species, the growth is inhibited in function to the toxic effect of the accumulation of solutes [41].

#### **9. Effect of potassium sources on the antioxidant activity**

**8. Changes in leaf proline protein induced by potassium**

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

higher level in this parameter occurred under potassium rate of K2

was a reduction in the content (**Figure 10B**).

**Figure 9.** Physiological anomaly called black bottom or apical rot.

this type of stress [40].

ylation) in water stress conditions [39].

shown in **Figure 10A**. As for the protein content, with the increase in K2

The proline concentration was significantly modified independently of potassium source, and

Under normal conditions, proline is produced using glutamate and arginine while glutamate is the main pathway in stress conditions [37]. When plant experiences stress such as inadequate situations of mineral, salt, and water, proline protects the cell against denaturation processes, because this organic compound is highly soluble in water. It is accumulated in the cytoplasm of cells present in leaves, stems, and roots. Abiotic stresses like salt stress to *Oryza sativa* plants showed several biochemical consequences at different proline levels [38]. Significant changes

Some authors affirm that proline has functions linked to processes of adaptation to water deficit; however, others point to proline as an indicator of stress. Although there is no clear evidence of proline accumulation in tolerant species, its accumulation in species sensitive to water deficit has been observed, and this mechanism seems to be part of the protection against

The synthesis of proline has special importance in plants, because it is closely related to the water potential of the tissues. Plants in conditions of water stress or saline have high levels of proline compared to plants under normal conditions. This phenomenon seems to be related to the mechanism of protection against lack of water, because proline helps reduce the water potential of tissues and thus retain water. It is not by chance that the solubility of proline is much superior (162 g in 100 mL) than that of the other protein amino acids (in the range of <1–25 g in 100 mL). Although the two proline synthesis pathways are equally important under normal conditions, the evidence is more favorable to direct glutamate pathway (without acet-

in *Glycine max* plants under water deficit as an abiotic stress [39] were also found.

O 1000 kg ha−1 [35], as

O concentration there

Plants have a high requirement for K for mainly maintaining a high K content in the cytoplasm, mainly to ensure enzyme activity [42]. A high concentration of K in cytosol and chloroplast stroma is also required to maintain anion neutralization and an appropriate pH level for cell functioning [21]. It can also participate in the control of stomatal opening and closing which is essential for photosynthesis. Despite the great importance of K, excess of it can reduce the osmotic potential of the solution, making the nutritious solution saline, resulting in a modified nutritious solution in which the growth of most species is prejudiced by the presence of high concentrations of soluble salts, exchangeable Na, or both in the rhizosphere [43]. Among the potassium fertilizers available on the Brazilian market, KCl is the most popular. Besides, K2 SO4 , K2 SO4 · 2MgSO4 , and other K sources are widely used in different agricultural segments in Brazil [44]. The above K source fertilizers produce different levels of salinity in nutritious solution, as, for example, KCl has a higher salt content than K2 SO4 . In the case of potato and eggplant, KCl application has resulted in lower yields compared to K2 SO4 [41].

The enzymatic activity of catalase (CAT) is an enzyme that increases the rate of dismutation of the superoxide radical in hydrogen peroxide and is considered as an antioxidant enzyme (reactive oxygen species—ROS). CAT activity increases with increasing K2 O concentrations (**Figure 11**). High rates of KCl and K2 SO4 increased the proline concentration at higher doses and reduced the protein concentration (**Figure 10**). The proline content of the leaf and the development of the eggplants are larger for the K2 SO4 source [41].

Salinity can restrict the absorption of water and nutrients, reduce photosynthetic processes, and increase respiration, inducing a reduction in plant growth [45]. In the case of water deficit, the activity of the enzyme system and the production of compounds related to the antioxidant system of plants are altered [46]. This plant response occurs due to excessive accumulation of ROS in plant cells, in particular of superoxide, hydroxyl radical, and hydrogen peroxide [47]. Salinity can promote an intense ROS production that can lead to the degradation of proteins and membranes, reducing photosynthesis and plant growth [48]. Among the enzymatic mechanisms involved in detoxification of ROS, there are the isoforms of the enzyme such as superoxide dismutase (SOD), CAT, ascorbate peroxidase (APX), and peroxidase phenols (POX). SOD acts by converting O2 into H2 O2 and is localized mainly in the mitochondria and chloroplasts. These organelles generate most of the ROS in plant cells [49]. Peroxidases and catalases convert

