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Integrating Plant Nutrients and Elicitors for Production of Secondary Metabolites, Sustainable Crop Production and Human Health: A Review.

Byline: Avila-Juarez Luciano, Torres-Pacheco Irineo, Ocampo-Velazquez Rosalia Virginia, Ana Angelica Feregrino-Perez, Andres Cruz Hernandez and Guevara-Gonzalez Ramon Gerardo

Abstract

Plants are essential sources of bioactive substances that promote health. For economic reasons, farmers usually focus on obtaining higher yields rather than crop nutraceutical quality. The application of non-essential elements (NEEs) is a technique used to increase secondary metabolites (SMs) in plants. This technique includes variations of the essential elements ratios in a nutrient solution or the inclusion of elicitors, such as salicylic acid or methyl jasmonate. Elicitor use is controversial because plants grow differently in inert substrates, in vitro and soil. Soil contains essential elements (EEs) and NEEs that can enhance SM synthesis and increase nutraceutical plant quality. However, any technique that modifies plant metabolism can decrease yields. Thus, developing techniques to increase both agricultural product yield and quality is necessary.

This review aims to demonstrate the necessity for a new recipe or "cocktail" of plant nutrients based on EEs and NEEs, and elicitors apply to achieve both a high yield and crops nutraceutical quality. (c) 2017 Friends Science Publishers.

Keywords: Elicitors; Essential chemical elements; Non-essential chemical elements; Secondary metabolites

Introduction

Since the "Green Revolution," agriculture has been influenced by land mechanization, genetically improved crops, and the excessive use of fertilizers and pesticides which are harmful to the environment (Floros et al., 2010). Such changes have increased agricultural yields (Dayan et al., 2009) because of the more rapid growth of crops (Stefanelly et al., 2010). Moreover, improved plant care and optimal climate handling have improved the conditions for plant growth (Bennett et al., 2012) and reduced the production of secondary metabolites (SMs).

Plants are an essential source of nutrients and secondary metabolites (SMs) (Patra et al., 2013), frequently referred as bioactive compounds, such as alkaloids, phenolic compounds (PCs) and terpenes (Jahangir et al., 2009). Certain SMs reduce the risk of disease, including colon cancer (Russell and Duthie, 2011) and, reduce blood pressure, serum lipids, diabetes mellitus, obesity (Perez-Vizcaino and Duarte, 2010) and cardiovascular diseases (Bernal et al., 2011).

Organic products contain additional SMs when compared with conventional products. The main difference between organic agriculture (OA) and conventional agriculture (CA) is that former uses more plant-friendly pesticides with lower residual effect, with crop growth medium as soil. Soil contains essential (EEs) and non-essential elements (NEEs) in inadequate ratios for plant nutrition, and under stress conditions, plants produce more SMs (de Costa et al., 2013). Moreover, SMs alter normal growth, resulting in decreased crop yields. However, organic products have better nutraceutical quality, but OA cannot always satisfy the demand for horticultural products because a) certain crop yields are approximately 30% of the obtained in CA (Ramos-Solano et al., 2010) and b) OA occupies less than 5% of the cultivable land (Connor, 2008).

Currently, various techniques are being used to increase SMs in plants, including varying ratios of EEs or adding NEEs in the nutrient solution (NS), soil or directly to the plant and applying elicitors such as jasmonic acid (JA), salicylic acid (SA) or nitric oxide (NO) and their derivatives. However, there is a risk of causing an increase or decrease in the yield and SM production by the use of these techniques. The application of elicitors is a widely used technique. However, the SM production in plants growing in soil, in vitro or in inert substrates varies, with more SMs usually produced in soil.

In a broad sense, "elicitor", for a plant refers to chemicals from various sources that can trigger physiological and morphological responses (Zhao et al., 2005). For instance: methyl jasmonate (MeJ) (Heredia and Cisneros-Zevallos, 2009), JA (Saw et al., 2010), SA, and hydrogen peroxide (H2O2) (Jeong and Park, 2005) act as elicitors. Elicitors mimic the action of plant signaling molecules (Ruiz-Garcia and Gomez-Plaza, 2013) and produce reactive oxygen species (ROS) (Yoshioka et al., 2011) that stimulate the plant to produce defense hormones and enzymatic or non-enzymatic antioxidant mechanisms to mitigate ROS effects. A similar effect is caused in the soil by NEEs.

