**2.3 Soil salinity and sodicity stress and its management and mitigation strategies**

Soil salinity and sodicity are abiotic stresses that prevent the growth of trees, especially citrus, by limiting water and mineral uptake. Citrus is one of the saltsensitive and salt-intolerant crops, and their response to soil salinity and sodicity depends on rootstock, scion, soil type, irrigation system, and climate. Changing these factors, under the same irrigation conditions, could generate entirely different results. Salinity reduces growth in citrus trees and causes physiological disorders [50]. Salinity damage happens when the dissolved salt in water is high enough to diminish crop growth. Indirect movement of water in the leaf tissues of citrus can cause accumulation of Cl<sup>−</sup> ions affecting transpiration and photosynthesis, and increasing Cl − ions concentrations, accelerates defoliation by enhancing leaf abscission and ethylene production [51]. The most common causes of soil salinity and sodicity are hydrological, geological, and soil processes. Other reasons are incompetent irrigation methods, improper drainage systems, dry weather or insufficient annual rainfall, and remaining salts accumulated in plant root areas [52].

The effect of soil salinity on the citrus growth parameters may be seen as decreased plant height, leaf area, stem diameter, fresh and dry weight, and increased tree senescence [50]. The decrease in growth indices may also be due to the salt osmotic effect on the roots and the toxic ions accumulated in plant organs. The addition of sodium chloride (NaCl) in growth media increments phosphorus, nitrogen, and potassium and alleviates calcium and magnesium in most citrus rootstocks [53]. Citrus physiological responses to salt stress might be attributed to changes in water relations. Citrus species, such as halophyte plants, cannot absorb salts from the soil solution and accumulate them in their tissues to regulate osmotic pressure. Instead, they accumulate secondary metabolites, such as pro and nontoxic inorganic ions, in their cells to overcome salinity stress. Salt accumulation in chloroplasts reduces chlorophyll content and directly affects and reduces photosynthetic activities and yield [50, 54].

Phytophthora root rot in citrus fruit is usually intensified under salinity stress conditions. Also, a reduction in number, weight, and fruit yield is observed in citrus fruit under salt stress. Salinity decreases plant growth and yield and reduces shoot and root biomass [55]. The plant pigment contents decrease in response to salt stress due to CL accumulation, destruction of chlorophyll biosynthesis, and the reduction in iron, magnesium, and manganese in several citrus rootstocks [56]. Reduced chlorophyll content in citrus rootstocks due to NaCl application was also reported [57, 58]. The figure below shows the adverse effects of salinity stress on the nutritional, physiological, and biochemical characteristics of four citrus species (**Figure 7**).

Since citrus fruits are one of the most sensitive plants, salinity stress reduces fruit yield and quality. So, improvement programs should be considered for better productivity of citrus fruits in soils affected by salt. One of the methods of managing salinity stress in citrus fruits is irrigation programming. Irrigation is an essential factor in managing salinity in areas with salinity stress. Increasing irrigation periodicity is recommended to leach the salts and minimize the salt concentration in the root zone [57]. Leaf and trunk damage related to the absorption of salt can be reduced by using micro-irrigation systems. Another strategy to diminish salinity stress is recommended repetitious fertilization or spreading dry fertilizers through fertigation. It is necessary to mention that nutrient fertilizers should not contain chloride (CL) or sodium (Na). In areas where the soil is sodium, calcium source application (gypsum; CaSO4)

**Figure 7.**

*Effect of soil salinity on nutritional, physiological, and biochemical parameters in four citrus species [59–61].*

reduces the adverse effect of sodium on shoot growth and improves plant growth in these conditions [50, 62].

Plant breeding and genetic manipulations are other ways of managing salinity and sodicity stress conditions, which increase the plant's adaptation to saline environments and salt tolerance. Currently, inorganic and organic conditioners like organic residues, phosphor-gypsum, and H2SO4, calcium application, drainage management, the genetic use of halophytic traits, arbuscular mycorrhizal, and avoiding cultivation of lands with high groundwater are being used to progress citrus productivities in salt-affected soils [62–65]. Quantitative trait locus (*QTL*) mapping is one way to identify salinity tolerance genes. This way has focused on characterizing genes encoding proteins involved in decreasing the amount of Na<sup>+</sup> from the root to the shoot [66].

Since drought stress is the beginning of creating salinity stress in the soils of dry and salty areas, therefore, methods, such as the use of superabsorbent, netting shades, the application of phytohormones, such as abscisic acid (ABA), polyamines, and chemical priming, will improve the performance of citrus fruits under salinity stress condition [34]. The application of nitrate and other compounds derived from nitrogen (urea or ammonium) has positively affected citrus morpho-physiological and biochemical responses under salt stress conditions [67]. Nitrate appears to stimulate photosynthesis and growth parameters as well as reduce leaf drop. Also, the increased nitrogen in leaf biomass leads to chloride dilution [68]. The beneficial effect of Paclobutrazol (PBZ) under salinity on the accumulation of photosynthetic pigments, phytohormones, and root morphology, such as size, number of lateral roots, and dry weight of roots, has also been documented [69].

