**5. Mineral element as a factor influencing quality of fruits in citrus production**

Inadequate mineral nutrition, whether due to deficiency or excess, may generate poor fruit quality. Therefore, it is necessary to achieve nutritional balance, in order to allow plants to grow vigorously and better tolerate biotic and abiotic stresses. Fresh size and mass, percentage of rind and juice, soluble solids content (SS), titratable acidity (TA), SS/TA ratio, and industrial yield, expressed in kg of sugar per 100 kg of processed fruit or SS per box (40.8 kg) are usually used to evaluate fruit quality in citrus fruits.

Nitrogen, phosphorus, and potassium are the most important nutrients that influence fruit quality of citrus fruits. However, deficiency or excesses in other nutrients have negative effects on fruit yield and quality [40]. Nitrogen (N) increases juice content, total soluble solid (TSS) per box and per acre, and acid content. However, excessive N can induce excess vigor and promote a vegetative rather than a flowering tree, and this can result in lower yields with lower TSS per acre. In contrast, poor fruit yields are produced when there is a low N level but promoted extensive flowering. Soluble solids: acid ratio increased with phosphorus but acid content reduced [40]. Fruit production, fruit size, green fruit, and peel thickness of citrus fruits are increased with potassium. Foliar spray of potassium nitrate or monopotassium phosphate in the spring often increases the fruit size of tangerine and grapefruit, and the fruit size and total pound solids of Valencia orange [40]. The use of urea can increase flowering and fruit set by foliar application (from 6 to 8 weeks before bloom) [40].

Productivity in low-fertility tropical soils can be enhanced by the use of fertilizer and this supports adequate mineral nutrition of citrus. Nitrogen (N) and potassium (K) fertilizers to some extent produce an increase in fruit yield and quality of citrus trees [41, 42]. Potassium also affects external fruit characteristics, in such a way that as the K supply increases, fruits become larger and coarse. But, K deficiency resulted in a reduction in the number and size of fruit in all citrus varieties and the soluble solids content of juice also decrease [42].

Citrus fruit requires high mineral nutrient amounts in order to express their full growth, yield, and fruit quality potential. In some cases, soil mineral concentrations are at sufficiency levels. However, it is necessary to apply acidity correctives for nutrients to become available and be used by the plant. The main macronutrient effects observed on citrus fruit quality include; an increase in juice color intensity, soluble solid (SS) and titratable acidity (TA), rind thickness increases, and color for nitrogen (N) [43]. They also observed reduction in titratable acidity and phosphorus (P) increases SS/TA ratio; potassium decreases SS, SS/TA ratio and juice color; while Mg was reported to have slight SS, SS/TA ratio, fruit fresh mass, and size increase and reduced rind thickness [43]. However, there is a need to validate these statements as citrus fruit quality is the product of complex actions that involved several factors, acting individually or together.

## **6. Roles of potassium in citrus production**

Potassium (K) alongside nitrogen is one of the most important nutrients for citrus production. It is needed for enzyme activation, cell division, photosynthesis, photosynthate transport, and osmoregulation. Potassium is required in more quantities in the meristematic tissues, buds, leaves, and root tips of citrus plants [44]. It plays important role in maintaining an anion-cation balance in cells, involved in protein synthesis, opening and closing of stomata, activation of enzymes, and in the turgidity of cells [45]. Potassium regulates the value of fruits through its influence on the size of the fruit, appearance of the fruit, fruit color, and vitamin contents [46].

This important nutrient plays a key role in stimulating photosynthesis, maintaining rapid root growth, and synthesis of protein from amino acids [47]. It helps to keep the stability of electric charges - an essential for ATP formation in plant chloroplasts. Stability in pH will counterbalance the insoluble and soluble macromolecular anions by the application of a large quantity of potassium. A wide range of potassium content in the leaves is required for the average vegetative growth of citrus plants [48]. The amount of K below 0.4% affects tree development; otherwise, over an extensive range of variation does not generally affect tree growth [49].

Potassium (K) is the most abundant inorganic cation and is key in ensuring optimal plant growth [50]. K is an activator of dozens of important enzymes, such as protein synthesis, sugar transport, nitrogen and carbon metabolism, and photosynthesis. It plays an important role in the formation of yield and quality improvement [51, 52]. K is also very important for cell growth, which is an important process for the function and development of plants [44]. K has strong mobility in plants, helps in regulating cell osmotic pressure, and balances the cations and anions in the cytoplasm [45, 53]. Through these processes, K is involved in the regulation of stomata opening and closing, cell elongation, and other important physiological processes.

The effects of K level on plant growth have been studied by many scientists. K applied to Red Fuji apple with 600 kg/ha K produces the highest yield and fruit quality [54]. More so, the application of 6 mM K promoted pear growth and improved photosynthetic efficiency [55]. Treatment of potassium with 500 kg/ha resulted in increased production of navel oranges with better fruit and quality parameters [56]. Potassium interacts with other nutrient ions. Magnesium (Mg) uptake in the soil is inhibited by high concentrations of K and this may lead to Mg deficiency in plants [22]. Some fruit disorders are likely to occur under low potassium conditions or high leaf N: K ratios, like plugging and creasing, and these result in less marketable fruit.

Potassium (K) has a significant role in juice acidity properties. High K concentration increases juice acidity, while a low K concentration causes a decrease in juice acidity properties. High K availability in the soil can reduce the uptake of magnesium, calcium, and ammonium for plants [42]. Nitrogen (N) and potassium (K) in the fruits are the most nutrients removed from the soil by citrus trees each year [42]. The roles played by potassium in the plant may include; root growth enhancement, drought


#### **Table 2.**

*Potassium at citrus growth stages [58].*

tolerance, decreasing water loss and wilting, improving pest and disease resistance, and reducing stalk lodging [57]. Plants required potassium in the range of 0.5 to 2% of dry matter, which is next to that of nitrogen. When the K level is in the ideal range, then a satisfactory yield of fresh fruit can be attained.

## **6.1 General guidelines in potassium application**

Potassium is more taken up by citrus fruits than any other nutrients. Potassium application rates can be done either upon a leaf or soil analysis but soil analysis is the most commonly used one. Potassium (also called potash) is listed on the fertilizer label as K2O and application rates of 0.8–1.4 lb. K2O/tree are commonly used in 3–5 splits during the growing season [58]. During late fruit growth, potassium uptake usually occurred and application rates should be increased to meet this need (**Table 2**).
