The Effect of Species and Cultivation Year on Phenolic Acids Content in Ancient Wheat

During the last decade older (ancient) wheat species, such as spelt (Triticum spelta L.), emmer (Triticum dicoccon Schrank), and einkorn (Triticum monococcum L.) have been recognised as an interesting option to increase the biodiversity of cultivated cereals. The aim of this study was to compare polyphenols content in the ancient species of cereals (including six accessions of spelt, four of emmer, and one of einkorn) cultivated in the three-year controlled plot experiment under organic management. It has been found that the content of almost all free and bound phenolic acids was significantly higher in einkorn than in emmer and spelt wheat species. Moreover, the concentrations of ferulic, p-coumaric, and caffeic acids in einkorn and emmer was higher in dry and very warm cultivation years. It is concluded that ancient wheat species, especially einkorn, could be an important source of phenolic acids in the human diet.


Introduction
Wheat, one of the most important cereals in the world, is not only a source of basic nutrients, such as carbohydrates, proteins, vitamins, and minerals, but also a source of antioxidants, such as phenolic compounds [1][2][3]. During the last decade, older (ancient) wheat species have been recognised as a possibility to increase biodiversity in cereal production [4], especially in organic agriculture [5]. Among these species are spelt (Triticum spelta L., hexaploid), emmer (Triticum dicoccon Schrank, tetraploid), and einkorn (Triticum monococcum L., diploid). These ancient wheats are a good source of valuable nutrients and have interesting sensory values [6]. Therefore, they are popular [5]. Despite their long cultivation history, today they are grown only locally on small areas [7]. Although the ancient wheat species have lower grain yields [8], they continue to attract growing interest because of their tolerance to abiotic and biotic stresses such as diseases [9,10], drought [11], and soil nutrient shortage [12]. At the same time, the consumption of ancient wheat species has been linked to the health benefits, partly attributed to the exceptionally high content of certain phytochemicals, especially phenolic acids [13][14][15].
Phenolic acids (PAs) are one of the groups of cereal crops metabolites with antioxidant effects [16]. In plants, they are involved in defence mechanisms against biotic and abiotic stresses [17]. In the human body, PAs may act directly as an antioxidant or exhibit anti-inflammatory and antimicrobial

Materials and Methods
The study was conducted at the experimental base of the Slovak University of Agriculture in Nitra in Slovakia (48 • 19 N, 18 • 07 E). The field trial was established as randomised block design with four replicates with plot area of 10 m 2 . The trial was located at an elevation of 177-178 m above sea level. The climate was continental with average long-term ) annual precipitations of 532.5 mm, and the average annual temperatures 9.8 • C. Weather data (precipitation and temperature) during wheat flowering and maturation was obtained from Department of Biometeorology and Hydrology in Nitra (Table 1). The soil of the experimental field was classified as a haplic luvisol developed on proluvial sediments mixed with loess.
Six spelt (Altgold, Oberkulmer Rotkorn, Ostro, Ebners Rotkorn, Rubiota, Schwabenkorn), four emmer (Farvento, Guardiaregia, Agnone, Molise sel Colli) and one einkorn (Ebners) winter accessions were cultivated according to organic farming principles with common to the region. No fertilisers (mineral or organic) nor pesticides were used during cultivation. Some nitrogen was provided through a crop of peas grown immediately prior to the ancient wheats, and 30 t of manure were applied three years prior to cultivation. The experiment was conducted during three consecutive growing years (2013)(2014)(2015). The plant material was acquired from cooperating institutions of Saatbau (Linz, Austria), South Bohemia University (České Budějovice, Czech Republic) and University of Molisé (Campobasso, Italy). Table 1. Precipitation (p) and average temperature (t) during cultivation period with calculations of temperature difference (∆t) and a log-term normal (n), and indication of characteristic (c) [33].

Year
Month After a manual harvest, all wheat samples were dehulled using KMPP300 laboratory dehuller (JK Machinery, Prague, Czech Republic) and then stored at 4 • C until further analyses. The samples were crushed into whole grain flour using FQC-109 laboratory mill (Kapacitív Kkt., Budapest, Hungary) equipped with 250 µm sifter. After milling, samples (10 g each) in four replications were transferred to 50 mL ST 50 tube from disintegrator ULTRA-TURRAX Tube Drive Control, IKA, Staufen, Germany) and defatted two times with hexane at a 2:1 ratio (v/w) at 1000 rpm, filtered through Whatman No. 1 filter paper and dried at room temperature.
