**8. Potassium deficiency and toxicity symptoms in vegetables**

As in other nutrients, K deficiency changes at different levels. The deleterious effects of deficiency begin in the dynamics of metabolism in biochemical plane, evolving at molecular, subcellular, and cellular levels, until reaching the tissues [23]. The visual perception of deficiency occurs when it reaches the level of tissue, that is to say that when verifying visual damages, a series of events deleterious to the development of plant have already occurred.

The first structures to experience potassium deficiency are the roots where there is a drastic reduction of nitrate levels, intermediates of glycolytic route (pyruvate), amino acids like malate and oxoglutarate, negatively charged amino acids such as glutamate and aspartate, increase in levels of soluble carbohydrates (sucrose, glucose, and fructose), and many amino acids with high C/N and/or positively charged amino acids such as glutamine, glycine, and arginine [81]. In addition, there is less transport of photoassimilates from the aerial part to roots, thus reducing root growth. In the strawberry, potassium deficiency increases the amount of root exudates favoring the infection and colonization of *Fusarium oxysporum* [82].

In the leaves, in the beginning of K deficiency, the content of this nutrient in the cytosol remains constant, although there is a decrease in the K content of vacuoles, probably because this reserve structure of K supplies the demand of cytosol. As deficiency persists and plant demand increases, cytosolic K reduction also occurs [83]. With this reduction, some enzymes such as pyruvate kinase are negatively affected, inhibiting glycolysis and causing a series of metabolic disorders following the tricarboxylic acid cycle [81].

There is an increase in the content of soluble carbohydrates in leaves due to reduction of their conversion to starch and reduction in the content of α-ketoglutarate derivatives [84–86]. Due to the decrease in the synthesis of compounds of higher-molecular weight, the leaves become more susceptible to attack by pests and diseases.

However, Carmona et al. [87] observed in cucumber plants the initial visual symptoms of K in the intermediate leaves (**Figure 2**). These authors suggest that the appearance of symptoms in intermediate leaves occurs as a function of time when the omission of K was applied (beginning of the fruiting), because the fruits in formation are strong drains of this nutrient, being the adjacent leaves to satisfy the demand of K of the fruits. In plants with undetermined growth, such as bean pod case (**Figure 3**) and in reproductive stages, potassium deficiency can also be observed in intermediate leaves, since leaves close to reproductive structures tend to supply K reproductive organs.

K is very mobile in plant being visible deficiency symptoms in the more mature leaves. It begins with a marginal chlorosis, which can also occur at leaf tips, followed by foliar limb necrosis and even necrotic part breakage (**Figure 4**) [88]. Carmona et al. [87] observed in cucumber plants the initial visual symptoms of K in the intermediate leaves (**Figure 2**). These authors suggest that the appearance of the symptoms in intermediate leaves occurs as a function of time when the omission of K was applied (beginning of the fruiting), because the fruits in formation are strong drains of this nutrient, being the adjacent leaves to satisfy a demand of K in fruits. In plants with undetermined growth, such as the bean pod case (**Figure 3**) and in reproductive stages, potassium deficiency can also be observed in intermediate leaves, since leaves close to reproductive structures tend to supply K reproductive organs.

Due to importance of K in plant physiology and nutrition in relevant literature, several studies report negative effect of K omission on growth (i.e., plant height, leaf area, shoot dry mass accumulation, and root) and mineral metabolism of plants (i.e., nutritional balance of plants),

As in other nutrients, K deficiency changes at different levels. The deleterious effects of deficiency begin in the dynamics of metabolism in biochemical plane, evolving at molecular, subcellular, and cellular levels, until reaching the tissues [23]. The visual perception of deficiency occurs when it reaches the level of tissue, that is to say that when verifying visual damages, a

