**5. Potassium salinity in nutrient solution and its effects on metabolism**

Potassium is an essential nutrient for all living beings, playing a key role in photosynthesis, which is the transformation of light energy into chemical energy (ATP and NADPH). As all vital plant functions depend directly or indirectly on ATP and NADPH, the influence of K on plant metabolism becomes evident. It also plays an important role in the activation of more than 60 enzymes, which act on several metabolic processes such as photosynthesis, protein synthesis, and carbohydrates, also affecting water balance and the growth of meristematic tissues [10].

K absorbed by the root is led to the aerial part by the xylem and phloem, its internal redistribution is quite easy. The element is directed from the older leaves to the younger leaves, to the growing regions and to the fruits. This is due, in part, to the fact that about 75% of plant potassium is soluble in tissues.

Cultures differ in their K requirements because of differences in the physiological functions in which K is involved. Cultures where the harvested part consists of young plant tissue, as is the case of leafy vegetables and fruits, have high requirements of K per unit of dry weight produced. When the same crop is harvested at the complete maturation stage, the requirement for potassium per dry weight unit is substantially lower. Cultures that produce fleshy fruits or storage organs have high K requirement when compared to cereals [11].

Among the various functions of potassium in plants, water use efficiency is better cited, as a consequence of the control of the opening and closing of the stomata, greater translocation of carbohydrates from the leaves to the other organs of the plant, and improved enzymatic efficiency and commercial quality of crops [12].

Relative quantitative evaluations for a particular mineral element can be achieved through the profile scanning of stomata. This type of comparisons between elements can only be made by applying calibration factors. In this way, K contents of guard cells of the opened and closed stomata can be measured. In opened stomata, there is more K, and there is more Cl than the closed one but the differences are not so great. On the other hand, P contents are almost the same.

The salinity of the nutrient solution is quantified by the electrical conductivity, which at varies function to the culture and nutrient balance in the solution. Once salts are diluted in the solution, the producer cannot identify which element is causing increasing osmosis power. The salinity in vegetables grown in the hydroponic system causes lower growth in plants, which is also due to the reduction in the absorption of some of the main nutrients, mainly Ca and K [15]. Plants are very sensitive to salinity where they absorb water having high contents of salts, which causes toxicity. This excess absorption promotes imbalances in the cytoplasm, causing damages to appear mainly at the edges and at the apex of the leaves, regions where the accu-

**Figure 3.** Profiles of relative amounts of K, Cl, and P across an open and a closed stoma. The traces are the result of scanning a 0.5–1 μm diameter beam across the stomata shown diagrammatically below the traces. In order to indicate the profile scanned, the images of the stomata have been cut off in this diagram where the beam crossed the guard [13].

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Imbalances may be the result of the salinity effect of nutrients above the required, or may be caused by physiological inactivation of an essential nutrient when it increases its internal

In a yield response curve, there is a point at which maximum production is reached and maintained at that level until an ionic concentration is reached in the solution, where production begins to decrease. This interval, between nutritional deficiency conditions and toxicity,

Lower absorption of K by vegetables has been attributed to the higher competition between Na and K by the absorption sites or a higher flow of K from the roots. The reduction in K concentration, under saline stress, is an additional complicator for plant growth, since in some situations this element is the main nutrient contributing to the decrease of osmotic potential [19].

depends particularly on the nutrient and nutritious solution salinity conditions [18].

mulation of absorbed salts occurs [16].

requirement in the plant [17].

Comparison of the traces and stomata indicates, as might be expected, that the P peaks coincide with the nuclei [13] (**Figure 3**).

Potassium also increases the natural resistance of the aerial part of plants, the fungal diseases, pests, damping-off and counter balances the effect of excess nitrogen absorption. However, excess potassium imbalances the nutrition of vegetables, making it difficult to absorb calcium and magnesium [13].

K is required for protein synthesis; when plants are deficient in K, there is less protein synthesis and accumulation of soluble nitrogen compounds, such as amino acids, amides, and nitrates. Thus, the proper use of nitrogen fertilizers depends, also, on an efficient supply of potassium to the plants. The supply of potassium fertilizers to the crops, besides affecting the production, also has an effect on the quality of the harvested fruit. Specifically for tomato, these qualitative characteristics are important both for use in industry and for consumption "in natura." In tomato, the fruit's flavor is determined by the amount of solids, mainly sugars and organic acids, and volatile compounds. Considering that, in the ripe fruit, 95% of its constitution is water, only a small amount of solid matter will determine its quality [14]. The decrease in the sugar contents correlates with high doses of nitrogen, which leads to the hypothesis that the apical pruning, associated to the various doses of N and K, can influence, in a certain moment, the level of substances in the fruits [14].

metabolism becomes evident. It also plays an important role in the activation of more than 60 enzymes, which act on several metabolic processes such as photosynthesis, protein synthesis, and carbohydrates, also affecting water balance and the growth of meristematic tissues [10]. K absorbed by the root is led to the aerial part by the xylem and phloem, its internal redistribution is quite easy. The element is directed from the older leaves to the younger leaves, to the growing regions and to the fruits. This is due, in part, to the fact that about 75% of plant

