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An in vitro tripartite culture system was used to monitor the effect of arbuscular mycorrhizal (AM) symbiosis on host plant water relations. We hypothesized that the root colonization by Glomus intraradices Schenck and Smith modifies the water's status, the control of water losses and the osmotic relations of leaves and roots of micropropagated strawberry (Fragaria ananassa Duch. cv. Kent) plantlets, under in vitro conditions of high humidity. Strawberry plantlets, AM or non-AM, disposed in a randomized complete block design, were assessed in order to measure relative water content (RWC of leaf discs, roots and complete plants, leaf stomatal conductance (Gs), leaf and root osmotic potentials (Psi(pi L), Psi(pi R)), chlorophyll fluorescence parameters Fv/Fm, fresh, dry and turgid weights and mineral content. Although Psi(pi L), Psi(pi R), Gs and RWC of foliar discs were not affected by the fungal inoculation, the RWC of the complete plants was 11% higher in AM colonized plantlets compared to cion-AM ones. This significant increase of RWC of whole plants was not related to an improved mineral nutrition nor growth stimulation of mycorrhizal plantlets. This is the first report that AM colonization enhances the RWC of entire strawberry microplants under axenic well-watered conditions. We have concluded that this effect is related to a higher water content in AM root systems not paralleled to a dilution in the root osmotic potential, Psi(pi R), suggesting that mycorrhizal roots must have a higher concentration of water soluble compounds or a different distribution in the cell compartments than non-AM roots.
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