The first structures to experience potassium deficiency are the roots where there is a drastic reduction of nitrate levels, intermediates of glycolytic route (pyruvate), amino acids like malate and oxoglutarate, negatively charged amino acids such as glutamate and aspartate, increase in levels of soluble carbohydrates (sucrose, glucose, and fructose), and many amino acids with high C/N and/or positively charged amino acids such as glutamine, glycine, and arginine [81]. In addition, there is less transport of photoassimilates from the aerial part to roots, thus reducing root growth. In the strawberry, potassium deficiency increases the amount of root exudates favoring the infection and colonization of *Fusarium oxysporum* [82]. In the leaves, in the beginning of K deficiency, the content of this nutrient in the cytosol remains constant, although there is a decrease in the K content of vacuoles, probably because this reserve structure of K supplies the demand of cytosol. As deficiency persists and plant demand increases, cytosolic K reduction also occurs [83]. With this reduction, some enzymes

like was observed in beet [78], cabbage [79], lettuce [80], and eggplant [81].

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

**Figure 1.** Physiological response of plants to normal and deficient potassium supply.

**8. Potassium deficiency and toxicity symptoms in vegetables**

series of events deleterious to the development of plant have already occurred.

**Figure 2.** Initial chlorosis (A and B) and advance to marginal necrosis (C) in intermediary leaves of "Nikkey" cucumber as an effect of the omission of K in the nutrient solution. Source: [87].

The deficiency of K leads to lower protein synthesis and accumulation of soluble nitrogen compounds such as putrescine, toxic to plants [88]. The central area of leaves may be dark green in color, similar to phosphorus deficiency and a thickening of nervure in leaves (**Figure 4**) [88].

However, the molecular changes triggered by K deficiency are not restricted only to old leaves, because in new leaves there is an increase in the levels of oxaloacetate and phospho-

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

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

35

In fruits, the deficiency of this macronutrient decreases the glucose, fructose, and sucrose levels, and in plants of Rosaceae family, it decreases sorbitol levels [90], and in species with large amounts of lycopene such as tomato [91], there is less red color intensity due to reduction in

Several studies have demonstrated the important role of K in plant growth and development, with a considerable decrease in the production of dry mass of plants when K is omitted from culture medium [92]. In sugar beet, the omission of K promoted a considerable reduction of the dry mass of the aerial part and radicular, besides promoting symptoms of nutritional disorders characterized by the appearance of chlorosis markedly red of margins of leaves and

In other study with cabbage, the omission of K was limiting for the vegetative growth of cabbage, considerably reducing the height of plants, the number of leaves, leaf area, and dry matter of shoot, roots, and whole plant. The deficiency of K, besides promoting a decrease of nutrient content in the aerial part, caused imbalance between the other nutrients and, consequently, morphological alterations, translated as characteristic symptoms of K deficiency [79]. The absence of K in lettuce plants led to reduced growth, with a considerable decrease in height, leaf area, dry mass of shoot, root, and emergence of nutritional disorders [80]. In cultivation under omission of K and eggplants a reduction of dry mass of shoot and root, a decrease of leaf area, height and number of leaves were observed, in parallel to nutritional

The great demand of plants for potassium and their capacity to accumulate this nutrient in the vacuoles makes the observation of symptoms of excess something very rare [23]. When K is excessed, it is also induced to plants; the use of sources such as potassium nitrate and potassium chloride (KNO3 = 74; KCl = 116) [94] can generate physiological disorders and affect the development of plant. Therefore, experiments with excess of potassium should be recommended sources with lower salt content. In addition, attention should be paid to the effect of accompanying ions that may cause toxicity, and excess K in the nutrient solution may increase or decrease the absorption of other nutrients. The increase of potassium provides an increase in the absorption of nitrate and, on the other hand, can lead to lower absorption of Mg and Ca [23, 94].

