**1. Introduction**

218 Artificial Photosynthesis

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Arid environments are extremely diverse in terms of their land forms, soils, fauna, flora, water balances, and human activities. Because of this diversity, no practical definition of arid environments can be derived. However, the one binding element to all arid regions is aridity. Aridity is usually expressed as a function of rainfall and temperature. A useful "representation" of aridity is the following climatic aridity index: p/ETP, where P = precipitation; ETP = potential evapotranspiration, calculated by method of Penman, taking into account atmospheric humidity, solar radiation, and wind. Three arid zones can be delineated by this index: namely, hyper-arid, arid and semi-arid. Of the total land area of the world, the hyper-arid zone covers 4.2 percent, the arid zone 14.6 percent, and the semiarid zone 12.2 percent. Therefore, almost one-third of the total area of the world is arid land.

Arid climate, is a climate that does not meet the criteria to be classified as a polar climate, and in which precipitation is too low to sustain any vegetation at all, or at most a very scanty scrub. An area that features this climate usually (but not always) experiences less than 250 mm (10 inches) per year of precipitation and in some years may experience no precipitation at all. In some instances an area may experience more than 250 mm of precipitation annually, but is still considered a desert climate because the region loses more water via evapotranspiration than falls as precipitation. Although different classification schemes and maps differ in their details, there is a general agreement about the fact that large areas of the Earth are arid. These include the hot deserts located broadly in subtropical regions, where the accumulation of water is largely prevented by either low precipitations, or high evaporation, or both. Abiotic disorders are associated with non-living causal factors such as weather, soils, chemicals, mechanical injuries, cultural practices and, in some cases, a genetic predisposition within the plant itself. Abiotic disorders may be caused by a single extreme environmental event such as one night of severe cold following a warm spell or by a complex of interrelated factors or events. A biotic plant problems are sometimes termed "physiological disorders" that reflects the fact that the injury or symptom, such as reduced growth, is ultimately due to the cumulative effects of the causal factors on the physiological processes necessary for plant growth and development (Schutzki & Cregg, 2007).

Abiotic stresses, such as drought, salinity, extreme temperatures, chemical toxicity and oxidative stress are serious threats to agriculture and the natural status of the environment. Increased salinization of arable land is expected to have devastating global effects, resulting

Instinctive Plant Tolerance Towards Abiotic Stresses in Arid Regions 221

Desert plants generally follow two main strategies i.e., they tolerate the stresses through phonologic and physiological adjustments referred to as tolerance or avoidance mechanisms contribute to the ability of a plant to survive stress but it also depends on the frequency and severity of the stress periods. Xeromorphic characteristics of desert plants have developed as the result of adaptation to drought, temperature divergence, salinity, poor nutrition, strong wind, sand movement and high light intensity (Fahn,1964,1990; Fahn and Cutler 1992; Huang et al.,1997). Plants in many habitats have various physiological mechanisms for responding to environmental changes, and the ability to tolerate environmental disturbances often contributes to their success in communities ( Gutterman, 2001). In addition to genetic adaptation, the survival of a certain species is often determined by its ability to acclimate to environmental changes ( Gutterman, 2002). Acclimation is known to be a widespread phenomenon in nature, and long-term responses can be observed in the

Fig. 2. Xeromorphic characteristics of desert plants in Arid environment (Ibrahim & El-

More than one abiotic stress including drought, dust, salinity, heavy metals and UV can occur at one time. For example, high temperature and high photon irradiance often accompany low water supply, which can in turn be exacerbated by subsoil mineral toxicities that constrain root growth. Furthermore, one abiotic stress can decrease a plant's ability to resist a second stress. For example, low water supply can make a plant more susceptible to damage from high irradiance due to the plant's reduced ability to reoxidize NADPH and thus maintain an ability to dissipate energy delivered to the photosynthetic light-harvesting reaction centers (Mark & Bacic, 2005). If a single abiotic stress is to be identified as the most common in limiting the growth of crops worldwide, it most probably be low water supply (Boyer, 1982; Araus et al., 2008). The Arabian peninsula is one of the five major regions where dust originates (Idso, 1976). The Sahara and dry lands around the Arabian peninsula are the main source of airborne dust, with some contributions from Iran, Pakistan and India

course of a season.

Gaely, 2011)

**2. Convergent abiotic stress**

in 30% land loss within the next 25 years, and up to 50% by the year 2050. Therefore, breeding for drought and salinity stress tolerance in crop plants (for food supply) and in forest trees (a central component of the global ecosystem) should be given high research priority in plant biotechnology programs(Wang et al., 2003).

Fig. 1. The Arabian Peninsula (land-surface image formatted and labelled by Bruce Rails back); National Geophysical Data Center (NOAA).

in 30% land loss within the next 25 years, and up to 50% by the year 2050. Therefore, breeding for drought and salinity stress tolerance in crop plants (for food supply) and in forest trees (a central component of the global ecosystem) should be given high research

Fig. 1. The Arabian Peninsula (land-surface image formatted and labelled by Bruce Rails

back); National Geophysical Data Center (NOAA).

priority in plant biotechnology programs(Wang et al., 2003).

Desert plants generally follow two main strategies i.e., they tolerate the stresses through phonologic and physiological adjustments referred to as tolerance or avoidance mechanisms contribute to the ability of a plant to survive stress but it also depends on the frequency and severity of the stress periods. Xeromorphic characteristics of desert plants have developed as the result of adaptation to drought, temperature divergence, salinity, poor nutrition, strong wind, sand movement and high light intensity (Fahn,1964,1990; Fahn and Cutler 1992; Huang et al.,1997). Plants in many habitats have various physiological mechanisms for responding to environmental changes, and the ability to tolerate environmental disturbances often contributes to their success in communities ( Gutterman, 2001). In addition to genetic adaptation, the survival of a certain species is often determined by its ability to acclimate to environmental changes ( Gutterman, 2002). Acclimation is known to be a widespread phenomenon in nature, and long-term responses can be observed in the course of a season.

Fig. 2. Xeromorphic characteristics of desert plants in Arid environment (Ibrahim & El-Gaely, 2011)
