**5. Defensive mechanisms of plants against abiotic stresses**

In response to these abiotic stresses, plant shows defensive mechanisms mainly by three ways: stress escape, stress avoidance and stress tolerance. Short growth period, seed dormancy, shedding of leaves etc. are some common escape mechanisms against abiotic stresses. Leaf rolling, reduced growth, reallocation of nutrients etc. are some common abiotic stresses avoidance strategies. In case of stress tolerance, certain cellular, physiological, biochemical and molecular changes such as osmotic adjustment, stiff cell wall, activation of stress tolerance compounds, metabolites and enzymes, expression of stress tolerance genes etc. occur inside the plant. For example, when photosynthesis is restricted, plant utilizes starch as energy source and in response to starch breakdown, synthesis of sugars, osmoprotectants, compatible solutes etc. occurs which helps the plant to tolerate stress [64]. Over expression of glutamine synthase, asparagine synthase genes etc. leads to tolerance of abiotic stresses by the plant. Besides, there are some detoxifying genes which alleviate various abiotic stresses through activating enzymes such as ascorbate peroxidase, glutathione peroxidase, glutathione reductase etc. Under abiotic stresses, abscisic acid (ABA) is produced inside the plants and this phytohormone helps the plant to cope up with abiotic stresses like drought, salinity, low temperature etc. Polyamines (low molecular weight aliphatic nitrogen compounds) also play important role in alleviating abiotic stresses through maintaining cell membrane integrity, reducing growth inhibitors, moderately expressing stress responsive genes and elevating antioxidant enzymatic activities. Endogenous polyamines can be increased by applying exogenous polyamines (putrescine, spermidine and spermine). Trace elements like selenium, silicon etc. and signaling molecules like nitric oxide also play positive roles in defending abiotic stresses.

#### **5.1 Water stress**

*Drought*: Plant reduces leaf area and number or modifies leaf into spine or other forms and leaf rolling to reduce transpiration loss of water. Stomata is closed and

leaf abscission and curling are seen under drought stress. Cuticle or wax deposition over leaf surface also restricts loss of water through transpiration. When water is limited in soil, plant's root is extended deeper into the soil in search of moisture, but shoot growth becomes limited (through reduction of growth promoters). Plant also mature comparatively earlier than normal when exposed to drought stress. Secondary metabolites are produced under drought stress, which further improve immunity of plant. Under drought condition, synthesis of ABA in roots and its transport to shoots through xylem regulates the stomata and thereby, controls transpiration loss of water. Further, reallocation of nutrients present in older leaves occurs. Accumulation of proline (osmoprotectant), arginine (compatible solute) etc. under drought (and/ or salinity stress) controls osmotic pressure and thus, helps in stress tolerance. Late embryogenesis abundant proteins are abundantly synthesized under drought stress during early embryogenesis to combat stress through improving water binding ability.

*Flood*: In plant like rice, adaption mechanism against flood mainly includes formation of gas filled spaces (aerenchyma) which can transfer oxygen from aerial parts to root zone of the plant.

### **5.2 Salinity**

Accumulation ABA under high salinity regulates stomatal opening and plays key role in maintaining leaf water potential. Further, there is a production of low molecular weight organic compounds (polyols: sorbitol, mannitol, glycerol, inositol and other forms of mono and dimethylated inositol), amino acids (proline, glutamate) and betaine (betaine glycine and alanine) inside the plant as defensive mechanism to alleviate salinity stress [65]. Plant detoxifies oxidative stress enzymatically or non-enzymatically. In enzymatic system, super oxide dismutase (SOD), catalase (CAT), peroxidase (guaiacol peroxidase, glutathione peroxidase and ascorbate peroxidase), monodehydroascorbate reductase (MDHAR), glutathione reductase (GR), glutathione transferase (GST), dehydroascorbate reductase (DHAR) etc. are produced which act as ROS scavengers. In non-enzymatic system, proline, glutathione, ascorbic acid, carotenoids, flavonoids, tocopherols etc. are synthesized which neutralize ROS.

#### **5.3 Temperature stress**

*Heat stress*: Plants adapt heat stress by changing leaf orientation, early maturation, rolling of leaf, transpiration cooling and synthesis of antioxidants and/or osmoprotectants or stress proteins. Elevation in transcription of genes (heat shock proteins) under heat stress is one of the common plant responses against heat stress.

*Cold stress*: Plant adapts cold stress through acclimation. Accumulation of ABA under cold stress plays key role in plant's adaptation to it. Plant enhances the level of unsaturated lipids which in turn improve fluidity and membrane stability and thereby, defend cold stress.

#### **5.4 Heavy metal toxicity**

Plant secretes various organic acids (oxalic acid, citric acid, tartaric acid, malic acid, succinic acid) under heavy metal toxicity and these acids chelates with heavy metals resulting in conversion of toxic to non-toxic elements. Metallothionein, Phytochelatins, glutathione etc. are synthesized by the plant and they act as chelates against heavy metals. Plant root also release amino acids which provide nutrition to fungi, bacteria and others. These microorganisms inhibit the uptake of heavy metals by the plant.
