1 The Need for Non-Food Energy Crops

Rapidly increasing energy costs, a foreseeable depletion of fossil fuel reserves and the pressing need to reduce greenhouse gas (GHG) emissions and mitigate global warming have made finding new sources of energy more urgent than ever before. Biomass, i.e. organic matter originating from plants (including algae, trees, crops and plant derived waste), has been used throughout human history as a source of heat and power. Indeed, biomass is estimated to be the fourth largest source of energy in the world, supplying 10–14% of primary energy, i.e. 46 EJ/year (Sims et al. 2006; Parikka 2004). Detailed reviews published by Sims et. al. (2006) and McKendry (2002a, b) provide thorough overviews of the different biomass feed stocks that are available currently and the diverse range of conversion technologies.

It is generally recognised that unless this potential increase in biomass production is carefully regulated there is likely to be very significant impacts on food production. At present, biofuel feedstock production occupies just 1% of cropland but the rising world population, changing diets and demand for biofuels are estimated to increase demand for cropland by between 17% and 44% by 2020 and, although sufficient suitable land is probably available, current policies do not ensure that additional production occurs in these areas (Renewable Fuels Agency 2008). Indeed, this and other reports (e.g. Davis et al. 2009; Robertson et al. 2008) highlight that, left unrestricted, production of biomass or fuels from traditional food crops will displace existing agricultural production, reduce biodiversity, and promote changes in land use that may even lead to extensive GHG emissions rather than savings (Gibbs et al. 2008; Environment Agency 2009; Upham et al. 2009; Buddenhagen et al. 2009). Conversely, other life cycle analysis studies have reported that, with careful consideration of the impacts of changing crops and land use, and also of the social, economic and environmental impact, the cultivation of energy crops is likely to be sustainable and have a mainly beneficial impact on GHG emissions (Hillier et al. 2009; Hastings et al. 2009; Monti et al. 2009; Haughton et al. 2009). Consequently, there is considerable pressure to identify non-food biomass crops that can meet the rising demand for biomass in a manner that is sustainable (Yuan et al. 2008; Gibbs et al. 2008; Smith 2008; Pauly and Keegstra 2008).

This chapter will focus specifically on the use and optimisation of non-food biomass crops that are suitable for cultivation in the United Kingdom and Northern Europe for energy production by combustion processes. It will briefly examine the range of combustion technologies, describe biomass composition and review how chemical composition influences the efficiency of energy conversion by combustion. It will then discuss the energy crops that are suitable for cultivation in the UK, highlight why they are fit for purpose and explore how these crops can be improved by approaches that include selective breeding and genetic manipulation (GM).

2 Biomass Combustion Technologies

2.1 The Combustion Process

There are three main thermal-conversion processes by which biomass can be converted to energy: combustion, gasification and pyrolysis; these three conversion processes are compared in Fig. 2.1. Combustion consists of burning biomass in air to convert the chemical energy stored in the biomass to heat, mechanical power or electricity (McKendry 2002b; Bridgwater 2003). Complete combustion requires sufficiently high temperature, strong turbulence of the air–gas mixture and a long residence time of the mixture in the fire chamber. Molecules of fuel are generally not reactive until they have undergone dissociation into reactive molecular fragments brought about by the high speed molecular collisions that are characteristic of high temperature. The latter two parameters increase the chance that molecules of pyrolysis gas have the opportunity to react with oxygen (Küçük and Demirbas 1997). The combustion of wood or woody biomass results in the production of hot gases, mainly carbon dioxide (CO2) and water vapour (H2O) through a complex series of reaction steps. An overall equation for the combustion of wood is presented below:

$${{\hbox{C}}_{{\rm{42}}}}{{\hbox{H}}_{{\rm{6}}0}}{{\hbox{O}}_{{\rm{28}}}} + {\hbox{ 43}}{{\hbox{O}}_{\rm{2}}} \to {\hbox{ 42}}{\hbox{C}}{{\hbox{O}}_{\rm{2}}} + {\hbox{ 3}}0{{\hbox{H}}_{\rm{2}}}{\hbox{O}}$$
(2.1)
Fig. 2.1
figure 1

Main thermal conversion processes for biofuels showing intermediates and final energy products (taken from McKendry 2002b)

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In contrast to gasification and pyrolysis, which produce intermediates that can be stored and used for subsequent energy or chemical production (syngas and bio-oil), the heat produced by combustion cannot be stored and must be used immediately for heat or the generation of power (Bridgwater 2003). Effective combustion of biomass requires temperatures of approximately 800–1,000°C and, whilst it is possible to burn any type of biomass, in practice it is only feasible to burn biomass with moisture content lower than 50% due to the additional energy expenditure of drying woody biomass to below 40% moisture content, although large-scale facilities are better able to cope with higher moisture contents. An additional loss of energy yield from biomass combustion results from the need to mill feed-stocks to dimensions compatible with commercial combustion technologies (McKendry 2002b; Royal Society 2008). In this context, the scale of combustion plants range from the small domestic scale to large-scale industrial plants capable of producing 100–3,000 MW (McKendry 2002b).

2.2 Biomass as a Feedstock for Combustion

Biomass has much higher ratios of hydrogen:carbon and oxygen:carbon compared to fossil fuels and therefore less energy content for thermal conversion; coal, for example, contains between 75% and 90% carbon (Jenkins et al. 1998) while biomass typically has a carbon content of the order of 50% (Ptasinski et al. 2007; Obernberger et al. 2006). However, biomass fuels contain a greater proportion of volatile components than coal and therefore are more reactive at high temperature. At temperatures of around 500°C, approximately 85% of wood biomass (by weight) is converted into gaseous compounds (Ptasinski et al. 2007; McKendry 2002a) and efficiencies of 15% for small power stations and up to 30% for larger and newer plants are typical (Bridgwater 2003). However, the availability of agricultural, pulp and paper waste materials has ensured that combustion has remained commercially viable despite problems due to high levels of emissions and ash handling (Bridgwater 2003). There are two possible options for biomass and waste utilisation by combustion processes: biofuels can be burnt as a single fuel in specially designed power stations of limited capability, e.g. Slough Heat and Power (http://www.sloughheatandpower.co.uk/) and the soon to be constructed power station at Stevens Croft, Lockerbie (http://www.eon-uk.com/generation/stevenscroft.aspx), or co-combusted with coal in existing power stations, e.g. Drax power station (http://www.draxpower.com/corporate_responsibility/climatechange/cofiring/) (Department of Trade and Industry 1998). The former option requires significant financial investment, as the low-energy density of biomass would dictate the construction of new biomass-specific power stations. In addition, the associated infrastructure cost for these stations would be high as a result of the requirement for them to be decentralised in order to minimise fuel transportation costs (Carroll and Somerville 2009). Such stations would require extensive storage areas because of the seasonal availability of most biofuels (Hein and Bemtgen 1998). In contrast, many large multi-partner studies carried out in the UK for the department of Trade and Industry (Department of Trade and Industry 1998; Woods et al. 2006), the European Union (EU) funded APAS project (Activite de promotion, D’Accompagnement et de Suivi) (Hein and Bemtgen 1998), in the United States for the Department of Energy (Segrest et al. 1998) and the Alliance for Global Sustainability (Massachusetts Institute of Technology, The University of Tokyo, Chalmers University of Technology and the Swiss Federal Institute of Technology) (Leckner 2007) have found that co-combustion of coal with up to 10% of biomass is possible using existing power stations and infrastructure (Carroll and Somerville 2009). Furthermore, this is unlikely to lead to increased emissions of sulphur dioxide (SO2), oxides of nitrogen (NOx) or hydrochloric acid (HCl) (Department of Trade and Industry 1998). Co-combustion using a greater proportion of biomass may be possible but this is likely to require the development of new combustion systems (Huang et al. 2006). Whilst the range of biomass feed stock for co-combustion is highly diverse, e.g. wood, olive and palm residues, tall oil (a viscous yellow-black odorous liquid by-product of the Kraft process of wood pulp manufacture), sunflower and cereal pellets, sewerage sludge, waste derived fuels, tallow and biomass from dedicated energy crops (Department of Trade and Industry 1998), this chapter will focus only on the latter.

