The greenhouse gas (GHG) emissions that result from burning fossil fuels are a major contributor to climate change. Energy usage is increasing globally and alternative forms of energy that are renewable and help reduce GHG emissions are urgently needed. There are many possible alternatives to fossil fuels, particularly for heat and power generation, e.g. wind, hydro, solar, as well as plant biomass, all of which are expected to play a role. However, there are few alternatives to replace transport fuels (e.g. battery-powered, hydrogen fuel cells) and the number of vehicles is increasing rapidly worldwide.
Plants are “biological solar panels”. Through photosynthesis, plants capture sunlight energy and use it to convert carbon molecules from atmospheric carbon dioxide to form carbohydrate. Plants use energy from carbohydrates for growth and the production of new dry matter (biomass). They also store carbohydrates in different forms as reserves. The carbon in plants can be used as source of heat/electricity (bioenergy) or transport fuels (biofuels).
Liquid transport biofuels can be produced from plant carbohydrates by biological conversion processes such as fermentation. These enzymatic processes operate best when the carbohydrates are in simple forms, such as sucrose and starch, as these are easily broken down by enzymes.
In the UK, bioethanol is currently produced from sugar and starchy food crops such as sugar beet and wheat, which store sugars and starches. This is sometimes called first generation biofuel as it relies on technologies that are already well developed. However, growing food crops requires high inputs of nutrients particularly nitrogen (N) fertilisers. As N fertilisers require fossil fuels to make this can lead to little overall energy saving and reduction in GHG emissions. Producing biofuels from the food parts of arable crops can also conflict with food production.
Perennial biomass crops, such as willows and the grass Miscanthus, are fast growing crops which can produce biomass with little N fertiliser. Biofuels from these crops could give high energy savings and GHG reductions. However, the carbon is not stored in an easy form. Instead, it is in the form of lignocellulose which makes up the plant cell wall. Complex linkages make it difficult for enzymes to access the carbon in this form.
In BSBEC-BioMASS we will bring together leading experts in plant biology, crop breeding, genomics, biochemistry, biomathematics and bioenergy to underpin the improvements needed in willows and Miscanthus to develop biofuels from lignocellulose. Wastes from food crops, such as straw, can also be used as a source of lignocellulose but this is the research focus of another BSBEC research hub.
Willows and Miscanthus are perennial crops which recycle nutrients and carbon during the growth cycle. Re-growth in the spring initially utilises the reserves laid down in the previous year after the growing season has ended. As the leaf canopy develops, the use of reserves is replaced by new photosynthate. Finally, growth ceases towards the end of summer but photosynthesis is still active and the photosynthate is now directed to the reserves. In BSBEC-BioMASS, we will focus on the processes which affect how well willows and Miscanthus are able to capture sunlight energy and convert carbon dioxide (and water) into carbohydrate. We will investigate how different growth forms could be bred to improve light capature. WE will also investigate how the carbon is assimilated and partitioned into different parts of the plant (stem, roots , leaves) during the perennial cycle and how the overall biomass composition and, more specifically, the structure and composition of the cell walls, influences the ease with which the biomass can be processed and converted to biofuel.
Willows
Willows are trees and shrubs of the genus Salix. They undertake the C3 photosynthetic pathway. They are extremely diverse, comprising circa 400 species, and vary from tall trees, to bushy shrubs and slow-growing alpine and arctic plants. Members of the Salix subgenus Vetrix (the sallows and osier shrubs) have been grown for baskets because they have flexible and coloured stems and because of their ease of cultivation. Willows from the same sub-genus have also become popular as biomass crops in temperate zones because they can be grown in short rotation coppice (SRC) cycles and produce high yields with low fertiliser inputs. Willows are vegetatively propagated. In SRC they are first planted as stem cuttings, 20 cm long and of approximately one cm in diameter. After the first years’ growth, the stems are cut back to elicit the coppice response which is characterised by the vigorous growth of multiple new stems in the spring. The SRC willow is then allowed to grow for a further three years before being harvested, after which it resprouts and an new SRC cycle begins. Willow is harvested at three year intervals in these cycles. The stems are harvested in winter, using specialised machinery, after all the leaves have dropped and nutrients have been recycled to the stem and stool. Current commercial willow plantations maintain viability for between 20-25 years and most provide 10-14 tDMha-1yr-1. Willows have a basis chromosome number of n=19. Ploidy levels up to dodecaploid are known but many biomass willows are diploid. Rothamsted has a breeding programme for willow which is funded by Defra.
Miscanthus
Miscanthus species are rhizomatous grasses that originate from Asian. There are several species but the diploid M. sinensis, the tetraploid M. sacchariflorus and particularly their triploid hybrid, M. x giganteus, have become of great interest as biomass crops due to their ability to produce high biomass yields with minimal nitrogen fertilisers. Although Miscanthus undergoes C4 photosynthesis, it is less sensitive to low temperatures than, for example, maize and is thus able to grow and yield well in large areas of Europe, including southern UK. As M. x giganteus is sterile, it can only be planted as rhizomes. Miscanthus is harvested annually, after winter. The crop is left standing during winter so that the nutrients can be recycled from the leaves to the rhizome. Once planted it takes circa three-four years to reach its maximum biomass potential. Yields appear to be maintained over long successive harvesting (trials up to 18 years old are still yielding well). It is harvested using standard machinery. The basic chromosome number of Miscanthus is n-19. IBERs has a breeding programme for Miscanthus, which is funded by Defra.