**5. Potassium nutrition and its relationship with abiotic and biotic stresses**

Among the factors that affect global food security are availability and quality of water resources [44]. Due to low availability of drinking water, the development of techniques that allow the lowest consumption of water or use of salt water in hydroponics is important to allow advancement in this mode of cultivation [45].

In this sense, an increase of K application in sweet potato plants irrigated with 50% of the field capacity was detrimental to the development of plants [46]. In relation to the use of salt water, one of negative consequences in hydroponics is a decrease in the accumulation of K [47]. However, it has been demonstrated in literature that, in tomato cultivars, the increase of K concentration in nutrient solution can be used to minimize salinity-induced oxidative stress, increasing the photosynthetic rate of plants and making them less sensitive to salinity [48]. Besides that, Ramadan and Shalaby 2016 [49] indicated that the foliar application of K resulted in an increased growth and yield of eggplants grown under conditions of salt stress.

A common situation in less-tech hydroponic crops is a lack of temperature control in production environment, because in these environments the temperature is not controlled, temperature variations are usually observed, which has harmful effects on plants. Thus, in an extensive compilation of data, Oosterhuis et al. [50] described that high K concentration in cells can improve cold stress tolerance by reducing the osmotic potential of cells and decreasing the freezing point of sap, preventing cell dehydration. On the other hand, considering that hydroponic crop plants experience high temperatures, but without water restriction, Römheld and Kirkby [51] indicate that an adequate nutrition results of K in an increase of plant's capacity to eliminate the reactive oxygen species. It is produced during the thermal stress and improving the efficiency of water use, these main factors are necessary to make plants less sensitive to heat.

In hydroponic crops, nitrogen is mainly supplied as nitrate (NO3−). For leafy vegetables, the accumulation of NO3− is a concern for humans, especially for children, it can be harmful to health, depending on the amount consumed in the diet [52]. Thus, the inclusion of ammoniacal N can promote a significant increase in productivity and contribute to growing demand for safer foods. However, several factors may alter the availability of ammonium (NH4 + ) to plants and increase their rate of absorption by plants, which may lead to phytotoxicity [53].

quality of seeds produced. For example, Marrush et al. [40] found that K deficiency benefits a high incidence of premature germination, in other words, viviparity in bell pepper plants

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

Consequently, the deficiency of K in mother plant, during the phenological stage of seed formation, may decrease the germination rate of harvested seeds [41]. However, the seeds of plants well-nourished with K may present the germination rate due to the accompanying ion of the source of K used, because of a negative correlation between germination percentage

In relation to the production of *Brassica oleracea* seedlings, Zhang et al. [43] have demonstrated that although it depends on a relationship with other nutrients, the enrichment of substrate with K provides more vigorous seedlings. However, there is little research to verify if invest-

Among the factors that affect global food security are availability and quality of water resources [44]. Due to low availability of drinking water, the development of techniques that allow the lowest consumption of water or use of salt water in hydroponics is important to

In this sense, an increase of K application in sweet potato plants irrigated with 50% of the field capacity was detrimental to the development of plants [46]. In relation to the use of salt water, one of negative consequences in hydroponics is a decrease in the accumulation of K [47]. However, it has been demonstrated in literature that, in tomato cultivars, the increase of K concentration in nutrient solution can be used to minimize salinity-induced oxidative stress, increasing the photosynthetic rate of plants and making them less sensitive to salinity [48]. Besides that, Ramadan and Shalaby 2016 [49] indicated that the foliar application of K resulted in an increased growth and yield of eggplants grown under con-

A common situation in less-tech hydroponic crops is a lack of temperature control in production environment, because in these environments the temperature is not controlled, temperature variations are usually observed, which has harmful effects on plants. Thus, in an extensive compilation of data, Oosterhuis et al. [50] described that high K concentration in cells can improve cold stress tolerance by reducing the osmotic potential of cells and decreasing the freezing point of sap, preventing cell dehydration. On the other hand, considering that hydroponic crop plants experience high temperatures, but without water restriction, Römheld and Kirkby [51] indicate that an adequate nutrition results of K in an increase of plant's capacity to eliminate the reactive oxygen species. It is produced during the thermal stress and improving the efficiency of water use, these main factors are necessary to make

ment in seedling production is offset by the final productivity of plant in the field.

**5. Potassium nutrition and its relationship with abiotic and biotic** 

(*Capsicum annum* L. cv. "California wonder").

and concentration of Cl in sweet pepper seeds [42].

allow advancement in this mode of cultivation [45].

**stresses**

ditions of salt stress.

plants less sensitive to heat.

Potassium nutrition is efficient in minimizing the phytotoxic effects of excess NH<sup>4</sup> + , since K and NH4 + are very similar in relation to valence and ionic radius, in addition to being absorbed by the same carrier. Thus, increased K concentration may inhibit or even decrease NH4 + uptake and thereby mitigate the phytotoxic effects of excess NH<sup>4</sup> + [54]. In addition, Hernandez-Gomez et al. [55] verified that the cultivation of peppers with high concentrations of NH4 + was possible to maintain plant productivity, increasing the K concentration of nutrient solution, which resulted in adequate K contents in the plant tissue. Also, in this study, the water relations, the photosynthetic rate, and the stomatal conductance were not affected, compared to plants cultivated with a high concentration of NH<sup>4</sup> + and a low concentration of K.

K plays a very important role in the mitigation of biotic stresses to which plants are susceptible, since it participates in the synthesis of high-molecular-weight compounds such as proteins, starch, and cellulose, reducing the accumulation of soluble sugars, organic acids, and amides, of which pathogens are fed [3]. In this context, Perrenoud [56] gathered a series of studies where the incidence of pests and diseases was reduced as a function of nutrition with K and diverse cultures. However, there are important relationships of K with other nutrients, and an adequate K/N ratio in plant tissue may be responsible for increased productivity, lower incidence of diseases, and increased quality of product harvested.

For example, Adams and Massey [57], in order to maximize productivity, fruit quality, and greater resistance to diseases in tomato, suggest a K/N ratio of 1.2/1 in vegetative stage and 2.5/1 in reproductive stage. Another important factor to consider is the nutrient concentrations, which must be adjusted, depending on several factors. This way, Nam et al. [58] demonstrated a concentration of K in strawberry; this element was responsible for a higher productivity, but was not the same that bring results about lower incidence of diseases, which indicates that in conditions of high productivity, the quality of food can be affected.
