Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 219

the concentrations of P and N in the shoots of the plants. However, other studies
indicate that the maximum benefits to the plants could be achieved with a single
efficient AMF species and that increasing the mycorrhizal diversity would not re-
sult in greater benefits for the plants. According to Santos ( 2008 ), increasing AMF
diversity in the community present in the soil can increase the chances for the estab-
lishment of a fungus species that is more efficient for plant growth. Thus, it is im-
portant to characterise the community structure of AMF in a particular environment
or ecosystem and assess the functional diversity of these symbionts to establish
whether there is a relationship between the AMF diversity and benefits to plants.
Physiological Aspects of the Arbuscular Mycorrhizal
Association in the Plant Tolerance to Stress
The establishment of AMs confers the plants with a range of benefits, primarily
through the extraradicular mycelium of the fungus that facilitates the absorption
of nutrients from areas located beyond the depletion zone of the roots, particularly
phosphorus, and it increases the availability and translocation of nutrients to cortex
cells in the plant roots. Other relevant effects of AMF are increased plant resistance
to pathogens of the root system and water absorption capacity. In soil, they favour
aggregate formation and stability, not only through the physical action of the my-
celium but also through the action of glomalin. Through the enhancement of the
hydric and nutritional status of plants, AMF can contribute the increased tolerance
to environmental stress conditions.
Thus, the symbiosis between plants and AMF also results in the reduction of
losses by stresses (Munier-Lamy et al. 2007 ) and consequently faster growth, with
economic inputs and the reduction of environmental contamination (Huang et al.
2009 ). Furthermore, these fungi can act as potential biological control agents, re-
ducing the effects or damage caused by plant pathogens through indirect means,
enhanced plant nutrition, or increased resistance in the root system.
The plant response to colonisation by AMF depends on the severity and frequen-
cy of drought, and other soil conditions. AMF can affect the growth and productiv-
ity of the host plant under conditions of high and low humidity (Borowicz 2010 ).
Thus, symbiosis could increase plant responses to moderate water deficit through
various mechanisms, including increased water absorption from the soil through
the hyphae (Augé et al. 2003 ), alteration of hormonal levels causing changes in sto-
matal conductance (Augé et al. 2008 ), increased leaf turgor and osmotic potential
reduction (Wu et al. 2006 ), and improved nutrition of the host plant (Chen et al.
2005 ).
Mycorrhizal plants develop a root system uses carbon more efficiently. Conse-
quently, these plants convert larger quantities of photosynthates in the root develop-
ment to increase their absorption capacity (Neocleous and Vasilakakis 2007 ).
The chlorophyll concentration in the leaves is an important physiological in-
dex for determining the degree of photosynthesis in plants. AMF can increase the