TABLE OF CONTENT
1.2 Definition of salinity
1. 3 How to measure salinity
1.4 Definition of Drought
1.4.1 Causes of drought
1.4.2 Types of drought
1.4.3 Consequences of drought
2.1 Definition of metabolite
2.1.1 Examples of metabolite
2.1.2 Types of metabolite
2.2 Primary metabolite
2.3 Secondary metabolite
3.1 Plant Physiology and Metabolism
3.2 Role of Secondary metabolites
4.1 Antioxidant system and Plant metabolism
4.2 Environmental stressors
4.3 Free radicals / Reactive Oxygen Species (ROS)
4.4 Effect of Drought on Plant Physiology and Metabolism
4.5 Effect of Salinity on Plant Physiology and Metabolism
4.6 Relationship between drought and Salinity
4.7 Effect of drought and salinity on Secondary Metabolites
It is widely accepted that food security around the world is decreasing due to detrimental effects of various unfavorable environmental conditions. To this effect, minimizing these effects continues to be top priority globally to ensure food security under changing climate.
Plants are frequently exposed to a plethora of conditions that may or may not be favourable to their optimum functioning. Some of conditions including micro-organisms, air, water, gases (O2 and CO2) among others contribute positively to the growth of plants (Gleadow and Woodrow 2002, Falk et al. 2007, Ballhorn et al. 2011). On the other hand, an excess or deficit of these environmental conditions may disrupt the normal and adequate functioning of plant which leads to the phenomenon of stress to the plant.
Generally, stress (with respect to plant metabolism) has been described as an adverse condition which inhibits the normal functioning and total well-being of a plant (Jones and Jones, 1989; Mahajan and Tuteja, 2005; Aksoy, 2008). In their environment, plants are vulnerable to numerous forms of stress yet it is known that under normal biological conditions, what constitutes stress for one plant may be optimum for another plant (Mahajan and Tuteja, 2005). All these stress factors (drought, salinity, extreme temperature etc.) collectively have been studied and known to be associated with negative consequences to the overall plant growth and productivity.
Cold, heat, salinity, drought (water-deficit condition), flooding (excess water), radiations (high intensity of ultra-violet and visible light), chemical and pollutants (heavy-metals, pesticides etc.), oxidative stress (reactive oxygen species, ozone), wind (sand and dust particles in wind) are categorized as abiotic stress while pathogen (viruses, bacteria, fungi), insects, herbivores, rodents are called biotic stress, as they are caused by plant’s interaction with other organisms. Additionally, abiotic stress in fact has been attributed to be the principal cause of crop failure worldwide, dipping average yields for most major crops by more than 50% (Bray et al., 2000; Mahajan and Tuteja, 2005).
In the presence of a potential stressor (biotic or abiotic), certain cellular responses are initiated primarily by interaction of the extracellular material with a plasma membrane protein. This extracellular molecule is called a ligand (or a stressor) and the plasma membrane protein, which binds and interacts with this molecule, is called a receptor. The stressor is perceived by the receptors present on the membrane of the plant cells, the signal is then transduced downstream and this results in the generation of second messengers including (but not limited to) calcium, reactive oxygen species (ROS) and inositol phosphates Mahajan and Tuteja, 2005). This further leads to a cascade of signal transduction, followed by a physiological response such as an increase in the production of secondary plant products (Selmar and Kleinwachter, 2013).
1.2 Definition of Salinity
Conceptually, salinity is the quantity of dissolved of salt content of the water. Salts could also be described as compounds like sodium chloride, magnesium sulfate, potassium nitrate and sodium carbonate, which dissolve into ions. For example, the concentration of dissolved chloride ions is sometimes referred to as Chlorinity (Anonymous, 2018).
Some researchers have grouped saline environments broadly as being either wet or dry. Wet saline habitats tend to occur in coastal regions and are dominated by salt marshes. Since these areas border the sea, they are subject to periodic inundations, and as a result the level of salinity fluctuates over time. Dry saline habitats are usually located inland, often bordering deserts (Flowers 2004; Bado et al., 2016). Other types of saline environments include seashore dunes, where salt spray is a salinizing factor, and dry salt lakes. Common features of saline environments are the salinity of the soil and/or of their associated water resources and specialised flora and fauna. The most abundant salts in saline soils are sodium chloride (NaCl) and sodium sulphate (Na2SO4), which may be associated with magnesium (Mg) salts (Bado et al., 2016).
