Host Response to Osmotic Stresses: Stomatal Behaviour and Water Use Efficiency of Arbuscular Mycorrhizal Plants

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Arbuscular mycorrhizal (AM) symbiosis can protect the host plants against the detrimental effects of the water deficit caused by osmotic stresses such as drought and salinity. Stomatal conductance (gs) and water use efficiency (WUE) are among the most studied water relations parameters in the mycorrhizal literature, since they are considered critical to the long-term performance of host plants in semiarid environments. Mycorrhizal effects on gs have been observed in about 50% of experiments involving AM and nonAM plants of similar size. In fact, gs rates usually are higher in AM than in nonAM plants, which implies that AM plants have a lower resistance to vapour transfer from inside the leaves to the atmosphere. AM and nonAM plants have also shown different critical points or thresholds of stomatal behaviour during drought episodes. The higher gs rates in AM plants have been associated with lower xylem-sap abscisic acid (ABA) and lower ABA fluxes to leaves in AM plants. On the other hand, it has been suggested that extraradical hyphae or increased root branching may allow mycorrhizal roots to better explore a particular soil volume, extending soil water depletion zones and giving a mycorrhizal root system more access to available water. In addition, it has been estimated that about half of the promotion of gs by AM fungi can be attributable to the soil colonization by AM fungi. Nevertheless, these results can vary when the host plant shows a water conservative strategy. Moreover, different AM fungal species have been shown to modulate also differently the physiological response, including gs, of host plant to drought. The AM influence on gs can also be modulated by environmental conditions such as irradiance, air temperature or leaf temperature. There are also several reports in the literature showing an increase of plant WUE by the AM symbiosis either under well watered or under osmotic stress conditions. The effects of AM symbiosis on WUE depend on the fungal species involved, without a correlation with the percentage of root infection. These effects have been rather related to higher net photosynthetic rate and optimal quantum yield of photosystem II in AM plants than in nonAM ones and with enhanced activities of carbon assimilatory enzymes such as Rubisco. In any case, specific studies dealing with the effect of AM symbiosis on leaf morphology are needed in order to ascertain how these parameters influence the WUE of the host plant.