**7. Cold acclimation**

In several species, the acquisition of freezing tolerance can be induced by exposure to low, nonfreezing, and non-injurious temperatures [124, 141]. Acclimation may be defined as changes that occur in a plant in response to chilling temperatures, which confer subsequent tolerance to the cold injury [113], especially during germination and early seedling growth [69]. Cold priming/acclimation is associated with multiple physiological and biochemical alterations, including membrane stabilization, increased ROS and methylglyoxal (MG) detoxifications, activation of cold-sensitive protein kinases, NO and hormone biosynthesis, and accumulation of antioxidants, HSPs, cold-regulated proteins (CORs), and dehydrins [141–149]. Cold acclimation makes plants capable of protecting themselves from freezing-induced injury [149, 150]. Gong *et al.* [69] reported that in maize seedlings, cold acclimation resulted in higher survival percentage, catalase, ascorbate peroxidase, superoxide dismutase activity, and lower electrolyte leakage, than in non-acclimated seedlings. Cold acclimation in *Zoysia spp.* resulted in higher ABA and H2O2 levels as well as regulated antioxidant metabolism, resulting in improved freezing tolerance [151]. Cold priming-induced proline and glycine betaine accumulations were found to be associated with freezing tolerance in barley and *Arundo donax* L. [152]. Cold acclimation in *Arabidopsis thaliana* L. induced the accumulation of endogenous NO, and increased proline levels, conferring freezing tolerance [54]. Cold acclimation also enhanced the expression of genes that play role in membrane stabilization against freeze-induced damage [153, 154]. Minami *et al.* [155] verified that plasma membrane subfractions, responded to cold, by considerably changing lipid and protein composition in Arabidopsis plants. The study indicated that the plasma membrane is restructured in order to resist different stresses that take place throughout a freeze–thaw cycle. Cold acclimation increased the abundance of ROS-scavenging proteins, LEA/ RAB proteins, and dehydrins in diploid wild wheat (*Triticum urartu* L.) [134, 156]. Studies have suggested that the activity of cold/chilling-induced genes may facilitate the metabolic changes that confer LT tolerance [156, 157]. Cold acclimation causes the synthesis of protective molecules, such as soluble sugars, sugar alcohols, proline,

#### *Perspective Chapter: Effect of Low-Temperature Stress on Plant Performance and Adaptation… DOI: http://dx.doi.org/10.5772/intechopen.110168*

and glycine betaine [106]. These molecules in conjunction with various proteins play a role to stabilize both phospholipids and proteins of the membranes and proteins of cytoplasm, maintain hydrophobic interactions between molecules and scavenge various types of ROS, which are produced under LT [158]. Some plants respond to LT by the synthesis of some specific proteins that are similar to plant pathogen-related (PR) proteins (particularly in winter rye), in response to cold and drought [159].

Kim *et al.* [23] reported that 14 days of LT stress killed most of the IR50 (sensitive) rice seedlings, while no negative effect was observed in M202 (tolerant) seedlings. Morsy *et al.* [66] showed that cold-tolerant seedlings of rice had 100% survival at 13/10°C regime in comparison to cold-sensitive seedlings, which suffered 50% mortality under the same conditions. Gong *et al.* [69] also reported that the percentage survival of maize seedlings increased after the pretreatment of seedlings at 1°C. Kargiotidou *et al.* [160] reported that the percentage survival of cotton is enhanced if plants are acclimated at low and nonfreezing temperatures prior to cold stress. Many studies in the literature have reported that LT stress affected seedling growth parameters viz. germination, and seedling growth, and caused chlorosis, wilting of leaves, reduced leaf expansion, and necrosis of tissue [12–14]. Low-temperature stress in *Elymus nutans* Griseb decreased shoot lengths of tolerant (DX) and sensitive (GN) genotypes by 88.8 and 91.7%, respectively compared to controls [15]. Jan *et al.* [161] showed that under cold stress one variety of rice SB showed no change in shoot length, while B-385 showed a slight decrease in average shoot length. Both the varieties showed some increase in root length under cold stress. Razmi *et al.* [162] reported that in sorghum (*Sorghum bicolor* L.) genotypes, LT reduced the germination percentage, root length, and shoot length of the seedlings, whereas increased the root/shoot ratio. Increased RL/SL under LT might be an indication of water deficit stress due to cold stress.