#### **2.4 Cold stress and its management and mitigation strategies**

Low temperature is the main restrictive factor for citrus growth and productivity worldwide. Citrus is grown at temperatures between 12.8 and 38°C; low temperatures

#### *Abiotic Stresses Management in Citrus DOI: http://dx.doi.org/10.5772/intechopen.108337*

below 13°C limit vegetative growth and fruit growth and delay maturity. The threshold temperature that destroys shoots is −12°C, but some citrus fruit can tolerate a temperature of −10°C [70]. *Poncirus trifoliata* (L.) Raf. is the most tolerant citrus rootstock to low temperatures [71]. The low ambient temperature that causes cold stress is a principal environmental abiotic stress. In these conditions, the production of reactive oxygen species (ROS), such as superoxide radicals, hydrogen peroxide, hydroxyl, and singlet oxygen is increased, which leads to ion leakage and water soaking **Figure 8**, and finally, leaves are destroyed. ROS produced in the chloroplast can destroy cellular components, such as proteins, pigments, membranes, lipids, and nucleic acids [39, 73].

Additionally, mechanical methods can decrease these damages to increase plant resistance against cold and frost damage. One of the strategies is the osmotic balance reaction to maintain plant water content. These activities are affected by osmotic pressure regulatory concentration compounds, such as secondary metabolites, including proline, and some inorganic ions, such as potassium. Potassium increases cells' tolerance against cold stress because it affects the freezing point of the liquid inside the vacuoles [74]. High potassium levels protected cells against freezing by lowering the freezing point of the cell solution [75]. Potassium can affect plant survival under different environmental stresses by creating osmotic balance and protection against oxidative damage. Potassium causes leakage and maintenance of water in plant tissues with increasing osmotic pressure and cell membrane fluidity; thus, it prevents cell membrane rupture and plant tissue damage and increases electrolyte leakage [76].

Potassium substantially affects stomata movement and water relation (turgor regulation and osmotic adjustment) in plants under cold conditions. The present results agree with those reported on Sour orange (*C. aurantium* L.) seedlings [77]. Numerous studies indicated that applying abscisic acid (ABA) may increase leaf water content under low-temperature stress [78]. The grafting technique and the use of tolerant rootstocks to cold stresses allow farmers to protect crops against various abiotic stresses, especially cold stress. A study in common clementine with a tetraploid Carrizo Citrange rootstock showed enhanced natural chilling stress tolerance [79].

#### **Figure 8.**

*Damaged leaves of sour orange (Citrus aurantium L.) seedlings were exposed to 0, −3, and − 6°C for 24 hours [72].*

## **3. Conclusions**

Citrus species is the world's most productive and widely consumed horticultural crops. The factors limiting citrus growth in tropical and subtropical climates are significantly different. Due to abiotic stresses, such as drought, salinity, and high and low temperature, citrus production confronts risks. For this reason, it is crucial using strategies to manage these stresses. Drought and extreme temperatures caused heavy fruit drops and a decline in yield, and it also increases the cracking and folding of fruit and reduces the product yield. Therefore, measures to prevent these problems should be taken in case of severe drought and high temperatures. To manage drought stress and high temperature, using superabsorbents, kaolin, and pure shade is less expensive than producing the product. Also, drought stress management and hightemperature in hot and dry areas reduce the possibility of salinity stress.

Kaolin particle film and shading net reduce water loss through the increasing reflectance of ultraviolet and infrared radiations, thereby reducing leaves and fruit tissue temperature. Kaolin application could be considered an implement to be used in tropical regions to improve plant acclimation to extreme temperatures and high radiation levels in citrus. Maintaining soil moisture, reducing water consumption, and taking advantage of suitable yield along with relative improvement of fruit quality of growth and fruiting period of citrus is essential. For this purpose, soil amendment with superabsorbent polymers to provide moisture in the root area and increase available water in water shortage conditions improve some physiological, biochemical, and phytochemical properties of citrus. Also, the uniformity of humidity during the plant growth period with the use of superabsorbent plays a role of importance in reducing other indicators, such as fruit bursting. Therefore, it is recommended to use this superabsorbent polymer in areas that face water shortages and improper distribution of precipitation. Exogenous application of phytohormones and hormones and even suitable nutrition of citrus fruits can also increase plant tolerance against abiotic stresses. Among the methods that can protect the plant from low-temperature stress are grafting techniques, plant breeding programs, osmotic balance reactions to maintain plant water content secondary metabolites, proline, and inorganic ions, such as potassium.

Citrus is salt-sensitive, and soil salinity reduces citrus growth and causes physiological disorders. Salt stress lowers stomatal conductance, net CO2 assimilation, and water potential of citrus leaves. Additionally, it causes an excessive concentration of sodium or chloride in citrus leaves. Increasing irrigation periodicity, fertilization and fertigation, Plant breeding, and genetic manipulations are among the methods of managing salinity stress in citrus fruits. In the presence of an adequate concentration of calcium in saline irrigation water, calcium improved the effects of saline on the plant's growth. Therefore, the plants could tolerate the effects of high salinity concentration. The authors' practical experience has shown that abiotic stresses occur worldwide in almost all citrus-growing areas. This research has attempted to explain citrus fruit management and improvement strategies to withstand abiotic stresses in citrus fruit production.

## **Conflict of interest**

The authors have no conflict of interest with any person or institution.

*Abiotic Stresses Management in Citrus DOI: http://dx.doi.org/10.5772/intechopen.108337*
