Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 220

chlorophyll concentration in the leaves. Indeed, mycorrhizal plants growing under
stress conditions possess greener leaves, suggesting that salt interferes with the syn-
thesis of chlorophyll (Colla et al. 2008 ). Mycorrhizal inoculation also increases the
absorption of phosphorus and magnesium and reduces the sodium content in the
plant, which in turn increases the chlorophyll content and consequently improves
the overall performance of mycorrhizal plants under stress conditions (Sheng et al.
2008 ).
Plants associated with AMF often have greater resistance to saline stress, per-
haps with greater consistency than the stress due to drought. Salinity negatively
affects the formation and functioning of the mycorrhizal symbiosis (Sheng et al.
2008 ). Studies indicate that AMF can increase plant growth and nutrient absorption,
reduce losses in productivity under salinity conditions and improve the tolerance to
salinity (Hajiboland et al. 2010 ). The colonisation of plant roots by some AMF is
reduced in the presence of NaCl (Giri et al. 2007 ), potentially due to the direct effect
of NaCl on the fungi (Juniper and Abbott 2006 ), indicating that salinity can inhibit
the formation of mycorrhiza (Sheng et al. 2008 ).
Many researchers have reported that AMF increases the ability of plants to ad-
dress saline stress (Jahromi et al. 2008 ) because of the enhanced absorption of
nutrients by the plants (Asghari et al. 2005 ) and the ionic equilibrium (Giri et al.
2007 ), which protects enzymatic activity (Rabie and Almadini 2005 ), and facilita-
tion of water absorption. However, there are few studies concerning the influence of
mycorrhizal inoculation on photosynthesis and water relations during saline stress.
Some reports indicate that mycorrhizal colonisation can improve the relative water
content in squash leaves (Colla et al. 2008 ), hydric potential and photosynthesis
of maize plants (Sheng et al. 2008 ), and chlorophyll concentration in the leaves of
various plant species (Sannazzaro et al. 2006 ; Colla et al. 2008 ).
Recent findings suggest that glomalin might indirectly influence the storage of
carbon in the soil through the stabilisation of soil aggregates (Zhu and Miller 2003 )
and soil stability. The stability of soil aggregates is one of the most important prop-
erties to control plant growth in arid and semi-arid environments through the con-
trol of the soil-plant hydric status.
The establishment of mycorrhizal associations results in increased tolerance of
plants to environmental stresses (Tang et al. 2009 ). However, little is known about
the physiological and molecular mechanisms responsible for this greater tolerance.
Increased activity and induction of new isoenzymes that participate in the anti-
oxidant system in inoculated plants allow the plant to tolerate excess superoxide
radicals generated during the prevalence of stress conditions.
Salinity induces oxidative stress in plants (Hajiboland and Joudmand 2009 ).
Plant cells contain an array of protection mechanisms and repair systems that can
minimise the occurrence of oxidative damage caused by reactive oxygen species
(ROS) (Abdel Latef and Chaoxing 2011 ). The induction of enzymes that eliminate
ROS such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and
ascorbate peroxidase (APX) is the most common mechanism for detoxifying the
ROS synthesised during a stress response.