1 Introduction 1.1 Photosynthesis in plants

Photosynthesis evolved early in the history of life (Blackenship, 2010). It is the process by which the chloroplast thylakoids of the leaf and other photosynthetic structures harvest light. The resultant chemical energy adenosine triphosphate and nicotinamide adenine dinucleotide phosphate (ATP and NADPH) are used to fix atmospheric carbon dioxide CO₂. In C₃ photosynthesis, CO₂ is fixed directly via ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco). In C₄ photosynthesis, CO₂ is fixed indirectly after primary fixation by phosphoenol pyruvate (PEP) carboxylase and subsequent re-release in adjacent cells not in direct communication with the atmosphere, where CO₂ is concentrated (Furbank, 2009). For the energy of sunlight to be stored, it must be absorbed by the pigments of the organism. Several types of pigments—such as chlorophylls, carotenoids, and phycobilins-serve this function in various photosynthetic organisms. A portion of the light energy absorbed by the pigments is eventually stored in chemical bonds. This energy conversion is a complex process involving interactions between several pigment molecules and electron-transport proteins. Collectively, these components are called a photosynthetic unit. Serving as antennae, most of the pigments collect light and channel the energy to the reaction center, where the chemical reactions leading to long-term energy storage take place (Blankenship, 1992).

Over time, photosynthetic research has enhanced our understanding of ecological phenomena and the global environment (Monsi and Saeki, 2005). Indeed, photosynthesis is now an integral component of simulation models that are used to predict the future of our planet. For a long time, improving the efficiency of photosynthesis by artificial modification of photosynthetic proteins and pathways has been considered impossible or unrealistic. Over evolutionary time, photosynthesis has become complex and tightly regulated. However, recent advances have made it possible to manipulate the process using molecular genetic engineering (Andrews and Whiney, 2003; Raines, 2006). The advancement in molecular engineering can undoubtedly have positive influences on crop productivity and yield (Parry et al., 2007 ) because photosynthetic rate depends on biomass accretion (Kruger and Volin, 2006).

To reawaken the interest of researchers in this line of study, we conducted a review of photosynthesis research in Nigeria, with a view for helping to utilize past work and to project future work for tackling new challenges and the modern application of photosynthesis in the areas of food productivity and climate change. This review is also intended to serve as a guide for possible research on C₃ and C₄ plants in Nigeria. Information used in this review was sourced from published research articles and books retrieved from scientific data published in reputable journals.

1.2 C₃ photosynthesis

The C₃ photosynthetic carbon-reduction cycle was first elucidated by Calvin, Bassham, and Benson in the 1950s in a series of experiments using the green alga Chlorella and radio-labelled carbon. This cycle, frequently referred to as the Calvin cycle, uses the products of the light reactions of photosynthesis, ATP and NADPH, to fix atmospheric CO₂ into carbon skeletons that are used directly for starch and sucrose biosynthesis (Figure 1).

The C₃ photosynthetic pathway is found in plants under moist or moderate environmental conditions (Winter et al., 1976 ). In the pathway, CO₂ is fixed by Rubisco and is synthesized into carbohydrates. This pathway operates only in the mesophyll cells (Boom, 2004), and the majority of plants use this photosynthetic pathway. Most photosynthetic organisms utilize this method of carbon fixation. Except for a few taxa, all trees and shrubs and most herbs are C₃ plants (Boom, 2004). These plants fix CO₂ using the enzyme Rubisco and convert it in the form of a three-carbon-atom molecule storage product, 3-phosphoglyceric acid (PGA), hence the name C₃ photosynthesis (Boom, 2004). In the long course of earth's history, there have been periods when conditions for C₃ plants were less favourable (Berner and Canfield, 1989). Although it is generally assumed that C₃ is the oldest photosynthetic pathway among higher plants, it is still surprising that far back in the evolutionary history of these plants there had been conditions that are considered unfavorable for the plants (Boom, 2004). This circumstances is because high O₂ concentrations cause photorespiration in this group of plants, which causes a considerable loss of energy for the plant (Boom, 2004).

