A drought is a natural disaster of below-average precipitation in a given region; resulting in prolonged shortages in the water supply, whether atmospheric, surface water or water. Many plants shave adapted to this drought conditions in many ways .Desiccation is delayed by all these mechanism that enable plant to maintain favorable tissue water content despite the unfavorable climatic condition. This is achieved by a number of ways which include improved uptake of water, reduce water losses and through a high capacity of water condition and storage of water.
There are many different mechanisms that plants used in order to tolerate drought. These mechanisms can be classified into three main groups which stress avoidance, stress tolerance and the efficiency mechanisms. Stress avoidance is those mechanisms than reduces the occurrence of damages in time of water deficit. For example, some plants species has adapted to drought by completing their reproductive and life cycle rapidly to escape periods of drought and grow when conditions are still favorable.
This mechanism is typical of the desert ephemeral that can complete the life cycles from germination to maturity in a short period ranging from 4 to 6 weeks. Not only does this mechanism apply to desert ephemeral, but similar mechanism has been noted in some annual plant. They show a dynamic response of this type by flowering earlier than usual if they are subjected to water stress (Jones,1992).
Another way by which plants has adapted to drought is the stress tolerance mechanism, which is those physiological adaptations that enable plants to continues function even though the conditions are unfavorable.
This include all water conservation adaptations which minimize transpiration and diffusion such as the presence of root hairs, reduced leaf size, reduced surface area of leaves, high leaf reflective and extreme development of structural tissues such as spine, wax and other protective structures.Small leaves have small surface area for transpiration to take place, hence reducing the amount of water vapor to be lost. This adaptation is mainly common in coniferous species which have narrow, needle like leaves. A more highly adaptation similar to this, is seen in the members of the cacti family which have developed spikes thus highly reducing transpiration .Some plants have leaf hairs which reduce transpiration by minimizing size of stomata pores and controls the opening and closing of the stomata. Apart from that, other plants species have adapted to drought through production of wax. These waxes form a cuticle in the epidermis of the leaves which is impermeable thereby reducing water loss by diffusion. Increased diffusion resistance of some species in drought environment is achieved by early closure of stomata and effective cuticle protection against transpiration (Daubenmire, 1974).
Lastly, is the efficient mechanism, which includes those adaptations that optimize the utilization of resources, especially water? This includes extreme development of structural tissues such as sclerenchyma and collenchyma, shorter internodes and succulent trunk. This help in the transportation and storage of water for a long period of time even though conditions are unfavorable. Succulent trunks are most comment in cacti species, as they have to adapt to the desert condition. Some plants, such as the baobab, have giant trunks, which give the tree the ability to store much water, which will make it surface during the drought season. The most important adaptation to drought in most species is the presence of well-developed roots. Many plants in dry environments rely on the development of a very deep and extensive root system that can obtain water from high depths and water tables. For example, may desert shrubs such as mesquite and Prosopis juliflora, have deep roots. Root development is very plastic, with more of plant resources such as carbohydrates being put into root growth relative to the shoot in dry conditions. Some of the seedlings of woody plants in dry region have taproots ten times as long as shoot, while grasses in such places elaborate a dense root system and also send their thread like roots to great depths (Larcher,1973).