Therefore, the agricultural industry should focus on obtaining high yields with greater nutraceutical quality to promote public health. In the present review, we present the relationship of EEs and certain NEEs in SM production and review the application of elicitors in plants cultivated in soil, inert substrates or in vitro and the relationship between elicitors, EEs and NEEs on secondary metabolite production in plant.

Sustainable Agriculture

Current agriculture practices require a shift towards sustainability models. Plant nutrition should focus on obtaining high yields and crops with greater nutraceutical value usually high in SMs than what are currently produced. Plants grown under OA conditions contain more SMs than produced under CA (Vallverdu-Queralt et al., 2012). Amendments containing NEEs as rare elements (REs) are used in OA. These elements cause stress in the plant (Wang et al., 2007a) and increase the amount of SMs (Challaraj et al., 2010a); however, the induced stress can also result in a lower yield. However, OA cannot adequately satisfy the demand for vegetables. An alternative could be to work with the CA techniques with a consideration for yield and nutraceutical quality. Therefore, sustainable agriculture must be promoted. In recent years, researchers have focused on the relationship between fruit and vegetable consumption and on identifying plant compounds that promote health benefits (Garcia-Mier et al., 2013).

These compounds are categorized into three groups: alkaloids, polyphenols, and terpenes broadly termed as secondary metabolites (Table 1).

Table 1: Categories of secondary metabolites in plants and their effects on health

SMs group###Health benefit###Reference

Alkaloids###Antioxidant###Herraiz and Galisteo, 2003

###Rheumatoid arthritis###Wang et al., 2007b

###Anticancer###Kabashima et al., 2010

###Anti-inflammatory activity###Yang et al., 2007

###Hypertension###Monteiro et al., 2012

Polyphenols###Antimutagenic###Feregrino-Perez et al., 2011

###Antioxidant###Krinsky and Johnson, 2005

###Anticancer###Fresco et al., 2006

###Antimicrobial###Veloz-Garcia et al., 2010

###Anti-inflammatory, Anti-itch###Sur et al., 2008

###Hypocholesterolemic###Jiao et al., 2010

###Antidiabetic activity###Kobori et al., 2009

Terpenes###Antitumor activity###Lage et al., 2010

###Protection against eye diseases###Krinsky and Johnson, 2005

###Antimicrobial###Mathabe et al., 2008

###Antidiabetic activity###Patil et al., 2011

The role of SMs is to protect the plant from stress. For example, ascorbic acid protects metabolic processes from damage caused by hydrogen peroxide (H2O2) and other toxic oxygen derivatives (Ahmad et al., 2010). Diets based SMs provide benefits by preventing or reducing certain diseases in humans. For example, green tea contains catechins that prevent chronic age-related disorders, such as cardiovascular disease (Hodgson and Croft, 2010), mediate vascular inflammation and atherosclerosis through different actions (i.e., anti-hypertensive, anti-lipemic, anti-inflammatory, anti-proliferative and anti-thrombogenic) (Moore et al., 2009; Naito and Yoshikawa, 2009) and prevent the invasion of certain cancers (Khan and Mukhtar, 2008).

Universal Nutrient Solutions in Agriculture

In 1939, Arnon and Stout published the "essential" elements for plants. Since then, recipes for "universal nutrient solutions" (UNSs) have been introduced, such as the by Hewitt (Steiner, 1961) and Steiner (1984). The latter recipe is widely used in agriculture research and is formed by 12 essential chemical elements: N, P, K, Ca, Mg, S, Fe, Mn, B, Cu, Zn and Mo. Currently, UNSs are produced with the maximum 12 essential chemical elements. However, differences exist between various chemical element concentrations. For example, FAO UNS has 34% more N than of Steiner. In contrast, Kilinc UNS has 70% and 77% less N and K, respectively than Steiner UNS (Table 2). Thus, choosing the most appropriate UNS for research, remains difficult because of variations between solutions. An ionic imbalance of elements in the solution could potentially affect the performance or production of compounds of interest.

Plant nutrition is a complex process that involves these essential elements in addition to carbon, oxygen and hydrogen. In the absence of these elements, plants cannot complete their life cycles (Arnon and Stout, 1939). Therefore, fertilization programs provide optimum amounts of fertilizer to increase visual quality and yield; however, such programs are insufficient. Changes in human populations have caused increases of chronic degenerative diseases, and a new method of producing crops is necessary that can potentiate yields but also produce food with high nutraceutical value capable of contributing to public health.

