Sugar, biomass and biofuel potential of temperate by tropical maize crosses

In order to shift society???s dependence away from petroleum the U.S. Department of Energy has set goals to replace 30% of petroleum based fuels with biofuel by the year 2030. This has produced an unprecedented interest in producing biofuels such as ethanol from plant biomass. A wide variety of potential biomass sources are currently under consideration, including grass species that employ the C4 pathway for photosynthesis, which include the perennial grasses, miscanthus (Miscanthus x giganteus) and switchgrass (Panicum virgatum L.), maize (Zea mays L.), sugarcane (Saccharum spp.), and sorghum (Sorghum bicolor L. Moench). These plants exhibit the highest potential for biofuel production because, depending upon the species, they can accumulate sugar, starch and/or lignocellulosic biomass. The perennial grasses are being considered for lignocellulosic fermentation, but this biofuel conversion process is not yet commercially viable. Current ethanol supplies are primarily produced via direct fermentation from the grain of maize in the U.S. and from sugarcane in Brazil. Similar to sugarcane, maize has the ability to store soluable sugars in the stalk, particularly when grain formation is impeded, either through mechanical ear removal, male sterility, or reproductive asynchrony. When tropically-adapted photoperiod sensitive maize varieties are grown in temperate climates, they produce tall plants that produce little to no grain because pollen sheds from the tassel before the silks emerge from the ears. As a result photoassimilates are retained in the stalk as sugar. The growth and use of maize for stalk sugar is believed to have been the impetus for its domestication, and has been reportedly used for this purpose by the Aztecs and colonial Americans, and has been the subject of modern academic evaluations throughout the current and last century. Like many traits in maize, genotypic variation for stalk sugar, as well as other desirable biofuel traits (i.e. biomass, nitrogen use efficiency, and drought tolerance) is clearly present in both temperate and tropical materials, which indicates the potential for improvement from genetic selection. When temperate and tropical maize are crossed, the progeny plants are better adapted to a temperate climate and are still tall, but exhibit minimal lodging, produce little grain and have the potential to accumulate large amounts of sugar in the stalk, similar to sugarcane. These traits indicate that the temperate x tropical maize (TTM) hybrids may potentially have high value as both as a sugar and lignocellulosic biofuel feedstock. In order to better determine the utility of TTM as a dedicated biofuel crop, we evaluated a series of TTM hybrids in 2008 and 2009 for their biological potential, genetic variability, the impact of nitrogen (N) on biomass and stalk sugar, and the subsequent biofuel potential. The TTM hybrids produced on average nearly 11 U.S. tons of biomass/acre, and when grain formation was prevented by ear shoot-bagging. TTM produced over 4,360 lbs/acre of sugar, which was three to four-fold greater than the non-ear shoot-bagged TTM and the ear removed hybrids. Taking into consideration the ethanol potential from sugar, stover, and grain, calculated estimates for ethanol production indicate TTM hybrids can yield the same amount of ethanol per acre as modern grain hybrids but with a lower requirement for supplemental fertilizer N. Temperate x tropical maize hybrids developed and selected for desirable biofuel attributes have the potential to be a sustainable biorefinery feedstock. This thesis is formatted in two chapters; the first provides a literature review pertaining to maize as a sugar and lignocellulosic biofuel feedstock. Elements of this were included in a book chapter review. The second chapter is a manuscript drafted in accordance with the guidelines set forth by the publication Global Change Biology Bioenergy, Wiley ??? Blackwell publishers