carbohydrate reserves of CAM plants would require
less energy for conversion to fuels and may therefore
result in a higher quality feedstock (Smith, 2008; Bor-
land
et al
., 2009).
The
Agave
genus is composed exclusively of obligate
CAM plants and includes species that are grown com-
mercially. Importantly, they thrive in conditions that are
unsuited to major food crops and pasture grasses. The
most widespread commercial uses of
Agave
spp. are for
fibers and beverages, derived usually from the leaves
and from the stem, respectively. At the peak of produc-
tion in 1964, over 1Mha of
Agave
were cultivated
globally for sisal fibers (FAO, 2010), the majority of
which were in Africa (Lock, 1969; FAO, 2010). Since
then, the widespread production of synthetic fibers
caused a decline in sisal production (Nobel, 1994) and
there were
o
0.5Mha of sisal planted by 2008 (FAO,
2010). In the 1990s,
70000ha of
Agave
, predominantly
the species
A. tequilana
, were cultivated in Me
́
xico for
the production of alcoholic beverages, along with
200000ha of
A. fourcroydes
, or henequen (sometimes
called sisal), grown for fiber production (Nobel, 1994).
Agave sisalana
(sisal) is also native to Me
́
xico, but is now
primarily grown in Brazil and Eastern Africa for fiber.
The potential of CAM plants for biofuel feedstocks
has been recently reviewed (Borland
et al
.,. 2009), yet
there is very little information about biofuel production
from CAM plants at a commercial scale. There is also no
systematic record of field-scale dry matter productivity,
which is crucial information for establishing the eco-
nomic and logistic viability of a biofuel enterprise.
Agave
species cultivated for fiber have quite different
production systems from those cultivated for bev-
erages, but both are examples of commercially scaled
agriculture that would be relevant to a bioenergy pro-
duction chain. This paper will synthesize information
from existing literature to evaluate the potential of
Agave
production systems for near-term sustainable
biofuel feedstocks. We review the form, diversity, and
ecology of
Agave
, the agronomy of the major crop
species, and finally the land availability for biofuel
production from this group of plants.
Review of
Agave
form, diversity and ecology
Agave
is a genus of some 200–300 species, within the
family Agavaceae, although it is sometimes placed in
the Liliaceae or Amarylidaceae (Purseglove, 1972;
Colunga-Garcı
́
a Marı
́
n
et al
., 2007). Gentry (1982) recog-
nizes two subgenera,
Agave
and
Littaea
, in North Amer-
ica. The natural distribution of the
Agave
genus is
limited to the Americas, with the greatest diversity of
species in Me
́
xico, although many species have been
distributed across mediterranean and dry tropical
regions for agricultural, hedgerow and horticultural
uses (Purseglove, 1972). All species of the genus that
have been analyzed use CAM and it is assumed that the
genus as a whole uses CAM physiology (Szarek & Ting,
1977; Szarek, 1979).
Agave
plants are perennial evergreen xerophytes,
ranging in size from several centimeters to 4m and
producing large flowering stalks 2–12m tall after 5–15
years (Gentry, 1982; Valenzuela-Zapata & Nablan,
2004). They have a basal rosette of large stiff, lanceolate,
succulent and persistent leaves, often with a terminal
spine, and sometimes with spiny margins. In transverse
cross-section, the leaves are crescent shaped. The epi-
dermis is highly cuticularized and the stomata are
deeply sunken at the base of hypostomatal cavities.
The mesophyll includes elongated water storage cells
as well as idioblasts, which contain calcium oxalate
crystals (Blunden
et al
., 1973). Stems are short and thick,
or basal with leaves formed around the terminal mer-
istem that they encircle. Shoots are typically monocar-
pic, i.e. they die after flowering, however side shoots
may allow the plant to persist. Flowers are usually
formed on a massive spike, sometimes termed a pole,
and have a paniculate inflorescence (Gentry, 1982).
Many
Agave
cultivars do not flower or are sterile, and
propagation is either by suckers or from bulbils pro-
duced on the flowering stem (Purseglove, 1972).