Nutritional Management: Is it the Right Tool to Increase SMs in Plants?

The increase in SMs is achieved by manipulating the ionic proportions of the chemical elements in the NS (Table 3). However, use of such techniques requires careful management because synergism or antagonism can be induced between chemical elements and can cause deficiencies or toxicity resulting in a decrease of yield.

Table 2: Universal nutrient solutions for hydroponics

Chemical Element###Hoagland and Arnon, 1950###Hewitt, 1966###Kilinc, 2007 (1)###Steiner, 1984###Kilinc, 2007 (2)###FAO, 1990###Jensen, 1985

mg L-1

N###210###168###50###167###150###150-225###106

P###31###41###26###31###31###30-45###62

K###234###156###66###277###234###300-500###156

Mg###48###36###10###49###30###40-50###48

Ca###160###160###33###183###100###150-300###93

S###64###48###5###111###15###NA###64

Fe###2.5###2.8###2.6###1.33###8###3-60###3.8

Mn###0.5###0.55###1.6###0.62###5###0.5-1###0.81

B###0.5###0.54###0.5###0.44###1.5###0.4###0.46

Cu###0.02###0.064###0.66###0.02###2###0.1###0.05

Zn###0.05###0.065###1###0.11###0.3###0.1###0.09

Mo###0.01###0.048###0.066###0.048###0.2###0.05###0.03

Plant Nutrition with Macronutrients

Nitrogen (N) is the only element used as a cation (4 ) or anion (NO-). Nitrogen influences growth and morphological development (Gifford et al., 2008), primary and secondary plant metabolism (Giorgi et al., 2009). The link between primary and secondary metabolic pathways in plants is considered to occur through phenylalanine ammonia-lyase (PAL), which explains the concurrent increase of flavonoid activity with increased PAL activity (Lillo et al., 2008). Nitrogen is highly consumed by plants, and non-optimal concentrations of N can lead to losses of yield.

Productivity is also limited under phosphorus (P) deficiency (Chen et al., 2008) and as part of energy rich molecules such as adenosine triphosphate (ATP), nucleic acids and phospholipids, it is involved in primary metabolism (Wu et al., 2003). The symptoms of plant P deficiency are the production of anthocyanins and decreases of development.

Potassium (K) is essential for the synthesis of proteins, glycolytic enzymes and for photosynthesis (Hu et al., 2005). It acts as a coenzyme and activates different precursor enzymes of metabolic pathways (Bussakorn et al., 2003), and its partial or total deficiency has been associated with increased antioxidant enzymes (AOEs). Potassium might play a special role in the process of carotenoid biosynthesis by activating several enzymes regulating carbohydrate metabolism as well as the precursors of isopentenyl diphosphate, pyruvate and glyceraldehyde 3-phosphate (Fanasca et al., 2006).

Calcium ions (Ca) have been adopted as a secondary messenger and represent a versatile signaling molecule in eukaryotic organisms (Dodd et al., 2010). It is involved in several plant physiological processes, acts as an indicator and translator, and is present in sensory proteins that decode specific stimuli (Batistic and Kudla, 2012). Low levels of Ca in the NS increase AOEs levels. However, within the cellular structure, non-optimal concentrations of Ca cause fruit damage; for example, "blossom end rot" in tomato results in the total loss of the product.

Magnesium (Mg) is involved in vital plant functions such as 1) phosphorylation for ATP formation in chloroplasts, photosynthetic fixation of carbon dioxide, protein synthesis, chlorophyll formation, phloem restoring, partitioning and assimilation of photosynthetic products, generation of oxygen reactive forms and photo-oxidation of leaf tissues and activation of enzymes such as ribulose-1.5-diphosphate carboxylase (RuBP) (Cakmak and Yazici, 2010). It is also part of the molecular structure of chlorophyll, and its absence causes severe plant stress that leads to increased AOEs production.

In certain cases, the absence of Mg in the NS reduces carotenes and increases AOEs, such as superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX) (Tewari et al., 2006).

Likely, sulfur (S) is converted to cysteine in plants, the main substrate for the synthesis of compounds that contain S (Nikiforova et al., 2005), such as methionine, glutathione, nicotinamide, phytochelatins and phytoalexins (Rausch and Wachter, 2005).

Plant Nutrition with Micronutrients

The application of micronutrients in plants has been strengthened, and the effects of these micronutrients on SM production depend mainly on the concentration and type of element (Table 4). Similar to macronutrients, inaccurate concentrations of micronutrients can cause crop damage related to toxicity because plants require micronutrients in small amounts.