Molecularly, the C₃ expression of photosynthetic carbon-reduction-cycle genes in mature leaves is sensitive to environmental and metabolic signals, providing long-term mechanisms for the plant to regulate primary carbon fixation. High levels of glucose and sucrose have been shown to be associated with reduced levels of a number of Calvin cycle mRNAs, including those encoding the small subunit of Rubisco, sedoheptulose-1,7-bisphosphatase, and fructose-1, and 6-isphosphatase. This feedback mechanism, which again appears to act at the level of transcription, might be important for source–sink regulation in the plant (Krapp et al., 1993 ). The signaling pathway involved in glucose repression of gene expression is not known although it has been suggested that the enzyme hexokinase might be involved. Recently, it has been shown that photosynthetic genes can respond at the transcript level to nutrient status (namely nitrogen and phosphorous levels) and this response is modulated by carbohydrate status. This process suggests that the interaction between carbohydrate status and nutrient status may function as a long-term strategy in the control of primary carbon metabolism.

1.3 C₄ photosynthesis

The C₄ photosynthetic pathway is found in plants growing in warm, sunny, and dry conditions (Winter et al., 1976 ). The leaves of C₄ plants display Kranz anatomy, in which vascular bundles are surrounded by an outer layer of mesophyll cells and an inner layer of bundle-sheath cells (Brown, 1999). The C₄ pathway fixes CO₂ initially by phosphenol pyruvate carboxylase (PEPC) (Figure 2), which is localized in mesophyll cells forming C₄ acids (malate and aspartate). The C₄ plants are among the most productive on the planet, and this trait is associated with high rates of photosynthesis and efficient use of water and nitrogen (Brown, 1999). In a typical C₄ plant, the initial fixation of HCO₃⁻ by PEPC and the re-fixation of CO₂ by Rubisco are compartmented into different cell types although segregation can occur within individual cells (Voznesenskaya et al., 2002 ).

C₄ photosynthesis is found in many plant species: monocots including Zea, Cyperus and Sacharrum; dicots such as Amaranth, Flaveria, Euphorbia, Cyperus, Cleome, and Boerhavia (Jørgensen and Ulloa, 1994; Ehleringer et al., 1997 ; Muhaidat et al., 2007 ). Table 1 presents some of the Nigerian plants with C₃ and C₄ features. Such a wide and diverse distribution of the occurrence of the C₄ pathway indicates that it has evolved independently at many different times (Langdale and Nelson, 1992). Because all C₄ plants share a number of characteristics in terms of leaf anatomy, physiology, and biochemistry, they stand as excellent examples of convergent evolution. The C₄ pathway provides a means for maintaining photosynthetic efficiency under conditions of high temperature or water limitation, by greatly reducing or eliminating photorespiration (Ehleringer et al., 1997 ). Photorespiration is problematic in these situations because increasing temperatures enhance the oxygenase activity relative to the carboxylation activity of the Rubisco enzyme (Ehleringer et al., 1997 ). In addition, C₄ plants have a selective advantage under arid conditions. To minimize water loss through transpiration, plants must reduce the opening of their stomata, which leads to reduced carbon CO₂ uptake and reduced release of oxygen. Because PEPC can fix carbon dioxide from relatively low intracellular concentrations, C₄ plants show higher rates of photosynthesis than C₃ plants under conditions that promote high transpiration rates (Sage, 2004). C₄ plants also show higher photosynthetic efficiencies under conditions of light saturation, such as occurs in open plains or Savannahs (Lattanzi, 2010). It is likely that the ancestors of most contemporary C₄ and C₃–C₄ intermediate plants evolved between 30 and 50 million years ago, in response to reduced carbon dioxide in the atmosphere, possibly in combination with elevated temperatures or water deficit (Ehleringer et al., 1997 ).