Agave
plants typically form a spreading fibrous root
system that arises adventitiously from the stem base
and, although shallow, may spread some distance. For
example, in the case of
A. sisalana
, anchor roots of
2–4mm in diameter may spread up to 5m from the
stem base, but vertically remain within the top 40cm of
soil. These roots become suberized, but produce smaller
feeder roots along their length, that can bare numerous
root hairs (Purseglove, 1972).
Agave
spp. often grow on rocky soils of poor quality
in regions with extreme drought and elevated tempera-
tures. Optimum growth can be achieved with high
annual rainfall amounts of 102–127cm, but high pro-
duction has also been observed in
Agave
spp. that grow
in regions with only 25–38cm of annual rainfall (Kirby,
1963). When considering drought, it is important to note
that total precipitation is not the only determinant. The
pattern of precipitation, i.e. a few high intensity events
vs. an even distribution of smaller events will have a
profound effect, the former being most typical of the
semiarid tropics and subtropics. Water vapor pressure
deficit, radiation and windspeed are the determinants
of potential evapotranspiration, and all three can be
high in the semiarid tropics and subtropics. Soil poro-
sity and topography determine drainage and can also
cause frequent droughts in otherwise high precipita-
tion zones. For example, the major growing region of
AGAVE
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henequen (
A. fourcroydes
) in the Yucata
́
n, Me
́
xico, is
characterized by shallow calcareous gravel soils over-
laying porous limestone. Even though precipitation
reaches 760mmyr
1
, the land is unsuitable for other
crops (Purseglove 1972; Colunga-Garcı
́
a Marı
́
n
et al
.,
2007).
Because droughts are frequent in semi-arid regions,
viable perennial crops must not only be efficient in their
use of water, but also capable of surviving without any
accessible water between rainfall events.
Agave
spp.
achieve this not by tolerance to low water potential,
but by hydraulic isolation. During a dry-down event the
roots shrink, leaving an air gap between the soil and
root surface. This prevents dehydration due to water
moving from the plant to soil as the soil water potential
declines. In parallel, the thick cuticle and closed sunken
stomata prevent water loss to the atmosphere and
maintain high plant water potential. Even though roots
of
Agave
are susceptible to cavitation, the high stem
water potential limits the occurrence of cavitation dur-
ing prolonged droughts (Linton & Nobel, 1999).
From 14 different studies that report annual
Agave
productivities, it is clear that eco-physiological
responses of species to different environments will
affect the success of new commercial plantations. There
are substantial differences in the reported productivities
of
Agave
spp. (Fig. 1), but these differences are con-
founded by regional variation (Fig. 2). Climate is likely
to be a major determinant of yields within and among
species. For example,
A. lechuguilla
produces only 3.8t
ha
1
yr
1
but occurs in areas with moderate annual
rainfall (427mm) while
A. mapisaga
produces 32tha
1
yr
1
in regions with 848mm of annual rainfall (Fig. 3).
Consider as another example that the yield of
A. salmi-
ana
ranges from a mean of 10tha
1
in an arid but
relatively cool region to 34tha
1
in a semiarid region
with moderately warm daytime temperatures (Fig. 3).
Much of the data on dry matter yield is based on
extrapolation from measurements on single plants,
rather than yields measured over large areas, which
adds uncertainty to the true yields that might be ob-
tained in large-scale plantings.
The relationship between nighttime temperature and
carbon assimilation has been described for
A. tequilana
(Luiz Corral, 2007), but is not necessarily consistent for
the majority of other
Agave
species. Using minimum
temperature data for the four locations shown in Fig. 3,
and assuming 8h of active CO
2
assimilation daily, the
range in estimated CO
2
uptake is less than actual
observations. We estimated that the theoretical CO
2
assimilation of
A. tequilana
only ranges from 39 to
42MgCO
2
yr
1
(or about 21–23tbiomassha
1
yr
1
)at
sites with mean nighttime temperatures that range from
9.6 to 20.6
1
C. The productivity of other
Agave
species
are far outside of this range, suggesting that the phy-
siological assumptions for
A. tequilana
, at least in
response to nighttime temperatures, may not be directly
transferable to other
Agave
spp.