Iron (Fe) is an essential element and its absence reduces productivity in photosynthetic organisms (Jeong and Guerinot, 2009). Fe is a co-factor for proteins involved in cellular processes such as respiration, photosynthesis and cell differentiation (Broadley et al., 2012). It is required by AOEs because it catalyzes the reactions of electron transfer (Halliwell, 2006).

Copper (Cu) is part of the structure of certain proteins, mainly those involved in photosynthesis (plastocyanins) and respiration (cytochrome oxidase) and in the electron transport chain (Pilon et al., 2006).

Zinc (Zn) is the only metal present in six enzyme categories: oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases (Auld, 2001). Zn is a co-factor of these enzymes groups involved in respiration, photosynthesis and hormone biosynthesis (Broadley et al., 2007).

The role of boron (B) in plants include sugar transport, cell wall synthesis and integrity, lignification, carbohydrate metabolism, ribonucleic acid (RNA), indoleacetic acid, phenolic metabolism, and it is incorporated in the cellular membrane (Ahmad et al., 2009).

Molybdenum (Mo) is necessary in biochemical and physiological processes (Sun et al., 2009) and is an essential component of mononuclear enzymes, metabolic processes and cycles of carbon, N and S (Liu et al., 2010). At high concentrations, Mo can induce the production of SMs (Yu et al., 2012).

Manganese (Mn) is involved in the metabolism of approximately 35 enzymes (Hebbern et al., 2009), and it acts as a metal catalyst and protein activator (Barber, 2003). Manganese participates in the following processes: activation of enzymes involved in N metabolism (i.e., glutamine synthase and arginase), gibberellic acid and RNA biosynthesis, polymerase activation and fatty acids biosynthesis (Hansch and Mendel, 2009).

Use of Non-essential and Beneficial Elements

Beneficial elements cause growth retardation, enzymatic activity changes (Gopal and Rizvi, 2008) and photosynthesis disorders (Ganesh et al., 2008). Beneficial elements are used to increase SMs; for example, the content of a-tocopherol, asparagine and tyrosine (Hediji et al., 2010), isocitrate dehydrogenase (ICDH), citrate synthase (CS), fumarase, malate dehydrogenase (MDH) and phosphoenolpyruvate carboxylase (PEPC) increases in tomato plants (Lopez-Millan et al., 2009) when 100 uM Cd is used in the NS. Hibiscus plants grown in soil with 20 mg kg-1 cobalt (Co), showed increased anthocyanins, and a similar effect occurs when 50 ppm nickel (Ni) is applied in the same crop (Aziz et al., 2007). In bean plants, 0.06 mM mercury (Hg) in the SN increases the contents of a-tocopherol, ascorbic acid and retinol, and this response appears to be concentration dependent (Zengin and Munzuroglu, 2005).

Silicon (Si) is often used as a beneficial elements in various crops because its effectiveness. Si increases biomass (Eneji et al., 2008) and provides resistance against plagues (Savvas et al., 2009) and heavy metals (Nwugo and Huerta, 2008). Si induces AOEs production (Soylemezoglu et al., 2009), such as SOD and catalase (CAT), which protect plant tissues (Al-Aghabary et al., 2004). In alfalfa plants with an NS that contains 1 mM Si, the content of SOD, CAT and POD increases and glutathione reductase (GR) decreases (Wang et al., 2011a). Selenium (Se) is another BE; however, its role has not been completely defined (Malik et al., 2010). Se promotes resistance to abiotic factors (Yao et al., 2009). For example, when Se is used in the NS of soybean at a concentration of 5 uM, an increase in SOD, CAT, APX and glutathione peroxidase (GPX) activities is observed (Malik et al., 2012).

Use of Rare Elements

Rare elements (REs) are homogeneous elements with similar chemical properties and include lanthanides, scandium and yttrium. Their use in agriculture is currently increasing, and mixtures of REs can be found in the market. These REs increase SOD, POD, total phenols (TP) and carotenoid content in corn (Challaraj et al., 2010a), modify plant enzymatic activity (Gopal and Rizvi, 2008), and promote the activation of antioxidant mechanisms such as AOEs or SMs. The effect of rare elements on plants varies depending on the element and its dosage. For instance, cerium (Ce), lanthanum (La) and neodymium (Nd) can increase the yield and fruit quality in certain concentrations and in some crops (Wang et al., 2007a); however, can cause toxicity in high concentrations.

