**7. Potassium and plant physiology**

Potassium is present in plant cells in cationic form K and plays an important role in the physiological activity of plants. In general, K has a close functional relationship with photosynthetic metabolism, enzyme activation, protein synthesis, osmotic regulation, ionic homeostasis, and regulation of stomatal movement [59].

The modulation of photosynthetic activity by K occurs at several levels; however, its role in ionic equilibrium shows to be a major one. For example, K is a dominant ion that promotes the balance of positive charges due to the light-stimulated H+ flux through the thylakoid membranes. In addition, it contributes to the generation of transmembrane pH gradient necessary for the synthesis of ATP by photophosphorylation. To maintain high pH (low H+ ) in the stroma during light, additional K influx from the cytosol is required, in a process mediated by an H<sup>+</sup> /K+ antiporter carrier [3]. However, the osmotic regulation of guard cells by K is a relevant factor in the control of gas exchange and water losses in plants, due to smaller or larger stomatal opening [72].

There is a set of experimental evidences that show a relationship between potassium nutrition and post-harvest quality of vegetables, since several studies report the effect of potassium nutrition on the post-harvest of fruits and leafy vegetables. In tomato fruits, the potassium nutrition provided under fertirrigation increased the concentration of lycopene in genotypes with contrasting production of lycopene [60]. This pigment or bioactive compound is associated with important antioxidant functions by acting on the detoxification of free radicals, reducing the appearance of cancers such as prostate [61, 62] and avoiding the onset of heart disease [63].

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

In another study about potassium fertilization in tomato plants, there is a linear increase in the concentration of lycopene with potassium fertilization [64]. This close relationship between potassium fertilization and concentration of lycopene in tomatoes seems to be related to enzymatic activation function exerted by K; more than one enzyme of metabolism of lycopene synthesis has the K activation cofactor, for example, phytoene desaturase or phytoene synthase, an enzyme that catalyzes the reaction of phytoene synthase from geranylgeranyl

Another important antioxidant, vitamin C or acid ascorbic, is positively influenced by potassium nutrition, since several studies report a higher concentration of vitamin C as a function of potassium nutrition, as observed in pepper [66] and chili. K is responsible for the uniform ripening and the increase of acidity of fruit that is an important characteristic for quality and

With the advancement of ripening process, tomato fruits present changes in their characteristics as flavor and color. The taste of tomato is attributed to the content of soluble solids [68], acids, and volatile compounds [69]. The total soluble solids present greater accumulation in the final phase of maturation also is constituted of 65% of sugars (sucrose and fructose).

Soluble solids present in fruits such as watermelon, melon, tomato, and strawberry include important compounds responsible for taste and consequent acceptance by consumers, and the most important are sugars and organic acids. In general, there is a close relationship between potassium nutrition and soluble solids content as evidenced in several studies such as tomato fruit [60, 70]. It should be considered that the production of soluble solids is a genetic charac-

Potassium is present in plant cells in cationic form K and plays an important role in the physiological activity of plants. In general, K has a close functional relationship with photosynthetic metabolism, enzyme activation, protein synthesis, osmotic regulation, ionic homeostasis, and

The modulation of photosynthetic activity by K occurs at several levels; however, its role in ionic equilibrium shows to be a major one. For example, K is a dominant ion that promotes the bal-

In addition, it contributes to the generation of transmembrane pH gradient necessary for the

flux through the thylakoid membranes.

teristic, but it is influenced by ambient temperature, irrigation, and fertilization [71].

diphosphate, which is the first step in the route of carotenoid synthesis [65].

flavor of fruit [67].

**7. Potassium and plant physiology**

regulation of stomatal movement [59].

ance of positive charges due to the light-stimulated H+

The K, as well as other univalent cations, activates enzymes by inducing conformational changes in their structures, making them biologically active and these changes are possible due to electrostatic bonding of K to enzyme [73]. Thus, K contributes to the occurrence of group of biochemical reactions of great physiological importance, such as the activation of carbohydrate metabolism enzyme in particular of pyruvate kinase (EC 2.7.1.40) and phosphofructokinase (EC 2.7.1.11), which catalyze the transference of phosphoric groups to pyruvate and D-fructose 6-phosphate, respectively [74].

Regarding protein synthesis, K is required at higher concentrations compared to its enzymatic activation function, which, for this function, the required K concentration is about 50 mM [3]. Due to osmotic properties, K plays an important role in the opening of stomata during the first hours of the day. This physiological phenomenon consists of influx of K in the guinea pigs, where the concentration of K increases from 100 to 400 mM or 800 mM, but decreasing throughout the day [72, 75].

K is also involved in the loading of sugars into phloem in a process coordinated by AKT3/3 like channels located on the brassicaea phloem *Arabidopsis thaliana* [76]. This process of loading sugars into phloem has great physiological importance because it allows the translocation of sugars from the source tissues to drainage tissues to supply the needs of growing organ such as roots system, fruits, and flowers [72].

Under conditions of adequate potassium availability, K uptake occurs through low-affinity transporters [77], with K translocation to plant tissues and redistribution to drain tissues. At the same time, photoassimilates are produced by the photosynthetic process and transport the same to drainage organs, with the consequent development and growth of plants (**Figure 1**). On the other hand, K deficiency triggers a set of changes in plants, which initially manifests in gene and molecular plane and then in physiological and morphological plane, culminating in the reduction of plant growth.

With low concentration of K in the plant growth substrate, carbohydrate metabolism disorders occur due to K being in low cell concentration to activate key enzymes of carbohydrate metabolism. With this, there is accumulation and inhibition of transport of photoassimilates from the source organs to draining organs, with the reduction of photosynthetic activity. In another way, K deficiency leads to changes in gas exchange, with reduction of stomatal conductance, reduction of CO2 diffusion, and photosynthesis.

As a consequence, the consumption of NADPH-reducing power by Calvin cycle decreases, with the superduction of electron transport chain and the generation of free radicals, culminating in photooxidation, followed by chlorosis and foliar necrosis (**Figure 1**). In the attempt to reestablish ionic homeostasis, plants promote redistribution of accumulated K; however, due to classical relationship between K, Ca, and Mg, these latter two macronutrients are absorbed in greater intensity, compared to K, under conditions of K deficiency (**Figure 1**).

such as pyruvate kinase are negatively affected, inhibiting glycolysis and causing a series of

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

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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

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.

**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].

metabolic disorders following the tricarboxylic acid cycle [81].

more susceptible to attack by pests and diseases.

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

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