LT stress increased the chlorophyllase enzyme activity in grapevine leaves and restrained the synthesis of total Chl [163]. Plants need to maintain a sufficient level of Chl a content to perform the photosynthesis process to some extent even under stress [17]. Yadegari *et al.* [83] reported that under LT stress at 5°C in soybean seedlings Chl a, b, and total Chl contents decreased. However, Yang *et al.* [17] reported that in bitter gourd genotypes, Chl a content increased, whereas total Chl and Chl b contents decreased under cold stress. Our results are supported by the work of Esra *et al*. [82] who reported that in two pepper varieties (*Capsicum annum* L.), Mert and KM-121, the content of Chl a and total Chl significantly decreased, while no significant change was found in the content of Chl b in response to LT stress. Tang *et al.* [20] also reported that under low LT stress in different sugarcane genotypes total Chl content decreased. Carotenoids are not considered photosynthetic pigments, but they play an important role in the protection of the photosystems and accumulate under LT. Carotenoids act as natural antioxidants by quenching triplet Chl and singlet oxygen species, which are potentially harmful to the chloroplast [24, 25]. Fu *et al.* [15] reported that under LT stress at 5°C in *Elymus nutans* seedlings, carotenoid content decreased. The decrease in carotenoid content was higher in GN (more sensitive) than DX (tolerant) seedlings. Gerganova *et al.* [164] reported that in tomato plants after cold treatment, a pronounced decrease was observed in carotenoids. Yadegari *et al.* [83] reported that Chl a, b, and total Chl decreased in both acclimated and nonacclimated soybean seedlings, but in cold-acclimated leaves, this decrease was lesser than in non-acclimated seedlings. It is well documented that photosynthetic apparatus is sensitive to several environmental stresses and PS II appears to be preferentially affected by chilling stress [165]. Fv/Fm reflects the susceptibility to damage of the photosystem II (PSII). Yang *et al.* [17] reported that in two bitter gourd genotype

seedling leaves, the Fv/Fm ratio was hardly affected by cold stress, suggesting that LT did not affect the efficiency of PSII. Grapevine seedlings grown under LT stress showed different effects on photosynthetic efficiency [166]. Tang *et al.* [20], however, reported that LT stress in different sugarcane genotypes affected Fv/Fm ratio, which decreased with temperature and stress period. Decreased Fv/Fm ratio indicated that the photosystem was affected under LT stress in sugarcanes. Many researchers pointed out that there was a significant decrease in Fv/Fm ratio under dark chilling stress and after the recovery period the values recovered to control levels [21, 22]. Mishra *et al.* [167] reported that Fv/Fm decreased in both acclimated and non-acclimated samples of *Arabadopsis thaliana*, but the decrease was more in sensitive than tolerant types. Hajiboland and Habibi [64] reported that cold and acclimation both did not affect the Fv/Fm of winter wheat "Sabalan" while causing a significant reduction of *Fv/Fm* in "Zagros" spring wheat. Khaledian *et al.* [37] reported an accumulation of H2O2 under cold stress in the leaves of chickpea plants.

Yang *et al.* [17] also reported higher electrolyte leakage for the sensitive bitter gourd genotype (Y-106-5) than the less sensitive one (Z-1-4). Liu *et al.* [42] reported an increase in electrolyte leakage in leaves of *Avena nuda* L. (naked oats) seedlings with cold stress and with prolongation of the stress period. LT stress leads to the destruction of cell membrane structure in maize plants [45], which caused increased permeability of membranes, and increased leakage of cell electrolytes and thus causing damage to plants. Electrolyte leakage was significantly (CD at 5%) lower in acclimated seedlings under LT stress, which probably suggested that membranes of acclimated seedlings were less affected under LT stress. Gong *et al.* [69] reported that electrolyte leakage from root tips of non-acclimated maize seedlings significantly increased after exposure to chilling stress, while cold shock pre-treatment remarkably reduced the leakage of electrolytes under chilling stress as compared to non-acclimated. Aaron *et al.* [168] reported that cold acclimation enhanced the freezing tolerance in *Petunia hybrida* and decreased the EL50 value. In conclusion, acclimation prior to LT stress results in the enhanced tolerance of plants. Different types of molecules accumulate under stress conditions, which are used as a potential acclimatizing agent for plants in the form of a spray.