In bean plants, gradually increasing La concentrations of the root NS (0.25, 0.5, 1, 2, 4, 8 and 12 mg L-1) results in increase of SOD, APX and GPX (Wang et al., 2011b). Taxol content increase when 1 mM of Ce4+ is applied to cells of Taxus cuspidata (Yang et al., 2009). In rice, the use of Ce4+ in the NS leads to an increase of SOD, CAT and malonyldialdehyde (MDA) (Xu and Chen, 2011). In radish plants, the use of terbium (Tb3+) (5 mg L-1) increases the activity of ascorbate and decreases guaiacol content (Wang et al., 2009).

Rare elements also increase the absorption of ions that may be beneficial for SM synthesis. For example, Ce3+ usage results in an increase of K, Mg, Ca, Cu, Fe and Mn content (Wang et al., 2008), and are applied to infertile soils to improve the availability of essential elements. Similar to beneficial elements, the increased dosage of rare elements can be toxic to plants.

Towards a New Cocktail of Necessary Nutrients (CNN)

In current agricultural practices, NS with essential elements, and the optimal EE concentrations required to develop NS for commercial crops are known and have produced increases in yield. However, to develop horticultural functional foods, new techniques that produce higher contents of SMs in crops are necessary. One method is to vary EE concentrations and another method incorporates non-essential elements such as rare elements in the nutrient solution.

Plants can absorb non-essential elements, and if present in an inadequate range, either in the soil or NS, stress will result in ROS production. Under normal and primarily under stress conditions, ROS are detoxified by a group of enzymatic antioxidants, such as SOD, APX and CAT, and non-enzymatic antioxidants (Fig. 1), such as ascorbic acid, glutathione, carotenoids and tocopherols (Miller, 2010).

Table 3: Effect of different concentrations of macronutrients in plants for the production of secondary metabolites

Element###Plant###Doses###Effect###Reference

N###Broccoli###0###Flavonoids|###Jones et al., 2007

###Cabbage###0###Flavonoids|###WeiFeng, 2009

###Lettuce###0###Flavonoids|###Chiesa et al., 2009

###Olive tree###0###Flavonoids|###Fernandez-Escobar et al., 2006

###9.58 meq L-1###Mannitol|###Boussadia et al., 2010

###Tomato###0###Flavonoids|###Simonne et al., 2007

###3.25 mM###Carotenoids|###Khavari-Nejad et al., 2013

P###Lentil###0###PC and anthocyanins|###Sarker and Karmoker, 2011

###Tomato###0.7 mM###- carotene and xanthophyll|###Khavari-Nejad et al., 2013

K###Millet###0###CAT, GPX and APX|###Heidari and Jamshidi, 2011

###Tomato###4 mM###Carotenoids|###Schwarz et al., 2013

###Basil###5 mM###Phenols, rosmarinic acid and anthocyanins|###Nguyen et al., 2010

###sunflower###soluble solids|

###25 Kg ha-1###SOD, CAT and GPX|###Soleimanzadeh et al., 2010

Ca###Millet###0###POD and CF|###Finger et al., 2006

###5 mM###PC|

###Tomato###0.1 mM###SOD and DAR|###Mestre et al., 2012

###CAT, APX and GR|

###Eggplant###0.5 meq L-1###Total phenols and PPO|###Pratima et al., 2002

###Tobacco###5 mM###Total phenols, POD and PPO|###Ruiz et al., 2003

###cherry###80 mM###Phenols, flavonoids, anthocyanins and ascorbic acid|###Aghdam et al., 2013

Mg###blackberry###0###Carotenoids|###Tewari et al., 2006

###Sunflower###SOD, POD and APX|

###Lettuce###0###Glutathione, SOD, APX, GPX and CAT###Chou et al., 2011

###60 mg L-1###Lactucopicrin|###Seo et al., 2009

S###Arabidopsis###0###-alanine, putrescence, raffinose, glutamine,###Zhang et al., 2011a

###-tocopherol and -sitosterol|

###Beans###0###Carotenoids|###Juszczuk and Ostaszewska, 2011

###Peas###200 mg plant-1###saccharose|###Scherer et al., 2006

Table 4: Effect of different concentrations of micronutrients on secondary metabolite production in plants

Element###Plant###Doses###Effect###Reference

Fe###Rapeseed###0###AP, POD, SOD and AA|###Tewari et al., 2013

###CAT|

###Plum###0###Asparagine, alanine, glutamine, and organic acids|###Jimenez et al., 2011

