Biofuels from crops are emerging as a Jekyll & Hyde -promoted by some as a means to offset fossil fuel emissions, denigrated by others as lacking sustainability and taking land from food crops. It is frequently asserted that plants convert only 0.1% of solar energy into biomass, therefore requiring unacceptable amounts of land for production of fuel feedstocks. The C 4 perennial grass Miscanthus  giganteus has proved a promising biomass crop in Europe, while switchgrass (Panicum virgatum) has been tested at several locations in N. America. Here, replicated side-by-side trials of these two crops were established for the first time along a latitudinal gradient in Illinois. Over 3 years of trials, Miscanthus  giganteus achieved average annual conversion efficiencies into harvestable biomass of 1.0% (30 t ha À1 ) and a maximum of 2.0% (61 t ha À1 ), with minimal agricultural inputs. The regionally adapted switchgrass variety Cave-in-Rock achieved somewhat lower yields, averaging 10 t ha À1 . Given that there has been little attempt to improve the agronomy and genetics of these grasses compared with the major grain crops, these efficiencies are the minimum of what may be achieved. At this 1.0% efficiency, 12 million hectares, or 9.3% of current US cropland, would be sufficient to provide 133  10 9 L of ethanol, enough to offset one-fifth of the current US gasoline use. In contrast, maize grain from the same area of land would only provide 49  10 9 L, while requiring much higher nitrogen and fossil energy inputs in its cultivation.

In the first side-by-side large-scale trials of these two C 4 crops in the U.S. Corn Belt, Miscanthus (Miscanthus 3 giganteus) was 59% more productive than grain maize (Zea mays). Total productivity is the product of the total solar radiation incident per unit land area and the efficiencies of light interception (« i ) and its conversion into aboveground biomass (« ca ). Averaged over two growing seasons, « ca did not differ, but « i was 61% higher for Miscanthus, which developed a leaf canopy earlier and maintained it later. The diurnal course of photosynthesis was measured on sunlit and shaded leaves of each species on 26 dates. The daily integral of leaf-level photosynthetic CO 2 uptake differed slightly when integrated across two growing seasons but was up to 60% higher in maize in mid-summer. The average leaf area of Miscanthus was double that of maize, with the result that calculated canopy photosynthesis was 44% higher in Miscanthus, corresponding closely to the biomass differences. To determine the basis of differences in mid-season leaf photosynthesis, light and CO 2 responses were analyzed to determine in vivo biochemical limitations. Maize had a higher maximum velocity of phosphoenolpyruvate carboxylation, velocity of phosphoenolpyruvate regeneration, light saturated rate of photosynthesis, and higher maximum quantum efficiency of CO 2 assimilation. These biochemical differences, however, were more than offset by the larger leaf area and its longer duration in Miscanthus. The results indicate that the full potential of C 4 photosynthetic productivity is not achieved by modern temperate maize cultivars.

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); PPFD ¢ , daily integral of PPFD; Rubisco, ribulose-1,5-bisphosphate carboxylase oxygenase; T leaf , leaf temperature ( ∞ C); TNC, total non-structural carbohydrate (mmol m -2 ); VPD, vapour pressure deficit (kPa).

Large areas of the tropics and subtropics are too arid or degraded to support food crops, but Agave species may be suitable for biofuel production in these regions. We review the potential of Agave species as biofuel feedstocks in the context of ecophysiology, agronomy, and land availability for this genus globally. Reported dry biomass yields of Agave spp., when annualized, range from o1 to 34Mg ha À1 yr À1 without irrigation, depending on species and location. Some of the most productive species have not yet been evaluated at a commercial scale. Approximately 0.6 Mha of land previously used to grow Agave for coarse fibers have fallen out of production, largely as a result of competition with synthetic fibers. Theoretically, this crop area alone could provide 6.1 billion L of ethanol if Agave were reestablished as a bioenergy feedstock without causing indirect land use change. Almost one-fifth of the global land surface is semiarid, suggesting there may be large opportunities for expansion of Agave crops for feedstock, but more field trials are needed to determine tolerance boundaries for different Agave species.