3 Lignocellulose

3.1 Structure and Composition of the Plant Cell Wall

By far the largest component of biomass from dedicated crops is lignocellulose, which forms the cell walls of plants. The composition of lignocellulose directly affects biomass quality for combustion and many efforts to improve biomass crops as feed stocks for combustion or other processes will focus on making specific modifications to cell wall composition. Cell walls are strong flexible composites of biological polymers that serve to maintain the structural integrity of the cell. The main components of cell walls, and the most abundant biopolymers on the planet, are cellulose (approximately 40–50% of most biomass by weight); hemicellulose (10–40%) and lignin (5–30% of biomass by weight; McKendry 2002a) with cellulose and lignin being the two most abundant biopolymers on Earth. Indeed lignin may account for 30% of all carbon fixed annually (Boerjan et al. 2003). Cellulose is made up of microfibrils (semi-crystalline bundles of 500–14,000 monomers of d-glucose joined linearly by β1–4 linkages); hemicelluloses (a hydrated matrix of cross-linked linear and branched polysaccharides composed of pentose and hexose sugars including glucose, mannose, xylose and arabinose) and pectin (structurally complex and often highly substituted linear and branched polymers rich in galacturonic acid; Somerville 2006; Mohnen 2008; Carpita and Gibeaut 1993).

Cells present in the vascular tissues of all higher plants contain high levels of lignin—a complex aromatic heteropolymer covalently bound to hemicellulose and which gives the strength and rigidity that allow plants to grow upright. Lignin also provides the vascular system with the hydrophobicity necessary for the transport of water and solutes (Vanholme et al. 2008). Lignin is formed from three hydroxycinnamyl alcohol monolignol monomers (hydroxyphenyl/ guaiacyl/ syringyl; H/G/S) differing in their degree of methoxylation (Boerjan et al. 2003; Boudet 1998). Lignin has a highly complex and somewhat random structure in which the three types of monolignol are linked by a variety of ether and carbon–carbon bonds. Current opinion holds that biosynthesis of lignin occurs in the extracellular milieu, where monolignols are oxidised by peroxide or laccase enzymes and coupled in a combinatorial fashion (Barsberg et al. 2006; Méchin et al. 2007; Morreel et al. 2004; Weng et al. 2008; Grabber 2005).

3.2 Plant Cell Wall Architecture

The primary cell wall is formed during cell elongation. In all dicotyledonous species (dicots) and many monocotyledonous species (type I monocots), the primary cell wall is composed primarily of cellulose microfibrils embedded in a hydrated matrix of xyloglucan hemicelluloses, pectins and structural proteins. The primary cell walls of type II monocots, i.e. grasses and related monocots (Poales), have a different composition. In this case, the major cross linking hemicellulose polymers are glucuronoarabinoxylans (GAX; Carpita and Gibeaut 1993). In addition, type II primary cell walls contain a higher proportion of cellulose and only negligible amounts of pectin (Carpita 1996).

Secondary walls are deposited during the differentiation of xylem, phloem and transfer cells once elongation is complete. Woody species and forest crops in particular, are rich in secondary cell walls. The molecular architecture of secondary walls is much less well characterised than that of primary walls (McCann and Carpita 2008; Boudet 1998). Secondary walls are generally thicker than primary walls, are enriched in xylans and cellulose, and contain only minor amounts of protein and pectin (Mellerowicz et al. 2001). Most importantly, in secondary walls, lignin replaces much of the water, making them impenetrable to solutes and enzymes (Pauly and Keegstra 2008).

Primary and secondary cell walls from grass species are also distinct from those of dicots in that they contain large amounts of cell-wall-bound hydroxycinnamic acids, namely p- coumaric acid (up to 3%) and ferulic acid (up to 4%; Allison et al. 2009a; Grabber et al. 1995; Waldron et al. 1996; Vogel 2008), which are bound to the arabinoxylan moieties of GAX and lignin by ether and ester bonds. Furthermore, ferulic acid forms a variety of dimers (Hatfield et al. 1999a) and, to a lesser extent, trimers through ether and ester linkages (Bunzel et al. 2003, 2004). These play an important structural role in the grass cell wall as they covalently cross-link adjacent GAX molecules by ester linkages and bind GAX to lignin by a combination of ester and ether bonds (Hatfield et al. 1999a).

4 The Effect of Chemical Composition on Feedstock Properties

Whilst biomass feedstocks are comprised primarily of carbon, hydrogen, oxygen and nitrogen, they contain additional components or ‘impurities’ that disrupt the combustion process. The presence and concentration of such substances is dependent on the plant material, agronomic and agricultural practices and geographic location and, in some cases, levels may also increase as a result of increasing crop yield (Royal Society 2008). Of particular concern for combustion efficiency are: calorific value; moisture content; the proportion of fixed carbon; and the content of ash, residues and alkaline metals (McKendry 2002a; Obernberger et al. 2006). Moisture content and carbon density have dramatic effects on calorific value, which is usually expressed as higher heating value (HHV) or lower heating value (LHV). HHV is the energy content when the material is burnt in air under standard conditions of temperature and pressure and the value includes the condensation enthalpy of water in contrast to LHV (Friedl et al. 2005). Biofuel quality could therefore be improved by breeding, agricultural or other interventions that decrease moisture content or increase carbon density — the most obvious route being to increase the proportion of lignocellulose, although, as will be discussed later, this is far from trivial. Typically, biomass fuels have moisture contents ranging from 16% to 30% and have LHVs of around 16–19 MJ/kg, in contrast to coal, which typically has a moisture content of approximately 11% and a LHV of 43 MJ/kg (McKendry 2002a).