###SOD and APX|

###sweet potato###9 mmol L -1###CAT|###Adamski et al., 2012

Cu###Poppy###2 mmol L-1###Carotenoids|###Cambrolle et al., 2011

###grapevine###2.5 mmol L-1###Carotenoids|###Cambrolle et al., 2013

###mustard seed###50 uM###Ascorbate and SOD|###Feigl et al., 2013

###rice###50 uM###Ascorbate and SOD|###Thounaojam et al., 2012

###100 uM###GPX, APX and GR|

Zn###wheat###3 mM###POD, CAT and APX|###Li et al., 2013

###Beetroot###50 uM###MDH, PEPC, ICDH and CS|###Sagardoy et al., 2011

###Tomato###100 umol L-1###Carotenoids, APX and GR|###Cherif et al., 2011

B###Tobacco###0###GDH, glucose and fructose, organic acids, phenols and amino acids| Beato et al., 2011

###Orange tree###2.5 uM###Carotenoids, saccharose, DHAR and CAT|###Han et al., 2008

###Carrot###5 uM###AA|###Eraslan et al., 2007

###Corn###4 mM###SOD and CAT|###Esim et al., 2013

###POD|

###Linen###450 mM###PAL, PPO and POD|###Heidarabadi et al., 2011

Mo###Glycyrrhiza uralensis Fisch 5.2 mg L-1###GA and squalene|###Wang et al., 2013

###Tomato###0.5-1 mg kg-1###Yield###Sandabe and Bapetel, 2008

Mn###Clover###5.2 uM###GPX|###Dorling et al., 2011

###Pea###50 uM###GOGAT, CAT and APX|###Gangwar et al., 2010

###Grape###30 mM###PPO, CAT and POD###Mou et al., 2011

Non-essential elements (mainly Res) activate plant response genes, alter plasmatic membrane potential (Kenderesova et al., 2012), and induce ROS and Ca-signaling (Rodrigo-Moreno et al., 2013) in response to ion effects. The production of antioxidant defenses triggered by the presence of certain non-essential elements depends on the type and concentration of the element but also on the plant species (Rodriguez-Serrano et al., 2009). Thus, the antioxidant mechanism can be inhibitory or stimulatory (Schutzendubel and Polle, 2002).

It has been shown that if non-essential elements are present in the soil or NS, can be absorbed by plants.

Sheppard et al. (2010) found the following non-essential elements in tomato fruit: Ag, As, Ba, Cd, Ce, Cl, Co, Cr, Cs, La, Li, Mo, Na, Nb, Nd, Ni, Pb, Pr, Rb, Sb, Se, Sm, Sn, Sr, Tb, Th, Tl, U, V, Y, Yb and Zr. Similar results were also found by Matos-Reyes et al. (2010), Demir et al. (2010) and Bressy et al. (2012).

Current common practice includes cultivating plants with recipes that contain non-essential elements in the NS. In China, rare elements have been used to increase the quality and yield of crops for several years. However, it is important to consider the adverse effect that NEEs have on crops because their optimal concentration and time of application and effect on each type of crop is currently unknown.

Non-essential elements most likely cause a hormetic effect, which is a plant response to doses with low dose-stimulation and high doses-inhibition of growth (Poschenrieder et al., 2013). By including NEEs such as As, Se, Cr, Al and Pb in the NS, yield increases, and it is likely stimulation in an adaptive compensation process (Poschenrieder et al., 2013). Studies have suggested stress-induced growth mainly because of excess metals; however, few studies have analyzed the physiological state and molecular mechanisms of the stimulant response to accurately assess the action of ions in the plant.

Based on these data, it may be possible to develop a recipe or "cocktail" of nutrients containing essential and non-essential element to increase plant SMs without affecting the yield. Physiologically, ROS can be induced in the plant to activate antioxidant mechanisms, thus generating functional foods. However, it is difficult to calculate the amount of SMs to induce at the expense of yield because an increase in ROS is usually accompanied by plant damage.