Salinity is one of the most important abiotic stresses in arid and semiarid regions (Zhang et al., 2010; Sahu et al., 2011; Olfati et al., 2012; Talebi et al., 2015). Salt stress retards plant growth and yield, and has become a serious problem in the world (Horvath et al., 2007; Kirdmanee, 2009; Moghbeli et al., 2012).
The impact of salt stress has been correlated with some morphological and physiological traits such as reduction in fresh and dry weight (Chartzoulakis and Klapaki, 2000). In fact, salinity affects plant metabolism by disturbing their physiological and biochemical processes of plants due to ionic and osmotic imbalances which slows down plant growth and productivity (Munns, 2005). The deleterious effects of salinity on plant growth are associated with low osmotic potential of soil solution, nutritional imbalance, specific ion effect, or a combination of these factors (Ashraf and Harris, 2004).
1.3 How to measure salinity
Plant growth responses to salinity vary with plant life cycle; critical stages sensitive to salinity are germination, seedling establishment and flowering (Ashraf and Waheed, 1990; Flowers, 2004). Criteria for evaluating salinity tolerance in crop plants vary depending on the level and duration of salt stress and the plant developmental stage (Shannon 1985; Neumann, 1997). In general, tolerance to salt stress is assessed in terms of biomass production or yield compared to non-stress conditions. In conditions of low to moderate salinity, the production capacity of the genotype is often the most important measure, whereas survival ability is often used at relatively high salinity levels (Epstein et al., 1980). The physiological mechanisms that play a major role in maintaining the production capacity of a genotype are not the same as those that contribute to tolerance at extremely high salt concentrations. Genotypes are generally evaluated using phenotypic observations. Phenotypic selection parameters include:
- Germination: Germination tests are easy to perform and may be important where the crops are required to germinate and establish in saline conditions. However, germination in saline conditions is not often associated with salinity tolerance in subsequent growth stages (Dewy, 1962; Shannon, 1985; Flowers, 2004).
- Plant survival: selection on the basis of plant survival at high salt concentrations has been proposed as a selection criterion for tomato, barley and wheat (Rush and Epstein 1976; Espstein and Norlyn 1977). The ability of a genotype to survive and complete its life cycle at very high salinities, irrespective of yield potential under moderate salinity levels, is considered as being tolerant in the absolute sense.
- Leaf damage: Since most crops are glycophytes, they are unable to restrict toxic salt ions being translocated from roots into shoots and leaves. Consequently, salinity damage may be readily observed by leaf symptoms of bleaching and necrosis. Screening for salt tolerance by leaf damage is therefore common (Richards et al. 1987; Gregorio et al., 1997).
- Biomass and yield: For plant breeders, yield and biomass are obvious parameters in assessing salt tolerance (Richards et al., 1987). These parameters, however, do not provide information on the underlying physiological mechanisms. In the past, plant breeders have not been interested in physiological mechanism; that a genotype was tolerant was sufficient, the physiological mechanisms were regarded as academic. However, with the emergence of gene function studies, this view is changing.
- Physiological mechanisms: Physiological mechanisms that confer tolerance to salt may be harnessed for screening. These may include measurements of tissue sodium content, ion discrimination and osmotic adjustment. Surrogates such as carbon isotope discrimination which give a general indication of plant stress may also be used (Flowers and Yeo 1981; Pakniyat et al., 1997).
1.4 Definition of Drought
Drought is one of the most significant environmental stressor that adversely disrupt plant physiology. It can be described as a temporal reduction of environmental moisture status relative to the mean state. Because of the complexity of drought, it is often studied only by separate aspects of the phenomenon (e.g. meteorological drought, soil drought, etc.) (Anonymous, 2018).