Alkaline metals (Na, K, Mg, and Ca) occur naturally in plants and their concentration in biofuels has major effects on combustion efficiency as they are involved in ash formation and decrease ash melting point, which can in turn cause blockage, erosion and/or corrosion of equipment through processes that are now well understood (Misra et al. 1993; Lewandowski and Kicherer 1997; Jenkins et al. 1998). In addition, high levels of nitrogen, chlorine and sulphur can lead to unacceptable emissions of NOx, HCl and SO2 and also boiler corrosion (Obernberger et al. 2006; Lewandowski and Kicherer 1997). Furthermore, the concentration of chlorides in the biomass has as much of an influence on the amount of alkaline metals vaporised during combustion as the concentration of the alkaline metals themselves, and is thought to act as a shuttle, transporting alkaline metals from the fuel to surfaces where they form stable sulphates (Jenkins et al. 1998). The continual removal of these minerals by harvesting can lead to soil degradation and non-sustainable production practices (El-Nashaar et al. 2009). At least one study has shown that the effect of alkaline metals in reducing conversion efficiency in several biomass fuels was much greater than the effect of differing lignin content. This latter study also showed that washing the biomass before conversion improved efficiency, presumably by leaching out chloride and alkaline metals (Fahmi et al. 2008).

Silica is another biomass component and whilst not posing a problem by itself it is involved in ash formation and is known to react with alkaline metals (Jenkins et al. 1998). It is abundant in the walls of grasses, where it is present mostly as inclusion bodies in the epidermis, periderm and other specialised root cells, rhizome and aerial shoots (Carpita 1996) and may be introduced as soil contamination during harvesting. The range of ash content in biofuels can vary between 1% and 20%, with wood typically having a low ash content of 1–2% (Misra et al. 1993); biomass from dedicated energy grass species lies within an acceptable range of 3–5% (Fahmi et al. 2008; Lewandowski and Kicherer 1997; McKendry 2002a). These levels are achieved by three key harvest management practices; namely, harvesting after senescence has occurred, harvesting after over-wintering in the case of Miscanthus species; or, for other grass species, allowing the mown crop to leach in the field for 1–4 weeks before baling (Cornell University 2006). These processes allow leaching of chlorine and alkaline metals so reducing ash content, and also reduce water content and the concentrations of protein and nitrogen in the foliar tissues. In addition, senescence, the natural process of winter die-back, allows nutrients and minerals to be mobilised to below-ground tissues for storage over the winter months (Jørgensen 1997). This decreases the requirement for fertiliser input and improves crop sustainability. In addition, harvesting after over-winter weathering dramatically reduces the proportion of leaf material in the biomass. Whilst leaves do not make a significant contribution to the composition of wood biomass, leaf material can make a significant contribution to grass biomass and, even after senescence, levels of ash, nitrogen, phosphorus, silica and alkaline metals are much greater in leaf material than in stem, resulting in a significant deterioration of biofuel quality (Monti et al. 2008).

5 Energy Crops for Combustion Processes in the European Union

Several studies have identified the importance of low external inputs as a key factor for energy crops and whilst this may result in poorer energy yields, emission balances are much more favourable (Kaltschmitt et al. 1997). In addition, suitable energy crops must be capable of growing on land that is marginally fertile in order not to displace current food production. These criteria have considerably narrowed the number of potential energy crop species suitable for production in the UK. Wood products and forest waste have obvious roles as biomass for combustion, and indeed wood chips from poplar have been found to be the most favourable when compared with other forms of bioenergy with the exception of rapeseed oil and wood chips from willow (Kaltschmitt et al. 1997). However, a study by Berndes et al. (2003), which reviewed 17 published studies on the contribution of biomass to the future global energy supply, reported that perhaps as much as half of the timber available in Europe may not be available for energy production (Berndes et al. 2003), and predicted that over the next 100 years it would be energy crops that would contribute the largest proportion to bioenergy supply.

All of the crops identified as having potential as biomass crops for Northern Europe and the UK in particular have high levels of lignocellulose (Table 2.1). Fast-growing woody C3 crops are attractive as sources of biomass in Europe as they meet with agronomic, environmental and societal requirements for successful deployment as energy sources, and much attention has been given to short rotation willow (Salix spp.; Smart and Cameron 2008) and poplar (McKendry 2002a). In the UK, Miscanthus x giganteus, a naturally occurring sterile hybrid of the South East Asian species Miscanthus sinensis and Miscanthus sacchariflorus, and, to a lesser extent, switchgrass (Panicum virgatum), a native of North America, are the two herbaceous species that have received most attention as commercially viable and environmentally sustainable biomass crops for combustion (Bullard 1999; Carroll and Somerville 2009; Bouton 2008; Price et al. 2004). Both of these perennial grass species provide easily harvestable annual crops with low moisture content and high dry matter yield. Furthermore, both M. x giganteus and switchgrass have C4 photosynthetic apparatus, which is common in species originating from tropical or dry locations. Both therefore have potential photosynthetic advantages over native C3 perennial grasses, e.g. temperate forage grasses such as Lolium, when CO2 is limiting, temperatures are high and water is scarce, and are able to convert a higher proportion of incident light into biomass (Ehleringer et al. 1997). In addition, perennial grasses generally have lower nitrogen content and have a lower requirement for nitrogen inputs when compared with annual species. They do not require annual tilling and this allows considerable amounts of carbon to remain sequestered in the soil, reduces soil erosion and decreases the energy inputs required for the operation of heavy farming machinery (Heaton et al. 2004a).

Table 2.1 Comparison of the compositions of biomass feed-stocks (from Pauly and Keegstra 2008, and IENICA 2009 crop database). Values adjusted to percentage dry weight (%DW)
Full size table