NEEs and Elicitors are Necessary for SMs Production

Several methods exist that increase SMs in plants, such as using elicitors (Table 5). Chemical elicitors, including SA, JA, NO, and MeJ, may interact with receptors in plants, activating defense response (Ruiz-Garcia and Gomez-Plaza, 2013). For instance, NO is involved in abiotic stress, as are heavy metals (Zheng et al., 2008). NO also interacts with ROS in various ways and may serve as an antioxidant and ROS scavenger during environmental stress (Zheng et al., 2010). Additionally, elevated NO down regulates K+/Cl-influx, and promotes K+/Cl-efflux and Ca2+ release during stomatal closure (Sokolovski and Blatt, 2004). NO regulates mineral absorption, particularly at concentrations of 50 uM, and enhances shoot uptake of Mg, Cu, Ca and Fe (Liu et al., 2015). Moreover, NO regulates genes related to plant growth and ion absorption (Besson-Bard et al., 2009). MeJ is a naturally occurring plant growth regulator that modulates chlorophyll degradation and anthocyanin biosynthesis (Ruiz-Garcia et al., 2012).

MeJ has been also involved in NH + accumulation in rice leaves (Hung and Kao, 2007). NH + is released through the action of PAL, the first enzyme in the phenylpropanoid biosynthesis pathway (Hahlbrock and Grisebach, 1979).

Table 5: Effects of chemical elicitors on plant antioxidant enzymes/secondary metabolites

Plant###Elicitor (dose)###Effect###Reference

Tomato###NO: 100 M###Chelate reductase|###Graziano and Lamattina, 2007

Tomato###NO: 20 uM###CAT, POD, SOD and APX|###Zhao et al., 2011

Tomato###NO: 100 M###Chelate reductase|###Graziano and Lamattina, 2007

Tomato###NO: 20 uM###CAT, POD, SOD and APX|###Zhao et al., 2011

Tomato###SA: 100 uM###CAT and POD|###Ortega-Ortiz et al., 2007

Tomato###SA: 10 mM###Vitamin C and degBrix|###Javaheri et al., 2012

Tomato###SA: 500 uM###Soluble solids|###Yildirim and Dursun, 2009

Tomato###MeJ: 0.1 M###Quercetin|###Horbowicz et al., 2011

Strawberry###MeJ: 300 M###Resveratrol|###Wang et al., 2007c

Cucumber###H2O2: 1.5 mM###SOD, GH and APX|###Zhang et al., 2011b

Cucumber###H2O2: 1.5 mM###POD, DHAR and APX|###Gao et al., 2010

Table 6: Production of bioactive compounds and/or antioxidant enzymes by elicitation in plants grown in soil, in vitro or in substrate

Cultivation###Plant###Elicitor###Bioactive Compounds/AOEs(difference from control)*###Reference

Medium

Soil###Artemisia annua###2 mmol NO###Total chlorophyll, artemisinin content, POD, SOD and CAT|###Aftab et al., 2012

Soil###Brassicacampestris###50 mmol H2O2###CAT and MDA|###Chun-Yan et al., 2007

Soil###Glycine max###2% (SO2+NO2)###PC|###Hamid and Jawaid, 2009

Soil###Lycopersicon esculentum###0.5 mM SA###Chlorophyll|###Yildirim and Dursun, 2009

Soil###Lycopersicon esculentum###10-4 M SA###Lycopene and vitamin C =###Javaheri et al., 2012

Soil###Syzygium samarangense###5 mM H2O2###Flavonoids, anthocyanins, total phenols and carotenoids|###Khandaker et al., 2012

Soil###Zea mays###100 ppm SA###Chlorophyll|###Rao et al., 2012

Substrate: UNS###Lycopersicon esculentum###100 uM SNP###Proline, chlorophyll, MDA, CAT, LOX, APX and GPX =###Kazemi, 2012

In vitro###Fagopyrum esculentum###10-6 M MeJ###Acids: caffeic, gallic, syringic, feluric, coumaric acid, and quercetin =###Horbowicz et al., 2011

In vitro: NS###Cucumis sativus###100 M SNP###SOD, CAT, GPX, APX, DHAR, AsA and GSH =###Lin et al., 2012

In vitro###Physalis peruviana###0.1 mg L-1 JA or 1 Mm SA###4- hydroxy-withanolides E =###Pineros-Castro et al., 2009

Substrate: UNS###Glycine max###100 uM SA or SNP###Flavonoids, anthocyanins, LOX and SOD =###Simaei et al., 2012

In vitro###Lycopersicon esculentum###1 mM SA###Total chlorophyll and carotenoids totals =###Shahba et al., 2010

Substrate: UNS###Lycopersicon esculentum###100 uM SNP###SOD, POD, CAT y APX =###Zhang et al., 2009