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

Cultures differ in their K requirements because of differences in the physiological functions in which K is involved. Cultures where the harvested part consists of young plant tissue, as is the case of leafy vegetables and fruits, have high requirements of K per unit of dry weight produced. When the same crop is harvested at the complete maturation stage, the requirement for potassium per dry weight unit is substantially lower. Cultures that produce fleshy

Among the various functions of potassium in plants, water use efficiency is better cited, as a consequence of the control of the opening and closing of the stomata, greater translocation of carbohydrates from the leaves to the other organs of the plant, and improved enzymatic

Relative quantitative evaluations for a particular mineral element can be achieved through the profile scanning of stomata. This type of comparisons between elements can only be made by applying calibration factors. In this way, K contents of guard cells of the opened and closed stomata can be measured. In opened stomata, there is more K, and there is more Cl than the closed one but the differences are not so great. On the other hand, P contents

Comparison of the traces and stomata indicates, as might be expected, that the P peaks coin-

Potassium also increases the natural resistance of the aerial part of plants, the fungal diseases, pests, damping-off and counter balances the effect of excess nitrogen absorption. However, excess potassium imbalances the nutrition of vegetables, making it difficult to absorb calcium

K is required for protein synthesis; when plants are deficient in K, there is less protein synthesis and accumulation of soluble nitrogen compounds, such as amino acids, amides, and nitrates. Thus, the proper use of nitrogen fertilizers depends, also, on an efficient supply of potassium to the plants. The supply of potassium fertilizers to the crops, besides affecting the production, also has an effect on the quality of the harvested fruit. Specifically for tomato, these qualitative characteristics are important both for use in industry and for consumption "in natura." In tomato, the fruit's flavor is determined by the amount of solids, mainly sugars and organic acids, and volatile compounds. Considering that, in the ripe fruit, 95% of its constitution is water, only a small amount of solid matter will determine its quality [14]. The decrease in the sugar contents correlates with high doses of nitrogen, which leads to the hypothesis that the apical pruning, associated to the various doses of N and K, can influence,

fruits or storage organs have high K requirement when compared to cereals [11].

potassium is soluble in tissues.

are almost the same.

and magnesium [13].

cide with the nuclei [13] (**Figure 3**).

efficiency and commercial quality of crops [12].

in a certain moment, the level of substances in the fruits [14].

**Figure 3.** Profiles of relative amounts of K, Cl, and P across an open and a closed stoma. The traces are the result of scanning a 0.5–1 μm diameter beam across the stomata shown diagrammatically below the traces. In order to indicate the profile scanned, the images of the stomata have been cut off in this diagram where the beam crossed the guard [13].

The salinity of the nutrient solution is quantified by the electrical conductivity, which at varies function to the culture and nutrient balance in the solution. Once salts are diluted in the solution, the producer cannot identify which element is causing increasing osmosis power. The salinity in vegetables grown in the hydroponic system causes lower growth in plants, which is also due to the reduction in the absorption of some of the main nutrients, mainly Ca and K [15].

Plants are very sensitive to salinity where they absorb water having high contents of salts, which causes toxicity. This excess absorption promotes imbalances in the cytoplasm, causing damages to appear mainly at the edges and at the apex of the leaves, regions where the accumulation of absorbed salts occurs [16].

Imbalances may be the result of the salinity effect of nutrients above the required, or may be caused by physiological inactivation of an essential nutrient when it increases its internal requirement in the plant [17].

In a yield response curve, there is a point at which maximum production is reached and maintained at that level until an ionic concentration is reached in the solution, where production begins to decrease. This interval, between nutritional deficiency conditions and toxicity, depends particularly on the nutrient and nutritious solution salinity conditions [18].

Lower absorption of K by vegetables has been attributed to the higher competition between Na and K by the absorption sites or a higher flow of K from the roots. The reduction in K concentration, under saline stress, is an additional complicator for plant growth, since in some situations this element is the main nutrient contributing to the decrease of osmotic potential [19].

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, with consequent loss of the absorption capacity of some ions, especially K+ [21]. Salinitytolerant varieties tend to have higher K+ transfer rates and only slight reduction in Ca2+ transfer to aerial part, in order to maintain a positive relationship between those nutrients and Na<sup>+</sup> and Cl<sup>−</sup> ions [22].

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

and burn on leaf margins [30] (**Figure 4**).

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

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,

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

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 suitable for the preparation of nutrient solutions [24].

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

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 vegetables to salinity in the nutrient solution [27].

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

When applied externally, Ca+2 decreases saline stress by means of an unknown function that preserves K+ /Na<sup>+</sup> selectivity and inhibits K+ absorption sites, which can reduce the Na<sup>+</sup> influx mediated by the K+ absorption low-affinity component. Calcium is usually maintained in the 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 can be used in conditions of high osmotic potential [24].
