**8. References**

Allen, J.F. (1992). Protein phosphorylation in regulation of photosynthesis. *Biochim Biophys Acta*. 1098: 275-335*.*

Barbagallo, R.P.; Oxborough, K.; Pallet, K.E. & Baker N.R. (2003). Rapid, non-invasive screening for perturbations of metabolism and plant growth using chlorophyll fluorescence imaging. *Plant Physiol.* 132:485-493.

fluorescence emission. However, at high illumination (300 mol·m-2·s-1), when the photochemical efficiency significantly decreased because of the light saturation, the sun

We conclude that fluorescence imaging is a useful method for the early assessment of photosynthesis tolerance to adverse conditions, such as drought, high light and heat, when there is still no visible damage to plants. However, not all fluorescence parameters are effective, and analysis of the maximum quantum yield in leaves adapted to darkness was unable to detect significant differences between control plants and plants exposed to stress photoperiods. In contrast, the analysis in illuminated leaves of the relative electron transport rate and the fluorescence parameters, Y(II), F and Nq, which are representative of the three processes of excitation energy dissipation (photochemistry, fluorescence and thermal dissipation, respectively) showed significant differences in the two species studied, indicating that sun species (*C. morifolium*) had greater tolerance to drought, heat and high

This work was supported by the Spanish Ministry of Science and Innovation (grant

Allen, J.F. (1992). Protein phosphorylation in regulation of photosynthesis. *Biochim Biophys* 

Barbagallo, R.P.; Oxborough, K.; Pallet, K.E. & Baker N.R. (2003). Rapid, non-invasive

fluorescence imaging. *Plant Physiol.* 132:485-493.

screening for perturbations of metabolism and plant growth using chlorophyll

species was more efficient in dissipating excess energy in the form of heat.

**5. Conclusions** 

**6. Acknowledgment** 

**7. List of abbreviations** 

Fv, variable fluorescence; FW, fresh weight;

LED, light-emitting diode;

RWC, relative water content;

Nq, non-photochemical quenching; PAM, pulse amplitude modulation; PPFD, photosynthetic photon flux density;

Y(II), effective PS II quantum yield.

*Acta*. 1098: 275-335*.*

BFU2008-00331).

DW, dry weight; F, fluorescence yield;

PS, photosystem;

TW, turgid weight;

**8. References** 

illumination than the shade species (*S. wallisii*).

Fm, maximal fluorescence yield in the dark adapted state; F0, minimal fluorescence yield in the dark adapted state; F'm, maximal fluorescence yield in the light adapted state;


**10** 

*Egypt* 

**Instinctive Plant Tolerance Towards** 

*Botany and Microbiology Department, Faculty of Science, Alexandria University* 

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,

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

**1. Introduction** 

2007).

**Abiotic Stresses in Arid Regions** 

Mohamed Mohamed Ibrahim

