Drought Resistance and Characteristics of Drought Resistant Plants
- Drought is a meteorological event defined as a period without precipitation of sufficient duration that soil moisture is depleted and injury to plants may result. It is permanent in arid regions, seasonal in regions with well-defined wet and dry seasons, and unpredictable or random in many humid regions.
- The response of plants to water stress is more appropriately termed dehydration tolerance.
THE CAUSES OF ARID REGIONS AND DESERTS:
- Arid regions have evaporation potential EP that is greater than precipitation P and this includes about one-third of the earth’s land surface. Deserts receive <25 cm/ yr precipitation and have a P/EP ratio of about 0.25.
- The major deserts of the world occur in the horse latitudes, or between 15-30 degrees north and south of the equator. The horse latitudes are an around-the-world band of arid regions. They are called the horse latitudes because of the historic abundance of these animals on the plains, grasslands and deserts that occur in these regions. Grasses prosper in arid regions because they can endure long periods of drought in dormancy, yet grow quickly when water is available.
- Air that ascended in other latitudes descends in the horse latitudes, and in so doing it becomes warmer because of compression. Warm air holds more moisture than cool air, and thus precipitation does not occur with descending air masses.
- The arid effect of the horse latitudes shifts northward in summer and southward in winter. In the northern hemisphere this causes summer drought in the higher latitudes, and increased summer precipitation in the lower latitudes.
- The other major cause of arid regions is mountain ranges. When air masses encounter mountain ranges the air is forced to ascend; it cools during ascension and precipitation may occur. The air mass descends on the lee side, warms, thus preventing precipitation in the “rain shadow” of mountain ranges.
- Because there is little water vapor in desert air, little incoming short-wave radiation or long-wave outgoing radiation is absorbed. For this reason deserts tend to be hot in the daytime and cold at night. Hot air rises up slopes and canyons during the day, and when cooled at night the air descends, causing winds that may, in the proper circumstances, build sand dunes. More commonly, “desert pavement” develops because the fine particles on the soil surface are blown away leaving larger stones on the surface.
- The stress of drought has induced a number of adaptive mechanisms in plants, and these are similar around the world. The same mechanism has evolved in very different plant groups (convergent evolution). For example the succulent form and appearance of some Cactaceae are very similar to the Euphoriaceae even though these families are unrelated.
- Desert soils are often saline because there is insufficient precipitation to wash the salts that have accumulated as a result of weathering away from the soil surface into ground water.
ADAPTATION TO DROUGHT BY DROUGHT AVOIDANCE:
- Desert ephemerals and geophytes are included in this group. Many desert annuals avoid drought by passing through the dry period in dormancy as a seed. After precipitation these annuals germinate, display photosynthetically active leaves, flower, and set seed very quickly, utilizing the water stored in the soil before it dries out.
- Geophytes, those plants that pass through the dormant season as in the soils as bulbs, rhizomes, corms, tubers, or similar organs, are also common in deserts. Because of their long vegetative life cycles, few if any trees are drought avoiders.
- Drought avoiders such as desert annuals and geophytes seldom have xerophytic characteristics such as extra wax layers on their leaves. The environment is mesic during the period of their vegetative and floral development, and there is adequate soil water because of the recent rain that induced their growth.
- Many drought avoiders have characteristics that prevent them from breaking dormancy or germinating until sufficient precipitation has occurred. A common characteristic is a germination inhibitor on the seed coat which must be washed away by the percolation of soil water past it. These seeds will not germinate if simply kept moist. This biological rain gauge often requires the passage of about 25 cm of water before the inhibitor is removed. This is sufficient precipitation to wet the soil to a depth of about 15 cm, enough soil water to assure that the seed can germinate and complete its life cycle.
ADAPTATION TO DROUGHT BY DEHYDRATION POSTPONEMENT:
- There are two general strategies of dehydration postponement: the water savers and the water spenders. Both categories may have a few features in common, but differ in others.