The occurrence of C₃–C₄ indicates that multiple independent steps are required for evolution of full C₄ capability. In most C₄ plants, efficient transfer of metabolites between mesophyll and bundle-sheath cells is as a result of higher plasmodesmatal linkage between these two cells, as compared to C₃ species (Botha, 1992). In the mesophyll cells of C₄ plants, chloroplast development is reduced; while in bundle sheath cells, chloroplasts are larger and undergo additional rounds of division (Hatch, 1987). According to Kanai and Edwards (1999), the operation of the C₄ cycle results in the increased concentration of CO₂ at the active site of Rubisco in the bundle-sheath cells by suppression of the oxygenase reaction of Rubisco. C₄ plants have evolved independently from C₃ plant in different taxonomic groups (Björkman, 1976), and the photosynthesis is more efficient than C₃ photosynthesis under some environmental conditions. C₄ plants fix atmospheric CO₂ through Phosphoenolpyruvate Carboxylase (PEPC) in the mesophyll cells to form oxaloacetate, which is converted to four-carbon decarboxylic acids (malate and aspartate) (Larcher, 1995). The four-carbon acids are then translocated to bundle-sheath cells and decarboxylated at the C₄ position with the release of CO₂ and C₃ compound (Larcher, 1995). C₄ plants have been reported to show higher export rates of photosynthate than C₃ plants (Leonardos and Grodzinski, 2000). To explain the higher rate of translocation, some researchers have pointed to the presence of a denser vascular system in C₄ leaves (Crookston and Moss, 1974) and a larger cross-sectional area of phloem (Gallaher et al., 1975 ) to be responsible for it. The vascular bundle is composed of two kinds of conducting tissues: xylem and phloem. Thus, it appears that C₄ leaves have a denser hydraulic network than C₃ leaves. The CO₂-concentrating mechanism of the C₄ pathway gives C₄ plants an efficient photosynthetic mechanism under low stomatal conductance and thus a higher water-use efficiency and photosynthetic ability under environments of low water availability than is the case for C₃ plants (Sage, 2004). Nevertheless, it is a generally accepted fact that C₄ plants evolved from C₃ plants; and the evolution was accompanied by modifications to anatomical and biochemical features of leaves. A change in the vein density of leaves undoubtedly occurred during the evolution from C₃ to C₄ plants (Sage, 2004; Ueno and Sentoku, 2006).

According to Hatch (1987), C₄ plants are divided into three subtypes, differing in the process of decarboxylation of C₄ acids; nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME), nicotinamide adenine dinucleotide malic enzyme (NAD-ME), and phosphoenolpyruvate carboxykinase (PCK). The difference in their biochemical function is associated with the structural features of the leaves. While the NADP-ME grasses have a bundle shealth that originated from the mestome sheath, both the NAD-ME and PCK grasses have one that originated from the parenchyma sheath (Dengler and Nelson, 1999). The bundle-sheath cells of the C₄ subtypes also differ in the structure, intracellular position, and quantity of chloroplasts and mitochondria (Yoshimura et al., 2004 ). The quantitative balance of photosynthetic tissues (Dengler et al., 1994 ) and organelles (Yoshimura et al., 2004 ) between the mesophyll and bundle-shealth cells reflects the difference in biochemical function of the photosynthetic subtypes.

2 Methods used in determining photosynthetic pathways in plants 2.1 Photosynthetic products

Initial product labeling with ¹⁴C is the only direct method for photosynthetic pathway determination. Hatch et al. (1967) reported that, in C₄ plants, as much as 93% of fixed radioactivity appeared in oxaloacetic, malic, and aspartic acids following exposure to ¹⁴CO₂ for approximately one second. In contrast, early products of the C₃ process were 3-PGA and hexose phosphates.

2.2 Carbon-dioxide compensation and photorespiration

The carbon-dioxide compensation point (the point at which photosynthetic CO₂ uptake equals respiratory CO₂ evolution when measured in a closed chamber) is an easily quantifiable characteristic. During photosynthesis, a light-induced release of CO₂ can occur and is referred to as photorespiration, as contrasted with CO₂ released by mitochondria or dark respiration. Plants with the C₄ pathway have a photosynthetic CO₂ compensation in the range of 0~10 ppm, indicating a lack of significant net photorespiration (Downton and Tregunna, 1968). Photorespiration occurs as a normal product of the Calvin cycle within the bundle-sheath cells of C₄ plants. However, because the mesophyll layer surrounds the bundle sheath, the C₄ pathway would rapidly refix any photorespiratory CO₂ and prevent leakage to the atmosphere (Bowes and Ogren, 1972). A much higher CO₂ compensation point (37~70 ppm) is characteristic of C₃ plants (Black, 1971). Carbon-dioxide compensation points provide a convenient means of identifying the type of photosynthetic pathway. The low CO₂ compensation point of C₄ plants indicates an ability to utilize more external CO₂, as compared to use by C₃ plants.