5.1 Miscanthus Species

M. x giganteus is propagated from rhizomes and, in the UK, typically grows to a height of approximately 3–4 m (see Fig. 2.2a). The crop takes two to three years to establish before yields are maximised. Weeds are controlled by soil-acting herbicide application after planting and again prior to emergence in the 2nd and possibly the 3rd year (Clifton-Brown et al. 2008a). M. x giganteus has a large root structure that extends approximately 1.8 m below the surface, where nutrients are stored in the rhizomes over the winter months (Carroll and Somerville 2009); recent studies suggest that there is considerable potential for these roots to sequester carbon and thereby decrease GHG emissions (Hillier et al. 2009; Clifton-Brown et al. 2007). It has also been grown successfully in the US and in European locations including Turkey, Ireland, Denmark, Germany, the UK, Switzerland, Spain and Italy (Lewandowski et al. 2000; Heaton et al. 2004b; Clifton-Brown et al. 2007; Acaroglu and Semi Aksoy 2005). In England and Wales, dry matter harvestable yields have been reported to range between 6.9 and 24.1 t ha–1 year–1 when the crop is grown on arable land (Price et al. 2004). One study in Ireland on marginal land reported average autumn and spring dry matter yields of 13.4 and 9.0 t ha–1 year–1 over a period of 15 years (Clifton-Brown et al. 2007) and modelling has predicted a peak output yield across Ireland of between 16 and 26 t ha–1 year–1 (Clifton-Brown et al. 2000). Yields reported in Europe range from 4 t ha–1 year–1 in Central Germany to 44 t ha–1 year–1 in Northern Greece and Italy (Angelini et al. 2009; Lewandowski et al. 2000). This considerable range of yield is most likely due to variations between sites in temperature and rainfall as well as differences in harvesting date, phenotypic type and possibly fertiliser treatment. In the UK, yields with current varieties of M. x giganteus are likely to be greater in the wetter west than in the drier east of the country (McKendry 2002a). One recent estimate has put the amount of Miscanthus (presumably mainly M. x giganteus) under cultivation in the UK in 2007 at 10,000 ha (Nix 2007), although the estimated area given over to Miscanthus made by the UK National Non-Food Crops Centre (2009) is somewhat lower, with only 4,032 ha of Miscanthus being grown in the UK in 2007 with the total area of new Miscanthus being planted each year rising from 302 ha in 2005 to more than 2,300 ha in 2006 and 2007. Most of this Miscanthus is used for co-firing with coal at large power-stations although an increasingly greater proportion is being used directly for the generation of combined heat and power at biomass dedicated stations, e.g. The Bluestone Holiday Village Project in Pembrokeshire, Wales (http://www.energycropswales.co.uk/opening_markets.php.en?subid=0). Dramatic increases in Miscanthus cultivation have been predicted for the next 20 years as the requirement for biofuel feedstocks increases. The area of land that is suitable for the cultivation of M. x giganteus in the UK alone amounts to more than 1.5 million ha (approximately 10% of agricultural land, J.C.C.-B., unpublished data), capable of yielding approximately 18.7 million t/year, and the area available in the 15 member states of the EU amounts to over 11.6 million ha, with a potential yield of more than 158 million t/year (Clifton-Brown et al. 2004). It is likely that these are indeed conservative estimates given that a primary goal of Miscanthus breeders is to radically increase yields by the development of “hi-tech” hybrid varieties (Hastings et al. 2009) as, despite the promising features of M. x giganteus, there is vast scope for genetically improving Miscanthus as a biomass feedstock by integrating desirable traits by exploitation of the huge genetic variation present in wild Miscanthus accessions, particularly those of M. sinensis (Stewart et al. 2009). This approach will therefore seek to increase the tolerance of Miscanthus to environmental stress, thereby opening up opportunity for cultivation on as yet unsuitable land.

Fig. 2.2
figure 2

a Photograph of a mature stand of Miscanthus giganteus being harvested (Pembrokeshire, Wales, February 2008). The stand height is between 2.5 m and 3.0 m. b Photographs of representatives of the two best represented Miscanthus species in the Aberystwyth collection: left M. sacchariflorus (canopy height 1.83 m), right M. sinensis (canopy height 1.24 m)

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Compared to many other lignocellulosic plants M. x giganteus has excellent combustion properties with low water (16–33%) and mineral content (Cl= 0.3–2.1 g kg–1; N= 0.9–3.4 g kg–1 and K= 3.7–11.2 g kg–1; Lewandowski and Kicherer 1997). Similar values were detected in a recent study in which 15 Miscanthus accessions were grown in five locations in Europe (Lewandowski et al. 2003). A major goal of breeding will be to increase yields at low levels of input (Moller et al. 2007). A substantial impediment preventing widespread cultivation of M. x giganteus in the UK is poor frost tolerance (Clifton-Brown et al. 2000; Farrell et al. 2006). An extensive breeding programme is underway at Aberystwyth University aimed at incorporating traits from wild genotypes of M. sacchariflorus and M. sinensis into new high yielding Miscanthus varieties (both novel hybrids as well as new varieties of M. sinensis and M. sacchariflorus) tailored for the commercial sector to improve drought and cold tolerance and stay-green characteristics (Clifton-Brown et al. 2008b). Figure 2.2b shows clearly the significant morphological differences that typically exist between these two species. M. sacchariflorus tends to have fewer but taller and thicker stems whilst M. sinensis has many smaller and thinner stems. In summary, improving the ability of new varieties to grow to high yields in the UK, and the development of varieties that could be propagated by seed are primary objectives in establishing wide-spread commercial cultivation but currently these are still some way off realisation.

5.2 Switchgrass

Switchgrass has received relatively little attention in Europe compared with M. x giganteus despite the former having been identified by the US Department of Energy as its main herbaceous dedicated energy crop because of its potential for high yields, low environmental impact and low input requirement (Bouton 2008; Carroll and Somerville 2009). It is a major component of the American prairies and many varieties grow in small dense clumps. In the US, like M. x giganteus, switchgrass may reach up to 3 m in height and its chemical composition and low moisture content make it ideally suited for a variety of bioenergy uses, including lignocellulosic conversion to bioethanol and combustion. Published annual switchgrass yields are sometimes lower than those of M. x giganteus, depending on the climate (Heaton et al. 2004a), and switchgrass is harvested annually or semi-annually (Bouton 2008). There are two distinct ecotypes available: lowland and upland, with the former, which has thicker stems, growing more sparsely as densely bunched plants. Furthermore, later maturity tends to result in higher mineral concentrations at harvest (DTI 2006). Switchgrass takes 3 years or more to reach maturity and is optimally grown as a highly managed single crop, generally sown using grassland drill; weeds are controlled using pre- and post-emergent herbicides. Commercial varieties can tolerate a wide range of soil and pH conditions and, with only limited fertiliser input, can produce a greater yield than other warm season grass species. In addition, the excellent seasonal yield distribution of switchgrass, especially for high spring yields, means that crops are of value to the live-stock industry in addition to its use as biomass (Vogel 2004). One study in Ardmore, Oklahoma, reported yields of between approximately 8 and 17 t ha–1 depending on the time of harvest (Bouton 2008) whilst a comprehensive comparison of switchgrass with M. x giganteus conducted in Illinois estimated an average yield of 10 t ha–1 from 77 separate observations (Heaton et al. 2004a). This latter review concluded that, in certain climates, M. x giganteus holds greater promise for biomass energy cropping than switchgrass. In the UK, yields of 9.63 t ha–1 year–1 have been reported for a lowland ecotype across three sites and two growth years in comparison with approximately 7 t ha–1 year–1 for typical upland varieties (DTI 2006). However, despite these lower yields and the difficulty of establishment, switchgrass is likely to have a role as a bioenergy crop in the UK as it can be sown from seed (rather than rhizomes, which require specialised equipment for planting) and harvested and bailed using equipment that is commonly available on farms familiar with growing perennial forage grasses (Vogel 2004). At present however, there is little evidence of commercial switchgrass cultivation in the UK and northern Europe, and most existing plantations are for research purposes.

