Sisal/Agave - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 332

analyzed to date use CAM [ 12 - 14 ]. CAM plants are unique in opening their
stomata at night to assimilate CO 2 into organic acids, primarily malic acid. During
the day, the CO 2 is released, by decarboxylation of the organic acids, into the
internal leaf air space. It is then assimilated into carbohydrate via the C 3 photosyn-
thetic pathway while allowing the stomata to remain closed [ 15 ]. Evaporative
demand is much lower at night in desert and semidesert environments, where
nighttime temperatures may be 20-30 C below daytime temperatures. In these
conditions, CAM plants can use a small fraction (0.05-0.1) of the water used by a
non-CAM plant for each CO 2 that is assimilated [ 16 ]. Water use efficiencies can
therefore be 10-20 times higher than for C 3 plants growing in the same environ-
ment. Although there is an additional energy cost in assimilating CO 2 first into an
organic acid, this is of little consequence in an environment where water is scarce,
but sunlight is plentiful. However, when grown in moist environments, CAM plants
will be at a theoretical disadvantage.
As perennial xerophytes, Agave spp. are adapted to survive in hot dry conditions.
The leaves have a waxy epidermis, sunken stomata, and large water storage cells in
the mesophyll [ 17 ]. The roots are retractile and shrink in response to low soil water
potential, leaving an air space between the soil and root surfaces. This, coupled with
the thick waxy cuticle covering the shoot, isolates the plant hydraulically from the
dry air and dry soil, allowing it to maintain a high water content through long
periods of drought.
There is a long history of Agave cultivation in Mexico and the southwestern US
desert, where the genus has been used for fiber production, sweet nonalcoholic
beverages, low-alcohol fermented beverages, distillation into tequila and mescal,
and as a food source, typically by baking the stem bases [ 3 , 18 ]. In their native
range, Agave plants hybridized naturally with the assistance of pollinators that
range from insects to bats. In cultivation, humans selected and cultivated varieties
for two separate traits: high sugar and long fibrous leaves. These varieties serve two
separate industries with the high sugar varieties supporting fermentation to alcohol
and the varieties with long leaves providing fiber for rope making and textiles [ 5 ].
Areas of Production
The Agave production system that supports the tequila industry parallels the fuel
ethanol production systems from corn in the USA and sugarcane in Brazil. It
similarly relies on the fermentation of sugars, and polysaccharides (fructans) that
are easily degraded to sugars (fructose), that are concentrated in the stem base [ 19 ].
Agave tequilana, like most Agave spp., is monocarpic [ 4 ], i.e., after producing
leaves for a period of time, the stem apex becomes reproductive and on the
completion of flowering and fruit formation, no further growth occurs and the
entire stem dies. During the vegetative stage, storage carbohydrates accumulate
in the stem base which, in the case of A. tequilana , swells into a large spherical
organ, termed a pi˜a (composition summarized in Table 15.1 ). When the flowering