Environmental Engineering Reference
In-Depth Information
8.4.3 Conserving Water
Succulents must be able to guard their water hoards in a desiccating environment and use
it as efficiently as possible. The stems and leaves of most species have waxy cuticles that
render them nearly waterproof when the stomates are closed. Water is further conserved
by reduced surface areas; most succulents have few leaves (agaves), no leaves (most cacti),
or leaves that are deciduous in dry seasons (elephant trees, boojums). The water is also
bound in extracellular mucilages and inulins that hold tightly onto the water.
Many succulents possess a water-efficient variant of photosynthesis called CAM. The
first word refers to the stonecrop family (Crassulaceae) in which the phenomenon was
first discovered.
Dudleya
is in this family, as are hen-and-chickens (
Sempervivum tectorum
)
and jade plant (
Crassula ovata
). CAM plants open their stomates for gas exchange at night
and store carbon dioxide in the form of an organic acid. During the day the stomates
are closed and the plants are nearly completely sealed against water loss; photosynthesis
is conducted using the stored carbon dioxide. At night the temperatures are lower and
humidity higher than during the day, so less water is lost through transpiration. Plants
using CAM lose about one-tenth as much water per unit of carbohydrate synthesized as
those using standard C3 photosynthesis. But there is a trade-off: The overall rate of photo-
synthesis is slower, so CAM plants grow more slowly than most C3 plants. (An additional
limitation is the reduced photosynthetic surface area of most succulents compared with
“ordinary” plants.)
The equilibrium between gaseous carbon dioxide and the organic acid is dependent
on temperature. Acid formation (carbon dioxide storage) is favored at cool temperatures;
higher temperatures stimulate release of carbon dioxide from the acid. Thus CAM works
most efficiently in climates that have a large daily temperature range (i.e., arid lands). Cool
nights allow much carbon dioxide to be stored as acid, and the warm days cause most of
the carbon dioxide to be released for photosynthesis. (A note of interest: A plant in CAM
mode will store enough acid to impart a sour taste in early morning; the flavor becomes
bland by afternoon when the acid is used up. But don't taste indiscriminately—many suc-
culents are poisonous!)
Many succulents possess CAM, as do semisucculents such as some yuccas,
epiphytic
(growing on trees or rocks) orchids and
xerophytic
(arid-adapted) bromeliads. Exceptions
are stem succulents with deciduous, non-succulent leaves, such as elephant trees,
limberbushes (
Jatropha
), and desert rose (
Adenium
). Succulents from hot, humid climates
that lack substantial daily temperature fluctuations also are usually not CAM. Some
succulents such as
Agave deserti
can switch from CAM to C3 photosynthesis when water
is abundant, allowing faster growth. Some 3.5% of all plant species spread among about
25 plant families use CAM.
Another crucial attribute of CAM plants is their idling metabolism during droughts.
When CAM plants become water-stressed, the stomates remain closed both day and night
and the fine (water-permeable) roots are sloughed off. The plant's stored water is essen-
tially sealed inside and gas exchange greatly decreases. However, a low level of respiration
(oxidation of carbohydrate into water, carbon dioxide, and energy) is carried out within the
still-moist tissues. The carbon dioxide released by respiration is recycled into the photo-
synthetic pathway to make more carbohydrate, and the oxygen released by photosynthesis
is recycled for respiration. Thus the plant never goes completely dormant but is metabo-
lizing slowly—idling. (This sounds like perpetual motion, but it isn't. The recycling isn't
100% efficient, so the plant will eventually exhaust its resources.) Just as an idling engine
can rev up to full speed more quickly than a cold one, an idling CAM plant can resume
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