Chapter 19 C4 Species as Energy Crops

Abstract

The cultivation and utilisation of energy crops has the potential to provide, in the coming decades, part of the solution to the twin issues of substituting for fossil fuels and protection from damaging climate change by reducing carbon emissions. The ideal energy crop should have sustained capacity to capture and convert solar energy into harvestable biomass with maximal efficiency and with minimal inputs and environmental impacts. C 4 plants, and in particular rhizomatous perennial grasses (PRGs), have many of the characteristics of the ‘ideal’ energy crop. Herbaceous perennial species require far fewer energy and capital inputs than annual crops and they also sequester more carbon in the soil. C 4 photosynthesis also allows greater efficiencies in the conversion of solar energy to biomass energy, and of nitrogen and water use. Currently the most important feedstocks for biofuels are maize in the USA and sugarcane in Brazil, both C 4 species. In temperate climatic regions, where there is the greatest current demand for renewable energy, few naturally occurring species have C 4 photosynthesis. However, there are some notable exceptions, such as Miscanthus and switchgrass ( Panicum virgatum ), which show significant cold tolerance and are currently being developed as energy crops. The unusual features of the C 4 pathway in these species which appear to confer cold tolerance are reviewed. The recent drive to exploit the energy production and carbon emission mitigation potentials of C 4 energy crops has been controversial because of the anticipated competition for use of land for food or fuel. Despite this, the yield benefits provided by C 4 photosynthesis suggest that these species will make a significant contribution to bioenergy production over the near- and longer-terms.