Desert Green Plants Manufacture their Food
The Desert Food Chain
by Jay Sharp
Q: How do green plants “manufacture their own food?”
A: Through the complex process called “photosynthesis,” a term that means, literally, “gathering of light.”
How It Works
In photosynthesis, a plant’s green leaves, fueled by sunlight a form of solar energy use water and carbon dioxide as raw materials to manufacture food (called “carbohydrates”) and oxygen. You can, in fact, think of the leaf of a plant such as a tree or a shrub as a miniature factory, which, like the real thing, uses fuel, raw materials and a manufacturing process to fabricate products.
A leaf captures and employs solar energy by utilizing a green photoreceptive pigment (a light-receiving chemical) called “chlorophyll,” which is embedded in light-gathering cells called “chloroplasts”in effect, microscopic solar collectors.
The leaf takes water from moisture that has been absorbed from the soil by the plant’s roots and delivered by vein-like structures. It takes carbon dioxide from the atmosphere, “inhaling” the gas through “stomata,” which are tiny specialized structures that compare with pores in your skin.
The leaf uses its solar energy to trigger and fuel its manufacturing process, a two-stage, highly complex chemical reaction that converts the water and carbon dioxide into carbohydrates and oxygen.
When it has completed the manufacture, the leaf ships the carbohydrates through vein-like structures for delivery to its parent plant, which uses some of the food immediately in producing new leaves, stems, roots, flowers and fruits. The leaf’s parent also saves some of the food, in the form of “potential energy,” which it will use as necessary to survive the stresses of a desert drought or a hard winter and to answer the awakening call of spring. The plant, as a “producer,” may also furnish carbohydrates to plant-eating animals, “consumers.” Plants serve as the first link and the foundation for food chains.
Meanwhile, the leaf expels the oxygen a byproduct of photosynthesis through the stomata into the atmosphere. It becomes a part of the air that animals breathe.
In an evaporative process called “transpiration,” the leaf also discharges water vapor through its stomata, a process that can be wasteful, possibly to the plant’s determent in the desert. Some plants, for instance, a large cottonwood tree along a drainage, may lose as much as 100 gallons of its water supply per hour to transpiration on a hot day in the desert, according to M. J. Farabee’s Internet article “Photosynthesis.”
As you might anticipate, our miniature factory, the leaf, functions most efficiently on clear warm days, when the sun’s light falls most intensely on its surface and absorbed water moves most freely through its tissues. Even on clear, warm days, however, the leaf may falter in the enterprise of photosynthesis should its water supply fail, as it often does in the desert. Its parent plant must then restrict the production of carbohydrates and oxygen. Some plants, for instance, the ocotillo, which grows in our Chihuahuan and Sonoran Deserts, sheds its leaves as a means of saving water used in photosynthesis or wasted through transpiration.
When it has adequate moisture, our leaf proceeds with the process of photosynthesis throughout the daylight hours. With the fall of darkness, it shuts down its solar collectors. It closes its stomata. It winds down the manufacturing process. It rests during the night.
In the desert , where moisture can be so scarce, some plants have adapted a water-saving modification to the process of photosynthesis.
These plants include, primarily, the succulents, for instance, agaves such as the Century Plant, which grows widely across our Southwestern deserts, or the yuccas such as the Joshua Tree, which grows in the Mojave Desert. The succulents are distinguished by waxy green leaves or stems that have tissues designed for storing water.
Unlike trees, which have leaves that open their stomata during the heat of desert days and close them during the cool of the night, the agaves and yuccas have leaves that close their stomata during the day and open them at night. The trees’ leaves take in carbon dioxide and expel oxygen and, wastefully, their water vapor during the hot day, as they carry out the process of photosynthesis. The thrifty agaves and yuccas, by contrast, take in and chemically store in an organic acid carbon dioxide and expel oxygen and water vapor at night. With dawn, when the sunlight becomes available as fuel, they close their stomata to minimize evaporation, release their carbon dioxide and proceed with photosynthesis.
Another type of succulent, the cacti, including for instance, various prickly pear, cholla and barrell species, which also grow widely across our deserts, has developed another water-saving approach to the process of photosynthesis. It is not their leaves but, rather, their green stems, that come equipped with stomata, which open at night and close in the day. They then follow the same course in photosynthesis and respiration as the agaves and yuccas. The cacti leaves are modified to serve as spines, which shield the plants from the desert sun and protect them from foraging animals. The leaves, or spines, have no stomata.
While the succulents have developed a means to conserve their water, they carry out photosynthesis with less efficiency than other plants because they must store and release their carbon dioxide, an extra and energy-costly step in the process. That means that the succulents may grow more slowly than other plants.
During periods of intense drought, the succulents may keep their stomata closed day and night to limit water loss, shutting down the process of photosynthesis. During such times, scientists say the succulent is “idling.” When moisture does become available, the succulents can open their stomata quickly, resuming the photosynthesis once again.
From Green to Gold
The plants of our Southwestern deserts, with their mild climates, fail to dress for autumn. You will find little color in the foliage of the region’s rocky and sandy basins. In fact, many desert plants, for example the Creosote Bush or the cacti, that live for more than one season simply retain their chlorophyll and therefore the green color in their leaves or stems through the fall and winter, although they do throttle back on the process of photosynthesis during the shorter and cooler days. Other plants may simply shed their leaves for the seasons, waiting for spring and new leaves before they resume the business of photosynthesis. Plants that live for only a season wither and die after a burst of photosynthesis, leaving behind an abundant seed bank but little color.
However, if you travel up into the mountain ranges overlooking our deserts during the fall, you will discover that aspen, maple and oak trees turn into a kaleidoscope of colors. This happens because the mountain plants, sensitive to the shortening of the days and the coming of winter snows, shut down the process of photosynthesis for the season. The green chlorophyll breaks down. Its color fades. Other leaf pigments, yellow and orange, emerge. Simultaneously, other chemical changes within the fall foliage yield still more colors, giving us the full splendor of autumn.
Plants and Animals
Plants and animals have an inseparable relationship, which is cemented by the process of photosynthesisperhaps the most important of all the biological processes. Plants produce carbohydrates. Animals consume carbohydrates. Plants take in carbon dioxide and expel oxygen. Animals inhale the oxygen and exhale the carbon dioxide. Plants and animals of the food chains depend on each other for life.
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