*Article* **Phenolic and Carotenoid Profile of Lamb's Lettuce and Improvement of the Bioactive Content by Preharvest Conditions**

**Virginia Hernández 1, M. Ángeles Botella 2, Pilar Hellín 1, Juana Cava 1, Jose Fenoll 1, Teresa Mestre 3, Vicente Martínez <sup>3</sup> and Pilar Flores 1,\***


**Abstract:** This study characterizes the phenolic, carotenoid and chlorophyll profile of lamb's lettuce, a vegetable whose consumption in salads and ready-to-eat products is constantly growing. The MS/MS analysis allowed the identification of thirty-five phenolic compounds including hydroxybenzoic and hydroxycinnamic acids, flavanones, flavanols and flavanones, many of which are reported here in lamb's lettuce for the first time. Chlorogenic acid was the principal phenolic compound found (57.1% of the total phenolic concentration) followed by its isomer *cis*-5-caffeoylquinic. Other major phenolic compounds were also hydroxycinnamic acids (coumaroylquinic, dicaffeoylquinic and feruloylquinic acids) as well as the flavones luteolin-7-rutinoside, diosmetin-apiosylglucoside and diosmin. Regarding carotenoids, seven xanthophyll and four carotenes, among which *β*-carotene and lutein were the major compounds, were detected from their UV-Vis absorption spectrum. In addition, chlorophylls a and b, their isomers and derivatives (pheophytin) were identified. Preharvest factors such as reduced fertilization levels or salinity increased some secondary metabolites, highlighting the importance of these factors on the final nutritional value of plant foods. Lamb's lettuce was seen to be a good potential source of bioactive compounds, and fertilization management might be considered a useful tool for increasing its nutritional interest.

**Keywords:** corn salad; leafy vegetables; phytochemicals; liquid chromatography; mass spectrometry

#### **1. Introduction**

The regular consumption of fruit and vegetables has many benefits for human health in terms of reducing the possibility of developing chronic diseases [1]. It has been estimated that a high intake of fruit and vegetables can reduce the risk of developing cardiovascular diseases [2] and several types of cancer [3]. Most of these health-promoting effects are related to the vegetable bioactive content. Phenolics are the most abundant antioxidants in the human diet [4], of which approximately a third correspond to phenolic acids and two thirds to flavonoids [1]. Several reports indicate that polyphenolic compounds are effective in the prevention of diseases caused by long-term diabetes such as cardiovascular disease, neuropathy, nephropathy and retinopathy [5]. Moreover, phenolic compounds seem to inhibit cell proliferation and tumor metastasis and induce apoptosis in various types of cancer cells, including colon, lung, prostate, hepatocellular, breast cancer or multiple myeloma [6,7]. While many plants contain phenolic compounds, their concentration and chemical forms depend on individual plant species. Vegetables and fruits are also rich in carotenoids, molecules with a high antioxidant capacity, and the consumption of a diet rich

**Citation:** Hernández, V.; Botella, M.Á.; Hellín, P.; Cava, J.; Fenoll, J.; Mestre, T.; Martínez, V.; Flores, P. Phenolic and Carotenoid Profile of Lamb's Lettuce and Improvement of the Bioactive Content by Preharvest Conditions. *Foods* **2021**, *10*, 188. https://doi.org/10.3390/foods 10010188

Received: 14 December 2020 Accepted: 14 January 2021 Published: 18 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in carotenoids is thought to reduce the risk of cancer, cardiovascular diseases, age-related maculopathy and cataracts [8]. The influence of chlorophylls on human health has not been as widely studied as that of phenolic compounds and carotenoids, although evidence of their benefits has been reported [9].