5.3 Willow and Poplar

Willow and poplar are promising candidates for woody energy crops and have received much attention in the US (Smart and Cameron 2008; Davis 2008). In the UK, willow (Fig. 2.3) has received comparatively much greater attention due to programmes such as the European Union funded “Willow for Wales” project (http://www.willow4wales.co.uk) and the National Willows Collection at Rothamstead Research. These collections each comprise approximately 1,300 genetically characterised clones. The availability of a complete genome sequence for poplar and its role as a model organism for plant biology will no doubt facilitate the development of improved varieties for bioenergy use (Carroll and Somerville 2009). Yields of 12.4 t ha–1 year–1 and 22.5 t ha–1 year–1 have been reported for poplar grown on non-irrigated and irrigated soils (Deckmyn et al. 2004) whilst a study in Quebec found average yields of 17.3 t ha–1 year–1 for poplar and 16.9 t ha–1 year–1 for willow without fertiliser or irrigation (Labrecque and Teodorescu 2005). These yields seem comparable with those which might be expected from M. x giganteus but there is obvious need for additional trials that will allow a direct comparison of these species with M. x giganteus and switchgrass at a variety of geographical locations in northern Europe to inform which crops would be most suitable in given locations. It is highly likely that some regions will be more suitable for the growth of trees rather than energy grasses. Large-scale commercialisation of short rotation coppice willow is already practiced in northern Europe (Moller et al. 2007) but in the UK, growth of tree bioenergy crops has been slow to take off, with only very limited amounts of willow under cultivation, and poplar not being cultivated on a commercial scale at all. One problem has been that cultivation requires considerable expenditure for establishment and subsequent harvesting ties up land for cultivation for considerable periods of time; indeed removing trees from land requires considerable expenditure. Furthermore, willow and poplar demand large amounts of water, which excludes them from growth in certain areas and, in addition, both are extremely susceptible to rust (Moller et al. 2007). Lastly, the availability of cheap forest chipped waste in the UK has undermined willow as a commercial crop at present but with increasing demand for clean chipped wood of high quality for domestic heating it is likely that willow cultivation will increase.

Fig. 2.3
figure 3

Photograph of a mature stand of coppice willow planted in 2004 and cut back in 2006. The stand (approximately 3.5–4.0 m in height) represents 2 years of growth. (Courtesy of Chris Duller, Field trial co-ordinator of Willow for Wales, Pembrokeshire, Wales, February 2008)

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5.4 Reed Canary Grass

Lastly, reed canary grass (Phalaris arundinace) is a rhizomatous C3 perennial grass that warrants mention as a potential bioenergy crop. This species is distributed widely across the temperate regions of Europe, Asia and North America and can grow as high as 2 m. Reed canary grass possesses two highly desirable traits: the ability to withstand drought and also to tolerate excessive precipitation. Like switchgrass it is propagated from seed and has the flexibility to be used for animal feed as well as for biomass, but unlike switchgrass it is relatively easy to establish, with full yields being reached in fewer years. One UK study has found that the crop requires nitrogen fertilisers for optimal growth (DTI 2006). One comprehensive study of 72 accessions at five locations in the US reported yields that varied with environment (mean of 9.2 t ha–1 year–1) and which remained high in wet locations and marginal land (Casler 2009). Yields of 10 t ha–1 year–1 have been reported in Sweden, where it is being evaluated as a bioenergy crop; however, in the UK much lower yields of approximately 4 t ha–1 year–1 (Chisholm 1994) and 5.3–5.5 t ha–1 year–1 (DTI 2006) are more typical. The ease and low cost of establishing and cultivating this crop suggest that in time there may be a role for reed canary grass as a secondary energy crop in the UK but currently there is little or no commercial cultivation of reed canary grass as a bioenergy crop (UK National Non-Food Crops Centre 2009).

6 Technologies for Crop Design

6.1 Modification of Hemicellulose and Cellulose

The modification of biomass crops for improved combustion can be divided into several key areas: (1) manipulation of the amount and structure of lignocellulose in the crop biomass; (2) altering the chemical composition of the biomass; and (3) altering quality parameters such as moisture content and particle size. Increasing cell wall polysaccharide concentrations would most likely also increase calorific value; however, efforts to modify hemicellulose or cellulose have been hampered by the extreme complexity of structural polysaccharide biosynthetic systems in plants. Hemicellulose and cellulose are synthesised in different cellular compartments by very different complex processes that are still not thoroughly understood (Somerville 2006). Cellulose is synthesised at the plasma membrane by rosette complexes that are thought to consist of 36 individual cellulose synthase proteins belonging to three or more different classes (Mutwil et al. 2008). In contrast, hemicellulose is synthesised in the Golgi, packaged into secretory vesicles and transported to the cell surface for incorporation into the cell wall matrix (Pauly and Keegstra 2008). However, the natural variability observed in the wall composition of several biomass feed stocks shown in Table 2.1 suggests that there is a great potential for altering wall composition without compromising the life cycle of the plant (Pauly and Keegstra 2008) but this goal may be difficult to achieve without a better understanding of the exact processes involved in biosynthesis.

Several investigations have shown the need for caution when altering wall composition in planta since this may cause changes that are detrimental for plant growth and lead to phenotypes including dwarfism (Desprez et al. 2007), lethality (Goubet et al. 2003) or compromised defence against pathogens (Sticklen 2006). There are several reports of increased polysaccharide concentration being effected by manipulation of growth regulators or insertion of genes to delay flowering (reviewed by Sticklen 2006). These studies, however, are still some way from being effective strategies for biomass improvement, and realistic options for biomass improvement by increasing wall polysaccharide content will depend on a more comprehensive understanding of the genes involved in cell wall biosynthesis. Over the last decade, considerable progress has been made in this area and genes have been identified that are involved in the biosynthesis of cellulose, hemicellulose and pectin, as well as genes responsible for the biosynthesis of the sugar nucleotide donors involved in polysaccharide biosynthesis (Zhong and Ye 2007; York and O’Neill 2008; Ye et al. 2006). This process has been greatly assisted by the availability of new model systems, e.g. maize (Zea mays), sorghum (Sorghum bicolor; Carpita and McCann 2008) and Brachypodium distachyon (Opanowicz et al. 2008), new data base resources, e.g. Maizewall (Guillaumie et al. 2007), better understanding of cell wall architecture (McCann and Carpita 2008) and by new techniques for identifying cell wall biosynthetic genes (McCann et al. 2007; Mitchell et al. 2007).

