Potassium sources and doses in coriander fruit production and essential oil content

Potassium (K) is one of the most required agricultural crop macronutrients, with potassium chloride being the most applied source. However, this fertilizer is not recommended for several crops due to its high chlorine content, promoting final product quality losses, thus being replaced by potassium sulphate. The aim of the present study was to evaluate the production and macronutrient, essential oil and linalool contents of coriander fruits submitted to different potassium sources and doses. The research was performed in a greenhouse, in plastic 46 kg boxes applying a mixture of soil and sand as substrate. The experimental design was of randomized blocks, applying a 2x4 factorial arrangement, with two potassium sources (potassium chloride and sulphate) at four doses (50, 100, 150 and 200 mg of K/kg substrate), performing four replicates. The highest fruit yields, and phosphorus and sulfur fruit contents were verified in plants fertilized with K2SO4. The application of increasing potassium doses, regardless of the source, resulted in increased K and decreased Ca contents and did not affect N and Mg fruit levels in the fruits. The highest essential oil concentration in fruits (0.15 g) and linalool in essential oils (0.42 mg) were verified when 153.8 and 131.3 mg of K/kg substrate using K2SO4 were applied, respectively.

Palavras-chaves: Coriandrum sativum, nutrientes, metabolismo secundário, linalol. that can increase their production and quality are indispensable. It is known that the composition and yield of essential oils can be affected by genetic, physiological and edaphoclimatic variations, with mineral nutrition being an important factor (El Gendy et al., 2015;Chrysargyris et al., 2017ab).
Among mineral nutrients, K is most required by crops, and its availability depends mainly on soil reserves and fertilizer applications (Zörb et al., 2014). Although not a constituent of any organic compound, this nutrient participates in enzymatic activation, the establishment of turgor and maintenance of cellular electroneutrality, and is involved in photosynthesis, carbohydrate transport, protein synthesis, cell expansion and stomatal movement, considered a quality-linked element (Nieves-Cordones et al., 2016). The most commonly applied K fertilizer is potassium chloride, which contains on average 60% water-soluble K 2 O. However, its high chlorine content causes losses in the final product quality and, thus, this compound is not recommended for use in certain crops, such as potato and tobacco, being replaced by K 2 SO 4 (Zörb et al., 2014). K 2 SO 4 , may be more appropriate for essential oil production, as it contains sulfur which, besides being a protein component, is a constituent of the acetyl-CoA molecule involved in terpene synthesis (Dubey et al., 2003).
According to Chrysargyris et al. (2017a) with Mentha spicata and Khalid (2013), with Calendula officinalis, increases have been reported in the content and quality of essential oils extracted from these plants due to potassium applications. Chrysargyris et al. (2017a) observed that the highest production of fresh matter and essential oils in Mentha spicata cultivation, as well as higher carvone content in the oils, was observed at 325 mg L -1 K applied in the nutrient solution, compared to 275 mg L -1 . Khalid (2013) observed an increase in plant growth, content and composition of Calendula officinalis essential oils, at increasing potassium doses up to 173 kg ha -1 using K 2 SO 4 as K source.
Information on the production and quality of essential oils in plants like coriander fertilized with K sources and doses are still unknown. Therefore, studies that relate K fertilization to the production and quality of these plants are required. In this context, the aim of the present study was to evaluate coriander fruit production, macronutrient content and essential oil and linalool content in fruits submitted to different K sources and doses.

MATERIAL AND METHODS
The research was performed in a greenhouse at the Research Support Unit of the Northern Fluminense Darcy Ribeiro State University campus, Campos dos Goytacazes, Rio de Janeiro, Brazil (21º19'S, 41º10'W, 14 m altitude). During the experiment, the temperatures inside the greenhouse ranged between 19 and 42ºC and relative humidity, between 37 and 98%.
The trial was performed in a randomized block design, as a 2x4 factorial scheme, with two potassium sources [potassium chloride (KCl) and potassium sulphate (K 2 SO 4 )] and four potassium doses (50, 100, 150, 200 mg K/kg of substrate), with four replicates. The experimental unit was a plastic box filled with 46 kg of a mixture of soil and sand as substrate, containing ten plants.
