Environmental Engineering Reference
In-Depth Information
I
NTRODUCTION
All over the world, soil salinity is becoming an increasing threat to plant growth (Zhu
2001; Rengasamy 2010). According to FAO, there are at least 800 million hectares land
subjected to salinity in the world (FAO 2008), accounting for as much as 6% of the world`s
total land area. Although some of the salt-affected influences are the result of the natural
causes, the considerable rest is derived from the degraded cultivated agricultural land (Munns
and Tester 2008; Lee et al. 2004). At present the world's cultivated land affected by salinity
has achieved 20% (Rhoades and Loveday 1990). Currently, the food supply is sufficient
enough but there are approximately 800 million people undernourished (Conway 1998). It
was reported that the food production is needed to increase to 150% to meet the over-growing
population by 2050 (FAO 2008; Rengasamy 2006). On the one hand, the cultivated
agricultural land is narrowed with the increasingly serious salinization, more and more food is
demanded with the increase of the population on the other hand. To meet these challenges, to
survive abiotic stress and to maximize performance under less favoring conditions crops seem
to be the major topics for research (Abreu et al. 2013).
Generally, the so-called saline soil is the soil with a high concentration of soluble salts.
The soils are regarded as saline on the basis of the soil solution ECs reaching 4 dS/m or more
(Brown 2008). The solution concentration is equivalent to 40 mMNaCl, generating an
osmotic pressure at about 0.2 MPa and reducing significantly the yields of most cereal crops
(Munns and Tester 2008). However, the deleterious effects vary according to several related
factors including climatic conditions, plant species and soil regime. For instance, rice is the
most sensitive specie among the grain crops, while barley displays a pronounced resistance
relatively (Aslam et al. 1993; Colmer et al. 2006). The salinity can threaten plant from two
aspects in general: hyperosmotic stress and hyperionic stress. (Türkan and Demiral 2009).
Based on the capacity of plants to grow on highly saline, plants are classified into two types:
glycophytes and halophytes (Flowers et al. 1977). Some of the halophytes have the ability to
exclude salts from their roots and shoots, some are able to endure high concentration salt, and
the others adopt measures such as ion compartmentation, synthesis of the compatible solute
and osmotic adjustment (Flowers and Colmer 2008). Yet most plants, almost all the crops are
glycophytes. Thus two key measures (producing salt-tolerant lines/cultivars through
conventional breeding or genetic engineering) to overcome this peril have been put forward
for a long time (Ashraf and Akram 2009). Growing halophytes and salt-tolerant crops on the
saline soil is regarded as the ―biotic approach‖, which is similar to the idea of the ―biosaline
agriculture‖(Wright and Ferrari 1976; Kingsbury and Epstein 1984; Ashraf and Wu 1994).
As we all know, other than animals, plants maintain a sessile lifestyle subjected to
various environmental stresses (Sarwat et al. 2013). Modern-day plants are all the products of
the primal livings, experienced eons of evolution with abiotic and biotic changes. Thus they
have acquired various adaptive mechanisms to optimize growth and development under
stresses (Xue et al. 2010). NaCl is the most common and widespread salt in soil, so the
evolved mechanisms of plants are all to regulate its accumulation and distribution (Munns
2005). For glycophytes high concentration salts can produce ionic stress, osmotic stress and
secondary stresses (Zhu 2002). In order to guarantee survival under such a detrimental
circumstance, plants have evolved a series of biochemical and molecular processes to
acclimatize themselves to the environment (Dufty Jr et al. 2002; Glombitza et al. 2004; Jaleel