K is an essential mineral nutrient for humans because of its important physiological role in conducting electrical impulses in nerve tissue, electrolyte balance of body fluids, and blood pressure control. Ingestion of food of plant origin (i.e., vegetables, grains, cereals, etc.) and animal (i.e., meat, milk, eggs, etc.) are the main sources of K in human food. However, the process of cooking, the increased consumption of processed foods, and the decreased consumption of vegetables and fruits implied a reduction in K intake by the population [95].

enolpyruvate derivatives [86].

the synthesis of this pigment [60, 64].

evolving toward necrosis until reaching the leaf apex [78].

disorders resulting from the omission of K [93].

**9. Potassium in human health and food consumed**

**Figure 3.** Potassium deficiency in bean pod plants (-K) compared to control (CS). Source: [89].

**Figure 4.** Progressive symptoms of K deficiency in the leaves of the cauliflower "Verona" after being supplied with a nutrient solution without K. Chlorosis and initial necrosis of the nervures (A), advanced stage of necrosis of the nervures (B), and advanced foliar necrosis (C). Source: [88].

However, the molecular changes triggered by K deficiency are not restricted only to old leaves, because in new leaves there is an increase in the levels of oxaloacetate and phosphoenolpyruvate derivatives [86].

The deficiency of K leads to lower protein synthesis and accumulation of soluble nitrogen compounds such as putrescine, toxic to plants [88]. The central area of leaves may be dark green in color, similar to phosphorus deficiency and a thickening of nervure in leaves (**Figure 4**) [88].

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

**Figure 4.** Progressive symptoms of K deficiency in the leaves of the cauliflower "Verona" after being supplied with a nutrient solution without K. Chlorosis and initial necrosis of the nervures (A), advanced stage of necrosis of the nervures

(B), and advanced foliar necrosis (C). Source: [88].

**Figure 3.** Potassium deficiency in bean pod plants (-K) compared to control (CS). Source: [89].

In fruits, the deficiency of this macronutrient decreases the glucose, fructose, and sucrose levels, and in plants of Rosaceae family, it decreases sorbitol levels [90], and in species with large amounts of lycopene such as tomato [91], there is less red color intensity due to reduction in the synthesis of this pigment [60, 64].

Several studies have demonstrated the important role of K in plant growth and development, with a considerable decrease in the production of dry mass of plants when K is omitted from culture medium [92]. In sugar beet, the omission of K promoted a considerable reduction of the dry mass of the aerial part and radicular, besides promoting symptoms of nutritional disorders characterized by the appearance of chlorosis markedly red of margins of leaves and evolving toward necrosis until reaching the leaf apex [78].

In other study with cabbage, the omission of K was limiting for the vegetative growth of cabbage, considerably reducing the height of plants, the number of leaves, leaf area, and dry matter of shoot, roots, and whole plant. The deficiency of K, besides promoting a decrease of nutrient content in the aerial part, caused imbalance between the other nutrients and, consequently, morphological alterations, translated as characteristic symptoms of K deficiency [79].

The absence of K in lettuce plants led to reduced growth, with a considerable decrease in height, leaf area, dry mass of shoot, root, and emergence of nutritional disorders [80]. In cultivation under omission of K and eggplants a reduction of dry mass of shoot and root, a decrease of leaf area, height and number of leaves were observed, in parallel to nutritional disorders resulting from the omission of K [93].

The great demand of plants for potassium and their capacity to accumulate this nutrient in the vacuoles makes the observation of symptoms of excess something very rare [23]. When K is excessed, it is also induced to plants; the use of sources such as potassium nitrate and potassium chloride (KNO3 = 74; KCl = 116) [94] can generate physiological disorders and affect the development of plant. Therefore, experiments with excess of potassium should be recommended sources with lower salt content. In addition, attention should be paid to the effect of accompanying ions that may cause toxicity, and excess K in the nutrient solution may increase or decrease the absorption of other nutrients. The increase of potassium provides an increase in the absorption of nitrate and, on the other hand, can lead to lower absorption of Mg and Ca [23, 94].