In defining drought, it is particularly important to distinguish between dryness and drought. Dryness is a constant feature of an arid area caused by the climate. The total area of arid climates is estimated at about 42% of the Earth’s land. Drought, on the other hand, is a temporary phenomenon related to the failure of usual precipitation. It always results in temporary loss of water and plant resources) (Anonymous, 2018).
Drought is often defined as a temporary situation when the water demand of a hydrological system (which may be an ecosystem or an anthropogenic system) exceeds the income of water from any sources.
1.4.1 Causes of drought
Drought is a complex physical and social process of widespread significance (American Meteorological Society, 2003; Owens et al., 2003). Plant experiences drought stress either when the water supply to roots becomes difficult or when the transpiration rate becomes very high (Anjum et al., 2011).
According to Hosseini et al. (2009) ; Drought severity is caused not only on the duration, intensity and spatial extent of a specific drought episode, but also on the demands made by human activities and vegetation on a specific region’s water supply.
Based on their respective; drought may be categorized as meteorological, agricultural, hydrological, and socio-economic droughts (Hosseini et al., 2009) which is discussed below.
1.4.2 Types of drought
Drought is considered the single most devastating environmental stress, which decreases crop productivity more than any other environmental stress (Lambers et al., 2008).
There are two types of drought based on their several etiologies including:
Meterological Drought: This is caused by a continuous shortfall in precipitation is coupled with higher evapotranspiration demand leading to agricultural drought (Hosseini et al., 2009; Mishra and Cherkauer 2010). Precipitation is a major source of drought in most ecosystems.
Agricultural Drought: This is the lack of ample moisture required for normal plant growth and development to complete the life cycle (Manivannan et al. 2008; Hosseini et al., 2009). It occurs when soil moisture is inadequate to meet the needs of a particular crop at a particular time.
Hydrological drought: refers to deficiencies in surface and subsurface water supplies (Hosseini et al., 2009).
Socioeconomic drought: occurs when physical water shortages start to affect the health, well-being, and quality of life of the people, or when drought starts to affect the supply and demand of an economic product (Moghaddas- Farimani and Hosseini 2004; Zamani et al. 2006; Hosseini et al., 2009).
1.4.3 Consequences of drought
The permanent or temporary water deficit has been demonstrated to severely hampers the plant growth and development more than any other environmental factor (Anjum et al., 2011).
Drought negatively impacts include growth, yield, membrane integrity, pigment content, osmotic adjustment, water relations, and photosynthetic process (Benjamin and Nielsen, 2006; Praba et al., 2009). Drought stress is affected by climatic, edaphic and agronomic factors. The susceptibility of plants to drought stress varies in dependence of stress degree, different accompanying stress factors, plant species, and their developmental stages (Demirevska et al., 2009). Acclimation of plants to water deficit is the result of different events, which lead to adaptive changes in plant growth and physio-biochemical processes, such as changes in plant structure, growth rate, tissue osmotic potential and antioxidant defenses (Duan et al., 2007).
With respect to plant growth ; drought can impair germination (Harris et al., 2002). Cell growth is considered one of the most drought-sensitive physiological processes due to the reduction in turgor pressure. Growth is the result of daughter-cell production by meristematic cell divisions and subsequent massive expansion of the young cells. Under severe water deficiency, cell elongation of higher plants can be inhibited by interruption of water flow from the xylem to the surrounding elongating cells (Nonami, 1998). Drought caused impaired mitosis; cell elongation and expansion resulted in reduced growth and yield traits (Hussain et al., 2008). Water deficits reduce the number of leaves per plant and individual leaf size, leaf longevity by decreasing the soil’s water potential. Leaf area expansion depends on leaf turgor, temperature, and assimilating supply for growth. Drought-induced reduction in leaf area is ascribed to suppression of leaf expansion through reduction in photosynthesis (Rucker et al., 1995). A common adverse effect of water stress on crop plants is the reduction in fresh and dry biomass production (Zhao et al., 2006).
