This is a perennial herbaceous plant that grows mainly in the United States. Its ethanol yield per hectare is the same as for wheat. It responds to nitrogen fertilizers and can sequester the carbon in the soil. It is a highly versatile plant, capable of adapting easily to lean soils and marginal farmland (Heaton et al., 2004). Like maize, it is a type C4 plant, i. e. it makes an alternative use of CO2 fixation (a process forming part of photosynthesis). Most of the genotypes of Panicum virgatum have short underground stems, or rhizomes, that enable them with time to form a grassy carpet. Single hybrids of Panicum virgatum have shown a marked potential for increasing their energy yield (Bouton, 2007), but genetic engineering methods on this plant are still in a developmental stage and for the time being only their tetraploid and octaploid forms are known; we also now know that similar cell types (isotypes) reproduce easily.

1.3 Sweet sorghum (sorghum bicolor L)

The grains obtained from this plant are rich in starch and the stems have a high saccharose content, while the leaves and bagasse have a high lignocellulose content. The plant can be grown in both temperate and tropical countries, and it tolerates drought, flooding and alkalinity. Sorghum is considered an excellent raw material because the methods for growing and transporting it are well established. Ethanol can be obtained from it by exploiting both its starch and its sugar content. Research is currently underway on the use of hybrid or genetically modified species, although those obtained so far are weaker and need to be further refined and tested as concerns energy conversion efficiency (Rooney et al., 2007).