Substrate: UNS###Cucumis sativus###1.5 mM H2O2###SOD, CAT, GSH-PX, GR and AsA =###Zhang et al., 2011b

Plants produce signaling molecules such as SA, JA and NO, and the content of these molecules increase when the plant is under stress. Compounds such as chitosan, harpin and 1-methylcyclopropane have been also identified, and provide benefits when exogenously applied to the plant. These benefits include protection against plague or diseases or support of metabolism. These compounds mimic the action of signaling molecules such as SA and JA and their derivatives. These also interact with plant receptors that activate defense mechanisms, such as TP and flavonoids (Liu et al., 2005). Signaling molecules such as methyl jasmonate (MeJ), SA, H2O2 and NO currently used exogenously to increase the SMs content in crops, and these molecules are known to regulate the production of AOEs and SMs (Fig. 1). These molecules have different characteristics (Fig. 2), but in some crops, these produce similar effects (see Table 5).

In hydroponic crops, the increased SMs in elicited plants are barely noticeable. However, the increase of SMs is significant when elicitors are used in plants growing in soil or compost (Table 6), and Turra et al. (2011) showed that compost contains REs. The increase of SMs may occur because soil contains certain NEEs and REs that helps to activate SM synthesis pathways. Rare elements are involved in plant metabolism and increase ion absorption, protein synthesis, chlorophyll a and b content, plant yield, and enzyme activity (POD and SOD) (Challaraj et al., 2010b).

Plants respond differently to elicitors i.e., certain SMs are activated in certain plants and the same SMs can be deactivated in others (Table 6). Signal perception is the first step in the elicitation process and leads to a transduction cascade by which plants respond to stimuli and activate kinases and produce ROS, ion flow and cytoplasm acidification (Vasconsuelo and Boland, 2007). However, if the plant is elicited and the necessary material (some chemical element) is not found in the soil or NS, the expected response to the stimuli will not occur. When the plant is elicited, one of two actions occurs: certain elements classified as non-essential are present in the ion flux, and they can stimulate SM synthesis; or NEEs present in a minimum quantity exert pressure in the cell that favors a secondary metabolism pathway.

Plants cultured in substrates or in vitro with NS, even when an elicitor is used, do not indicate an increase in SMs (compared to the non-elicitor control), which may result from missing a certain metabolite biochemical pathway chemical element that is necessary for activation (Table 6). However, the elicitation of plants grown in soil usually produce a favorable response in terms of SM production, which may be explained by the presence of NEEs, such as EBs or REs, in soils, and these NEEs participate directly and indirectly in the production of SMs.

Conclusions

Plants generate SMs to protect cells from the harmful effects caused by ROS, and SMs also have beneficial health effects. OA produces horticulture products with greater amounts of SMs, but such agricultural techniques are inadequate to satisfy the global demand, whereas the NS used in CA are insufficient to produce fruits and vegetables with high nutraceutical value. The use of a technique that may increase plant SMs, such as varying the ionic EE ratio or adding NEEs in the NS, results in lower yield. Applying only elicitors, such as MeJ, NO and SA, forces the plant to produce SMs but causes lower yields. Certain NEEs can be included in the NS, and elicitors can be applied to plant foliage. Thus, NEEs could enhance ROS production and elicitors could activate antioxidant mechanisms. Thus, the production of ROS and bioactive compounds, such as terpenes, alkaloids and phenols, would be equilibrated.

According to reports found in the literature, the application of elicitors and the presence of NEEs in the NS or soil are necessary to increase and potentiate SM production. Therefore, the coordinated combination of these two techniques is required for the production of SMs. However, ions must be identified that can be added to the NS without being transferred to the edible part of the plants or concentrations of such ions must be determined that are low enough to avoid health damage. In addition, this new cocktail must not have a negative impact on the environment. Thus, the new NS should increase yield and produce food with higher nutraceutical qualities capable of preventing human diseases.

Acknowledgements

The first author acknowledges the financial support from Universidad Autonoma de Queretaro, Queretaro, Mexico. Additionally, authors thanks to FORDECYT (193512), FOMIX-Qro and Ciencia Basica SEP-CONACYT 2012, for partial support of this research.

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Author:Avila-Juarez, Luciano; Torres-Pacheco, Irineo; Ocampo-Velazquez, Rosalia Virginia; Feregrino-Perez,
Publication:International Journal of Agriculture and Biology
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Date:Jun 30, 2017
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