Khan et al. (2001) conducted a study comprising of six treatments, namely, control (six irrigations), five, four, three, two and one irrigation in maize. It was concluded that plant height, stem diameter, leaf area decreased noticeably with increasing water stress. The reduction in plant height could be attributed to decline in the cell enlargement and more leaf senescence in the plant under water stress (Manivannan et al., 2007a). Drought led to substantial impairment of growth-related traits of maize in terms of plant height, leaf area, number of leaves/plant, cob length, shoot fresh and dry weight/plant. Furthermore, Kamara et al. (2003) revealed that water deficit imposed at various developmental stages of maize reduced total biomass accumulation at silking by 37%, at grain-filling period by 34% and at maturity by 21%.
With respect to plant yield ; plant yield often is the result of the expression and association of several plant growth components. The deficiency of water leads to severe decline in yield traits of crop plants probably by disrupting leaf gas exchange properties which not only limited the size of the source and sink tissues but the phloem loading, assimilate translocation and dry matter portioning are also impaired (Farooq et al., 2009).
Drought stress inhibits the dry matter production largely through its inhibitory effects on leaf expansion, leaf development and consequently reduced light interception (Nam et al., 1998). Drought at flowering commonly results in barrenness. A major cause of this, though not the only one, was a reduction in assimilate flux to the developing ear below some threshold level necessary to sustain optimal grain growth (Yadav et al., 2004). When maize plants were exposed to drought stress at teaseling stage, it led to substantial reduction in yield and yield components such a kernel rows/cob, kernel number/row, 100 kernels weight, kernels/cob, grain yield/plant, biological yield/plant and harvest index (Anjum et al., 2011a). Drought-related reduction in yield and yield components of plants could be ascribed to stomatal closure in response to low soil water content, which decreased the intake of CO2 and, as a result, photosynthesis decreased (Chaves, 1991; Cornic, 2000; Flexas et al., 2004). In summary, prevailing drought reduces plant growth and development, leading to hampered flower production and grain filling and thus smaller and fewer grains. A reduction in grain filling occurs due to a reduction in the assimilate partitioning and activities of sucrose and starch synthesis enzymes.
With respect to photosynthetic activities ; the ability of crop plants to acclimate to different environments is directly or indirectly associated with their ability to acclimate at the level of photosynthesis, which in turn affects biochemical and physiological processes and, consequently, the growth and yield of the whole plant (Chandra, 2003). Drought stress severely hampered the gas exchange parameters of crop plants and this could be due to decrease in leaf expansion, impaired photosynthetic machinery, premature leaf senescence, oxidation of chloroplast lipids and changes in structure of pigments and proteins (Menconi et al., 1995). Anjum et al. (2011a) indicated that drought stress in maize led to considerable decline in net photosynthesis (33.22%), transpiration rate (37.84%), stomatal conductance (25.54%), water use efficiency (50.87%), intrinsic water use efficiency (11.58%) and intercellular CO2 (5.86%) as compared to well water control.
Many studies have shown the decreased photosynthetic activity under drought stress due to stomatal or non-stomatal mechanisms (Del Blanco et al., 2000; Samarah et al., 2009). Stomata are the entrance of water loss and CO2 absorbability and stomatal closure is one of the first responses to drought stress which result in declined rate of photosynthesis. Stomatal closure deprives the leaves of CO2 and photosynthetic carbon assimilation is decreased in favor of photorespiration. Considering the past literature as well as the current information on drought-induced photosynthetic responses, it is evident that stomata close progressively with increased drought stress. It is well known that leaf water status always interacts with stomatal conductance and a good correlation between leaf water potential and stomatal conductance always exists, even under drought stress. It is now clear that there is a drought-induced root-to-leaf signaling, which is promoted by soil drying through the transpiration stream, resulting in stomatal closure. The "non-stomatal" mechanisms include changes in chlorophyll synthesis, functional and structural changes in chloroplasts, and disturbances in processes of accumulation, transport, and distribution of assimilates.
With respect to the effect of drought on chlorophyll content; chlorophyll is one of the major chloroplast components for photosynthesis, and relative chlorophyll content has a positive relationship with photosynthetic rate. The decrease in chlorophyll content under drought stress has been considered a typical symptom of oxidative stress and may be the result of pigment photo-oxidation and chlorophyll degradation. Photosynthetic pigments are important to plants mainly for harvesting light and production of reducing powers. Both the chlorophyll a and b are prone to soil dehydration (Farooq et al., 2009). Decreased or unchanged chlorophyll level during drought stress has been reported in many species, depending on the duration and severity of drought (Kpyoarissis et al., 1995; Zhang and Kirkham, 1996). Drought stress caused a large decline in the chlorophyll a content, the chlorophyll b content, and the total chlorophyll content in different sunflower varieties (Manivannan et al., 2007b).
