**2. Introduction of abiotic stresses types in citrus**

Citrus species, the most consumed fruit products in the world, are mainly produced in coastal areas in several countries and Mediterranean areas. Production in these areas is affected by abiotic stresses, such as drought, extreme temperature,

salinity and sodicity, and others [12]. These stresses cause many reactions in the plant, including the destruction of the photosynthesis system and changes in gene expression and metabolic processes, such as increasing the synthesis of secondary metabolites and production and accumulation of reactive oxygen species (ROS) [13, 14]. It is necessary to entirely understand the reaction of citrus to abiotic stresses at different levels to apply strategies to increase stress tolerance. Plant metabolism is affected by environmental stresses, and studying at such a level is necessary for using stressreducing compounds. **Figure 3** summarizes the adverse effects of abiotic stresses and some physiological and biochemical responses of plants to these stresses, which we will describe below.

#### **2.1 Drought stress and its management and mitigation strategies**

Citrus growth and production are affected by various environmental agents, one of which is drought. Drought, or, in other words, limited access to water is considered one of the most limiting citrus growth and production factors with adverse effects. This stress negatively affects citrus production and will be progressively more intense in some areas due to global climate changes [15, 16]. Climate change event and, consequently, global warming has exacerbated the destructive effects of drought stress with various impacts on temperature and rainfall patterns in different areas [10]. In such circumstances, water scarcity and precipitation consider limiting factors for citrus productivity in some countries, especially in Mediterranean regions.

Drought causes physiological and biochemical changes in the citrus, associated with decreased osmotic potential on the cell surface [17, 18]. Citrus growth and

#### **Figure 3.**

*It summarizes the adverse effects of abiotic stresses and some physiological and biochemical responses of plants to these stresses. Though the consequences of various abiotic stresses are different, the physiological and biochemical responses seem approximately similar. It is noteworthy that the adaptive strategies of plants against abiotic stresses are analogous (the above-summarized information is extracted from the work of [14]).*

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

performance are decreased under drought-stress conditions by changing photosynthesis rates. Also, with the increase in the duration of drought stress, the amount of secondary metabolites changes. In this situation, osmotic pressure-regulating substances, such as proline (Pro), accumulate in the plant by spending much energy and reducing the osmotic potential. Research on lemon [19] and "Nagami" kumquat [20] leaves show a decrease in leaf water potential under drought stress conditions. Another physiological effect of drought stress in citrus is the reduction of leaf chlorophyll. In such conditions, various types of reactive oxygen species (ROS) are produced in photosystem II of photosynthesis. ROS produce compounds, such as malondialdehyde, which cause cell damage. With the accumulation of malondialdehyde, the permeability of the plasma membrane and, thus, ion leakage increases [10, 15]. The more significant influence of fruits, compared to other vegetative organs, in response to drought stress is an exciting object. Drought stress affects the quantity and quality of citrus fruit and causes a decrease in yield because available water is one of the crucial factors for increasing yield [21]. Also, in drought stress conditions, maintaining cell integration through osmotic regulation and then the accumulation of soluble solids in the fruit during the growth period increases the quality properties, such as soluble solids (SS) and titratable acidity (TA). The increase of these quality indicators in drought stress conditions has been reported in Salustiana orange fruit [22].

One of the adverse effects of drought stress in citrus is the fruit-cracking after drought stress, which is evident in citrus cultivars with navels, such as "Navel" and "Valencia" orange (*Citrus sinensis* (L.) Osbeck) and mandarin hybrids (*Citrus reticulata* Blanco). Cracked fruits are prone to decay and lose their ability to be stored. As a result, their economic value decreases. In order to reduce the bursting and the economic loss caused by it, the water required by the plant should be provided in different stages of growth [23]. Executing suitable agricultural practices is generally considered obligatory for overcoming the adverse effects of drought stress [24]. Today, advanced techniques are used in citrus production to increase the soil's water-holding capacity and improve the application of limited water resources. One of the new methods is using superabsorbent polymers (SAP) or hydrophilic polymer gels. These polymers can quickly absorb large amounts of water and gradually provide the water stored in their structure to the plant while drying the environment. In this way, the soil remains wet for a long time without rewatering [25, 26]. Modifying the citrus root system surrounded by these polymers will increase water retention and minerals in the plant growth environment, improve soil texture, increase water penetration and germination, and faster plant growth. Research conducted on Carrizo citrange and Cleopatra mandarin seedlings showed that the application of superabsorbent polymer increased water absorption and leaf chlorophyll content [25]. Superabsorbent polymers provide moisture to the soil and plants and reduce quality indicators of the fruit, such as soluble solids and titratable acidity. The results of research on sweet orange confirm these findings [27]. Another strategy proposed in recent years to deal with the harmful effects of drought stress and the high summer temperature is using antiperspirant and sunlightreflective compounds, such as kaolin. Kaolin is a natural clay with an aluminum phyllosilicate (Al2Si2O5 (OH)4) structure, which is chemically neutral in a wide range of pH changes and does not harm living organisms [28, 29].

High temperature causes drought stress in plants. So, net shading is another technique to improve tree water status and water use efficiency in water deficiency conditions [30, 31]. The most common shade used for this purpose is made of aluminum, polypropylene, polyethylene, and polyester, as well as thin or linen sacks woven together in the form of webs [32, 33]. These nets do not change the natural

composition of light; only the light spectrum passing through the net is changed. The benefits of using net shading on plants in drought stress conditions are improving plant water status and yield, delaying fruit maturation to ripen more fruit, and reducing photo-inhibition. In severe water stress conditions, net shade is effective in the long term [34, 35]. Net shading may also harm fruit growth capacity because it reduces the amount of light received by the plant.

Nevertheless, single or multi-layer shades with different light transmission percentages is used [31]. An example of polyethylene shade with layers and different light transmission percentages is presented in **Figure 4**. As mentioned in **Figure 4**, shading was carried out with commercial green and reticular polyethylene shade, which transmits about 70, 50, and 0% of incident light from left to right, respectively.

Other drought stress management methods include exogenous application of plant growth regulators (PGRs), such as abscisic acid (ABA), auxin, gibberellic acid, brassinolide, jasmonates, benzyl-adenine, salicylic acid (SA), and biostimulants. PGRs are nonnutritive organic compounds that control different phases of plant growth and development as secondary stress messengers and play an essential role in reducing these abiotic stress conditions. For example, a study on citrus has shown that jasmonic acid (JA) signaling pathways are effective in water stress. In citrumelo CPB 4475 (*Citrus paradisi* × *Poncirus trifoliate*), a hybrid citrus genotype used as a rootstock, it was found that the exogenous JA application can also effectively diminish the damage caused by drought to plants [37]. Also, rootstock breeding and biotechnological approaches can mitigate climate change effects and increase drought tolerance in citrus [33, 38].