2.3 Oxygen suppression

Oxygen differentially affects CO₂ exchange in C₃ and C₄ plant species, primarily because of differences in photorespiration. In soybean (Glycine max), and probably other C₃ species as well, the total O₂ inhibition consists of two discernible effects (Waller and Lewis, 1979). Oxygen substitutes for CO₂ in the carboxylase reaction to yield P-glycolate, a C₃ photorespiratory intermediate. As a result of this substitution, O₂ competitively inhibits the carboxylase with respect to CO₂ (Ogren, 1976). During photorespiration, glycolate is oxidized, releasing CO₂. Consequently, oxygen depletion reduces glycolate oxidation, thereby increasing photosynthetic CO₂ assimilation by 40% ~ 50% in species possessing the C₃ pathway, while having no effect on C₄ plants (Downes and Hesketh, 1968). The re-fixation of photorespiratory CO₂ allows C₄ plants to utilize all of the fixed CO₂, thus increasing photosynthetic efficiency. Chollet (1976) postulated that the enzyme complement of the C₄ pathway increased CO₂ concentration at the site of the C₃ carboxylase, reducing the competitive inhibition of O₂ and minimizing photorespiration. The CO₂ concentration at the site of the C₃ carboxylase, coupled with a specialized leaf anatomy allowing recapture of photorespiratory CO₂, was apparently responsible for the lack of photorespiration and the absence of an inhibitory effect of 21% O₂ on net photosynthesis in C₄ plants (Waller and Lewis, 1979).

2.4 Carbon-isotope discrimination

Carbon-isotope ratio (¹³C/¹²C) in plant tissue is characteristically is known to be less than that of atmospheric CO₂, indicating that plants preferentially assimilate the lighter of the two isotopes (Troughton et al., 1974 ). Carbon-isotope values are defined as the difference in per 13 mil (1 mile = 0.0254 mm) of the ¹³C/¹²C ratio of the sample relative to a standard and reported as ¹³C‰ (Smith and Epstein, 1971). Details of the procedure are described elsewhere (Park and Epstein, 1960). Higher plants are placed into two categories, those with low ¹³C‰ values (−24‰ to −34‰) and those with high values (−6‰ to −19‰) (Smith and Epstein, 1971). The distinctive difference apparently results from differences in affinity of the enzyme systems of the two pathways for the two isotopes of carbon (Whelan et al., 1973 ). Thus, the carbon-isotope technique was cited as a reliable method of distinguishing between C₃ and C₄ plants (Bender, 1971). This method was successfully used by Cornet and Bonhomme (2007) to characterize 23 genotype of Dioscorea; they indicated that the δ¹³C values for all yam samples studied ranged from −25.39‰ to −30.07‰, which indicated all species had a C ₃ photosynthetic type.

2.5 Leaf anatomy

Leaf anatomy provides easily distinguished differences between C₃ and C₄ plants. C₄ photosynthesis is characterized with leaf property called "Kranz" anatomy (after Haberlandt's description in German of a wreathlike arrangement of cells). Kranz anatomy can be described as two distinct concentric layers of chlorenchyma cells formed by a bundle sheath containing most of the chloroplasts, surrounded by an outer layer consisting of a small number of mesophyll cells (Carolin et al., 1973 ). The visual identification of such arrangements in transverse section has been used in numerous anatomical surveys of leaves to identify the photosynthetic pathway for hundreds of species (Renvoize, 1987; Faniyan et al., 2013; Ayeni et al., 2015a ; Ajao et al., 2017 ). Plants with the C₄ photosynthetic pathway generally have well-developed parenchymatic bundle sheaths containing high concentrations of chloroplasts and starch. Bundle-sheath cells utilize the C₃ photosynthetic process; however, they are surrounded by mesophyll cells containing chloroplasts utilizing the C₄ photosynthetic process, which fix and then supply CO₂ for the C₃ pathway.

