**4. Salinity stress**

Salinity stress is mostly caused by the high concentration of NaCl which induces abiotic stress in plants in irrigated and non-irrigated conditions. According to a global estimation, 20% of cultivated land and 50% of irrigated land is under salinity stress [33]. Salinity stress retards plant growth and productivity, mainly due to ion toxicity and osmotic stress. Thus, induced osmotic stress decreases stomata opening, reducing photosynthetic ability [34].

Besides limitation in photosynthetic ability, salinity stress causes degradation of enzymatic proteins in photosynthetic apparatus and chlorophyll degradation [34]. Furthermore, salinity stress causes secondary stress, in particular oxidative stress, mainly caused by ion toxicity and osmotic stress, which damage plant cells by excessive accumulation of Reactive Oxygen Species (ROS) [35]. ROS causes significant damage to proteins, nucleic acids, lipids, and photosynthetic pigments. As a result, antioxidant capacity and photosynthetic capacity are two important factors to consider in salinity stress studies [36].

Application of exogenous selenium (1 μM) alleviates inhibitory effects caused by salt stress. In an experiment, Jiang [36] studied different concentrations of Na2SeO3 (0, 1, 5 and 25 μM) on 15 days old maize plants. This study found that the application of 1 μM Se increases net photosynthetic rate, improves antioxidant defense mechanism and reduces chloroplast ultrastructure damage caused by NaCl.

## **5. Temperature stress**

At leaf temperatures greater than 38°C, maize plants demonstrated a drop in net photosynthesis (Pn), and the decrease was particularly severe when the temperature was increased suddenly rather than slowly [37]. The reduction in photosynthesis was not due to stomata closure, as the transpiration rate rise in response to the increase in temperature. An increase in temperature greater than 32.5°C decreased the activation state of rubisco, which guided to complete inactivation at 45°C [38]. With the increase in leaf temperature, the level of 3-hosphoglyceric acid decreased. Rubisco activation acclimatized with increased leaf temperature and the acclimation process was associated with the expression of new activase polypeptide. Crafts-Brander and Salvucci concluded that the primary constraint responsible for the decrease in net photosynthesis at a temperature greater than 30°C was the inactivation of rubisco [38].

Maize is sensitive to chilling injury (below 15°C) and shows less adaptation growing in low temperatures [39]. Miedema found that 36% of the imbibed seeds died when exposed to 4°C for 28 days [40]. Sugar and amino acid exudation at lower temperatures may be linked to cell membrane failure. Young seedlings died after six days at 1°C and 8 days at 2.5°C. After 3 days of cooling, the Golgi bodies and inner mitochondrial membrane were destroyed, the endoplasmic reticulum was decreased, and lipid bodies accumulated.

Maize leaves are most sensitive to chilling injury. Chilling injury induces premature leaf senescence [39]. The combined exposure of Maize leaves to low temperature (10°C) and high light decreases CO2 assimilation and leads to irreversible inhibition of photosynthesis [40].

Janda et al. [41] treated young maize seedlings grown in hydrophobic conditions with 0.5 mM salicylic acid, which protected plants in subsequent application of low-temperature stress. Salicylic acid pretreatment lowered catalase activity, which boosted antioxidant enzyme activity such as peroxidases and glutathione reductase, resulting in higher freezing tolerance in immature maize plants, according to Janda et al. [41]. Another research found a significant reduction in lipid peroxidation in Glycinebetaine (GB) cells compared to control during chilling [42]. This result implies that an increase in chilling tolerance may be caused by reducing lipid peroxidation of the cell membrane in the presence of GB.

Maize appears to have a harder time adjusting to low temperatures. This adaptation necessitates the capacity to germinate, grow, and mature at low temperatures and resistance to frost, chilling, and soil fungi during germination. Breeding for low-temperature adaptability has grown more essential as feed maize in northern areas has increased. Appropriate selection criteria are required for a sensible approach to this breeding task. As a result, we need to know first which plant characteristics limit maize output in a chilly climate, and second, how genetically variable those characteristics are.

## **5.1 Damage by low nonfreezing temperature**

Temperatures below and near the germination and growth minimums in Maize can induce various physiological problems. Chilling damage is the medical term for these low-temperature effects. Chilling is not to blame for all of the negative impacts of cold weather. At temperatures above the chilling range, for example, low-temperature chlorosis occurs.