- This category includes the phraeatophytes such as Prosopis and some Populus species. They are common as riparian vegetation along water courses, and the roots of some may extend fifty meters into the soil to tap deep ground water sources. Some, such as Prosopis are so efficient at extracting soil water that the growths of more desirable species such as grasses are inhibited.
- The stomata of water spenders remain fairly open, and transpirational water may be used for evaporative cooling of their leaves. In order to supply large volumes of transpiration water these plants develop extensive root systems, both in depth and in spread. They also maintain favorable root to shoot ratios. The water conduction system of water spenders is efficient, having more vessels in the xylem, dense leaf venation, shorter internodes, and more sapwood.
- These plants may also have extensive root systems and efficient conduction systems, but may also have water storage capacities as well as characteristics designed to reduce water loss.
- Succulents such as cactus and Manyh euphorbes store water, often enough to supply plant needs for several months. The succulent stems of many of these species are pleated, allowing the stem to expand when absorbing water, and to contract as water is used. The no of these succulents is generally low. After precipitation, roots develop to tap soil water, and then as the soil dries out the roots abscess and scar tissue forms sealing the succulent stem from the dry soil.
- The African baobab (Adamsonia digitata) is the best example of water storage in the tree form. The tree grows to a diameter of several meters, and is used as a water (and food) source by the African elephant. Douglas fir sapwood can supply several days of the trees water needs that of Scotch pine about one-third of the daily requirement, and 24% of the daily requirement of spruce. Current investigations indicate that a substantial portion of the water withdrawn from storage in trees comes from the bark.
Reduced Water Loss
- Many species reduce water loss during water stress. Olea europea reduces its water loss by 28% after acclimatization, Quercus ilex by 11%, and many desert species by 30-70%. Sclerophyllous trees and shrubs may reduce their water loss to 10-20% the rates before water stress.
- This reduction is accomplished in a number of ways. Leaf hairs and additional waxy layers may develop; some leaves fold or roll to reduce water loss. There is generally a reduction in leaf size during water stress, and there may be a change in leaf orientation to reduce radiation absorption.
- Leaves may also be shed during drought. Aesculus california, Prunus persica, and Quercus species are known to shed leaves during drought. Leaf abscission reduces water loss, and preserves the buds and cambium for a longer period.
- Low humidity induces stomatal closure in some species even if soil water is adequate. This characteristic has been shown to vary among province collections in species such as Populus, and Juglan nigra, the black walnut.
ADAPTATION TO DROUGHT BY DEHYDRATION TOLERANCE
- Dehydration tolerance is the capacity of the protoplasm to endure dehydration without damage to its fine structure. There very few examples of dehydration tolerance among higher plants, but many thallophytes are remarkably dehydration tolerant. These include some fungi, algae, and the lichens which can resume photosynthesis within an hour after rehydration from an air-dry condition. The resurrection plant, Seleginella, a few ferns, mosses, and a few grasses can also tolerate air-dry dehydration.
- Among higher plants such dehydration is tolerated at one stage of the life cycle, the seed. Indeed, seed preservation depends upon, in part, maintenance in a dry condition.
- Dehydration tolerance can be determined by measurement of the relative water content RWC which is lethal to 50% of the cells or tissue. Cells generally die if allowed to equilibrate with air at a relative humidity of 92 – 96%. This corresponds to ψ -5.5 to -11 MPa. The organs of most species (e.g. leaves) are fatally damaged if the relative water content falls to 50-75%, but there are some exceptions. The desert shrub creosote bush (Larrea divaricata) of North and South America, and Acacia aneura of the Australian desert can withstand reductions of RWC to 30%.
Trees may be damaged by drought in late winter in regions where the soil and/or stem is frozen, but air temperature and humidity is conducive to evaporation. Water loss from buds, even though covered with protective bud scales, may cause damage, as well as the excessive dehydration of evergreen leaves.
Image: Tamarix Forests at Killa Saifullah Balochistan
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