Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 225

Because mycorrhizae are compartmentalised biological systems, they are in-
fluenced by the effects of the environment and countless edaphic factors of each
component that directly or indirectly regulate the formation, operation, and occur-
rence of AMs. The components and controlling factors have constant and intense
interactions, such that a change in any of these factors influences the occurrence of
mycorrhizae and AMF propagules.
Establishment of AMF in Extreme Temperature Conditions
Arbuscular mycorrhizal fungi (AMF) may respond to high temperature conditions
by changing their morphology, modifying their external environment, or adapting
their internal metabolism, although the degree of phenotypic plasticity might vary.
Because AMF obtain carbon from autotrophic host plants, fungi can also be ex-
posed to stress through changes in carbon allocation from the host plant. Results
obtained from the refinement and application of molecular identification methods
in recent years has revealed that the degree of host specificity by some mycorrhi-
zal fungi might be greater than expected. This result implies that the availability
of compatible roots influences the survival of the fungus and changes in species
composition in plant communities. Restricting the supply of assimilates from the
compatible host root could limit the growth of certain fungi in rehabilitation areas.
Therefore, in many situations, mycorrhizal colonisation appears to be more depen-
dent on the host plant than on the temperature (Hawkes et al. 2008 ), and normally
high temperatures such as 35 and 40 °C show no significant effects on mycorrhizal
development (Zhu et al. 2010 ).
There are few studies that examine in detail the factors that affect the survival
of specific AMF in their natural habitats. Instead, the effects of physical-chemical
factors, especially temperature, on plants are widely reported.
Few plant species survive under continuous temperatures above 45 °C. Both
photosynthesis and respiration are inhibited at supra-optimal temperatures. How-
ever, as the temperature increases, the photosynthetic rate decreases more rapidly
than the respiration.
The structure and stability of cell membranes are important during high tempera-
ture stress. The excessive fluidity of lipid membranes at elevated temperatures is
correlated with the loss of physiological function. In some species, the acclimatisa-
tion to high temperatures is associated with the increased saturation of fatty acids
in the lipids, which makes membranes less fluid. The strength of hydrogen bonds
and electrostatic interactions between the polar groups of the aqueous phase of the
membrane decreases, which results in a stronger association between integral pro-
teins of the membrane and its lipid phase. Thus, high temperatures modify the com-
position and structure of membranes, resulting in the loss of ions and the inhibition
of metabolic processes such as photosynthesis and respiration.
One aspect common to fungi and plants when subjected to high temperature
stress is the generation of reactive oxygen species. The uncontrolled accumulation