Arbuscular Mycorrhizal Colonization Promotes the Tolerance to Salt Stress in Lettuce Plants through an Efficient Modification of Ionic Balance

Soil salinity is the biggest problem which hinders the productivity of agricultural crops, causing adverse effects on plant growth and development. In this regard, it has been shown that the arbuscular mycorrhizal fungi (AMF) can establish a symbiosis with most agricultural plants improving water and nutrient absorption under salinity stress conditions. The functional contribution of AMF strains (Claroideoglomus claroideum (Cc) and a native consortium of AMF (HM) isolated from saline soils) on the growth and nutrition of lettuce plants (Lactuca sativa var. longifolia) was evaluated under increasing salt stress conditions (0, 40, and 80 mM NaCl). At 60 days of growth, biomass production, nutrient content (N, P), ions (Ca²⁺, Mg²⁺, Na⁺, K⁺), chlorophyll, proline content, and AMF propagules were evaluated. The highest growth was observed in plants inoculated with Cc, which produced a higher percentage of root colonization and hyphal length at all levels of salinity, compared to plants inoculated with HM or non-inoculated plants. These results were directly related to higher biomass production, increased synthesis of proline, increased N uptake, and noticeable changes in ionic relations, based in a diminishing Na⁺, compared to non-mycorrhizal plants. Our results suggest that this improved ionic balance is due to a filtering effect of AMF structures both in the soil and in the root that prevents the entry of toxic Na⁺ ions, which is important due to the level of lettuce production on saline soils improving the crop by means of directed inoculation with efficient AMF strains.