Drought can be defined as the absence of rainfall and water or irrigation for a period of time sufficient to deplete soil moisture and injure plants (Plantlifeonline.net, 2007). In short, drought is a period of time without rainfall. The drought is one of the most serious global issues for agriculture field and need to harsh precaution need to be taken immediately. Four-tenths of the world's agricultural land lies in arid or semi-arid regions especially in Africa.
Meanwhile, drought stress is defined as effects of some period of plants that involve plant water relationships. According to Farooq et al. (2008) drought stress reduces and decreases the size of plant's leaf, extension of stem and proliferation of roots that disturbs plant water relations and reduces water-use efficiency. While ForestryNepal(n.d.) defined that drought stress occurs when the available water in the soil is reduced and atmospheric conditions cause continuous loss of water by transpiration or evaporation. Drought stress tolerance can be found in almost all plants but its extent varies from species to species and even within species. It is characterized by reduction of water content, diminished leaf water potential and turgor loss, closure, nutrient metabolism and growth promoters. Plants display a variety of physiological and biochemical responses at cellular and whole organism levels towards prevailing drought stress, thus making it a complex phenomenon. Based on Bishop(n.d) report, many variables play a part in reaching drought conditions, these include lack of natural rainfall, types of soil, air temperature, humidity, conditions of wind, exposure of sun, and also plant type or root depth that increase pant water loss. Drought stress can affect the growth of plants in various ways.
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One of the effects is the priming on seed performance of several plants due to lack of natural rainfall. For example of plant that can be affected from this problem is Canola plant (Brassica napus L.). Based on the report of Mohammadi and Amiri(2010) , Canola plant is one of the most important oil seed crops which its production has been notably extended during recent years in Iran and due to lack of rainfall at planting time and the seeds are common planted in seedbeds having unfavorable moisture. The drought stress is responsible for both inhibition and delayed seed germination and seedling establishment of Canola. Consequently, this stress adversely affects growth and development of crop and also results into low Canola yield. There is a decrease in water uptake both during imbibition and seedling establishment under this stress condition. Tutorvista(n.d.) stated that imbibitions process is the phenomenon of adsorption of water by the solid particles of a substance without forming a solution. Furthermore, inhibition of radicle also occurs due to the effect of stress condition. The inhibition emergence is mainly because of a decrease in water potential gradient between the external environment and the seeds. In addition, the seed priming has been successfully proved and demonstrated to improve germination and emergence in seeds of many crops and plants, especially under stress conditions. The seed priming is a technique that starts the germination process in the lab or plant. Moreover, the basic chemical reactions or framework for the seed to germinate and for the process to occur efficiently in the lab or plant, high moisture and ideal temperature condition are needed (Hariss, n.d.).
Secondly is that the drought stress can affects the photosynthetic rate and leaf gas exchange of plants. Siddique et al. (1998) reported that, drought stress effects on photosynthetic rate and leaf gas exchange characteristics. The experiment had been done to four wheat (Triticum aestivum L.) cultivars which were evaluated under semi-controlled conditions. According to Siddique's observation, four cultivars which were Kanchan, Sonalika, Kalyansona, and C306, grown in pots and were subjected to four levels of water stress. However, cultivars that showed the highest photosynthesis rates both at vegetative and at anthesis among others is the Kalyansona. They had concluded that the exposure of plants to drought stress led to noticeable decrease in photosynthesis rate, stomatal conductance and mesophyll conductance and a concomitant increase in intercellular CO2 concentration. The plants that were subjected to drought at the early vegetative stage displayed similar physiological characters subsequently under well-watered conditions as compared with control. Therefore, the photosynthesis rates decreased with decrease in stomatal conductance, but a weak relationship between them implied that non-stomatal limitations to photosynthesis might have been in operation. The involvement of CO2 concentration and assimilation was described in Farooq et al. (2008) report. The CO2 assimilation by the leaves is reduced mainly by the closure of the stomata, damaged the membrane and disturbed activity of various enzymes in the plants, especially those of O2 fixation and adenosine triphosphate(ATP) synthesis. Moreover, the enhancement of metabolite flux through the photorespiratory pathway had increased the oxidative load on the tissues as both processes generate reactive oxygen species. The damage and injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth.
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