The phenolic acid extracts were prepared according to the method of Wang et al. [34] with modifications. The defatted samples (2 g) were mixed with 15 mL of 60% methanol, sonicated for 15 min. (Bandelin DT 100, Germany), and the suspension was centrifuged at 8965 g (9000 rpm) (Universal 320, Hettich, Tuttlingen, Germany). The residual was extracted twice and all supernatants were combined. The free PA extracts were concentrated with vacuum evaporator RVO 400 (Ingos, Prague, Czech Republic) and reconstituted with 10 mL 60% methanol. Samples were stored in a freezer (−20 • C, 4-5 days) until further use. Bound PAs were released from wheat residual by alkaline hydrolysis. The 15 mL of 4M NaOH was added and each sample was shaken 60 min. at room temperature. After sonication (15 min), acid hydrolysis with 6M HCl was performed to acidify the samples to pH 2. Finally, the PAs were extracted using diethyl ether, evaporated to dryness, and bound PAs were reconstituted in 10 mL 60% methanol. Samples were stored in a freezer (−20 • C, 7 days) until quantification procedure.
The quantitative and qualitative analyses of the PAs were determined using HPLC/MS/MS system Agilent 1260 (Agilent, Santa Clara, CA, USA) equipped with DAD detector and Triple Quadrupole 6410 MS/MS system, autosampler and multicolumn thermostat. The analytical column Symmetry C18 (4.6 × 250 mm ID, 5 µm; Waters, USA) was used. Mobile phase consisted of 0.2% formic acid in water (v/v) (solvent A) and 0.2% formic acid in methanol (v/v) (solvent B). The gradient program (10−26% B 0-5 min, 26-65% B 5-10 min, 65% B 10-25 min, 65-10% B 25-30 min) was performed with a flow rate of 1 mL/min. with injection volume 40 µL. The PAs detection was based on the retention times and confirmed by the ion mass from MS/MS detector.
The HPLC/MS/MS system was controlled by the MassHunter software (Agilent, Santa Clara, CA, USA). For peak quantification, the calibration curves were constructed for sinapic acid, syringic acid, trans-ferulic acid, caffeic acid, salicylic acid, and hydroxybenzoic acid. All standards in concentration 10, 50, 100 and 200 µg/mL solution (60% MeOH). The calibration curves were linear and on the basis of the calibration curves the detection limits were: 0.09, 0.08, 0.94, 0.03, 0.05, 0.06 µg/mL respectively. The concentrations of individual PAs in whole grain flour were expressed in µg/g of a dry matter basis (DM).
The effects of wheat species and cultivation year on phenolic acids concentration were assessed using two-way analysis of variance (ANOVA) derived from linear mixed-effects model. The ANOVA considered species and year as fixed factors, and field replication as a random factor. The difference between significant main effects and interaction means were determined using the Fisher's least significant difference (LSD) test. All statistical analyses were done using the Statistica software version 10.0 (StatSoft, Tulsa, OK, USA). The set of data used for the statistical evaluation is available online in the Supplementary Materials (Table S1).

Results
Species of wheat significantly affected phenolic acids content ( Table 2). While there was no effect of cultivation year on the free, bound and total PAs, significant interactions between species and year were found for bound PAs and total PAs. The phenolic acids content (total, bound and free) was found to be significantly higher in einkorn (T. monococcum L.), and lower in spelt (T. spelta L.) and emmer (T. dicoccon Schrank) ( Table 2). At the same time, the contribution of free PAs in the total phenolic acids ranged from 5.9% in spelt and 6.0% in emmer, up to 6.3% in einkorn. There were two significant interactions between species and cultivation year for bound and for the total of all phenolic acids content ( Table 2). Since the total phenolic acids consists mostly of bound forms, the interactions for both were the same. While in the first cultivation year both spelt and einkorn had a higher content of bound PAs than emmer, in the following two years einkorn was found to have more bound PAs in total than other two wheat species examined. Table 2. Effect of wheat species and cultivation year on phenolic acids (PAs) concentration (µg/g dry mass) in whole grain flour. All values are means ± standard deviations for four experimental replicates. Values in rows (among species or year) followed by the same letter are not significantly different at p < 0.05. Main effects and interaction F-test from two-way ANOVA at *** p < 0.001, and ns-not significant.

