**5.2 Drought stress**

Drought is another frequent abiotic factor that negatively influences plant growth, survival, and development [40]. The main symptoms of drought stress include wilting as well as a decline in the net rate of photosynthesis, transpiration rate, water usage effectiveness, relative moisture content, and overall chlorophyll content [41]. Also, drought compromises the electron transport system, which results in the generation of activated oxygen and the shutting of plant stomata, consequently reducing CO2 absorption [42].

AMF play the following crucial functions in the response to drought stress; several studies have demonstrated how crucial plant symbiosis is for reducing the detrimental effects of drought:


It was reported that cassava growth could be stimulated under drought stress by inoculating with an AMF. For instance, according to the experiment by Ekanayake et al. [48] the inoculation with *G. clarum*, and *G. mosseae* significantly increased the *Recent Advances in Plant: Arbuscular Mycorrhizal Fungi Associations and Their Application… DOI: http://dx.doi.org/10.5772/intechopen.108100*

photosynthetic photochemical efficiency of the photosystem 11 light reactions in cassava. The PS11 photochemistry's maximal quantum yield (Fv/Fm) was similarly significantly decreased (p 0.05) by water stress, whereas AMF inoculation significantly (p 0.05) decreased the negative effects of water stress on cassava cultivars grown under a water deficit regime.

Similarly, Pea et al. [49] investigated whether intraspecific variation in *R. irregularis* affects the physiological responses of cassava to water stress because it frequently experiences seasonal drought where it is cultivated. Two genetically distinct *R. irregularis* isolates were inoculated into cassava to promote recovery, which was then subjected to a drought situation before being re-watered. Cassava samples treated with the two distinct fungi had considerably different physiological stress reactions to drought. However, after being re-watered, both plants recovered but at different rates. They concluded that intraspecific genetic variation in AMF considerably affects the physiological reactions of cassava under water stress. This shows the opportunity to increase cassava resistance to water stress by utilizing naturally occurring polymorphism in AMF [49].

#### **5.3 Heavy metal stress**

For several enzyme-catalyzed or redox reactions, as well as the metabolism of nucleic acids, plants require specific mineral elements, such as copper (Cu), iron (Fe), nickel (Ni), and zinc (Zn) amongst others [50]. However, high concentrations of these heavy metals, can alter the protein structure or cause mutations in the plant's genetic makeup [51]. This may cause indications of deficiency such as chlorosis, diminished germination, and slow growth. Diffusion of these heavy metals into the plant's roots after a groundwater and soil surface deposition is mostly caused by anthropogenic activities including fertilizer application and the diffusion of industrial waste, which pose a significant stressor for these plants [50, 51].

However, AMF had been reported in supporting plants in heavy metal-contaminated environments using these mechanisms.


Recently, the benefits of AMF as reviewed by Riaz et al. [52], highlighted their potential as plant-based remediation techniques for extremely heavy metal-contaminated soils [52]. Additionally, according to Dushenkove et al. [54], certain plants, like cassava, can remove heavy metals from soils that have been contaminated by crude oil by using high biomass crops in tandem with a system of soil amendments using the rhizofiltration approach. Rhizofiltration is a technique that uses plants to remove toxins from aqueous streams [54].

For the first time, the capacity of cassava to phytoextract mercury (Hg) and gold (Au) from biosolids and mine tailings that contain these metals was successfully proven [55]. Pre-rooted cassava cuttings with 5–7 nodes were grown in a blend of 25% mine tailings and 75% biosolids. Plant cuttings were additionally grown in

hydroponics samples containing Hg and/or Au to gauge the two metals' root uptake. Up to 12.59 g/kg of Hg and 1.89 g/kg of Au were discovered in the cassava's fibrous roots. Hence, due to the cultivation simplicity, cassava provides a sustainable choice for Hg removal and Au recuperation.