Anatomical characteristics that have been used to distinguish plant species with both C₃ and C₄ photosynthetic pathways include stomatal density (Oguro et al., 1985 ; Kim, 2012; Ajao et al., 2017 ); interstomata distance (Oguro et al., 1985 ); stomatal index and stomatal size (Taylor et al., 2010 ; Ajao et al., 2017 ); Kranz tissue (Bruhl and Wilson, 2007; Martins and Alves, 2009); interveinal distance (Nelson et al., 2005 ; Soros and Dengler, 1998; Ajao et al., 2017 ); leaf thickness (Nelson et al., 2005 ; Marshall et al., 2007 ; Ajao et al., 2017 ); mesophyll thickness (Nelson et al., 2005 ); intercellular air spaces (Marshall et al., 2007 ; Ajao et al., 2017 ); one-cell criterion (Soros and Dengler, 1998; Ajao et al., 2017 ); and maximum lateral cell count (Soros and Dengler, 1998; Bruhl and Wilson, 2007; Ajao et al., 2017 ). Leaf vascular traits such as leaf-vein density and vascular spacing are also known to be diverse across C₃ and C₄ flowering plant species (Nelson and Dengler, 1997; Ajao et al., 2017 ) because of their importance in leaf physiological functions such as photosynthesis and water-use efficiency (Mckown et al., 2010). The most important among these anatomical features is the occurrence of Kranz anatomy.

A number of researchers have used anatomical characteristics to classify plants along C₃ and C₄ photosynthetic pathways. For example, Welkie and Caldwell (1970) worked on the leaf anatomy of species in some dicotyledon families, as related to the C₃ and C₄ pathways of carbon fixation. Also, Hattersley et al. (1982) reported on leaf anatomical variations in Neurachne and its relatives, in relation to C₃ and C₄ photosynthesis. Oguro et al. (1985) compared leaves of C₃ and C₄ species of Panicum (Poaceae), using anatomical and morphological characteristics. In addition, Dengler et al. (1994) studied the quantitative leaf anatomy of C₃ and C₄ species of Poaceae, considering the bundle sheath and mesophyll surface-area relationship. Another interesting report is that of Ueno et al. (2006) , which used the criteria of differentiation of some amphibious species of Eleocharis in Cyperacea. The paper reports that Eleocharis vivipara has a unique nature that expresses C₄ characteristic under terrestrial conditions and C₃ characteristics under submerged conditions. In a similar way, Fisher et al. (1997) subjected all North American species of the halophytic genus Suaeda (Chenopodiaceae) to C₃ and C₄ photosynthetic pathways classification using their leaf anatomy, which agrees with already established classification based on morphological characters. In their report, they showed that C₃ species belong to the Chenopodina group, while C₄ species are in the Linbogermen group. Nelson et al. (2005) studied functional leaf anatomy of some dicots in Canada, using characteristics such as leaf thickness, mesophyll thickness, and intercellular air space to compare crassulacean acid metabolism (CAM) plants with C₃ and C₄ plants. Soros and Dengler (1998) studied quantitative leaf anatomy of C₃ and C₄ Cyperaceae and comparisons with Poaceae. Huxman and Monson (2003) worked on stomatal responses of C₃, C₃–C₄ intermediates, and C₄ Flaveria species to light and intercellular CO₂ concentration.

2.6 Leaf and inflorescence morphology

Studies on morphological characteristics (vegetative or floral) that could possibly be used as indicators in the classification of plant species into C₃ and C₄ photosynthetic groups are rare (Ayeni et al., 2015b ). Interestingly, previous reports have suggested that some morphological characteristics are related and can be linked with the two identified photosynthetic pathways in the Cyperus genus. Muasya et al. (2009) reported that in this genus, those belonging to the C₃ photosynthetic pathways tend to possess spikelets arranged in digitate clusters, while the C₄ counterpart spikelets are usually spicately arranged. This finding is also in line with the report of Larridon et al. (2011) , that genus Cyperus is most commonly divided into two main infrageneric units, determined by the state of a set of anatomical and inflorescence characteristics. Ayeni et al. (2015b) reported the first known work on the use of morphological characteristics to delimit Nigeria species of Cyperus into C₃ and C₄ pathways.