Due to their importance in human health, the characterization of bioactive compounds contained in fruits and vegetables and their related beneficial effects need to be studied in greater depth. Specifically, green leafy vegetables can be a source of ascorbic acid, flavonoids, phenolic acids and carotenoids, besides minerals, fiber and many trace elements. Lettuce and escarole have long been the most common vegetables used in salads, partly due to their healthy attributes, and several studies about characterization of polyphenols in lettuce can be found in the literature [10,11]. However, new leafy vegetables are increasingly consumed in salads, especially as ready to eat products. Among them lamb's lettuce (*Valerianella locusta* L. Laterr.) has special relevance due to its pleasant taste and texture and nutritional value. However, information on *V. locusta* in the literature is scarce and mostly focuses on the shelf-life and quality changes that may take place during postharvest storage [12] or by some preharvest conditions [13–16]. Plant development and yield are strongly affected by mineral nutrition and environmental stresses that reallocate resources from primary to secondary metabolism with a direct effect on product quality [15]. In some species it has been shown that salt stress induced the synthesis of substances in proportion to the increase in NaCl concentrations, confirming the important role of these molecules for the tolerance to stress conditions in plants and salinity as an efficient technique for increasing the secondary metabolite content in plants [17]. However, little information on the phytochemical profile of *V. locusta* can be found, with the exception of some recent works [18]. Moreover, despite the well-documented impact that mineral nutrition and irrigation water quality have on the biochemical composition of plants, and hence on the nutritional value of vegetables, there is hardly any information about how these preharvest aspects can affect lamb's lettuce quality.

The main objective of the present study was the characterization of the phenolic, carotenoid and chlorophyll profile of lamb's lettuce. Taking into consideration mineral nutrition and salinity as two of the preharvest factors that most affect the quality of plant foods, the impact of fertilization and salinity (NaCl content) on the bioactive compound content of lamb's lettuce was also evaluated.

#### **2. Materials and Methods**

#### *2.1. Plant Material*

Lamb's lettuce (*Valerianella locusta* L. Laterr. cv. Favor) plants were grown in a greenhouse equipped with a dynamic root floating system that pumped the nutrient solution from a tank into different containers (trays). Plants were supported through a floating board made of high-density polyethylene. The roots were fully submerged in the nutrient solution that circulated back to the tank through a drain for reuse. The pH of each nutrient solution was adjusted to between 5.5 and 6.0 every day. Water lost by transpiration was replaced every two days and nutrients were added every week to restore their initial concentrations. In order to study the impact of fertilization and salinity on the bioactive compound content, the control nutrient solution [1 ⁄2 Hoagland solution, electrical conductivity (EC)1 dS cm−1, 7 mM N, 2 mM Ca and 3.5 mM K] was modified to obtain different treatments in four consecutive experiments with different levels of nitrogen (0.1, 1 and 7 mM N), calcium (0.5, 2, and 5 mM Ca), potassium (0.1, 0.5 and 3.5 mM K), and salinity (0, 15, 30 and 60 mM NaCl), respectively. In every experiment, the plants were distributed in two blocks with three replicates per treatment and block. Each replicate consisted of a tray (3.6 m−2) containing 100 plants m−2. Salinity treatments consisted of applying 15 mM NaCl on one, two or four days (for the 15, 30 and 60 mM NaCl treatments, respectively) in order to avoid an osmotic shock. Final ECs of the different saline treatments were 1 (control), 2.7, 4.0 and 6.5 dS cm<sup>−</sup>1. Thirty days after transplanting (DAT), when the plant had five fully expanded leaves, fifty plants per replicate were harvested and weighed

after being washed and gently dried. They were then powdered with liquid N2 and kept at −80 ◦C until subsequent analysis. Each sample was analyzed in triplicate.

