3.1 Diverse abiotic stresses and their response toward medicinal plants and the defense mechanisms adopted by plants.
Another type of stress called heat shock, which is normally brought on by an increase by 10–15°C above ambient temperature also influences plant growth and metabolism. On the other hand, the highest photosynthetic rate was determined at 30°C. Besides other symptoms, the prominent symptoms are inhibition in photosynthesis at high-temperature stress (Camejo et al. 2005). Temperature stress also affects several changes at the cellular level and metabolism. High and low temperatures incite the biochemical and physiological changes in plants. These stresses lead to lipid peroxidation and membrane disruption. Moreover, chlorophyll and protein degradation were also recorded due to high-temperature stress. High-temperature stress showed a negative effect on photosynthesis as well as reduced various enzyme activities in rice (Mohammad and Tarpley 2009). Moreover, RUBISCO activity was also reduced due to the inhibition of photosynthetic activity in Festuca arundinacea under high-temperature stress. High-temperature stress ranges between 25°C and 40°C encouraged not only the activities of reactive oxidative species (ROS) scavenging enzymes but also increased the activity of lipoxygenase and the content of 3,4-methylenedioxyamphetamine (MDA) and cysteine as well as that of protein and non-protein thiol in the leaf and root of Phalaenopsis (Ali et al. 2005). Low temperature also acts as a growth-limiting factor to limit the productivity and distribution of crop plants. When a plant is subjected to low temperature, ROS produced repeatedly and may disrupt normal metabolism through oxidative damage to lipids, nucleic acid, and protein. Low-temperature stress affects the physiological and biochemical characteristics of Crofton weed (Eupatorium adenophorum) (Ma et al. 2020). As a result, MDA, total soluble protein contents, soluble sugar, chlorophyll changes, and fluctuation in SOD activity increase (Li et al. 2008). Exposure of sweet potato (Ipomoea batatas L.) to chilling stress reduced transpiration rate and stomatal conductance while enhancing peroxidase activity and electrolyte leakage (Islam et al. 2009).
3.3.1 Effect of Plant Genotype on Rhizosphere Microbiome Assembly
A large number of bacteria associated with the rhizosphere of plants produce phyto-therapeutic compounds that increase the growth of medicinal plants. Rhizosphere microbes of Matricaria chamomilla L., and Calendula officinalis L growing naturally in desert ecosystem showed abundant occurrence of bacteria (gram positive), which is very important for pathogens suppression. A large difference in endophytic bacterial diversity in medicinal plants, i.e., Lamiophlomis rotate, Codonopsis pilosula, and Ephedra sinica was recorded by Li et al. (2008). In contrast, different varieties of microbial groups are present in the soil. About 50 strains of 47 genera Stigmatella (1), Archangium (8), Myxococcus (18), Corallococcus (11), Cystobacter (7), Pyxidicoccus (1), with dominant genera of Corallococcus and Myxococcus. The incessant cropping of Rahmannia glutinossa decreases the productivity of some medicinal plants due to plant–microbe interaction activity in the rhizosphere (Qi et al. 2009). Different medicinal plant–microbe interactions in the rhizosphere is presented in Table 3.2.
Table 3.2 Rhizosphere Microbes Interactions in Medicinal Plants.
| Plant species | Microorganism | References |
|---|---|---|
| Angelica sinensis | Terriglobus saanensis, Mucilaginibacter polysacchareus, Mucilaginibacter myungsuensis, Mucilaginibacter ximonensis | Whang et al. (2014) |
| Matricaria chamomilla, Calendula officinal, Solanum distichum | Bacillus sp. | Koeberl et al. (2013) |
| Rumex patientia | Proteobacterium Bacteroidetes, Acidobacteria, Gemmatimonadetes, Verrucomicrobia, Planctomycetes, Actinobacteria, Firmicutes, Chloroflexi | Qi et al. (2012) |
| Atractylodes lancea | Gram-negative bacteria | Dai et al. (2013) |
| Origanum vulgare | Pseudomonas, Stenotrophomonas | Bafana (2013) |
| Typhonium giganteum | Kribbella flavida, Kribbella karoonensis, Kribbella alba | Xu et al. (2012) |
| Ginseng plants | Actinomycetes | Zhang et al. (2013) |
| Ajuga bracteosa | Pseudomonas | Kumar et al. (2012) |
| Nerium indicum | Pontibacter | Kumar et al. (2012) |
| Nerium indicum | Pontibacter | Raichand et al. (2011) |
| Fritillaria thunbergii | Proteobacteria Acidobacteria Actinobacteria Bacteroidetes | Shi et al. (2011) |
| Astragalus membranaceus | Geodermatophilus obscurus | Zhang et al. (2011) |
| Phytolacca acinose | Aspergillus fumigatus | Guo et al. (2010) |
| Agathosma betulina | Cryptococcus laurentii | Cloete et al. (2010) |
| Ocimum sanctum, Coleus forskohlii, Catharanthus roseus, Aloe vera | Azospirillum, Azotobacter, Pseudomonas | Karthikeyan et al. (2008) |
| Annona squamosa, Eclipta alba, Cassia auriculata | Bacillus, Pseudomonas, Enterobacter, Corynebacterium, Micrococcus, Serratia | Tamilarasi et al. (2008) |
3.4 Microbe-Mediated Mitigation of Abiotic Stresses
Crop plants need to manage external environment pressure exerted by edaphic factors with intrinsic biological mechanisms, as a result of which growth, development, and productivity fail. Microorganisms are the natural populations of complex environments that have various metabolic