Biodiversity of
Agave
, at the species and genetic
levels, has been affected by land management choices
in the last century. There has been steady selection
pressure on the genetic diversity of
Agave
species that
are grown for tequila and fiber (Vargas-Ponce
et al
.,
2009). There has also been a decline in the husbandry
practices that historically promoted populations of
many species for food, fiber, and forage (Valenzuela-
Zapata & Nablan, 2004).
A. fourcroydes
and
A. sisalana
were selected for leaf length to provide long fibers and
Fig. 1
Productivity of nine cultivated
Agave
species under
nonirrigated conditions (Clary & Jameson 1981; Nobel 1984,
1985, 1990, 1991, Nobel & Meyer 1985; Nobel & Hartsock, 1986;
Nobel & Quero, 1986; Nobel & Valenzuela-Zapata, 1987; Nobel
et
al
., 1992; Idso & Kimball, 1995). Bars represent means
SE.
Fig. 2
Productivity of various cultivated
Agave
species under
nonirrigated conditions from six locations. Bars represent means
SE. (Clary & Jameson, 1981; Nobel 1984, 1985, 1990, 1991,
Nobel & Meyer 1985; Nobel & Hartsock, 1986; Nobel & Quero,
1986; Nobel & Valenzuela-Zapata, 1987; Nobel
et al
., 1992; Idso &
Kimball, 1995)
70
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A. tequilana
was selected for the high sugar content of
the pin
̃
as (Fig. 4). Pin
̃
as are the swollen and nonstruc-
tural carbohydrate rich stem bases that are harvested as
the fermentation feedstock for the production of tequila
(Fig. 4). There are other varieties of
A. tequilana
that may
produce greater biomass, but less sugar, that could be a
better choice of feedstock for cellulosic fuels. Published
productivities of species vary spatially, and there is no
one species that appears to have the greatest productiv-
ity across all regions of Me
́
xico or elsewhere. The
Agave
Temperature (°c)
Precipitation (mm)
Month
40
(a)
(b)
(c)
(d)
35
30
25
20
15
10
5
0
450
400
350
300
250
200
150
100
50
0
Jan
Fab
mar
Apr
May
Jun
Jul
Aug
Sep
oct
Nov
Dec
Temperature (°c)
Precipitation (mm)
Month
40
35
30
25
20
15
10
5
0
450
400
350
300
250
200
150
100
50
0
Jan
Fab
mar
Apr
May
Jun
Jul
Aug
Sep
oct
Nov
Dec
Temperature (°c)
Precipitation (mm)
Month
40
35
30
25
20
15
10
5
0
450
400
350
300
250
200
150
100
50
0
Jan
Fab
mar
Apr
May
Jun
Jul
Aug
Sep
oct
Nov
Dec
Temperature (°c)
Precipitation (mm)
Month
40
35
30
25
20
15
10
5
0
450
400
350
300
250
200
150
100
50
0
Jan
Fab
mar
Apr
May
Jun
Jul
Aug
Sep
oct
Nov
Dec
Fig. 3
Climatograms of four locations in Mexico identified in the map legend (upper left) where the biomass of five species (shown in
table at lower left) has been measured: (a) Mazatlan, Sin. (b) Merida, Yuc. (c) Morelia, Mich. (d) Tacubaya, D.F. (Nobel 1985; 1990; 1991;
Nobel & Hartsock, 1986; Nobel & Valenzuela-Zapata, 1987; Nobel
et al
., 1992)
Fig. 4
Commercial scale
Agave
production:
A. tequilana
photo courtesy of Alejandro Vela
́
zquez Loera (a), pin
̃
as from
A. tequilana
(b),
A.
fourcroydes
photo courtesy of Abdo Magdub-Me
́
ndez (b), fiber from the leaves of
A. fourcroydes
photo courtesy of Abdo Magdub-Me
́
ndez (d).
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