Population Growth and Environmental Stresses

The serious change in environmental condition due to the high growth rate of population. Stresses are one of the biggest problems globally and harmful for field crops growth and then for food security (Jaleel et al. 2009); (Lesk et al. 2016).

Stress is an aspect that decreases crop yield or aborts biomass. Stress can be biotic caused by insects and diseases and abiotic factors like drought, salinity, flooding, heat, wind, and cold are very important accounting for about 70% reduction in crop yield (Boyer 1982).

It has been calculated, due to partial availability of water, cultivation on the earth is only possible on 16% of the potentially arable land (Alexandratos and Bruinsma 2012). The current situation is agriculture scientists are facing the challenge of drought also with water shortage which may affect negatively the arable area.

Drought is a prolonged abnormal dry period, which occurs in a region where it generally receives less than average precipitation. Agriculture is the world’s largest consumer of water, accounting for almost 70% of all withdrawals, and as much as 95% in developing countries (Tekle and Alemu 2016).

The average global combined temperature of Land and sea surface has been elevated by 0.85°C between 1880 and 2012 (Team et al. 2014). A recent study has analyzed the data from studies published 1980 -2015 to report up to 21 and 40% yield reduction in wheat (Triticum aestivum L.) and maize (Zea mays L.) , on a global scale due to drought (Daryanto et al. 2016) In crops drought stress tackles by various morphological, physiological, biochemical, and molecular responses.

Drought tolerance is a complex trait that is controlled by polygenes and this expression is influenced by various environmental factors.

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Qualitative Trait Loci (QTL) mapping stratergies and use of genes expression pattern to produce drought tolerant genotypes (Nezhadahmadi et al. 2013). Plant use different mechanisms to deal with the drought. The escape strategy use through accelerating flowering before the onset of drought season (Deikman et al. 2012), or improve the water use efficiency (WUE) through closing stomata and increasing root development (Gowda et al. 2011) or can also use some tolerance mechanisms such as osmotic adjustment and production of antioxidants (Umezawa et al. 2006), yet there is no unified drought mechanism at the whole plant or gene level (Blum 2005).

Three main mechanisms reduce crop yield by soil water deficit: (i) reduced canopy absorption of photosynthetically active radiation, (ii) decreased radiation-use efficiency and (iii) reduced harvest index (Earl and Davis 2003). Improvement of crops through conventional breeding and modern biotechnology both are offer potential for substantial progress (Cominelli and Tonelli 2010). In crops, drought resistance is a key factor for the improvements of drought tolerance and also understanding the mechanism of components. Drought resistance mechanisms have been extensively reviewed and summarized from crop physiology, plant breeding and molecular perspectives for different crops (Manavalan et al. 2009; Mittler and Blumwald 2010; Nguyen et al. 1997; Turner et al. 2001)