3 An overview of C₃ and C₄ plants research in Nigeria

The literature survey conducted on previous work such as Gentry (1993), Jorgensen and Ulloa (1994) and Pinto-Escobar and Mora-Osejo (1966) revealed that C₃ and C₄ plants are distributed among 21 genera and 11 families in Nigeria (Table 1).

Out of the 21 genera, 3 general including Euphorbia, Cyperus and Boerhavia have been classified into C₃ and C₄ by Faniyan et al., 2013; Ayeni et al., 2015a , b; Ajao et al., 2016 , 2017. This thus infers that research into this line of thought is still not optimally explored, despite the fact that Nigeria is endowed with vast array of C₃ and C₄ plants. Although many of the species listed in the table have been classified based on stable anatomical characters such as stomata type, trichomes, and arrangement of the cell without putting into consideration the photosynthetic pathways exhibited by the plants. All the previously reported work on C₃ and C₄ plant in the country employed the use of anatomical character to delimit the species into their respective pathways, with pioneering one being that of Faniyan et al. (2013), where they used the occurrence of Kranz structure, one cell distant count and interveinal distance to classify four species of Euphorbia. Their study revealed that E. hirta and E. hyssopifolia are C₄ while E. heterophyla and E. graminea are C₃. They also found out that characters such as stomatal density, cell sizes, vein-stomatal distance, inter-stomatal distance, leaf thickness, mesophyll thickness, and intercellular air spaces were not found to be useful in their classification.

Two years later, Ayeni et al. (2015a) employed leaf anatomical characters to delimit twelve species of Cyperus into C₃ and C₄. The study revealed the usefulness of a combination of anatomical characters, rather than isolated ones, in the grouping of plant species according to their photosynthetic pathway. The combination of characters that was reported to be useful in their study are Kranz tissue, maximum cell distant count, maximum lateral cell count, interveinal distance, and to some extent, leaf and mesophyll thickness, were also found to be useful to classify Cyperus species into their respective photosynthetic pathways. This study confirmed the photosynthetic pathway utilized by C. difformis and C. haspan to be C₃, while that of C. articulatus, C. compressus, C. distans, C. esculentus, C. imbricatus, C. iria, C. rotundus, C. sphacelatus and C. tenuiculmis is C₄.

Interestingly, Ayeni et al. (2015b) also reported that floral morphological characteristics have a potential to be useful in the grouping of Cyperus species as either C₃ or C₄ species. They found that compound umbellate inflorescence, digitate spikelets, and spikelet length not over 1 cm are may be peculiar to C₃ species, while C₄ species have simple umbellate inflorescence, spicate spikelets, and spikelet length over 1 cm. The two papers demonstrated a reliable basis for assessing photosynthetic pathways of the investigated Cyperus species in Nigeria, using inflorescence morphology and anatomical characters.

Lastly, in continuation of our effort to classically delimit our native species along the photosynthetic lines, Ajao et al. (2016 , 2017), reported the first known photosynthetic delimitation of genus Boerhavia into C₃ and C₄ pathways. In the study, characters such as stomata index, stomata size, inter-stomatal distance, stomatal density, interveinal distance, intercellular air spaces, leaf thickness, mesophyll thickness, Kranz tissue, one cell distant count criterion, maximum lateral cell count criterion, vein density and vein distance was useful in the grouping three species B. erecta, B. coccinea and B. repens into C₄ plants and B. diffusa into C₃ plant. Also in the study, the characters including interveinal distance less than 166 μm and maximum lateral count ranging 2~6 were touted to be diagnostic for C₄ dicotyledons species. The submission is due to the fact that higher interveinal and maximum lateral counts has been reported for monocotyledons. To the best of our knowledge, there is a dearth of information to date on the physiological and biochemical characterization of C₃ and C₄ plants in Nigeria. Research into this direction is imperative to complement the previous studies.