The mixture of soil and washed sand at a 70:30 (v/v) ratio, respectively, was used as substrate, with the following physico-chemical characteristics: pH in water, 4.2; P, 4 mg dm -3 ; K + , 50 mg dm -3 ; S, 37 mg dm -3 ; Ca 2+ , 5.2 mmol c dm -3 ; Mg 2+ , 4.4 mmol c dm -3 , Na + , 0.90 mmol c dm -3 ; Al 3+ , 3.8 mmol c dm -3 ; H+Al, 20.4 mmol c dm -3 ; sand, 480 g kg -1 ; silt, 60 g kg -1 and clay, 460 g kg -1 . For soil acidity correction, 33.5 mg of dolomitic lime was applied with 80% PRNT per 46 kg box and, after 30 days, 30 mg dm -3 of P was applied in the form of triple superphosphate alongside the potassium doses. For the treatments, the initial potassium content in the substrate was considered and the necessary dose was added, so that the dose reached the final value for each treatment, except for the 50 mg kg -1 K treatment, where no potassium source addition was necessary. The soil was subsequently incubated for 10 days.
The coriander cv. Verdão seeds were sown directly in the boxes and, after the appearance of the first pair of leaves, 20 mg kg -1 N in the form of urea was applied. During the experiments, daily watering with deionized water was carried out to maintain soil moisture.
At 80 days after sowing, the mature fruits (Msaada et al., 2007) were collected and evaluated regarding number of fruits, fresh fruit mass, dry fruit mass and macronutrients content, as well as essential oil content in fruits and linalool levels in essential oils.
To evaluate nutrient content, the fruits were dried at 65 o C in a forced ventilation oven for 72 hours and then ground in a Willey-type knife mill. For N content determinations the plant material was submitted to a sulfur digestion and N was determined by the Nessler method (Jackson, 1965) and the other macronutrients contents were determined using plasma spectrometry (ICPE-9000, Shimadzu, Kyoto, Japan) after digestion with HNO 3 and H 2 O 2 , in an open digestion system. ICPE-9000 conditions were 8.0 L min -1 plasma gas, 0.70 L min -1 auxiliary gas and 0.55 L min -1 carrier gas (Peters, 2005). Essential oils were extracted by hydrodistillation by steam stripping in a Clevenger-type apparatus for two hours using 50 g of fresh fruits (Msaada et al., 2007). Following extraction, essential oils were collected using Pasteur pipettes and their mass was used to determine essential oil content, calculated by the formula: The chemical analysis of the essential oils was carried out using a Gas Phase Chromatograph coupled to a Mass Spectrometer (Shimatzu 17A). The samples were diluted in hexane and then subjected to gas chromatography as follows: using a DB5 30 m capillary column with 0.25 mm internal diameter, 220 o C temperature in the injector and 240 o C in the detector, initial temperature of 60ºC, maintained for one minute, increasing at 3 o C per minute, up to 240 o C, which was maintained for another 30 minutes, at a 1:20 split ratio (Msaada et al., 2007).
The data were submitted to analysis of variance. For the quantitative factor, a polynomial regression analysis, an F test of the regression variance analysis and the coefficient of the statistically significant model and higher R 2 were used, while the Tukey test (p<0.05) was performed on the qualitative factor.

RESULTS AND DISCUSSION
Interactions between K doses and sources for number of fruits, fresh fruit mass, essential oil content, linalool oil content and S and P content in C. sativum fruits ( Figures 1A, 1B, 2A, 2B, 3B and 4A) were observed. K and Ca levels in the fruits were influenced by potassium doses (Figures 3A and 4B). N and Mg contents were not influenced (p≤0.05) by K sources and doses in coriander fruits, with mean content of 28.1 g kg -1 and 4.18 g kg -1 , respectively.
