**2.2 Extreme temperature stress and its management and mitigation strategies**

Climate change has caused severe problems, especially global warming, which has increased the earth's average temperature by four degrees fahrenheit compared to the ice age. **Figure 5** shows the increase in the earth's temperature from 1884 to

#### **Figure 4.**

*Green and reticular polyethylene shades with one (A), two (B), and four (C) layers on page mandarin seedlings, from left to right, respectively [36].*

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

2021. Climate change and agriculture are interrelated activities and affect citrus as one of the largest fruit crops in the world. The growth and yield of citrus begin to change when the environmental temperature increases; the water required by the plant decreases [39]. Extreme temperature causes biochemical, morphological, physiological, and genetic changes in citrus. High temperatures in citrus growing regions can lead to significant leaf-to-air vapor pressure difference (D) and excessive leaf temperature in sun-exposed leaves. This high-temperature stress can diminish net CO2 assimilation, growth, fruit yield, and citrus quality. The critical stages of citrus phenology increase the abscission of reproductive structures and the dropping of young fruit [40]. In citrus, high temperatures cause the generation of reactive oxygen species (ROS), the decline in chlorophyll content, an accumulation of carotenoids, and delayed coloration [41]. Research conducted on Carrizo citrange and Cleopatra mandarin showed that heat increases the phenolic compounds, the composition of secondary metabolites, and the antioxidant capacity of the leaves [42].

The plant's nutritional requirements are higher during the flowering and fruiting of citrus, and trees produce and accumulate the most carbohydrates during the growth stage. Citrus trees have a lot of nutritional requirements during the flowering and fruiting period; therefore, in the growth stage, trees produce and accumulate carbohydrates more than other materials. If nutrient accumulation is insufficient, high temperature may lead to increased respiration and diminishing photosynthesis, which causes nutritional imbalance and thus exacerbates flower and fruit drops [43]. The high temperature increased the fruit drops of Nagami Kumquat [43] and Tosa Buntan pummelo (*Citrus grandis* (L.) Osbeck) trees [44].

Global warming, and, as a result, extreme temperature stress is a dangerous challenge for agricultural products, including citrus fruits, and requires efficient strategies to ensure the production of quality products. One of the effective strategies is kaolin particle film application (**Figure 6**). The kaolin application is a suitable strategy to deal with the increasing temperature of the environment caused by global warming. It can also significantly help to expand the growth of citrus fruits in areas

#### **Figure 5.**

*Global temperature changes from 1884 to 2021 (adapted from NASA/GISS).*

#### **Figure 6.**

*Kaolin clay particles application on page mandarin (Citrus reticulata) trees. The page mandarin trees were treated with 7% (A) and 5% (B) kaolin clay [36].*

that suffer from summer heat stress. The main effect of kaolin is to increase solar radiation reflection and reduce, consequently, the temperature of the leaves exposed to light in the hottest hours [29, 34]. By reflecting a part of the light irradiated to the plant, kaolin significantly reduces the temperature of the leaves. It also prevents the accumulation of Pro by increasing the potential and maintaining the relative water content of the leaves in the summer. As a result, it reduces the adverse effects of drought stress. The effects of lowering temperature are reducing water consumption, increasing chlorophyll content, maintaining the quantitative and qualitative characteristics of the product, and preventing bursting and sunburn of citrus fruit, which was found in research conducted on Balady mandarin (*Citrus reticulata* Blanco) [29] and grapefruit [28] have been evident. **Figure 6** shows page mandarin (*C. reticulata*) trees sprayed with kaolin.

In citrus-producing regions, net shading could ameliorate leaf water use efficiency, photosynthesis, and fruit quality, especially in citrus seedlings, where most leaves are exposed to sunlight. The advantages of using these shades include reducing sunlight radiation intensity, especially in hot seasons, soil water maintenance, reducing wind speed, reducing leaf temperature, and photo-inhibition [32, 45]. Growth improvement, increasing fruit yield and quality and net gas exchange, mitigating fruit dropping, and other advantages, such as protecting the plant against bird attacks, insects, rain, and high daily, and night temperatures [46]. Compost application can also reduce soil temperature, allowing better root growth and ultimately decomposition and adding significant organic matter to the soil [30, 47]. Some research were carried out on the effect of methyl jasmonate (MeJA) on citrus under high-temperature stress. Findings found that the JA, and JA-isoleucine accumulation, were induced in heat stress conditions, which can further counteract the harmful effects of extreme temperature stress by closing the stomata and reducing transpiration [21, 48, 49]. In addition, in citrus-growing regions, high temperatures will increase irrigation requirements. Therefore, the pressure on the underground aquifers increases the soil salinity. All these stress factors have undesirable results in citrus cultivation.
