DEALING WITH ARIDITY: Conserving and Obtaining
Water
Annual (or ephemeral) plants escape drought conditions
by surviving as seeds. They take advantage of moisture during the
rainy season to quickly germinate, grow, and reproduce. Perennial species
of plants must survive dry conditions. Some species lose some or all of
their aboveground structures (and perhaps some of their roots) during extended
dry periods. The ocotillo (Fouquieria splendens), for example, will
shed its leaves during dry periods and replace them during wet periods,
perhaps more than once each year. To help conserve water, plants also have
thick waxy cuticles or resins covering the surface of their leaves or stems.
But plants cannot completely seal themselves off from the air around them.
Leaves of Ocotillo (Fouquieria splendens)
Photograph by Mark Eberle, March 2003
Most water leaves a plant through transpiration from openings
on their leaves called stomata. These openings through the epidermis also
permit carbon dioxide (CO2),
an essential molecule for photosynthesis, to enter the plant, and they
allow oxygen, a by-product of photosynthesis, to exit. Although the stomata
must be open for the plant to obtain CO2,
desert plants need to minimize the loss of water through open stomata.
This can be accomplished by hairs (trichomes) that grow on the surface
of some leaves and help to maintain a vapor layer that reduces the amount
of transpiration. Sunken stomata (stomatal crypts) also can reduce the
transpiration gradient. Leaf curling, which reduces the surface area of
the leaf, is a more drastic means of reducing transpiration. These adaptations
are advantageous to the survival of the plants; however, the cost in metabolic
energy is high. It also has been suggested that plants that do not protect
their stomata are better able to take in CO2
during times when stomata are open. For example, a stomatal crypt, rather
than reducing transpiration losses, might protect the guard cells from
dry winds that could cause them to close the stoma, thereby allowing the
stoma to remain open longer, replenishing CO2
levels inside the leaf.
DEALING WITH ARIDITY: Photosynthesis
Photosynthesis is a complex chemical process, and incorporated
within this complexity are adaptations that help desert and semidesert
plants to survive. Most plants use C3
photosynthesis, in which CO2
enters leaves through stomata, and mesophyll cells absorb this CO2
from the intercellular spaces. The enzyme Rubisco (ribose biphosphate carboxylase/oxygenase)
catalyzes the reaction that joins the CO2
with RuBP (ribulose biphosphate, a 5-carbon molecule). Through a series
of chemical reactions in the Calvin cycle, this 6-carbon molecule is split
and modified into 3-carbon molecules of PGAL (phosphoglyceraldehyde), which
are converted into larger carbohydrates, lipids, amino acids, or nucleotides).
In environments where water is limited, the stomata of plants remain closed
more of the time to conserve water that would otherwise leave the plant
through transpiration. However, this also prevents CO2
from entering the plant and prevents O2,
a by-product of photosynthesis, from leaving. Thus, while the stomata are
closed, the levels of CO2 inside
the plant decrease and the levels of O2
increase. This can lead to photorespiration. When the level of CO2
is low and the level of O2 is
high, Rubisco joins O2, rather
than CO2, with RuBP, but the
oxidation of RuBP in C3 plants
exposed to hot, dry conditions does not result in the production of organic
molecules.
Some plants in arid regions conserve water and avoid photorespiration
spatially
through a process called C4
photosynthesis. As with C3
photosynthesis, mesophyll cells absorb CO2
from intercellular spaces, but the CO2
is combined with the 3-carbon molecule of PEP (phosphoenolpyruvate), producing
a 4-carbon molecule (hence the name C4
photosynthesis). The 4-carbon molecules move from mesophyll cells into
adjacent bundle-sheath cells. Here, the 4-carbon molecule is split into
CO2 and a 3-carbon molecule,
which returns to the mesophyll cell and is converted back into PEP. This
concentrates the CO2 in the bundle-sheath
cells, where O2 concentrations
are relatively lower, so the CO2
is more likely to enter the Calvin cycle, thus limiting photorespiration.
