**3. Potassium interaction with other nutrients**

The positive effect between nutrients can be enhanced by the balanced application of two nutrients i.e. synergistic interaction. Antagonistic effects occur where an increase in one nutrient reduces the uptake and function of the other, thereby resulting in reduced crop quality. For example, the rate of Mg uptake can be depressed by Ca and vice versa [18]. This is due to higher affinity to Ca than to Mg in root plasma membrane binding sites [19]. The application of phosphorus is reported to reduce plant uptake and utilization of zinc [20, 21]. More so, an increase in the use of nitrogen fertilizer led to increasing in the uptake and utilization of zinc in plants [21]. Nitrogen and potassium in the fruits account for most of the nutrients removed from the soil by citrus trees each year and the interaction of K with N is considered the most important interaction. The process of converting inorganic nitrogen to organic nitrogen compounds is energy-consuming. Therefore, inorganic nitrogen absorbed by plants must be converted into amino acids and protein as much inorganic nitrogen is of no use to the plant. Good K nutrition favors the rapid turnover of inorganic nitrogen into proteins and consequently, potassium improves the effect of nitrogen fertilizer.

Mg uptake by the plant can be inhibited by the presence of high K concentrations in the soil and this may also induce Mg deficiency in plants [22]. K may also be reducing Ca uptake, where the soil is low or deficient in Ca despite the strong predominance of Ca on the exchange sites of the soils [23]. It is evident that K affects significantly the absorption and utilization of other nutrients by plants, and the appropriate K level differs in different crops.

K and N metabolism relationship has been evaluated in some studies. In contrast to the antagonistic relationship between K<sup>+</sup> and NH4 + nutrition, a positive correlation was found to exist between the acquisition rates of K<sup>+</sup> and NO3 − [24, 25] and the synthesis of amino acids and proteins can be enhanced by a sufficient supply of K, which promoted N metabolism [26, 27]. Potassium (K) deficiency was found to reduce Nitrate reductase (NR), Glutamine synthetase (GS), and Glutamate synthetase (GOGAT) activities and this inhibited nitrate absorption in certain plants [28]. More so, K deficiency was reported to up-regulate the activities of GS and Glucose dehydrogenase (GDH) in Arabidopsis [29]. The metabolism of N affected by K appears to vary in different types of plants. Meanwhile, the K concentration has a significant impact on C metabolism, and the metabolic process and energy level that exist between C metabolism and N metabolism show a strong interaction [30]. K supply of 6 mM to apple dwarf rootstock seedling was optimal; as it promoted photo-assimilate transport from leaves to roots and increased nitrogen use efficiency (NUE) which influences photosynthesis. This also enhances C and N metabolizing enzyme activities, nitrate assimilation gene activities, and nitrate transport [31].

## **4. Effect of deficiencies of mineral elements in citrus production**

When plants are potassium (K) deficient, it affects the rate of photosynthesis. Nitrogen in large quantities with a little amount of K has resulted in having drops in protein used as building blocks, disease start setting up, reduction in carbohydrates production, reduction in fruiting, increasing fruit split, creasing of fruit, and drop in plugging. A decrease in yield and low fruit quality can be as a result of a shortage of K. Negative effects of low K generally occur on fruit yield and quality before leaf deficiency symptoms. K in the leaf range of 0.5–0.8% has been observed to have decreased yield and small fruit while K concentrations of 1.2% or more produced the maximum yield of high-quality fruit [32]. No visual deficiency symptoms were observed with moderately low concentrations of K in the tree which cause a general reduction in growth [32] and production is seriously impaired when there is an onset of visual deficiency symptoms in leaves.

Mineral deficiencies in certain plant may develop into leaf chlorosis symptoms. Different mineral deficiencies in citrus can result in distinct chlorosis patterns. Mg deficiency symptoms may appear as leaf interveinal chlorosis, with chlorotic development and necrotic lesion which occurs in later stages, particularly under high light intensity [33, 34]. Citrus trees with inadequate Mg may have no symptoms in the spring growth flush, but leaf symptoms develop as the leaves age and the fruit expand and mature in the summer and fall. Magnesium deficiency symptoms occur on mature leaves following the removal of Mg to satisfy fruit requirements.

Potassium-deficient plants as observed [35] do not develop leaf chlorosis but resulted in less biomass and some changes in nitrogen metabolism. The result of the experiment also shows that magnesium deficiency produced leaf chlorosis symptoms and loss of chlorophyll in the leaf. Reduction in nitrate concentrations brought about a partial impairment of the nitrate reductase system and this is due to effects on nitrogen metabolism [35].

*Citrus Mineral Nutrition and Health Benefits: A Review DOI: http://dx.doi.org/10.5772/intechopen.107495*

Calcium deficiency was the only treatment that revealed profound effects on the nitrogen economy of citrus leaves. Remarkable lower nitrogen level in leaves and reduced nitrate concentration was observed in calcium deficient [35]. Calcium deficiency with nitrogen metabolism interference resulted in an extreme reduction of the free amino acid pool. The most abundant free amino acid in citrus is proline [36] and glutamic acids, the precursor of proline synthesis were most severely affected [37]. More so, a change in the protein level of Ca-deficient leaves might be caused by the decline in the level of ribulose bisphosphate carboxylase (RuBPcase) [35]. High concentrations of NH4 + , K+ , Ca2+, Mn2+, and SO4 2− in the soil can induce magnesium deficiency [38]. The uptake of Mg, Ca, and other cations by the citrus plant are usually interfered with by the high concentration of potassium (K) available in the soil either due to excessive use of fertilizer or natural soil minerals [39].