In other African countries such as Ghana and South Africa, research into C₃ and C₄ plant has reached considerable level. In Ghana, awareness has been created on the need to cultivated C₄ plants in other to meet the demand for fossil fuel in the next few decades, as it is touted that there will be rise in fossil fuel demand and prices (Black et al., 2012 ). In South Africa, Stock et al. (2004) used δ¹³C values of herbarium specimens of 68 southern African species from 22 genera and eight tribes to delimit the species to either the C₃ or C₄ photosynthetic pathway. Out of the 68 species studied, 28 exhibited C₄ photosynthesis distributed among nine genera of four tribes (Cypereae, Scirpeae, Abildgaardieae and Rhyncosporeae). Their study also revealed the absence of strong relationships in the abundance of C₄ plant and climatic factors such as altitude and rainfall in the country. Vogel et al. (1978) investigated the distribution of C₃ and C₄ grasses in South Africa. They found out that C₃ and C₄ grasses co-occupied areas that have minimum of 100 mm of annual rainfall and maximum of 1000 mm. Furthermore, low temperatures below 25 also favors C₃ grasses when compared to C₄ counterparts. And, studies focusing on energy dissipation of C₄ plants under water stress have been reported (Lal and Edwards 1996; Ripley et al., 2007 ). The phylogenetic issue on Alloteropsis semialata, a South African grass with C₃ and C₄ photosynthesis has also been resolved (Ripley et al., 2007 ).

In West Africa at large, the influence of C₃ and C₄ vegetation on soil organic matter dynamic in semi-natural tropical ecosystems including Sahel savanna, Sudan savanna, savanna woodland and the dry forest has been investigated (Saiz et al., 2015 ). They found out that current environmental conditions encourage the distribution of C₃ plants over their C₄ counterparts in more mesic savanna ecosystems of West Africa. In addition, δ¹³C was also found to be varied with soil across the ecosystem. In addition, the interdependence between biotic and abiotic factors undoubtedly has influence on whether soil organic matter dynamics of C₃ and C₄ derived vegetation will vary in ecosystems where both vegetation types coexist. In Southern Africa, nitrogen isotopic (δ¹⁵N) abundances on soils and C₃ and C₄ plants along land use gradients of varying aridity has been investigated (Aranibar et al., 2008 ). The δ¹⁵N values of soils and plants were found to be higher in the areas with significant land use intensity. However, their study also revealed C₃ plants significantly have higher δ¹⁵N than C₄ counterparts from the same area. In another study conducted by Swap et al. (2004) , the mean annual precipitation and the δ¹⁵N of the C₃ vegetation of southern African were found to be inversely proportional whereas, on the contrary, no relationship was found between mean annual precipitation and the δ¹³C and δ¹⁵N signatures of C₄ vegetation.

4 Future research and its application in food security, global climate change, and biofuel in Nigeria

Photosynthesis is one of the main research objects in many studies on the impact of water deficit on plants (Ghannoum, 2009). Water deficit is one of the worst disasters that affect health and activities in the world. The changes in precipitation patterns and the expansion of waterlogging or of drought-affected areas are expected in the future due to global climate change (Xoconstle-Cazáres et al., 2010 ). The response of C₃ photosynthesis to water stress has been well studied (Flexas et al., 2004 ). In general, the literature points to the fact that C₃ photosynthesis is negatively affected by water stress, measured as changes in leaf-water potential or relative water content. In contrast, the response of C₄ photosynthesis to water stress has been less studied. In spite the fact that C₄ plants make a significant contribution to the global carbon budget, and C₄ crops, such as maize and sorghum, are pivotal to current and future global food security (Lloyd and Farquhar, 1994; Brown, 1999; Pingali, 2001). The CO₂-concentrating mechanism, compared to the C₃ photosynthetic pathway, ensures a higher carbon-assimilation rate and dry-matter production under drought conditions, when stomata closure reduces CO₂ supply (Larcher, 1995). Consequently, higher stomatal resistance allows a low transpiration rate and high water-use efficiency in C₄ plants (Zhang and Kirkham, 1995).

Breeding of drought-resistant plants can be an advantageous strategy for avoiding drought-induced damage to agriculture. A suitable alternative to ordinary crops is C₄ plants, such as maize, sorghum, millet, and amaranth. These plants are characterized by high productivity (Svirskis, 2009), nutritive value, and resistance to water deficit (Liu and Stutzel, 2004; Osborne and Freckleton, 2009). They dominate in arid and hot regions; approximately half of the world's grasses and only from 4%~10% of all plant species use the C₄ photosynthetic pathway. Nevertheless, high photosynthetic capacity and productivity of these plants determine their essential contribution to global primary production (Osborne and Freckleton, 2009).

