One of the important characteristics of some of the perennial grasses is the possession of C4 metabolism rather than C3. C4 and C3 metabolism refer to the pathways used

to assimilate carbon dioxide during photosynthesis. C4 plants are more efficient at higher light and temperatures compared to C3 plants. The C4 plants have a lower moisture content, require less fertilizer input and are twice as efficient with water. C4 assimilation of carbon is theoretically 350 kg/ha/day compared with 200 kg/ha/day for C3 plants (Venturi and Venturi, 2003). All these features make C4 plants more suitable for biomass fuel planting than C3 plants.

The development of the C4 metabolism of carbon dioxide assimilation evolved from the Calvin cycle in C3 plants to avoid the loss of carbon dioxide through photorespiration. The fixation of carbon dioxide during photosynthesis takes place in three stages. The addition (carboxylation) of carbon dioxide to ribulose-1,5-bisphosphate (Fig. 4.3) is fol­lowed by the reduction of 3-phosphoglycerate to glyceraldehyde-3-phosphate. Ribulose-1, 5-bisphosphate is then regenerated from glyceraldehyde-3-phosphate. The first step of the Calvin cycle is catalysed by the chloroplast enzyme ribulose bisphosphate carboxylase/ oxygenase known as rubisco. However, another property of the rubisco enzyme is to catalyse the oxygenation of ribulose-1,5-bisphosphate which is the start of light-dependant oxygen uptake and carbon dioxide release, known as photorespiration, which reduces plant yield.

In the C4 metabolism two different types of cell are involved in photosynthesis, the mesophyll and bundle sheath cells (Fig. 4.4). In the mesophyll cells carbon dioxide is used to carboxylate phosphoenolpyruvate (PEP) forming oxaloacetate. The oxalo — acetate is converted to malate (C4) and this is transferred to the bundle sheath cell where the malate is converted into pyruvate and carbon dioxide. The carbon dioxide is then used in the Calvin cycle. The C4 cycle has a higher energy demand but the cycle reduces photorespiration and water loss. The phosphoenolpyruvate (PEP) carboxylase enzyme has a high affinity for the carboxyl ion such that it is saturated and in equilibrium with carbon dioxide gas. Oxygen is not a competitor in the PEP

carboxylase reaction because the substrate is a carboxyl ion. The high activity of the PEP carboxylase allows the plants to reduce the stomatal opening, reducing water loss, while fixing carbon dioxide at an undiminished rate. The high concentration of carbon dioxide in the bundle sheath cells allows the cells to carry out photosynthesis at high temperatures.

Animal and municipal waste

Animal wastes consist of excess slurry and dung from cattle, chickens and pigs. These wastes can be used to generate biogas through anaerobic digestion and there are a number of farm-sized units available. There are also a number of small electricity power stations which run on chicken slurry from large battery chicken farms.