Loss of chlorophyll contents under water stress is considered a main cause of inactivation of photosynthesis. Furthermore, water deficit induced reduction in chlorophyll content has been ascribed to loss of chloroplast membranes, excessive swelling, distortion of the lamellae vesiculation, and the appearance of lipid droplets (Kaiser et al., 1981). Low concentrations of photosynthetic pigments can directly limit photosynthetic potential and hence primary production. From a physiological perspective, leaf chlorophyll content is a parameter of significant interest in its own right. Studies by majority of chlorophyll loss in plants in response to water deficit occurs in the mesophyll cells with a lesser amount being lost from the bundle sheath cells.
In terms of the role of drought in the accumulation of osmolytes : plants accumulate different types of organic and inorganic solutes in the cytosol to lower osmotic potential thereby maintaining cell turgor (Rhodes and Samaras, 1994). Under drought, the maintenance of leaf turgor may also be achieved by the way of osmotic adjustment in response to the accumulation of proline, sucrose, soluble carbohydrates, glycinebetaine, and other solutes in cytoplasm improving water uptake from drying soil. The process of accumulation of such solutes under drought stress is known as osmotic adjustment which strongly depends on the rate of plant water stress. Wheat is marked by low level of these compatible solutes and the accumulation and mobilization of proline was observed to enhance tolerance to water stress (Nayyar and Walia, 2003). Of these solutes, proline is the most widely studied because of its considerable importance in the stress tolerance. Proline accumulation is the first response of plants exposed to water-deficit stress in order to reduce injury to cells. Progressive drought stress induced a considerable accumulation of proline in water stressed maize plants. The proline content increase as the drought stress progressed and reached a peak as recorded after 10 day stress, and then decreased under severe water stress as observed after 15 days of stress (Anjum et al., 2011b).
Proline can act as a signaling molecule to modulate mitochondrial functions, influence cell proliferation or cell death and trigger specific gene expression, which can be essential for plant recovery from stress (Szabados and Savoure´, 2009). Accumulation of proline under stress in many plant species has been correlated with stress tolerance, and its concentration has been shown to be generally higher in stress-tolerant than in stress-sensitive plants. It influences protein solvation and preserves the quaternary structure of complex proteins, maintains membrane integrity under dehydration stress and reduces oxidation of lipid membranes or photoinhibition (Demiral and Turkan, 2004). Furthermore, it also contributes to stabilizing sub-cellular structures, scavenging free radicals, and buffering cellular redox potential under stress conditions (Ashraf and Foolad, 2007).
2.1 Definition of Metabolite
Plant metabolism is defined as the complex of physical and chemical events of photosynthesis, respiration and the synthesis and degradation of organic compounds (Anonymous, 2018). Photosynthesis produces the substrates for respiration and the starting organic compounds used as building blocks for subsequent biosynthesis of nucleic acids, amino-acids, and proteins, carbohydrates and organic acids, lipids and natural products. Plants undergo both primary and secondary metabolism. In fact, while secondary metabolism facilitates the primary metabolism in plants; the primary metabolism consists of chemical reactions that allow the plant to live.
Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, catalytic activity of their own (usually as a cofactor to an enzyme), defense, and interactions with other organisms (Tiwari and Rana, 2015).
It is important to note here that in order for the plants to stay healthy, secondary metabolism plays a pivotal role in keeping all the plants’ systems working properly. A common role of secondary metabolites in plants is defense mechanisms.
2.1.1 Examples of Metabolite
According to Buchanan et al. (2015), primary metabolites includes lactic acid, certain amino acids while examples of some of secondary metabolites present include peptides / growth factors, antibiotics, terpenoids, alkaloids, nucleosides among others.