As the world population races toward 10 billion, agricultural scientists are realizing that another "green revolution" is needed for crop yields to meet demands for food. In rice, yield potential is limited by the photosynthetic capacity of leaves that, as carbohydrate factories, are unable to fill the larger number of florets of modern rice plants. One potential solution is to introduce a higher-capacity photosynthetic mechanism like the C₄ pathway into rice. This is the goal of researchers in the international C₄. Rice Consortium: to identify and engineer the genes necessary to install C₄ photosynthesis in rice (Hibberd et al., 2008 ). Rubisco, the primary CO₂-fixing enzyme in rice, is a poor catalyst of CO₂ under present-day atmospheric conditions. It has a tendency of confusing its substrate CO₂ with the more abundant O₂, as well as being a very slow catalyst of CO₂, turning over only once or twice per second. Rubisco's oxygenase activity requires the re-cycling of phosphoglycolate in the photorespiratory pathway, resulting in an energy cost and loss of previously fixed CO₂. Many plants have developed active CO₂-concentrating mechanisms to overcome Rubisco's inefficiencies among land plants; and this has led to the development of C₄ photosynthesis, a biochemical CO₂-concentrating mechanism. C₄ photosynthesis arose multiple times in the past 60 million years in warm, semi-arid regions, with early occurrences coinciding with low atmospheric CO₂ in the late Oligocene (Sage et al., 2011 ).

A C₄ rice is achievable with a subset of C₄ genes, but it will require substantial fine-tuning of biochemistry and anatomy. Particularly intriguing is the need for additional metabolite transport across membranes of organelles in C₄ photosynthesis (Langdale, 2011). A functional C₄-concentrating mechanism in rice would allow for an approximately two-thirds reduction in Rubisco levels, relative to wild-type rice; but Rubisco would be sequestered in bundle-sheath cells and ideally have a greater catalytic turnover rate (Badger et al., 1998 ). However, it is expected that another few years of research is required for optimization of the phenotype and field-testing for C₄ rice to become ready for cultivation in farmers' fields.

Another driver of the current C₄ research agenda is the global focus on biofuels. Two of the current major biofuel crops, sugarcane and maize, are both C₄ species. Whereas the future of sugar cane as a fuel crop is almost certain, the use of maize can be defended only in a future in which lignocellulosic fermentation means that grain is not used to produce ethanol. However, another C₄ species may hold the key to biofuel demands, for example, in the United States of America. The perennial grass Miscanthus giganteus is capable of producing a higher biomass than maize, primarily because it can photosynthesize efficiently for a longer period during the growing season. This increased efficiency is achieved in two ways. First, Miscanthus can photosynthesize at cooler temperatures than maize, as a consequence of cold-tolerant pyruvate phosphate dikinase (PPdK) activity (Wang et al., 2008 ). Second, its perennial habit makes it possible for the plant to capture more light early in the season because at that time the canopy is bigger than that of annual crops such as maize (Dohleman and Long, 2009). Current estimates suggest that 9.7 million hectares (1 hectare = 10,000 m²) of Miscanthus would provide enough biomass to meet the annual U.S. energy mandate (Somerville et al., 2010 ). Given that long-term field trials have shown that Miscanthus produces high yields even on poor soils and that 14 million hectares (1 hectare = 10,000 m²) of land dropped out of agricultural use in the U.S. between 1997 and 2007 (http://www.ers.usda.gov/statefacts/us.htm), this C₄ perennial could resolve the food versus fuel dilemma in the U.S. for the foreseeable future.

This trending C₄ research has resulted in one of the largest consortia of plant biologists pursuing a common goal. Nigeria can also optimistically take on this challenge, anticipating that advances in our understanding of C₃ and C₄ photosynthesis will better serve humanity in years to come by rejuvenating our economy in the areas of food production and green energy.

Acknowledgments:

The authors thank Dr. Sabiu Saheed for encouraging us to write this review; also, the support of Mr. Alayande Kazeem and Dr. Balogun Fatai Oladunni is greatly appreciated.