6.2 Modification of Lignin

In contrast to cellulose and hemicellulose, the biosynthesis of lignin is better understood (Boerjan et al. 2003) and lignin has proved to be highly amenable to manipulation by genetic engineering (Boudet 1998; Li et al. 2008, Vanholme et al. 2008; Weng et al. 2008). Several reviews have been published that describe in detail efforts to alter lignin quantitatively and qualitatively to improve the efficiency of lignocellulosic fermentation to liquid transport fuels and biorefinery intermediates (Hatfield et al. 1999b; Grabber 2005, Weng et al. 2008; Chang 2007; Sticklen 2006). Generally, this involved modifying lignocellulose to improve degradation and facilitate enzymic deconstruction, and only rarely has the focus been on increasing calorific value. However, although lignin content is positively correlated with calorific value (Demirbas 2001), there is growing evidence linking lignin concentration to soot formation (Fitzpatrick et al. 2008). Furthermore, changes in cell wall composition may affect particle size in the processed biomass, which has been shown to have implications for combustion efficiency (Bridgeman et al. 2007). Therefore, in some cases it may be desirable to breed varieties of Miscanthus with reduced lignin content.

Our understanding of lignin biosynthesis has been furthered by the study of the brown-midrib mutants of maize (Li et al. 2008), sorghum and millet (Vogel 2008), all of which are characterised by a reddish-brown pigmentation of the leaf midrib and are associated with altered and often lowered lignin content (Barriere et al. 2004; Marita et al. 2003). Most of the work to characterise these mutations was carried out on maize, the first species in which these mutations were identified. A pathway showing the biosynthetic pathway of lignin is shown in Fig. 2.4. Two of these phenotypes are due to lowered activity of specific lignin biosynthetic genes. The bm1 phenotype is due to a mutation affecting cinnamyl alcohol dehydrogenase (CAD), but it is not yet clear whether the mutation actually lies within this gene (Halpin et al. 1998); bm3 mutants are defective in caffeic acid O-methyltransferase (COMT; Vignols et al. 1995). The molecular basis of the other two phenotypes bm2 and bm4 is not yet understood (Marita et al. 2003); bm2 mutants contain fewer guaiacyl and syringyl residues and have altered patterns of lignin deposition (Vermerris and Boon 2001), whilst the lignin composition of bm4 mutants resembles that of bm2 (Barriere et al. 2004). There is now considerable evidence to suggest that lignin biosynthesis is highly plastic; normal maize lines and hybrids display substantial genetic variability for lignin and degradability traits, at times rivalling the extremes associated with bm mutants (Argillier et al. 1996; Deinum and Struik 1989; Dhillon et al. 1990; Jung et al. 1998; Jung and Buxtono 1994; Lundvall et al. 1994; Méchin et al. 2000; Roth et al. 1970) and given the genetic similarity of Miscanthus with model C4 grass species it may be possible to breed successfully for brown-midrib traits in this energy crop.

Fig. 2.4
figure 4

Schematic showing the main biosynthetic pathway of monolignol biosynthesis (based on Boerjan et al. 2003). In order of appearance: PAL phenylalanine ammonia-lyase, C4H cinnamate 4-hydroxylase, 4CL 4-coumarate:CoA ligase, CCR cinamoyl-CoA reductase, COMT caffeic acid O-methyltransferase, CCoAOMT caffeoyl-CoA O-methyltransferase, CAD cinamyl alcohol dehydrogenase, F5H ferulate 5-hydroxylase, P/L peroxidise and laccase

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Another, more adaptable, approach to modifying lignin content in energy crops is GM. Evidence from studies in model species, e.g. tobacco, alfalfa, maize and poplar (Vanholme et al. 2008; Li et al. 2008) suggest that altering lignin content in Miscanthus species and coppice tree species using molecular approaches is quite feasible, although in practice it may be easier to decrease or modify lignin rather than to increase total lignin content. Changing the expression of many of the lignin biosynthetic genes often results in altered lignin monomer composition and or reduced total lignin content (Li et al. 2008; Vanholme et al. 2008); however, all too frequently, transgenic phenotypes are less dramatic or possibly contrary to expectation. Many lignin biosynthetic genes are members of multigene families and other homologues may be involved in other important cellular processes (Campbell and Sederoff 1996); furthermore, this leads to enormous plasticity of plant metabolism, often resulting in surprising and unexpected phenotypes. For example, decreasing the activity of CAD might be expected to result in lower levels of monolignols available for incorporation into lignin and reduced lignin content. In practice, this is often not the case as other intermediates, e.g. cinnamaldehydes, may be incorporated into lignin in their place (Boerjan et al. 2003). This aside, genetic engineering can result in dramatic changes in lignin content; in one study, down-regulation of 4-coumarate: CoA ligase (4CL) in aspen was shown to reduce lignin content by up to 45% (Hu et al. 1999). Caffeoyl-CoA O-methyl transferase (CCoAOMT) seems to be a major hub in controlling lignification (Ye et al. 1994), and probably also cross-linking in grasses, making this enzyme an ideal target for digestibility improvement by lignin reduction and altered composition. Indeed, in alfalfa, reducing CCoAOMT activity to a residual 5% increased cell-wall digestibility by 34% (Guo et al. 2001). Strongly reduced lignin content with radically altered composition has also been reported in an Arabidopsis ref8 mutant defective in p-coumarate 3-hydroxylase (C3H). This intervention resulted in drastically altered phenyl propanoid metabolism, the formation of lignin composed almost entirely of H units and significant developmental defects (Franke et al. 2002). Down-regulation of COMT leads to decreased synthesis of synapil alcohol and compensatory deposition of 5-hydroxyconiferyl alcohol (M15H). This alteration of lignin composition also results in the incorporation of large amounts of novel benzodioxane structures (Atanassova et al. 1995; Ralph et al. 2001, 2000). Simultaneous downregulation of 4CL and over-expression of ferulate 5-hydroxylase (F5H) has been reported to result in lower lignin content, higher S/G level, and increased cellulose in aspen plants (Li et al. 2003).

New power plants designed specifically for biomass combustion may negate the emission problems associated with elevated lignin content and make high-lignin energy crop varieties with increased calorific value practical. Although this has so far been difficult to achieve in a predictable manner by simply over-expressing biosynthetic genes, further studies using alternative gene promoter sequences and different construct architectures may have more success. Perhaps a better approach for increasing lignin content in biofuels may be to over-express appropriate regulatory genes (Weisshaar and Jenkins 1998; Zhong and Ye 2007; Patzlaff et al. 2003). This approach has been utilised with great success in other phenylpropanoid pathways, e.g. condensed tannins and anthocyanins (Robbins et al. 2003; Nesi et al. 2000; Kubo et al. 1999; Quattrocchio et al. 1999; Spelt et al. 2000) and is likely to deliver biomass with higher lignin levels, increased structural strength, and improved pest and disease resistance. In turn, these modifications would also increase the value of timber crops as construction materials, reduce the need for toxic wood preservatives and also increase the unit calorific value of the biofuel.