In the present study, higher K supplies resulted in increased fruit production ( Figures 1A and 1B) and nutrient content in coriander fruits ( Figure 3A). El-Bassiony et al. (2010) and Afzal et al. (2015), investigating sweet pepper and tomato plants, verified positive effects of K application on harvested fruit yield and quality.
The highest fruit yields were obtained at 146.2 mg K kg -1 substrate K 2 SO 4 used and 126.2 mg K kg -1 substrate KCl used, with 41% increase in the number of coriander fruits when fertilized with K 2 SO 4 ( Figure 1A). The highest fresh matter masses were estimated at 140.6 mg K kg -1 substrate K 2 SO 4 and 143.7 mg K kg -1 substrate KCl used, with 27% increase observed when potassium sulphate was applied ( Figure 1B). Potassium participates in important plant metabolism processes, such as sugar and water transport, protein and starch synthesis and stomatal opening and closing, as well as in the activation of enzymes involved in the photosynthetic process, such as pyruvate kinase and phosphoenolpyruvate (Prajapati & Modi, 2012;Nieves-Cordones et al., 2016). Potassium has been the target of some researchers mainly because it is essential for enzyme activation such as enzyme of essential oil (Khalid, 2013).
However, KCl as the K source led to lower fruit production and fresh fruit mass values compared to K 2 SO 4 ( Figures 1A and 1B). This may be related to the presence of S, since the contents of this nutrient were higher in plants fertilized with the sulfate ( Figure  4A). S is an essential mineral nutrient for plants, is found in amino acids such as cysteine and methionine, proteins (Marschner, 2012) and as acetyl-CoA molecule component that is involved in the synthesis of terpenes (Dubey et al., 2003).
In this context, one study evaluated the effects of K 2 SO 4 and KCl on Tagetes erecta flower production and plant growth, and the highest values for these variables were obtained for the highest K dose (240 kg K 2 O ha -1 ) using K 2 SO 4 as source (Sanghamitra et al., 2015). Similar results were observed in the cultivation of foraging species by testing different K sources and doses, where the highest growth was observed at the highest K 2 SO 4 dose (Sima et al., 2013).
Although Cl is considered an essential plant micronutrient, required in small amounts, when present in high concentrations, it can result in negative effects both on the growth and quality of the harvested product, depending on the plant species (Marshner, 2012;Geilfus, 2018). Thus, despite being a highly recommended source of K, mainly due to the market price, KCl can be detrimental to the production quality of certain crops, such as persimmon, tobacco, potatoes and wheat, among others. According to Geilfus (2018), excess Cl induces dysfunctions that hinder crop quality and impair starch partitioning, nutrient absorption, protein biosynthesis and photosynthesis. Thus, it is often suggested that KCl could be replaced by K 2 SO 4 in crops (Khan et al., 2014).
The essential oil (0.15 g) and linalool (0.43 mg) content in the fruits increased when applying K 2 SO 4 , with the highest value detected at the estimated dose of    153.8 and 131.3 mg K kg -1 substrate, respectively (Figure 2A and 2B), contrary to what was observed on KCl doses, linear decrease for the linalool present in the oils extracted from C. sativum fruits was observed. Essential oil production can be influenced by both the source and the amount of nutrients applied (El Gendy et al., 2015;Chrysargyris et al., 2017a). Increasing K doses in coriander cultivation resulted in higher essential oil production (Figure 2A). The same was observed for Lavandula angustifolia, where increased K content in the nutrient solution, from 275 mg L -1 to 300 mg L -1 , led to higher oil production (Chrysargyris et al., 2017b). Khalid (2013), assessing K doses (K 2 SO 4 ) in Calendula officinalis, reported that the highest essential oil accumulation (0.29% and 0.095 g plant -1 ) was observed at a K treatment of 173.2 kg ha -1 compared to the control treatment (0.13% and 0.015 g plant -1 ).