Free PAs Bound PAs Total PAs
Triticum spelta L. 35 In this study, the concentrations of individual phenolic acids were measured for free and bound fractions (Table 3). These included ferulic acid, p-hydroxybenzoic acid, caffeic acid, p-coumaric acid, salicylic acid, sinapic acid, and syringic acid. As expected, the predominant compound across phenolic acids in all wheat species was ferulic acid, which accounted for over 72% of free PAs and almost 95% of bound ones ( Table 3). The significantly highest average concentrations of free and bound ferulic acid were found in einkorn (T. monococcum L.), while spelt (T. spelta L.) and emmer (T. dicoccon Schrank) contained lower and statistically equal values. Similarly, einkorn wheat showed the highest concentration of p-coumaric and sinapic acids, both free and bound. Spelt wheat was characterised by the highest content of p-hydroxybenzoic and syringic (bound only) acids and at the same time the lowest content of caffeic (free only), p-coumaric and sinapic acids. Emmer wheat had a lower content of salicylic and syringic (free only) acids.
When the concentration of phenolic acids was calculated as a percentage of the highest value across all wheat species, the ferulic acid and therefore also total of phenolic acids contents were found be the highest in einkorn (Figure 1). This was observed for both free and bound phenolic acids. In comparison to einkorn, the content of ferulic acid and total phenolic acids in spelt and emmer was about 20% lower. Similarly, einkorn wheat had also the highest content of free and bound caffeic, p-coumaric, sinapic, and synergic (free only) acids. The spelt wheat was found to have the highest concentration of free and bound p-hydroxybenzoic, caffeic (bound only), salicylic and syringic acids. None of the phenolic acids were in the highest percentage content in emmer.  When all species of wheat were analysed together by the year of cultivation, the free ferulic, caffeic, p-coumaric acids were found in higher amounts in the first year of the experiment ( Table 3). The p-hydroxybenzoic acid was found at the lowest concentration in the first or last year of the experiment, as free or bound PAs respectively. The most important factor significantly affecting the individual PAs content was wheat species, however, the effect of cultivation year was also found for most of the individual PAs, both free and bound (Table 3). Only bound caffeic acid concentration was not different between wheat species. In the opposition to the total PAs concentration, the content of most of the individual PAs was found to be significantly affected by the cultivation year. This effect was observed more frequently for free PAs (except for syringic acid; Table 3), compared to bound PAs for which the effect of year was found for p-hydroxybenzoic acid, p-coumaric acid, salicylic acid and syringic acid (Table 3). Moreover, the significant interaction between wheat species and cultivation year were found for most of the individual PAs, both free and bound (Table 3). There were also found frequent interactions between species and cultivation year for bound and for free individual phenolic acids content (Table 3). For caffeic and p-coumaric acids, both free and bound, the interactions were resulted from lower and not significant differences between wheat species in the second year while significant differences in the first and third year (Table 4). Only bound syringic acid concentration was lower in the first year across species, while higher in subsequent two years, especially in spelt. Free p-hydroxybenzoic acid as well as bound ferulic and p-hydroxybenzoic acids were found to be in higher concentration in spelt but only in the first year. In another two years, content on these acids were higher in einkorn or no significant difference between species were noted. The opposite pattern was found for free salicylic and syringic acids. A significantly higher concentration of these compounds was found in einkorn wheat in the first year and in spelt during second in the second and third year. When all species of wheat were analysed together by the year of cultivation, the free ferulic, caffeic, p-coumaric acids were found in higher amounts in the first year of the experiment ( Table 3). The p-hydroxybenzoic acid was found at the lowest concentration in the first or last year of the experiment, as free or bound PAs respectively. The most important factor significantly affecting the individual PAs content was wheat species, however, the effect of cultivation year was also found for most of the individual PAs, both free and bound (Table 3). Only bound caffeic acid concentration was not different between wheat species. In the opposition to the total PAs concentration, the content of most of the individual PAs was found to be significantly affected by the cultivation year. This effect was observed more frequently for free PAs (except for syringic acid; Table 3), compared to bound PAs for which the