6.3 Breeding Strategies

In addition to modification approaches based upon GM, other plant breeding strategies may have value for Miscanthus. Building upon the range of Miscanthus biomass currently available, traditional methodologies are immediately available as Miscanthus sinensis has previously been bred primarily for ornamental applications, e.g. Miscanthus sinensis “Zebrina”. Therefore, as this crop has not been subjected to selection for combustion characteristics, initial selections from germplasm (combined with accurate chemical phenotyping) may well produce improved cultivars in short- to medium-term time periods. An example of this type of approach has been outlined by Clifton-Brown et al. (2008b), who reported phenotypic variation in a replicated spaced trial containing 249 genotypes grown in Aberystwyth. Many genotypes are hard to cross for various reasons including sexual incompatibility. Clearly, future approaches for lines derived from high yielding accessions may well rely upon the identification of quantitative trait loci (QTL) for combustibility traits derived from Miscanthus mapping families (Atienza et al. 2002).

6.4 Chemical Phenotyping and High-Throughput Screening

The development of improved varieties of energy crops requires extensive phenotype analysis and therefore analytical methods that are robust, cost effective and capable of coping with large numbers of samples. However, many traditional methods that are commonly used for measuring chemical composition are time-consuming and costly (Giger-Reverdin 1995; Bridgeman et al. 2007; Friedl et al. 2005) and unsuitable for large-scale analysis at high rates of through-put. By contrast, methods based on spectroscopic analysis e.g. near infrared (NIRS) and Fourier transform infrared (FTIR) spectroscopy, offer practical solutions for the inexpensive, robust and accurate analysis of parameters including cell wall structure (Chen et al. 1998), the concentration and composition of aromatic cell wall components including lignin and hydroxycinnammic acids (Allison et al. 2009a; Stewart et al. 1997; Alves et al. 2006), cell wall carbohydrates (Fairbrother and Brink 1990), digestibility (Decruyenaere et al. 2009), nitrogen content (Gislum et al. 2004), and fixed carbon, nitrogen, alkali index and ash content (Allison et al. 2009b; Huang et al. 2007). Spectra can usually be acquired within approximately a minute, and often sample preparation is considerably simplified to drying and grinding. Early use of this approach sought to correlate the absorbance at specific wave lengths to the concentrations of specific cellular components determined using gravimetric, analytical or chromatographic techniques, e.g. lignin strongly absorbs at 1,510 cm–1 (Monties 1989); however, for analysis of non-purified samples this simplistic approach is prone to inaccuracies caused by the presence of additional compounds with overlapping absorbencies. This problem can be overcome by using multivariate regression methods such as partial least squares or multivariate regression (Labbé et al. 2008; Gislum et al. 2004; Allison et al. 2009a, b).

Pyrolysis gas chromatography/mass spectrometry has great potential for the high throughput chemical analysis of lignocellulose composition. This approach has been used extensively by engineers to profile biomass, but much less so by biologists (del Rio et al. 2007; Fahmi et al. 2007; Galletti and Bocchini 1995). The method requires rapid gasification of biomass, usually in an oxygen-free atmosphere, separation of the pyrolysis volatiles on the gas chromatography column and detection by mass spectrometry. One particularly flexible pyrolysis unit is the CDS Pyroprobe 5200 pyrolyser; this unit is temperature programmable and pyrolysis products generated between specified temperatures are first trapped and then introduced onto the gas chromatograph. The instrument can then be heated to a higher temperature with similar sample trapping allowing the sequential analysis of cell wall components in order of thermal decomposition and therefore discrimination of products originating from hemicellulose from products originating from cellulose or lignin. Both quadrupole and ion trap gas chromatograph/ mass spectrometers have application, the former offering more quantitative data with the ability to clearly distinguish between known products and the latter offering more qualitative discrimination and identification of unknown products.

Several methods are available for determining the elemental composition of biomass. Two of these rely on analysis of hot gaseous plasmas; namely inductively coupled plasma mass spectrometry (ICP MS) or optical emission spectrometry (ICP OES), and over recent years instrument stability has improved significantly, reducing the number of calibration standards required in addition to the samples being analysed, with parallel decreases in instrument cost. Both methods require samples to be ground and digested with concentrated acids overnight but subsequent analysis is largely automated. Analysis by mass spectrometry allows quantitative data to be collected on approximately 100 elements but instrumentation is generally more expensive. In contrast, analysis of emission spectra is less costly but usually only groups of 4–5 elements can be analysed at any one time and considerable method development is often required (Conte et al. 1999). Silica and chloride present special difficulties, the former requiring samples to be dissolved in concentrated hydrofluoric acid and the latter being intractable by this approach and requiring analysis by ion chromatography. X-ray fluorescence (XRF) is an alternative approach and although there are few reports of the technique being used to analyse biomass feed-stocks (Robinson et al. 2009) it has been used to determine elemental composition in biological samples, e.g. in mycobacteria (Gresits and Könczöl 2003) and mussel shells (Kurunczi et al. 2001) and it would appear that XRF has great potential for the analysis of biomass. XRF is used widely in the cement industry to measure elemental composition — an application where the concentration of chlorine is of particular interest — and the method has the advantage of stability, thus negating the requirement for frequent recalibration. However, the precisely defined particle size required for XRF analysis requires lengthy milling to the required size. XRF is therefore a time consuming process and this obstacle would need to be addressed before it could be applied at the high rates of through-put necessary for application to chemical phenotyping on a bioenergy crop breeding programme.

6.5 Case Study: Variation in Cell Wall Composition Between 249 Miscanthus Genotypes

As part of a large growth trial at Aberystwyth we have measured the amount of cellulose, hemicellulose and lignin in several triploid M. sacchariflorus X sinensis hybrids, including the hybrid recognised as M. x giganteus (64 observations), M. sacchariflorus (272 observations) and M. sinensis (1,629 observations) over two consecutive growth years. Planted in 2005 at 1.5 m intervals in four replicate plots, the plants were harvested after over-wintering in the field in the February following the 2006 and 2007 growth years. Sampling entailed removing the entire above-ground foliage and shredding of the material through a modified forage harvester. This material was then weighed, and approximately 200 g removed for cell wall analysis. This material was oven-dried at 60°C and then ground using a rotary mill to pass through a 1 mm mesh. The results of these measurements are presented in Table 2.2. These values were predicted using partial least squares models from the NIR spectra of the samples. Spectra were collected and manipulated using standard procedures (Barnes et al. 1989) and multivariate regression models to predict neutral detergent fibre (NDF; a measure of total cell wall), acid detergent fibre (ADF; a measure of total cellulose and lignin), and acid detergent lignin (ADL) were trained and validated on compositional data obtained using standard gravimetric methods (Van Soest 1963, 1967).

