One of the most overlooked issues in the selection of sustainable biofuel feedstocks is the issue of "water use" or "water footprint". While the net energy yields and "carbon footprints" (i.e., lower carbon dioxide emissions) of feedstocks may be good, the water consumption during cultivation of the bioenergy crop might be high. Furthermore, some feedstocks may need to be planted in areas with high frequency of rainfall. Bioenergy crops with low water footprints, and which can be cultivated in semiarid areas with little rainfall has received recent attention. A journal article by Sarah Davis and co-authors from the University of Illinois (United States) reports the global bioenergy crop potential of a plant belonging to the genus, Agave. According to the article, Agave plants have low water requirements because they utilize what is known as the "Crassulacean Acid Metabolism (CAM) pathway". Under this type of metabolism, carbon is assimilated at night and thus decreases the diffusive gradient of water out of the leaves. By assimilating carbon at night, potential evapotranspiration of water is reduced, and water loss per amount of CO₂ assimilated is reduced. The article also mentions that "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." The full report is published in the journal, Global Change Biology: Bioenergy (URL above)