Saline Soils: Strategies and Perspective to Counteract Salt Stress in Crops
Dipartimento di Biologia, Universita di Roma Tor Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy.
Salinization of soil is increasing with a perspective to lose arable lands. The presence of salt in the soil can damage seriously the crops, causing considerable losses in their yields. To solve this critical issue, the improvement of the knowledge about the response mechanism in plants exposed to salinity is necessary to better select tolerant genotypes that can be used in breeding programs.
This paper recapitulates some aspects of plant response to salinity and the current perspective in solving the problem related to the improvement of plant tolerance to salinity. An overview of salt tolerance studies in Brassica is presented.
Salt-affected soils are spread in several countries with different extent and severity (Ruan et al., 2010). Soils containing excess salts occur naturally in arid and semiarid climates and coastal lagoon systems, that is, natural or primary salinity, whereas secondaiy salinization is related to inappropriate land management and agricultural practice, like irrigation systems that use water containing trace amounts of sodium chloride (NaCl) and seawater (Tester and Davenport, 2003; Munns, 2005). In saline soils, NaCl can be considered the most prevalent salt but the presence of other soluble salts has been also reported (Munns and Tester, 2008).
Electrical conductivity (ECe) of the soil is commonly considered as an important indicator: A soil is classified as saline when ECe is 4 dS/rn or more, which corresponds to 40 mM NaCl (Munns and Tester, 2008). According to FAO (www.FAO.org), a soil with ECe higher than 8 dS/m has to be considered strongly saline, whereas the threshold of ECe for saline soil has been reported to a reference value of 15 dS/m by World Soil Resources Reports (IUSS Working Group WRB, 2007). Depending on the values of ECe of the soils, the crop cultivation ranges from restriction to many crops (moderately saline) to only a few tolerant crops (strongly saline land).
Excess of salts hinders plant growth by affecting the soil-water balance, thus affecting plant nutrient availability and both crop suitability and yields. In agriculture the sensitivity of crops to stress is usually related to losses in yield (Dolferus, 2014); therefore, species with an improved salt tolerance represent a key component in keeping the yields at a good level to sustain food and biofuel production. In saline land, the activity and biodiversity of soil micro-organisms are also affected influencing key soil processes (Canfora et al., 2014).
PLANTS AND SALINE STRESS
The presence of salt in the soil elicits a stress, that has a negative impact on plant physiology by impairing metabolic processes and decreasing the photosynthesis. Plants try to cope with stress by activating different genes, thus reprogramming their metabolism (Fomi et al., 2017). The efficacy of these responses is the basis of plant tolerance to salinity, a rather complex character that shares common traits with the response to other stress (Forni et al., 2017). Intense efforts have been dedicated to elucidating the complicated regulatory mechanisms of plant salt tolerance. The tolerance is also based on the limitation of the take up of salts by the roots or on controlling its concentration and distribution within the plant organs. The network and the regulation of transcription of genes involved in stress response have been well described and reviewed by Balderas-Hernandez et al. (2013) and Denlein et al. (2014).
Plant species can be divided into two groups: (1) halophytes, that is, plants tolerant to salinity that naturally grow in saline soils; (2) glycophytes, that is, veiy sensitive plants whose growth can be severely inhibited by 100-200 mmol/L NaCl (Mahajan and Tuteja, 2005; Zhu, 2007). Variability in salt tolerance has been detected within the species and even genotypes. The major effects of salt on plant morphology and physiology have been described and reviewed by several authors (Munns 2002, 2005; Denlein et al., 2014; Forni et al., 2017). The results obtained in the different experiments have evidenced that the plant response to salinity can depend on the phenological stage, the severity and the length of exposure.
Time frame plays a role in the response to salinity, being very important for the screening of genotypes for tolerance. Salt acclimation or gradual step acclimation (Sanchez et al., 2008) can be induced by gradual exposure to saline conditions (Zhu 2001; Bartels and Sunkar, 2005). This approach can induce salt tolerance in sensitive genotypes, suggesting that the latter may possess a genetic program for tolerance to some extent. The dynamism of acclimation in salt response has been highlighted by the data of Skiiycz et al. (2010,2011), that is, leaves exposed to mannitol, an elicitor of osmotic stress, showed temporally dynamic genetic and morphological changes, depending upon the developmental stage and length of treatment. Vice versa, salt shock is the extreme form of salt stress, when plants are suddenly exposed to a high level of salinity. Salt shock is rare in agriculture or in natural ecosystems, where usually plant exposure to salinity occurs gradually. According to Shavrukov (2013), in experimental condition, the more gradual application of NaCl would result in the more closely mimic of the salt stress response of plants in saline field conditions.
The effects of salts on plants are in some cases shows common traits to other stress, for example, drought. In plants, salt exposure induces osmotic shock or plasmolysis, mostly in the root cells (Munns, 2002). In fact, root is the first target organ where the water uptake decreases with increased salt concentrations. The expression of 5590 genes was found to be related to salt regulation in roots of Arabidopsis seedlings, mainly in cortex cells (Geng et al., 2013). After salt exposure, root growth can decrease together with changes of growth direction in the attempt to avoid highly salinity; such directional change is based on an active redistribution of auxin in the root tip. This phenotype, defined as halotropism, has been detected in different species (Galvan-Ampudia et al., 2013).
For the resistance to salt stress, Na~ and K~ membrane transporters play a pivotal role (Schroeder et al., 2013). When Na+ and Cl' are taken up in large amounts by the root, where Na+-influx pathways are located, they negatively affect growth by impairing metabolic processes and decreasing the photosynthesis.
Osmotic stress, caused by salt, may induce a water removal from the cytoplasm, thus, reducing cytosolic and vacuolar volumes; the organelle proteins may have decreased activity or even undergo to complete denaturation (Bartels and Sunkar, 2005). Component of stress response, like autophagy, can be the extreme consequence of osmotic stress (Kroemer et al., 2010). Genes involved in autophagocytosis (ATG genes, AuTophaGy-related genes) are functioning in response to salt stress in Arabidopsis (Slavikova et al.,
2008). Moreover, in barley roots the strong salt shock-induced apoptosis-like cell death (Katsuhara, 1997).
The leaf growth is quite sensitive to osmotic stress; the changes in this organ are quite rapid with the increasing of salinity (Bartels and Sunkar, 2005; Munns, 2002). The reduction of water content and stomata conductance reduce the rate of transpiration, impacting on photosynthetic rate. Therefore, the reduced leaf size, a process known as leaf area adjustment, is considered beneficial. In the meantime, the roots continue to grow, leading to a change in the ratio shoot/root.
The decreased photosynthetic efficiency is caused by salt injury in young photosynthetic leaves that may lead to the acceleration of their senescence (Forni et al., 2017). Moreover, photosynthesis decline and reactive oxygen species (ROS) production result in a decrease of plant growth. In fact, ROS overproduction damages thylakoid membranes and photosystems reduces the concentration of chlorophyll and carotenoids and changes the ratio of chlorophyll a and b (Parida and Das, 2005).