Sisal/Agave - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 331

support alcoholic beverage and fiber industries. Archaeological findings and reports
of early Spanish explorers suggest that pre-Columbian communities in the south-
western USA and Mexico cultivated Agave on a large scale. This practice provided
not only beverages and fibers but also an insurance food crop, rich in carbohydrates,
during periods of drought when corn and bean crops failed [ 3 ]. The Agave genus has
been cultivated on four different continents, but the geographic range may be even
larger than previously realized because species of this genus are tolerant of a wide
range of temperatures, precipitation patterns, and soils.
Adapted to semiarid and arid climates, the Agave genus has the potential to grow
on lands that have been labeled marginal or unsuitable for other crops. Crops grown
on semiarid lands are often heavily irrigated and are thus likely to have a greater
environmental impact and lower profitability than crops grown in rain-fed agricul-
tural regions. These dryland agricultural systems, such as those in western Texas and
other states in the southwestern USA, are less stable agricultural production systems
that move in and out of production depending on climate trends and market condi-
tions for different products. A plant like Agave requires little or no irrigation and
would be less vulnerable to drought and extreme temperatures as well as monsoon
events than irrigation-dependent crops used in semidesert areas across the globe.
There are 208 known species of Agave , most of them native to Mexico [ 4 ], and
more research is needed to determine which of these have optimum traits for
bioenergy feedstock. More research is also needed to determine the geographic
range across which commercial plantations of Agave varieties would be viable.
Despite the novelty of this crop, there is enough potential as a successful bioenergy
feedstock that research focusing on Agave varieties has begun at sites that span the
globe. This chapter reviews the current state of knowledge and the traits that make
Agave a hopeful source of biomass for bioenergy.

Taxonomy, Domestication, and Breeding History

The genus Agave is classified in the monocotyledon family Agavaceae, but is
sometimes included in either the Liliaceae or Amaryllidaceae [ 4 - 6 ]. There are two
subgenera [ 7 ] and 200-300 species that are all native to the new world, with most
originating from M´xico. Ploidy levels vary, even within species, from 2 n to 8 n
( n

30) and hybridization has occurred frequently in wild cultivars [ 8 - 10 ]. Higher
chromosome numbers are often found in species with larger leaves and more dense
fiber tissue [ 8 , 10 ]. Recent molecular analysis of Agavaceae also shows that the genus
Agave is paraphyletic with respect to three other genera [ 4 ]. Little variation in
genome size of diploid varieties in cultivation has been observed [ 9 ].
All Agave species are xerophytes, but they range in size from a few cm to 4 m in
height [ 7 , 11 ]. The plants consist of a basal rosette of stiff, evergreen leaves that are
succulent and usually lanceolate in shape with a terminal spine. Most species have
leaves with spiny margins. All species in the genus are expected to use
Crassulacean Acid Metabolism (CAM) photosynthesis because all Agave spp.

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