2.2 Types of Metabolite
Plants require both primary and secondary metabolism for their optimum functioning.
2.2.1 Primary metabolite
Plant primary metabolism in a plant comprises all metabolic pathways that are essential to the plant’s survival. Meanwhile, primary metabolites are compounds that are directly involved in the growth and development of a plant.
2.2.2 Secondary metabolite
Secondary plant metabolites are produced by other metabolic pathways, although important are not essential to the functioning of the plant. They are used in signaling and regulation of primary metabolic pathways. For instance; plant hormones which are secondary metabolites are often used to regulate the metabolic activity within the cells and oversee the overall development of the plant.
3.1 Plant Physiology and Metabolism
Plants have the most sophisticated chemical system in the world (Stitt et al., 2010). They use light energy to convert CO2 into carbohydrates in their leaves. They absorb nutrients like nitrate, phosphate, and sulfate via their roots and convert them to amino acids and nucleotides, using light energy in the leaves in the day and energy derived from respiration in leaves in the dark and in non-photosynthetic tissues. Carbohydrates, amino acids, and nucleotides are then transported to growing tissues, where they are converted into macromolecular cellular components like proteins, nucleic acids, cell walls, pigments, and lipids.
Plants also synthesize a number of secondary metabolites, including phenylpropanoids and flavonoids, terpenoids, glucosinolates, and alkaloids (Stitt et al., 2010). These have important roles in cellular function, in signaling, and in adaptation to abiotic and biotic stress. Their unique synthetic ability is the result of a highly complex and sophisticated metabolic apparatus. Previous report has pointed out the complexity and flexibility of plants as revealed in the discoveries of several pathways like glycolysis, the oxidative pentose phosphate pathway, and organic acid metabolism, present in more than one compartment (Lunn, 2007). This is in addition to the unveiling of the diversity of plant secondary metabolite.
Plants metabolites are known to be unique sources for pharmaceuticals food additives, flavors and others industrial values (Tiwari and Rana, 2015).
3.2 Role of Secondary metabolites
Plants possess several natural products. These Natural products are those chemical compounds or substances that are isolated from living organism. It can be in form of primary or secondary metabolites.
It has been well-established that the primary metabolite is directly involved in normal Growth, development and reproduction. For example, carbohydrate, protein, fat and oil, alcohol etc.
On the other hand, secondary metabolites are not directly involved in growth, development and reproduction of an organism, but they have an ecological function (Nwokeji et al., 2016). Plant secondary metabolite can be found in the leaves, stem, root or the bark of the plant depending on the type of secondary metabolite that is been produced (Hill, 1952; Nwokeji et al., 2016). Unlike the primary metabolites, absence of secondary metabolites does not result in immediate death, but rather in long term impairment of the organisms’ survivability (Nwokeji et al., 2016).
Although from various sources, plant secondary metabolites are generally classified into three distinct groups namely; Terpenes, Phenolic compounds and Nitrogen-containing compounds. Their biosynthesis are known derived from primary metabolism pathways, which include tricarboxylic acid cycle (TCA), methylerithrotol phosphate (MEP) pathway, mevalonic and shikimic acid pathway.
Abbildung in dieser Leseprobe nicht enthalten
Figure 3.1: With N [1. Alkaloids; 2. Non-protein amino acids; 3.Amines; 4.Cyanogenic glycosides; 5.Glucosinolates]; Without N [6.Monoterpenes; 7.Sesquiterpenes; 8. Diterpenes; 9.Triterpenes, Saponins, Steroids, Tetraterpenes; 10.Flavonoids; 11.Polyacetylenes; 12.Polyketides, Phenylpropanes]
This constitutes the largest class of secondary product. They are also called terpenoids. The diverse substances of this class are generally insoluble in water. All terpenes are derived from the union of five-carbon atoms that have the branched carbon skeleton of isopentane. The basic structural element of terpenes is sometimes called isoprene unit because terpene decompose at high temperature to give isoprene. Terpene are toxic and feeding deterrents to many plants feeding insect and mammals. Thus they appear to have important defensive role in the plant kingdom (Gershenzon and Croteau, 2012).