The higher oil production observed in coriander fruits fertilized with K 2 SO 4 ( Figure 2A) can also be linked to increased S ( Figure 3B) and P contents (Figures 4A) when using the same K source. Increments in S and P levels in the coriander seeds of 40% and 17.3%, respectively were observed when comparing the lowest and highest K 2 SO 4 doses. The effect of S on essential oil production has also been observed in Cymbopogon martinii, where plants grown at 40 kg ha -1 S presented higher essential oil yields compared to plants cultivated without fertilization . Hani et al. (2015), found that coriander plants fertilized with 24 kg P per hectare increased essential seed oil content by 16% when compared to nonfertilized plants.
C. sativum essential oils consist of monoterpenes, including linalool. Geranyl diphosphate (GPP, C 10 ) is the universal precursor in monoterpene synthesis and is synthesized from the fusion of isopentenyl diphosphate (IPP, C 5 ) with its isomer, dimethyl diphosphate (DMAPP, C 5 ). ATP, NADPH and acetyl-CoA molecules are required for IPP and DMAPP synthesis. Thus, essential oil biosynthesis is dependent on S and P, since these nutrients are a constituent part of these molecules (Dubey et al., 2003). Freitas et al. (2004), studying phosphate fertilization in Mentha arvensis, verified that increased P doses from zero to 50 mg per kg of soil resulted in a 74% increase in essential oil concentrations.
In addition to essential oil content, essential oil composition is also affected by mineral nutrients. In the present study, linalool levels in fruits and oils were higher with increasing K doses ( Figures 2B and 3A), which may be due to both the increase of fruit K content ( Figure 3B) and S and P contents ( Figures 4A and 4B). In this context, one study reported that the proportion of essential constituents of C. officinalis essential oils (α-cadinol, β-cadinene and α-cadinene) was altered by K doses (Khalid, 2013). In another study, increasing K concentrations in Mentha x gracilis nutrient solution increased myrcene, α-pinene, β-pinene and limonene productions and decreased linalool and pulegone levels (Garlet et al., 2013).
The higher P and S contents in coriander fruits of plants fertilized with K 2 SO 4 led to higher linalool levels in comparison to plants fertilized with KCl ( Figure 2B and 3A). Some studies highlight the important role of S and P in isoprenoid synthesis, since they are a part of several molecules that participate in this process, such as acetylCoA, ATP and NADPH (Dubey et al., 2003). P effects on linalool synthesis in coriander fruits was also observed by Hani et al. (2015), where increasing doses of this nutrient led to increased linalool levels. In another study, the highest S dose in C. martinii cultivation increased cisβ-ocimene, linalool, geraniol, geranyl acetate and geranyl hexanoate levels in oils , and S increasing doses in the cultivation of Cymbopogon flexuosus reduced citral content in essential oils (Zheljazkov et al., 2011).
As predicted, increasing K doses led to increased nutrient content in fruits ( Figure 3A). On the other hand, Ca content was reduced ( Figure 4B). Ca is absorbed by un-suberized root system cells, in the form of Ca 2+ , and the increase of other salts, such as K + , can decrease Ca absorption by the roots (Marschner, 2012).
An increase in S and P contents in coriander fruits (Figures 4A and 4B) was observed with increasing K doses in the form of K 2 SO 4 . The use of K 2 SO 4 source elevated P content up to the estimated dose of 143.4 K mg kg -1 of substrate ( Figure 4A). During amino acid synthesis, the reduction of S requires considerable amounts of energy, which may explain the increased P absorption observed herein, since this nutrient plays a fundamental role in energy transfer in plant metabolism. On the other hand, plants fertilized with KCl presented decreased P contents ( Figure 4B). Because this is a nutrient only necessary in small amounts, Cl in excess in the root environment can cause toxicity, due to increased salinity, which then decreases P concentrations in plant tissue, due to decreased phosphate activity in the soil solution (Geilfus, 2018).
The highest fruit yields and phosphorus and sulfur contents were observed in coriander plants fertilized with potassium sulfate. Essential oil contents in fruits increased when applying K 2 SO 4 , with the highest value obtained at an estimated dose of 153.8 mg kg -1 K, while for linalool this was estimated at 131.3 mg kg -1 K. Thus, K 2 SO 4 application increases the number, fresh mass and essential oil content of coriander fruits.