#### *2.2. Metabolite Analyses*

#### 2.2.1. Phenolic Compounds

Phenolic compounds were extracted with methanol:formic acid (97:3) according to Cantos et al. [19] and analyzed using an Agilent 1200 liquid chromatograph (Santa Clara, CA, USA) equipped with a G6410A triple quadrupole mass spectrometer detector (MS/MS) equipped with an electrospray ionization (ESI) interface, operating in negative ion mode. A Lichrosphere C18 analytical column of 250 mm × 4 mm and 5 μm particle size was used (Agilent Technologies, Waldbronn, Germany). The mobile phase was 0.1% formic acid in water (solvent A) and 0.1% formic acid in acetonitrile (solvent B) at a flow rate of 1 mL·min−1. The gradient began with 5% B, raised to 10% B in 9 min, 30% in 50 min, and 100% in 2 min and held at 100% B for an additional 3 min before returning to initial conditions in 1 min and remaining isocratic for 6 min. The following operation parameters were used: 2000 V capillary voltage, 60 psi nebulizer pressure, 13 L/min drying gas flow and 350 ◦C drying gas temperature. Fragmentor voltages (F) from 20 to 200 V and collision energies (CE) from 2 to 50 V were used for optimizing selective reaction monitoring (SRM) transitions. Myricetin (Sigma-Aldrich, St. Louis, MO, USA) was used as internal standard. The phenolic compounds were identified by MS/MS experiments: full scan and neutral loss (NL), precursor ion (PreI) and product ion (ProdI) scan modes. Protocatechuic, luteolin−7*-O-*glucoside, diosmetin, diosmin, apigenin−7*-O-*glucoside, hesperidin (hesperetin−7*-O-*rutinoside) (Extrasynthese, Genay, France) and chlorogenic acid (5*-O-*caffeoylquinic acid), caffeic acid, *p-*coumaric acid, luteolin, and quercetin were quantified with respect to their standards (Sigma-Aldrich, Steinheim, Germany). Chlorogenic isomers were quantified with respect to chlorogenic acid; caffeic acid*-O-*hexosides, dicaffeoylquinic and caffeoylferuloylquinic acid with respect to caffeic acid; sinapic-hexose with respect to sinapic acid (Sigma-Aldrich, Steinheim, Germany); coumaroylquinic isomers with respect to *p-*coumaric acid; feruloylquinic isomers with respect to ferulic acid (Sigma-Aldrich, Steinheim, Germany); isorhamnetin-rutinoside with respect to isorhamnetin (Sigma-Aldrich, Steinheim, Germany); luteolin-apiosylglucoside and luteolin−7– rutinoside with respect to lutein-7*-O-*glucoside; quercetin-glucuronide, quercetin-glucoside with respect to quercetin; apigenin-rutinoside and acacetin-rutinoside with respect to apigenin−7*-O-*glucoside; diosmetin-apiosylglucoside with respect to diosmin.

#### 2.2.2. Carotenoids

Carotenoids and chlorophylls were extracted with methanol/tetrahydrofuran (1:1, *v*/*v*) containing MgO (Merck, Darmstadt, Germany) and 0.1% (*w*/*v*) butylated hydroxytoluene (BHT) (Sigma-Aldrich, St. Louis, MO, USA) following the methodology validated by Motilva et al. [20]. For that, an Agilent Series 1100 liquid chromatograph (Santa Clara, CA, USA) equipped with a photodiode array detector (DAD) and a 250 mm × 4.6 mm i.d., 3 μm two serially coupled Prontosil C30 columns Bischoff (Leonberg, Germany) were used. The mobile phase was methanol (solvent A) and methyl tert-butyl ether (solvent B) eluted at a flow rate of 1.0 mL/min, as follows: (1) initial conditions 15% solvent B and 85% solvent A, maintained for 20 min (2) a 20-min linear gradient to 30% solvent B, then maintained for 10 min (3) a 80-min linear gradient to 90% solvent B. Compounds were eluted and recorded for 70 min and the subsequent gradient allowed the column to be cleaned. All-*trans*-violaxanthin, 9 *cis*-neoxanthin, antheraxanthin, all-*trans*-lutein, zeaxanthin, *β*-cryptoxanthin, all-*trans*-*β*-carotene and all-*trans*-α-carotene were quantified using commercially available external standards (DHI LAB, Hoersholm, Denmark). Luteoxanthin was quantified with respect to antheraxanthin. The *cis* isomers of *β*-carotene were quantified with respect to all-*trans*-*β*-carotene. *β*-apo-8 -carotenal (Sigma-Aldrich, St. Louis, MO, USA) was added as internal standard.
