By Ramish Sajid and Zahoor Ahmad : Impact of desert on plants

Deserts cover more than one fifth of the Earth’s land, and they are found on every continent. Desert is defined as a place that receives less than 10 inches (25 centimeters) of rain per year is considered a desert area. Desert plants may have to go without fresh water for years at a time. Some plants have adapted to the arid climate by growing long roots that tap water from deep underground. Other plants, such as cacti, have special means of storing and conserving water. Many desert plants can live to be hundreds of years old. Desert plants tend to look very different from plants native to other regions. They are often swollen, spiny and have tiny leaves that are rarely bright green. Their strange appearance is a result of their remarkable adaptations to the challenges of the desert climate. Aridity is the sole factor that defines a desert.

Drought tolerance (or drought dormancy) refers to a plant’s ability to withstand desiccation without dying. Plants in this category often shed their leaves during dry periods and enter a deep dormancy. Most water loss is from transpiration through leaf surfaces, so dropping leaves conserves water in the stems. Some plants that do not normally shed their leaves have resinous coatings that retard water loss (e.g., creosote bush). The roots of drought tolerant shrubs and trees are extensive compared to those of plants in wetter climates, covering an area up to twice the diameter of the canopy. They exploit the soil at greater depth than the roots of succulents; sometimes they extend to extreme depths (e.g., mesquite). Most of a mesquite’s roots, however, are within three feet (0.9 m) of the surface.

Annual plants escape unfavorable conditions by not existing. They mature in a single season, then die after channeling all of their life energy into producing seeds instead of reserving some for continued survival. Seedlings rapidly produce rosettes of leaves during the mild fall weather, remain flat against the ground as they grow more slowly through the winter, and bolt into flower in the spring. Since the plants are inconspicuous until they begin the spring bolt, many people mistakenly think that spring rains produce our wildflower displays. Desert environment create a negative impact on the plants by altering its morphological and physiological structure and adapting them to survive under harsh conditions. Certain adaptations are developed to acclimatize the plant and adapt them to survive under unfavorable conditions. Certain adaptations are as under:
Many desert plants have smaller, fewer, and deeper pores than other plants. With such pores, hot and dry winds are prevented from blowing directly across the pores and evaporating so much of the plants’ water. Plants not only lose water through their pores, they also lose it through the cell walls on their leaves. The leaves and stems of many desert plants have a thick covering that is coated with a waxy substance, allowing them to seal in and protect what moisture they already have. Plants typically lose a large amount of water through transpiration, especially on hot, sunny days when they are doing photosynthesis. So if transpiration occurs during daytime hours, high temperatures can cause water to evaporate quickly. But if the process occurs at night, less water is lost. Succulent plants store water in fleshy leaves, stems or roots. All cacti are succulents, as are such non-cactus desert dwellers as agave, aloe, elephant trees and many euphorbias. Several other adaptations are essential for the water storing habit to be effective. Many succulents, as well as semi succulents such as most yuccas, epiphytic orchids and xerophytes’ bromeliads, possess a water-efficient variant of photosynthesis called CAM, an acronym for Crassulacean Acid Metabolism (CAM). CAM plants open their stomata for gas exchange at night and store carbon dioxide. By day, while the stomata are closed, photosynthesis is conducted using the stored carbon dioxide. Because of the lower temperatures and higher humidity at night, CAM plants lose one-tenth as much water per unit of carbohydrate synthesized as standard C3 plants.

During the hottest part of the day many desert grasses and other plants “roll up” their leaves (hide and rest) to reduce the amount of surface area exposed to sun and wind. Some plants simply position themselves so they have less exposure to the climatic elements on a hot, sunny day. One way desert plants, trees and shrubs suck up as much water as possible is by growing very deep taproots. Sometimes these roots can get to be more than 100 feet long. The above-ground parts of a plant may remain small for years simply because the plant puts most of its energy into developing its taproot system.
Desert plants can soak up water, store it and prepare to use it during drought. For example, cacti and many other desert plants store water in their fleshy leaves and stems. Desert plants may also have other adaptations for water storage, such as pleats or folds that will allow the plant to swell with added water when it can. The pleats or folds can almost disappear if the plant soaks up a lot of water; then the plant can shrink. Though many desert plants die to the ground during the hottest part of each year, the water they have stored in underground roots, tubers and bulbs will sustain them until the next moist period. Desert plants produce and give off chemicals from their leaves or roots that keep other plants from growing nearby. It is thought that plants do this to reduce competition, especially when water is scarce.

The authors are from Department of Life Sciences, The Islamia University of Bahawalpur.

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