effect of year was found for p-hydroxybenzoic acid, p-coumaric acid, salicylic acid and syringic acid (Table 3). Moreover, the significant interaction between wheat species and cultivation year were found for most of the individual PAs, both free and bound (Table 3). There were also found frequent interactions between species and cultivation year for bound and for free individual phenolic acids content (Table 3). For caffeic and p-coumaric acids, both free and bound, the interactions were resulted from lower and not significant differences between wheat species in the second year while significant differences in the first and third year (Table 4). Only bound syringic acid concentration was lower in the first year across species, while higher in subsequent two years, especially in spelt. Free p-hydroxybenzoic acid as well as bound ferulic and p-hydroxybenzoic acids were found to be in higher concentration in spelt but only in the first year. In another two years, content on these acids were higher in einkorn or no significant difference between species were noted. The opposite pattern was found for free salicylic and syringic acids. A significantly higher concentration of these compounds was found in einkorn wheat in the first year and in spelt during second in the second and third year. Table 3. Effect of wheat species and cultivation year on free and bound individual phenolic acids concentrations (µg/g dry mass) in whole grain flour. All values are means ± standard deviations for four experimental replicates. Values in rows (among species or year) followed by the same letter are not significantly different at p < 0.05. Main effects and interaction F-test from two-way ANOVA at * p < 0.05, ** p < 0,01, *** p < 0.001, and ns-not significant. ns *** ns *** *** ns *** p species × year *** *** *** *** ns * *** When all species of wheat were analysed together by the year of cultivation, the free ferulic, caffeic, p-coumaric acids were found in higher amounts in the first year of the experiment ( Table 3). The p-hydroxybenzoic acid was found at the lowest concentration in the first or last year of the experiment, as free or bound PAs respectively. The most important factor significantly affecting the individual PAs content was wheat species, however, the effect of cultivation year was also found for most of the individual PAs, both free and bound (Table 3). Only bound caffeic acid concentration was not different between wheat species. In the opposition to the total PAs concentration, the content of most of the individual PAs was found to be significantly affected by the cultivation year. This effect was observed more frequently for free PAs (except for syringic acid; Table 3), compared to bound PAs for which the effect of year was found for p-hydroxybenzoic acid, p-coumaric acid, salicylic acid and syringic acid (Table 3). Moreover, the significant interaction between wheat species and cultivation year were found for most of the individual PAs, both free and bound (Table 3). There were also found frequent interactions between species and cultivation year for bound and for free individual phenolic acids content (Table 3). For caffeic and p-coumaric acids, both free and bound, the interactions were resulted from lower and not significant differences between wheat species in the second year while significant differences in the first and third year (Table 4). Only bound syringic acid concentration was lower in the first year across species, while higher in subsequent two years, especially in spelt. Free p-hydroxybenzoic acid as well as bound ferulic and p-hydroxybenzoic acids were found to be in higher concentration in spelt but only in the first year. In another two years, content in these acids was higher in einkorn or no significant difference between species were noted. The opposite pattern was found for free salicylic and syringic acids. A significantly higher concentration of these compounds was found in einkorn wheat in the first year and in spelt during second in the second and third year.

Discussion
Wheat is a rich source of phenolic acids [16] but its content might be strongly affected by climatic conditions, agronomic management, and plant genetic traits [35,36]. In this study, concentration of phenolic acids (PAs) in three wheat species (einkorn: T. monococcum L., emmer: T. dicoccon Schrank, spelt: T. spelta L.) grown under organic management during three subsequent cultivation years was evaluated and compared. Similarly to the previous studies, results obtained here showed that the free PAs consisted only up to 10% of total PAs [13,32,34,37], while the bound forms are the majority of all PAs in these cereals [13,38]. Spelt and emmer showed similar content of free, bound, and total PAs. All phenolic acid forms however were found to be in the significantly highest concentration in einkorn. This was also reported in several other research in which grain and flour of einkorn wheat were compared to spelt and emmer [24,32,39].