Derivatives of terpene called saponins are steroid and triterpene glycoside, so named because of their soap like properties. Another derivative of terpene is carotenoids which give the yellow, red and orange colour in some plants like carrot. Examples of plant that contain terpenoids include polypodiumvulgare, Digitalis spp, pine and fir, peppermint plant, lemon, basil, sage,corn ,Gossypiumhispida (cotton),wild tobacco (Kessler and Baldwin, 2001).
3.2.2 Phenolic Compounds
Plants produce a large variety of secondary product that contains a phenol group- a hydroxyl functional group on an aromatic ring. These substances are classified as phenolic compounds. Plant phenolic are a chemically heterogeous compound, some soluble only in organic solvents, some are water soluble, while others are insoluble polymers. Some simple phenolic are activated by ultra violet light. Phenolic are wide spread in vascular plants and appear to function in different capacities. The derivatives of phenolic compounds include simple phenyl propanoid, benzoic acid derivatives, anthocyannin, isoflavones, tannins, lignin, and flavonoid compound beginning with phenylalanine. Lignin is generally formed from three different phenyl propanoid alcohols namely coniferyl, coumaryl, and sinapyl (Davin and Lewis, 2005). The flavonoids are one of the largest classes of plant phenolics, the basic structure contain 15 carbon arranged in two aromatic ring connected by a three carbon bridge (Taiz and Zeiger, 2005). The basic function of the flavonoids is for pigmentation and defence. The red, pink, purple and blue colours observed in plants parts are as a result of anthocyanins (Li et al., 2003). Tannins were first used to describe compound that could convert raw material hides into leather in the process of tanning. Tannins are generally toxic that significantly reduce the growth and survivorship of many herbivores when added to their diets. Tannins can be seen in fruits like apple, black berries, tea and red wine (Taiz and Zeiger, 2005). Tannins are mainly constituent of woody plants especially heart wood. Some derivatives of tannin include Gallic acid.
3.2.3 Nitrogen-containing compounds
A large variety of secondary metabolites have nitrogen in their structure. These include the alkaloids, cyanogenic glucoside, glucosinate (Taiz and Zeiger, 2005).
Alkaloids are large family of more than 15,000 nitrogen containing secondary metabolites found in approximately 20% of the species of vascular plant. The nitrogen atom in these substances is usually part of the heterocyclic ring, a ring that contain both nitrogen and carbon atom. They show striking pharmacological effect on vertebrate animal.as their name would suggest. Most alkaloids are alkaline, at pH value commonly (7.2). Alkaloids were once thought to be nitrogenous wastes. Most alkaloids are now believed to function as defense against especially mammals, because of the general toxicity and deterrence capacity (Hartmann, 2013). One group of alkaloid, the pyrrolizidine alkaloid illustrates how herbivore can become adapted to tolerate plant defensive substance and even use them in their own defence (Hartmann, 2013).
In addition to the classes of secondary metabolites; it is important to consider the biosynthesis of secondary metabolites.
Consequently, the biosynthesis of the various three classes of secondary metabolite varies depending on the class involved. The terpenes are biosynthesized from primary metabolite through the pathway of mevalonic acid and methyerithrotol phosphate (Taiz and Zeiger, 2005).
In the mevalonic acid pathway, three molecules of acetyl-coA are joined together stepwise to form mevalonic acid. This key six-carbon intermediate is then pyrophosphorylated, decarboxylated and dehydrated to yield isopentenyldiphosphate (IPP). The IPP is the activated five carbon building block of terpenes. Although all the details have not yet been elucidated, glyceraldehyde-3-phosphate and two carbon atoms derived from pyruvate appear to combine to generate an intermediate that is eventually converted to IPP.
In biosynthesis of phenolic compound, two basic pathways are involved, the shikimic acid pathway and the malonic acid pathway (Taiz and Zeiger, 2005). The shikimic acid pathway convert simple carbohydrate precursors derived from glycolysis and the pentose phosphate pathway to the aromatic amino acid (Hartmann, 2013). One of the intermediate is shikimic acid, which has given its name to this whole sequence of reaction. The well-known broad spectrum herbicide glyphosphate kill plant by blocking a step in this pathway.