The trade-off is in the additional energy cost of C4
photosynthesis compared to C3
photosynthesis.
Some succulents (e.g., some cacti and agaves) avoid photorespiration
temporally
through the process of crassulacean acid metabolism (CAM).
These plants open their stomata at night, when transpiration losses will
be lower, and store CO2 as malic
acid or isocitric acid. The succulents are able to dilute the acids in
water-storing vacuoles. During daylight hours, the acid molecules are split
to release CO2, establishing
a relatively high CO2 concentration
and a relatively low O2 concentration,
which limits photorespiration. The amount of CO2
that can be stored as acids is limited, which limits photosynthesis and
growth potential, but the process is facultative. When water is plentiful,
the plants open their stomata during day and assimilate CO2
through the process of C3 photosynthesis.
Leaves of Lechuguilla (Agave lechuguilla)
Photograph by Mark Eberle, March 2003
DEALING WITH HEAT
Perennial plants have several means of reducing leaf
heating. Small leaves or leaflets have a smaller, insulative boundary layer
of calm air, so they can be cooled more efficiently than large leaves at
a given wind speed. Increasing albedo with light-colored trichomes, spines
(e.g., cacti), or salt excretions causes more light to be reflected. Orienting
leaves to get maximum sunlight in the cooler morning or evening but less
direct sunlight during the heat of the day also reduces leaf heating. Ephemerals,
on the other hand, need to maximize their productivity during their short
period of growth, so they tend to have larger leaves oriented toward the
sun.
DEALING WITH HERBIVORES
In addition to protecting themselves from water loss
and heat, desert plants also must protect themselves from herbivores. Some
herbivory defenses include spines, bad tastes, pungent odors, nutrient-poor
tissues, and poisonous compounds. All of these efforts are designed to
discourage animals from consuming the energy-expensive structures and precious
water resources found in a desert plant.
Leaves of Sotol (Dasylirion)
Photograph by Mark Eberle, March 2003
REPRODUCTION
When reproduction from seed is unlikely, clonal reproduction
allows plants to survive or spread in an area. However, this does not provide
genetic diversity among the offspring as sexual reproduction through cross-pollination
of flowers does. Thus, desert annuals and most perennials use a variety
of methods to promote successful reproduction from seeds.
Some species of desert plants produce large numbers of
flowers, which increases the likelihood that they will attract animal pollinators.
The often low density of plants of the same species in deserts reduces
the likelihood that wind-blown pollen would reach the flower of another
individual of the same species, so bees, beetles, butterflies, moths, hummingbirds,
and bats are among the animal pollinators in southwestern deserts. Each
species of desert plant typically attracts a relatively broad range animal
pollinators in a certain group (e.g., bees, moths) rather than a single
species (e.g., yucca and yucca moth). Attracting a small group of animal
pollinators reduces the likelihood that the animal pollinator will deposit
pollen on the wrong species, but having only 1 pollinator species means
that the flowers and the particular species of animal pollinator must both
be present at the same time.
Some desert plants produce large numbers of seeds to increase
the probability that some will not be consumed. Some seeds are carried
by the wind, some stick to animals, some are carried underground, and some
are deposited with animal feces. Seed dormancy seems to be particularly
beneficial to plants that germinate during the brief, unpredictable conditions
in desert ecosystems. In some cases, seed dormancy can be broken when the
seeds are exposed to certain levels of moisture, warmth, or oxygen. For
example, water can leach chemical inhibitors from the seed. For the seeds
of some species, the seed coat must be scarified by sand, fungus, or some
other agent to break dormancy. The length of seed dormancy might vary in
a seed crop of a particular species to stagger germination and reduce the
possibility of catastrophic loss of a population. In contrast, seed dormancy
could be detrimental if predation is high.
Several species of desert plants (e.g., saguaro, Christmas
tree cholla) develop under nursery plants (e.g., palo verde, creosote bush),
where the shrub provides shade, reduces herbivory, and protects the young
plant from cold temperatures. Eventually, the plant will replace its nurse
plant, perhaps through the more efficient use of water or some other resource.