Table 2.2 Average of hemicellulose, cellulose and acid detergent lignin expressed as %DW in 254 independent accessions of mature individual genotypes of Miscanthus sinensis x sacchariflorus hybrids (values for Miscanthus giganteus are shown for the purpose of comparison), M. sacchariflorus and M. sinensis grown in 4 replicated plots at Aberystwyth University, UK and harvested after over-wintering in the field for two successive growth years. Each genotype is replicated four times within the experiment and abundance of cellulose and hemicellulose have been calculated using near infrared spectroscopy (NIRS)-predicted values of acid detergent fibre (ADF), neutral detergent fibre (NDF) and acid detergent lignin (ADL). Significant differences between the Miscanthus species are denoted by lower case letters
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Whilst M. sacchariflorus and M. sinensis have very similar mean levels of cellulose and hemicellulose, there is considerable difference between the mean ADL content of these two species. Furthermore, these data suggest that the several new Miscanthus hybrids are similar to M. x giganteus in that they have significantly higher levels of cellulose and ADL, but lower levels of hemicellulose than M. sacchariflorus or M. sinensis. The mean concentration of ADL in the Miscanthus hybrids (including M. x giganteus) is much greater than in either M. sacchariflorus or M. sinensis. Whether this similarity between the triploid is due to a dominant ploidy effect or the result of only very similar M. sinensis and M. sacchariflorus parents being compatible is currently under investigation. Statistical analysis of these data by unbalanced analysis of variance (Table 2.2) detected significant differences between the hybrids, M. sacchariflorus and M. sinensis for cellulose (P< 0.001; s.e.d. 0.862), hemicellulose (P< 0.001; s.e.d. 0.425) and ADL (P< 0.001; s.e.d. 0.297). Significant differences were also detected between the two growth years for cellulose [means of 42.11 and 43.39 % dry weight (DW) for 2006 and 2007 respectively; P< 0.001; s.e.d. 0.280] and hemicellulose (means of 33.84 and 33.18% DW for 2006 and 2007 respectively; P< 0.001; s.e.d. 0.142) but not for lignin (means of 9.32 and 9.30% DW for 2006 and 2007, respectively). Analysis of these data by Pearson correlation shows that ADL shows weak positive correlation with cellulose (R=0.492) and weak negative correlation with hemicellulose (R = –0.523). There seems to be no correlation between cellulose and hemicellulose content (R = –0.171). The composition in cell wall measured in M. x giganteus in this experiment are somewhat different from those presented in Table 2.1; the specimens analysed in this trial contained greater amounts of cellulose and hemicellulose but less lignin. This is possibly due to differences in environmental conditions and genetic differences between genotypes classified as M. x giganteus. Indeed, in another study (de Vrije et al. 2002), M. x giganteus was reported to have 38% DW cellulose, 24% DW hemicellulose and 25 % DW Klason lignin (which in grass species is typically twice the amount of ADL; Hatfield and Fukushima 2005). This study has enabled genetic mapping families to be devised in order to map QTL relating to cell wall composition and assist in the breeding of Miscanthus varieties with optimised cell wall composition.

7 Conclusions and Future Perspectives

The need for developing a sustainable non-fossil fuel economy is self evident and has been highlighted in many recent reviews, including two publications by Nicholas Stern (Stern 2007, 2009). In the EU, the undertaking to reduce GHG emissions, decrease our carbon foot-print, increase security of energy supply and to steer our economies towards sustainable economic growth is stimulating the expansion of renewable energy resources, of which biofuels are part of the overall portfolio. The development of new, and improvement of existing, energy conversion technologies will enable effective and efficient conversion of biomass feedstocks to energy, and facilitate growth of dependency on biomass as a renewable source of energy. Given the changes to climate that are forecast over the next century, a key obstacle to optimal reliance on bioenergy crops will be the inevitable competition for land use between food and fuel, and it is imperative that bioenergy crops are capable of delivering sufficient yield under local climatic and soil conditions. Furthermore, the chemical composition of the biomass must be matched to the final energy conversion process. The precise goals for each species will, to some extent, be case-specific although in all cases there will be a drive to increase sustainable yield at low levels of inputs. In cases where the bioenergy crop originated in warmer wetter climates, e.g. Miscanthus, there will be a need to increase cold-, frost- and drought-tolerance (Oliver 2009) and develop varieties capable of germinating from seed. Due to the sterility of M. x giganteus, inclusion of these traits will most likely require extensive rebreeding. For switchgrass an important goal to facilitating widespread deployment will be to improve establishment time, which with current varieties may be 3 years or greater depending on the climate. It may also be possible to develop varieties capable of being grown on more marginal land for use as a secondary bioenergy crop, or for use as silage, hay or pasture. Changing biomass chemical composition may be achievable using conventional breeding technologies such as integration of traits from other Miscanthus accessions, or from related species, or may involve more indirect approaches such as mutation breeding or GM, perhaps by using either regulatory transgenes (Demura and Fukuda 2007) or by the stacking of multiple transgene interventions (Halpin and Boerjan 2003) to develop stable viable phenotypes with improved lignocellulose quality traits. After many years of reluctance, the EU seems to be gradually losing its objections to the cultivation of GM crops, and it might be envisaged that, given the non-food nature of these crops, this last avenue may soon be open to the development of commercial varieties once adequate proof has been amassed of trait and crop safety to humans, live-stock and the environment. An additional hazard will be that the very traits that increase yield, improve tolerance to environmental stress and allow these crops to be grown on ever more marginal land also have the effect of making energy crops potentially invasive weeds. In a recent study in Hawaii, Buddenhagen et al. (2009), using a widely accepted weed risk assessment system, reported 70% of regionally sustainable biofuel crops had a high risk of becoming potentially invasive weeds compared to only 25% of non-biofuel species. Assuming that these proportions are not the result of geographical location, these results should alert European plant breeders to be vigilant. These concerns aside, plant breeding objectives may include the initial development of low lignin–high lignocellulose varieties for combustion in existing power stations and later, as biomass specific power stations become more wide spread, high lignin–high lignocellulose varieties. Other improvements reached by breeding or GM routes might involve changing plant architecture, not only to improve light interception, but also to alter composition by increasing or decreasing grass internode length (to modulate lignin and particle size) or reduce stand height whilst increasing stand density in the case of coppice forestry to ease extraction of the mature crop from what is all too often water-logged sites. In addition, it is possible that society will once more regard itself as being dependent on the land and that work in the agro-energy sector will provide new employment opportunities. The future for energy crops in the UK and northern Europe is therefore at once challenging, dynamic and exciting. What is important is that we currently have options that are fit-for-purpose and have no excuse to prevent us moving towards meeting our GHG reduction targets in 2020 and beyond.