H2 O2

 into water and molecular oxygen, which are harmless to plants. Although the salinization leads to the production of ROS, at certain concentrations, K has an effect of reducing the harmful effects of salinization and ROS, mitigating stress effects [50]. This effect has been widely investigated in view of the need to understand its relationship with salinity and stress tolerance better. **Figure 12** [51] shows the general scheme of salt and drought stress tolerance in plants. Some osmolytes are involved in salt and drought stress tolerance through osmoprotection and

K is usually the most abundant cation in the cultures, being found in the tissues in greater

content of carbohydrates, oils, fats, and proteins; stimulates the filling of the grains, reducing the chopping; promotes storage of sugar and starch; helps symbiotic N fixation; increases the use of water; and increases resistance to droughts, frosts, pests, and diseases. As K improves the quality of agricultural products, it is described as the "quality nutrient." It is interesting to note the high correlation of K and proteins in the seeds of several cultivated plants, since cultures with high protein contents require (and export) large amounts of K through the grains. Among the essential mineral nutrients for plants, K stands out for its influence in quality attributes that affect the concentration of phytonutrients critical for human health. However, many

). K stimulates vegetation and tillering (grasses); increases the

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ROS detoxification. They protect the plant from osmotic and ionic stresses [51].

**10. Potassium increases crops quality**

**Figure 12.** A general scheme of salt and drought stress tolerance in plants.

proportion in the ionic form (K+

**Figure 11.** Catalase activity (mKat mg−1 of protein) as a function of potassium sources and doses.

**Figure 12.** A general scheme of salt and drought stress tolerance in plants.

high concentrations of soluble salts, exchangeable Na, or both in the rhizosphere [43]. Among the potassium fertilizers available on the Brazilian market, KCl is the most popular. Besides,

in Brazil [44]. The above K source fertilizers produce different levels of salinity in nutritious

The enzymatic activity of catalase (CAT) is an enzyme that increases the rate of dismutation of the superoxide radical in hydrogen peroxide and is considered as an antioxidant enzyme

and reduced the protein concentration (**Figure 10**). The proline content of the leaf and the

Salinity can restrict the absorption of water and nutrients, reduce photosynthetic processes, and increase respiration, inducing a reduction in plant growth [45]. In the case of water deficit, the activity of the enzyme system and the production of compounds related to the antioxidant system of plants are altered [46]. This plant response occurs due to excessive accumulation of ROS in plant cells, in particular of superoxide, hydroxyl radical, and hydrogen peroxide [47]. Salinity can promote an intense ROS production that can lead to the degradation of proteins and membranes, reducing photosynthesis and plant growth [48]. Among the enzymatic mechanisms involved in detoxification of ROS, there are the isoforms of the enzyme such as superoxide dismutase (SOD), CAT, ascorbate peroxidase (APX), and peroxidase phenols (POX). SOD

These organelles generate most of the ROS in plant cells [49]. Peroxidases and catalases convert

SO4

source [41].

and is localized mainly in the mitochondria and chloroplasts.

solution, as, for example, KCl has a higher salt content than K2

(**Figure 11**). High rates of KCl and K2

acts by converting O2

development of the eggplants are larger for the K2

into H2

O2

**Figure 11.** Catalase activity (mKat mg−1 of protein) as a function of potassium sources and doses.

eggplant, KCl application has resulted in lower yields compared to K2

(reactive oxygen species—ROS). CAT activity increases with increasing K2

SO4

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

, and other K sources are widely used in different agricultural segments

SO4

increased the proline concentration at higher doses

SO4 [41].

. In the case of potato and

O concentrations

K2 SO4 , K2 SO4 · 2MgSO4

> H2 O2 into water and molecular oxygen, which are harmless to plants. Although the salinization leads to the production of ROS, at certain concentrations, K has an effect of reducing the harmful effects of salinization and ROS, mitigating stress effects [50]. This effect has been widely investigated in view of the need to understand its relationship with salinity and stress tolerance better. **Figure 12** [51] shows the general scheme of salt and drought stress tolerance in plants. Some osmolytes are involved in salt and drought stress tolerance through osmoprotection and ROS detoxification. They protect the plant from osmotic and ionic stresses [51].