Among all PAs, both free and bound, the ferulic acid had the highest and caffeic acid the lowest content in all three wheat species examined. The second predominant phenolic compound in bound form was p-coumaric acid, followed by the syringic acid. These two were also reported in similar proportions in other studies [32,34]. Ferulic and p-coumaric acids were shown in significantly high concentrations in wheat and oat and accounted for about 70-90% of the total PAs. Higher syringic acid content is generally observed for oat, spelt and wheat grain without glume [40]. Ferulic acid, p-coumaric acid and total phenolic acids were also found in significantly higher content in grain and bread from einkorn than from durum wheat [41]. As described by Shewry and Hey [7] modern wheat, spelt, einkorn, emmer and durum wheats have similar contents of phenolic acids. The authors also pointed out limited data available on the content of bioactive components in ancient wheats, and recommended further detailed studying, using various species of ancient and modern wheat [7]. Moreover, we could add that to date, no systematic literature review or meta-analysis have been published on this topic and definitive conclusions should not be drawn yet.
Metabolic pathways in plants are affected by climatic conditions, agricultural practices and underlying genes. It was shown a significant correlation between the content of bioactive components and environmental factors, such as precipitation and temperature [42]. Also, crop rotation and tillage can possibly affect the polyphenol and flavonoid contents differently depending on the plant species [36]. The effect of drought stress was previously reported to have a stimulating effect on the content of total polyphenols in fresh leaf samples of eight modern wheat genotypes [43]. In our study higher content of ferulic, p-coumaric and caffeic acids in einkorn and emmer was found in the year 2013 and 2015, when the lower precipitation and higher cumulative temperature occurred. At the same time, the quantity of p-hydroxybenzoic, syringic and sinapic acids was the lowest. The significant differences in phenolic acids content in wheat between two agronomic seasons were found by Lachman et al. [27]. They concluded that the higher concentration of free total polyphenols recorded in emmer, einkorn, and modern wheats was a consequence of lower rainfall and higher temperatures during the ripening stages of the cereals [27]. In another study, minimum rainfall during heading and maturity stage in wheat was associated with an increased level of phenolic concentration, supporting the result of the current study [24]. Similarly, in a recent study higher total PAs content in einkorn, emmer, spelt, and modern wheats was observed in the very dry year 2018 [39].
Wheat is a major source of calories in the human diet and provides essential amino acids, minerals and vitamins, also by the beneficial phytochemicals and dietary fibre [2,44]. All these effects are strongly associated with the presence of bioactive compounds, such as phenolic acids, exhibiting the antioxidative abilities, inhibit lipid oxidation by trapping peroxyl radicals [45]. The most potent compounds are caffeic, ferulic, and sinapic acids. It was also shown that the bioavailability (use efficiency by animal/human organism) of PAs is strongly determined by the form compounds and the ability to release of free PAs [46]. In the study by Mateo Anson et al. the content of almost all free phenolic acids was about 20% higher in einkorn than in emmer and spelt wheat species. Only p-hydroxybenzoic acid concentration was higher in spelt. Moreover, most of the bound individual PAs shown the highest content in einkorn whole grain flour, placing this wheat species as the most beneficial for health. The ancient wheat species were reported previously to have more other nutrients as well [47]. Einkorn also shows benefits of reduced reactivity for individuals with coeliac disease and some forms of non-coeliac wheat sensitivity [48]. The more recent experiments found higher carotenoid levels in einkorn than in modern wheat and has shown better sourdough fermentation [49]. Fermentation allows bound, more bioavailable phenolic acids to be released [50]. The future experiments involving fermentation and baking methods and their effects on bound phenolic acids release would be an interesting topic for research, expanding results of studies like ours.

Conclusions
Based on the results from this study together with reports from other research, we conclude that ancient wheat species, especially einkorn, could be an important source of phenolic acids in the human diet. It was demonstrated that there is a significant effect of both the species and cultivation year on both free and bound phenolic acid content in ancient wheats. Einkorn grains were richer in the most potent antioxidants, such as ferulic, caffeic, and sinapic acids. The lowest concentration of all phenolic acids was found in emmer. Our results highlight also that the concentrations of ferulic, p-coumaric and caffeic acids in einkorn and emmer rise in dry and very warm cultivation years. Among other species examined, PAs measured in spelt were more variable during years studied. Despite this fact, all three ancient wheat species can be recommended for cultivation as they increase diversity in cereal production and human nutrition.